evolução tectônica de um fragmento do cráton são francisco ... · contribuiÇÕes Às...

Evolução tectônica de um fragmento do Cráton São Francisco

Meridional com base em aspectos estruturais, geoquímicos (rocha

total) e geocronológicos (Rb-Sr, Sm-Nd, Ar-Ar, U-Pb).

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FUNDAÇÃO UNIVERSIDADE FEDERAL DE OURO PRETO

Reitor

Dirceu do Nascimento

Vice-Reitor

Marco Antônio Tourinho Furtado

Pró-Reitor de Pesquisa e Pós-Graduação

Newton Souza Gomes

ESCOLA DE MINAS

Diretor

Antônio Gomes de Araújo

Vice-Diretor

Antenor Barbosa

DEPARTAMENTO DE GEOLOGIA

Chefe

César Augusto Chicarino Varajão

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EVOLUÇÃO CRUSTAL E RECURSOS NATURAIS

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CONTRIBUIÇÕES ÀS CIÊNCIAS DA TERRA – VOL. 7

TESE DE DOUTORAMENTO Nº 009

Evolução tectônica de um fragmento do Cráton São Francisco

Meridional com base em aspectos estruturais, geoquímicos (rocha

total) e geocronológicos (Rb-Sr, Sm-Nd, Ar-Ar, U-Pb).

Arildo Henrique de Oliveira

Orientador Dr. Maurício Antônio Carneiro

Co-orientadores (University of Queensland) Dr. Keneth D. Collerson

Dr. Balz S. Kamber

Tese apresentada ao Programa de Pós-Graduação em Evolução Crustal e Recursos Naturais do

Departamento de Geologia da Escola de Minas da Universidade Federal de Ouro Preto como requisito

parcial à obtenção do Título de Doutor Ciência Naturais, Área de Concentração: Geologia Estrutural e

Recursos Naturais.

OURO PRETO

2004

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Universidade Federal de Ouro Preto – http://www.ufop.br

Escola de Minas - http://www.em.ufop.br Departamento de Geologia - http://www.degeo.ufop.br/ Programa de Pós-Graduação em Evolução Crustal e Recursos Naturais Campus Morro do Cruzeiro s/n - Bauxita 35.400-000 Ouro Preto, Minas Gerais Tel. (31) 3559-1600, Fax: (31) 3559-1606 e-mail: [email protected] Os direitos de tradução e reprodução reservados. Nenhuma parte desta publicação poderá ser gravada, armazenada em sistemas eletrônicos, fotocopiada ou reproduzida por meios mecânicos ou eletrônicos ou utilizada sem a observância das normas de direito autoral.

ISSN 85.230.0108-6

Depósito Legal na Biblioteca Nacional

Edição 1ª Evolução tectônica de um fragmento do Cráton São Francisco Meridional com base em aspectos estruturais,

geoquímicos (rocha total) e geocronológicos (Rb-Sr, Sm-Nd, Ar-Ar, U-Pb). Catalogação elaborada pela Biblioteca Prof. Luciano Jacques de Moraes do

Sistema de Bibliotecas e Informação - SISBIN - Universidade Federal de Ouro Preto

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Catalogação: [email protected]

O48e Oliveira, Arildo Henrique de. Evolução Tectônica de um Fragmento do Cráton São Francisco Meridional com base em aspectos Estruturais, Geoquímicos (rocha total) e geocronológicos (Rb- Sr, Sm-Nd, Ar-Ar, U-Pb)[manuscrito]. / Arildo Henrique de Oliveira.– 2004. xxii, 134f.: il. Color., grafs. , tabs; mapas – (Contribuições as Ciencias da Terra. Série D; v.7) ISSN: 85-230-0108-6 Orientador: Prof. Dr. Maurício Antônio Carneiro. Co-Orientadores: Prof. Dr. Keneth D. Collerson e Dr. Balz Kamber Área de concentração: Geologia Estrutural. Tectônica. Tese (Doutorado) – Universidade Federal de Ouro Preto. Escola de Minas. Departamento de Geologia. Programa de pós-graduação em Evolução Crustal e Recursos Naturais. 1.Geologia Estrutural - Teses. 2.Geoquímica - Teses. 3.Tempo geológico - Teses. Campo Belo (MG). I. Universidade Federal de Ouro Preto. Escola de Minas. Departamento de Geologia. Programa de pós-graduação em Evolução Crustal e Recursos Naturais. II. Título. CDU: 551(815.1)

mailto:[email protected]

Com todo carinho

dedico essa Tese

a minha e sempre querida Mãe Delcia,

à família

e a minha eterna Andreia

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Agradecimentos

Ao final desse trabalho que, perdurou por alguns anos, inúmera são as pessoas que fizeram parte dessa

caminhada e, de forma direta ou indireta contribuíram e ajudaram na superação dos obstáculos. Torna-

se difícil em citar nomes, pois são tantos que tenho receio de esquecer de alguns. Sendo assim, fica

aqui meu sincero agradecimento aos colegas, amigos, professores e funcionários tanto do

Departamento de Geologia da Universidade Federal de Ouro Preto, quanto do Department of Earth

Sciences da Universidade de Queensland/Austrália.

O meu agradecimento especial ao meu amigo e orientador Dr. Maurício Antônio Carneiro pelo apoio,

amizade e ajuda em várias etapas do desenvolvimento desse projeto.

Ao Co-orientador Professor Ken Collerson da Universidade de Queensland por abrir as portas de outra

Universidade e de outro país.

Ao Co-orientador Dr. Balz Kamber que foi fundamental na obtenção dos dados U-Pb e, em algumas

etapas de meu aprendizado na Universidade de Queensland.

Ao pessoal do ACQUIRE Centre (Alan Greig e Irina).

Especial agradecimento ao CNPq (Processo 200714-1/05) pela concessão da bolsa Sandwish por um

período de 18 meses e, pela assistência de seus funcionários nos momentos em que precisei.

Agradeço também a CAPES pela concessão de bolsa de Doutorado pelo período que estive no Brasil.

Como não seria justo continuar a citar nomes de uma grande lista e, incorrer no erro e deixando muitos

de fora, fica aqui meus sinceros agradecimetos a todos os amigos do Brasil e de Brisbane que direta ou

indiretamente deram sua contribuição.

Um grande abraço a todos e meu muitíssimo obrigado

Valeu....

OM SHIVA

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Sumário

AGRADECIMENTOS..........................................................................................................................ix

LISTA DE FIGURAS ..........................................................................................................................xv

LISTA DE TABELAS.........................................................................................................................xix

RESUMO .............................................................................................................................................xix

ABSTRACT .........................................................................................................................................xxi

CAPÍTULO 1. CONSIDERAÇÕES GERAIS.....................................................................................1

1.1. Introdução .........................................................................................................................................1

1.2. Objetivos da Tese ..............................................................................................................................3

1.3. Metodologia ......................................................................................................................................3

1.3.1. Revisão Bibliográfica 1........................................................................................................4

1.3.2 Revisão Bibliográfica 2.........................................................................................................4

1.3.3 Trabalho de Campo ..............................................................................................................4

1.3.4 Análise Petrográfica..............................................................................................................4

1.3.5 Preparação de Amostras........................................................................................................4

1.3.6 Geoquímica ...........................................................................................................................5

1.3.6.1 Metodologia ICP-OES.............................................................................................5

1.3.6.2 Metodologia ICP-MS...............................................................................................6

1.3.7 Geocronologia.......................................................................................................................7

1.3.7.1 Rb-Sr........................................................................................................................7

1.3.7.2 Sm-Nd......................................................................................................................8

1.3.7.3 Ar-Ar .......................................................................................................................9

1.3.7.4 U-Pb.........................................................................................................................9

1.3.8 Tratamento dos Dados ........................................................................................................10

1.3.9 Elaboração da Tese .............................................................................................................11

CAPÍTULO 2. CAMPO BELO METAMORPHIC COMPLEX: TECTONIC EVOLUTION OF AN ARCHAEAN SIALIC CRUST OF THE SOUTHERN SÃO FRANCISCO CRATON IN MINAS GERAIS (BRAZIL) ...............................................................................................................13 2.1. Abstract ...........................................................................................................................................13

2.2 Introduction ......................................................................................................................................13

2.3. Geologic Context ............................................................................................................................15

2.4. Geology of the area .........................................................................................................................16

2.4.1. Gneissic Units ....................................................................................................................16

2.4.1.1. Cláudio Gneissic Unit...........................................................................................18

2.4.1.2 Itapecerica Gneissic Unit ......................................................................................18

2.4.1.3 Candeias Gneissic Unit .........................................................................................19

2.4.2. Amphibolitic Unit ..............................................................................................................19

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2.4.3 Supracrustal Unit................................................................................................................ 19

2.4.4 Fissure Mafic Unit.............................................................................................................. 21

2.5 Metamorphism................................................................................................................................. 22

2.6 Structural Analysis .......................................................................................................................... 23

2.6.1 Foliations............................................................................................................................ 24

2.6.2 Shear bands ........................................................................................................................ 24

2.6.3 S/C Structures..................................................................................................................... 26

2.6.4 Folds................................................................................................................................... 27

2.6.5 Foliation Sigmoids ............................................................................................................. 27

2.6.6 Pegmatites .......................................................................................................................... 28

2.6.7 Quartz Veins....................................................................................................................... 28

2.6.8 Amphibolite Boudins ......................................................................................................... 28

2.6.9 Faults .................................................................................................................................. 28

2.7 Tectonic Evolution .......................................................................................................................... 29

2.7.1 Event 1 ............................................................................................................................... 29

2.7.2 Event 2 ............................................................................................................................... 29

2.7.3 Event 3 ............................................................................................................................... 30

2.7.4 Event 4 ............................................................................................................................... 31

2.7.5 Event 5 ............................................................................................................................... 31

2.8 Conclusions ..................................................................................................................................... 31

CAPÍTULO 3. ORIGIN OF CHARNOCKITES AND ENDERBITES INFERRED FROM INCOMPATIBLE ELEMENT LOSS DURING PROGRADE DEHYDRATION....................... 33 3.1 Abstract ........................................................................................................................................... 33

3.2 Introduction ..................................................................................................................................... 33

3.3 Brief Review of relevant geochemical research .............................................................................. 35

3.4 São Francisco Craton: Relevant Geologic Record .......................................................................... 36

3.5 Samples Selection............................................................................................................................ 38

3.6 Petrography Features ....................................................................................................................... 38

3.7 Analytical Methods ......................................................................................................................... 40

3.8 Results ............................................................................................................................................. 46

3.8.1 Similarities and differences from our rocks compared with typical Archaean

granitois from Barberton normalized by N-MOR....................................................................... 47

3.8.2 Deviations of expected trace element abundances ............................................................. 48

3.9 Discussion ....................................................................................................................................... 50

3.10 Summary ....................................................................................................................................... 55

CAPÍTULO 4 RECENT ADVANCES AMONG Sr AND Nd CONSTRAINTS CONCERNING TO THE TECTONIC EVOLUTION OF CAMPO BELO METAMORPHIC COMPLEX, SOUTHERN PORTION OF THE SÃO FRANCISCO CRATON, BRAZIL................................ 59 4.1 Introduction ..................................................................................................................................... 59

4.2 Geochronological overview............................................................................................................. 60

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4.3.Sample selection and analytical procedures.....................................................................................63

4.4. Results .............................................................................................................................................63

4.4.1 Outline of petrography features ..........................................................................................63

4.4.2 Rb/Sr sytem.........................................................................................................................69

4.4.3 Sm/Nd sytem.......................................................................................................................73

4.5 Discussion ........................................................................................................................................73

4.6 Summary ..........................................................................................................................................79

CAPÍTULO 5 IMPLICATIONS FOR TRANSAMAZONIAN AGE RELATED EFFECT 40Ar/39Ar FOR THE CAMPO BELO METAMORPHIC COMPLEX, SOUTHERN SAO FRANCISCO CRATON, BRAZIL ....................................................................................................81

5.1 Abstract ............................................................................................................................................81

5.2 Introduction ......................................................................................................................................81

5.3 Problems to be addressed .................................................................................................................83

5.4 Geological setting.............................................................................................................................84

5.5. Sample selection and analytical procedures....................................................................................85

5.6 Results ..............................................................................................................................................85

5.6.1 Petrograph features .............................................................................................................85

5.6.2 40Ar/39Ar System .................................................................................................................87

5.7 Discussion ........................................................................................................................................97

5.8 Summary ........................................................................................................................................104

CAPÍTULO 6 GEOCRONOLOGIA U-Pb......................................................................................107

6.1 Introdução ......................................................................................................................................107

6.2 Resultados U-Pb.............................................................................................................................108

6.2.1 Amostra AH 11 .................................................................................................................110

6.2.2 Amostra AH 14 .................................................................................................................112

6.2.3 Amostra AH 15 .................................................................................................................114

6.2.4 Amostra AH 07 .................................................................................................................117

6.2.5 Amostra AH 08 .................................................................................................................118

6.3 Sumário ..........................................................................................................................................120

CAPÍTULO 7 CONSIDERAÇÕES FINAIS ...................................................................................121

7.1 GENERALIDADES.......................................................................................................................121

7.2 QUESTÕES ACERCA DA EVOLUÇÃO TECTÔNICA DO CRÁTON SÃO FRANCISCO MERIDIONAL ....................................................................................................................................122

7.3 Discussão........................................................................................................................................123

7.4 Sumário ..........................................................................................................................................126

REFERÊNCIAS BIBLIOGRÁFICAS .............................................................................................127

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Lista de Figuras

CAPÍTULO 1

Figura 1.1 - (a) Mapa geológico da porção meridional do Cráton São Francisco (modificado de Machado Filho et al. 1983; (b) Mapa geológico simplificado da area estudada (modificado de Oliveira 1999, Oliveira & Carneiro 2001)............................................................................................................. 2

Figura 1.2 - Mapa esquemático dos zircões das 5 amostras selecionadas (a numeração das amostras selecionadas está em negrito) ................................................................................................................ 10

CAPÍTULO 2

Figura 2.1 - Geologic map of southern São Francisco Craton (modified from Machado Filho et al. 1983)...................................................................................................................................................... 14

Figure 2.2 – Geologic map of the studied area (modified from Oliveira 1999); ................................. 15

Figure 2.3 - Stereographic diagrams representing the polar projections of the foliations of the study area ........................................................................................................................................................ 25

Figure 2.4 - Kinematic indicators (A-G) and schematic models explaining one of the event active in the study area (H). ................................................................................................................................. 26

CAPÍTULO 3

Figure 3.1 - Panel (a) Photograph of massive charnockite with incipient foliation taken in Alemão dimension stone quarry (Candeias Unit). Panel (b) Photograph taken at Marilan dimension stone quarry (Itapecerica Unit) illustrating the quality of rocks collected from unweathered core.. ................................................................ 37

Figure 3.2 - Panel (a) Geological map of the southern São Francisco Craton (modified from Machado Filho et al. 1983); Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001)................................................................................................................................... 39

Figure 3.3 - (a) Chemical classification diagram in Na2O x CaO x K2O space (Glikson 1979) used to divide the studied rocks into: 1 - thondhjemites, 2 - tonalites, 3 - granodiorites, 4 – adamellites, and 5 – granites. (b) (MgO + Fe2O3 + MnO) vs SiO2 showing the expected trend from fractionation of ferromagnesian silicates and Fe-oxides. (c) (Na2O + K2O – CaO) vs K2O diagram shows compositional relationship for Archaean gneisses and granitoids, in which studied samples plot along the calc-alkaline trend. (d) Plot of La/Yb vs Yb comparing rocks from Candeias, Claudio and Itapecerica Units.. .................................................................................................................................. 46

Figure 3.4 - N-MORB-normalized trace element patterns of average gneisses from A – Candeias, B – Itapecerica, and C – Claudio Units. Elements arranged in order of decreasing incompatibility in MORB-melting (after Sun & McDonough 1989). Normalizing values were taken from Sun & MacDough (1989) except W where the average of Indian, Pacific and Atlantic MORB was calculated from data presented by Newson et al. (1996). ....................................................................................... 48

Figure 3.5 – Upper Continental Crust normalization of same data (average of Candeias Unit) as shows in Fig. 3.4. Normalizing values were taken from McLennan (2001). ................................................... 50

Figure 3.6 – Detail of Upper Continental Crust normalized average patterns of Candeias, Itapecerica, and Claudio Units. Panel (a) highlights depletion of Cs, Rb, U, Nb, Ta, W and Sr by comparison with average of typical Archaean granitoid rocks from Barberton (Kleinshanns et al. in press). ................. 51

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Figure 3.7 - Selected trace element rations of gneiss from Candeias (solid diamonds), Itapecerica (solid square) and Claudio (open triangle) unit: (a) Th/U vs La/W, (b) Th/U vs Hf/W, (c) Th/U vs Th/Cs, (d) Cs/Rb vs Cs/Th, and (e) Nb/Ta vs Hf/W. .............................................................................56

Figure 3.8 - Selected relationship between major and trace element from Candeias (solid diamonds), Itapecerica (solid square) and Claudio (open triangle) samples: (a) Eu/Eu* vs Sr/Nd, (b) Eu/Eu* vs Al2O3, (c) Pb vs (Na2O + K2O – CaO), and (d) Ce/Pb vs (MgO + Fe2O3 + MnO)........................................57

CAPÍTULO 4

Figure 4.1 - Panel (a) Geological map of the southern São Francisco Craton (modified from Machado Filho et al. 1983); Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001) ...................................................................................................................................61

Figure 4.2 – Panel of pictures from gneisses quarry .............................................................................64

Figure 4.3 - Panel of pictures of amphibolites rocks.............................................................................65

Figure 4.4 -Rb-Sr isochron diagram showing the nearby correlation between Marilan quarry and Fazenda Corumba quarry. The three point isochron are considering be of uncertain realiability .........70

Figure 4.5 - Rb-Sr isochron diagram showing in close proximity correlation between the three gneisses units..........................................................................................................................................71

Figure 4.6 - Rb-Sr isochron diagram showing amphibolites from Kinawa and Corumbá quarry.........72

Figure 4.7 - Sm-Nd errochron diagram showing nearby correlation between the three gneisses units as demonstrated for Rb-Sr ..........................................................................................................................74

Figure 4.8 - Positive trend correlation between Cs;Rb vs Cs;Th from Cláudio (open triangle), Itapecerica (solid square) and Candeias (solid diamond) Units. ............................................................77

Figure 4.9 - Nd versus time (DePaolo 1981) for the studied samples (see table 4.2 for reference) ............78

CAPÍTULO 5

Figure 5.1 - Panel (a) Geological map of the southern São Francisco Craton (modified from Machado Filho et al. 1983); Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001) ...................................................................................................................................82

Figure 5.2 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles from samples OPU 1436 (Fig. 5.1b-point A) ...............................................................................................................91

Figure 5.3 Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles from samples OPU 1197 (Fig. 5.1b-point B)................................................................................................................92

Figure 5.4 - Plot 40Ar/39Ar apparent age vs cumulative argon released for biotites of amphibolites from Corumbá quarry (Fig 5.1b, point A) ......................................................................................................94

Figure 5.5 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of amphibolites from Corumbá quarry (Fig 5.1b, point A)..............................................................................................95

Figure 5.6 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of amphibolites from Supracrustal Unit (Fig 1b, point C). ..............................................................................................96

Figure 5.7 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbronorite from fissural mafic Unit (Fig 5.1b – point D). .......................................................................................98

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Figure 5.8 - Plot 40Ar/39Ar apparent age vs cumulative argon released for biotites of gabbronorite from fissural mafic Unit (Fig 5.1b – point D). ............................................................................................... 99

Figure 5.9 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbro from fissural mafic Unit (Fig 5.1b – point E)............................................................................................... 100

Figure 5.10 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbro from fissural mafic Unit (Fig 5.1b – point F)............................................................................................... 101

CAPÍTULO 6

Figura 6.1 – Mapa geológico modificado por Oliveira & Carneiro (2001), mostrando os pontos amostrados........................................................................................................................................... 108

Figura 6.2 – Fotografias das amostras onde foram coletados os zircões datados: a) Gnaisse da pedreira Kinawa; b,c) Gnaisse da pedreira Corumbá; d) Gnaisse da pedreira Oliveira e, e) Pedreira de rocha ornamental Alemão.............................................................................................................................. 110

Figura 6.3 Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH11. Os zircões pertencem a um gnaisse migmatítico da pedreira Kinawa (Figura 6.1C) .............. 111

Figura 6.4 – Diagrama Concórdia U-Pb para os zircões da mostra AH 11 da Unidade Gnáissica Cláudio ................................................................................................................................................ 112

Figura 6.5 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH14. Os zircões pertencem a um gnaisse migmatítico da pedreira Corumbá (Figura 6.1 D)........... 113

Figura 6.6 – Diagrama Concórdia U-Pb para os zircões da amostra AH 14 da Unidade Gnáissica Cláudio ................................................................................................................................................ 114

Figura 6.7 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH14. Os zircões pertencem a um gnaisse migmatítico da pedreira Corumbá (Figura 6.1 D)........... 114

Figura 6.8 – Diagramas Concórdia U-Pb para os zircões da amostra AH 15 da Unidade Gnáissica Cláudio (a, b) e para as amostras AH 14 e AH 15 da Unidade Gnáissica Cláudio ............................. 116

Figura 6.9 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH07. Todos os cristais são de um enderbito da pedreira Oliveira (Figura 6.1 B)............................. 117

Figura 6.10 – Diagrama Concórdia U-Pb para os zircões da amostra AH 07 da Unidade Gnáissica Candeias .............................................................................................................................................. 118

Figura 6.11 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH08 e pertencentes ao charnockito da pedreira Alemão (Figura 6.1 A)........................................... 119

Figura 6.12 – Diagrama Concórdia U-Pb para os zircões da amostra AH 08 da Unidade Gnássica Candeias .............................................................................................................................................. 120

CAPÍTULO 7

Figura 7.1 – Diagrama mostransdo a perda de Pb com relativo ganho de U .................................... 125

Figura 7.2 – Também mostra o distúbio que as rochas foram submetidas desde o Arqueano até o Neoproterozóico/Paleozóico................................................................................................................ 126

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Lista de Tabelas

CAPÍTULO 1

Tabela 1.1 - Cálculo para determinação da razão 87Rb/86Rb ...................................................................8

Tabela 1.2 - Cálculo para determinação da razão 147Sm/144Nd................................................................9

CAPÍTULO 2

Table 2.1 - General characteristics of the rocks types of the study area and their probable ages and tectonothermal events. ......................................................................................................................17

Table 2.2 - Simplified table of the structural elements associated with the tectonothermal events. .....27

CAPÍTULO 3

Table 3.1 - Major (wt%), trace element (ppm) concentrations and selected trace element ratios of Campo Belo Metamorphic Complex. ...............................................................................................42

CAPÍTULO 4

Table 4.1 - Rb-Sr whole-rock analytical data for representative samples from the Campo Belo Metamorphic Complex .....................................................................................................................66

Table 4.2 - Sm-Nd whole-rock analytical data for representative samples from the Campo Belo Metamorphic Complex. The ε(T1) was calculated fro 2.72Ga (supposed age for the formation of the rock) and ε (T2) was calculated for 2.62Ga (supposed age for the “metamorphic” event) ...................................................68

Table 4.3 - Summary of the Archaean tectonomagmatic events in the southern part of the São Francisco Craton in the light of U/Pb and Sm-Nd data. This table was summarized by Teixeira et al. 2000..............................................................................................................................................75

CAPÍTULO 5

Table 5.1 – Ar-Ar results for amphibolites and gabbros from Campo Belo Metamorphic Complex ......... 87

CAPÍTULO 6

Table 6.1 – Resultados analíticos ................................................................................................................ 109

xvii

Resumo

Os estudos abordados nessa tese envolveram os gnaisses, anfibolitos, gabros e gabronoritos

das unidades gnáissicas, anfibolíticas, supracrustal e máfica fissural. Os resultados obtidos foram

importantes no trato da evolução tectônica do Complexo Metamórfico Campo Belo (CBC) na porção

meridional do Cráton São Francisco.

Os resultados, ora apresentados, foram balisados em petrografia, geologia estrutural,

geoquímica em rocha total (elementos maiores, traços e REE) e geocronologia (Rb-Sr e Sm-Nd em

rocha total, Ar-Ar em anfibólios e biotitas de gnaisses, anfibolitos, gabros e gabronoritos e U-Pb em

zircões de gnaisses). Esses dados mostraram que a região foi submetida a múltiplos episódios

tectonotermais que vão do Arqueano ao Proterozóico.

Os estudos petrográficos mostraram três picos metamórficos bem distintos: o primeiro atingiu

a fácies granulito e afetou as três unidades gnáissicas e anfibolítica. O segundo situa-se na fácies

anfibolito e está bem caracterizado nas rochas máfica-ultramáficas e pelíticas da unidade supracrustal.

O terceiro pico esta caracterizada por um retrometamorfismo regional para fácies xisto-verde.

O padrão estrutural mais significativo que afetou a área estudada está associado ao vigoroso

evento de migmatização e à geração da Zona de Cisalhamento Cláudio (NE/SW com cinemática

dextral). As estruturas mais tardias são o fraturamento crustal de direção NW-SE onde se

posicionaram os diques de gabros e gabronoritos.

Os estudos geoquímicos foram focados apenas nos gnaisses das unidades de Cláudio,

Itapecerica e Candeias. Os resultados mostraram que as rochas da região sofreram múltiplas perdas de

alguns elementos, principalmente de incompatíveis. Os resultados mostraram que o padrão

geoquímico das rochas félsicas, principalmente em se tratando dos gnaisses de fácies granulito

(enderbitos) e charnockitos, mostram particularidades distintas, principalmente quando comparados

com metagranitóides arqueanos ou com a média das crostas superior e inferior e crosta total. Foi

observado também que para alguns dos elementos traços incompatíveis (e.g. Cs, U, Nb, Ta e W), a

mobilidade foi muito alta. Alguns dos elementos (e.g. Cs, Rb e U) são considerados móveis com a

presença de fluidos, porém outros (i.e. Nb, Ta, W) são considerados imóveis. Esses elementos também

se encontram com razões baixas.

Já os dados geocronológicos dos gnaisses, anfibolitos, gabronoritos e gabros mostram

resultados por volta de: 2.9-2.7 Ga (U-Pb), 2.6 Ga (Rb-Sr), 2.05 Ga (U-Pb), 2.0-1.9 Ga (Ar-Ar), 1.7

Ga, 1.0Ga (Ar-Ar), 0.5 Ga (U-Pb).

Este conjunto de informações permitiu caracterizar pelo menos sete eventos tectonotermais

para a região de Cláudio e imediações.

O evento de 2.9-2.7 Ga é interpretado como a idade do protólito das rochas estudadas. O

evento de 2.6 Ga foi caracterizado como um possível evento metamórfico. A esse evento foi indexada

a formação da Zona de Cisalhamento Cláudio.

O evento de 2.05 Ga ficou bem evidenciado nos charnockitos e mostrou que a região esteve

xix

sob eventos que atingiu a fácies granulito de metamorfismo nesse período.

Já os eventos de 2.0-1.9Ga, ~1.7Ga e 1.0 Ga foram bem caracterizados através dos dados de 40Ar/39Ar. As idades de 2,0-1,9 Ga foram correlacionadas ao resfriamento do evento de granulitização

de 2.05Ga e não apenas o soerguimento da crosta siálica. Os eventos de 1.7Ga e 1.0Ga estão

associados ao magmatismo máfico fissural (gabronoritos e gabros respectivamente). Com esses

resultados caracterizam-se pelo menos dois períodos de resfriamento crustal que devem estar

relacionados ao posicionamento / cristalizações desse magmatismo fissural.

O primeiro evento de magmatismo fissural pode ser correlacionado ao evento estateriano/rifte

do Espinhaço. Já o segundo evento de magmatismo, com idade de aproximada de 1,0 Ga, foi o

responsável pela reativação de falhas pré-existentes de direção NW-SE e pode ser correlacionada ao

rifte Macaúba. Em suma, os resultados ora apresentados mostram que a estabilização da crosta siálica

do Cráton São Francisco Meridional, pelo menos na região de Cláudio, ocorreu no final do

Mesoproterozóico e não no Arqueano.

Por fim o evento de 0.5 Ga que representa apenas um intenso distúrbio isotópico com

conseqüente perda de Pb. Os dados ora apresentados foram de grande importância para o

entendimento de algumas das lacunas até então pouco evidentes para a evolução da porção meridional

do Cráton São Francisco. Entretanto, uma evolução similar já foi mostrada para a porção setentrional

do Cráton São Francisco.

xx

Abstract This thesis present the results of gneisses, amphibolites, gabbronorite and gabbros from

Cláudio, Supracrustal and Mafic Fissural Units and display imperative information for the evolution of

the Campo Belo Metamorphic Complex (CBC) in the southern part of the São Francisco Craton.

Using petrography, structural, geochemistry in whole rock, Rb-Sr and Sm-Nd in whole rock,

Ar-Ar for amphibole and biotite, and U-Pb for zircons, we have provide important information about

the timing and tectono-metamorphic history from the Archaean to the Proterozoic.

Based on petrography features, three distinct and anachronic metamorphic peaks were

registered: the first, of highest metamorphic grade related to the granulite facies, is present in the rocks

of the Gneissic Units and of the Amphibolitic Unit. The second, in the amphibolite facies, is observed

in the mafic-ultramafic and clastic-pelitic rocks of the Supracrustal Unit. The third is characterized by

regional retrograde metamorphic processes, which reached the greenschist facies and cover the whole

region.

The mainly and the most expressive structural pattern is characterized by a vigorous regional

migmatization process and by the generation of the NE/SW Claudio Shear Zone, presenting dextral

kinematic movement. The later expressive event is represented by a fissure mafic magmatism placed

in the NW-SE regional structures.

For geochemistry analyses were chosen gneisses from the three units: Cláudio, Itapecerica and

Candeias. By combining major and trace elements evidence it was possible to identify element losses

caused by prograde dehydration in the gneisses rocks (essentially in the charnockites and enderbites).

Those rocks display relative depletion in some of the most incompatible elements (i.e. Cs, Rb, U, Nb,

Ta and W) in multi-element spider diagrams. Some elements (i.e. Cs, Rb and U) have traditionally

been considered mobile with fluids but we found that others (i.e. Nb, Ta, W), which are often

considered immobile, are also deficient in our samples.

The geochronological results of gneiss, amphibolites, gabbronorite and gabbros displayed

results at about 2.9-2.7 Ga (U-Pb), 2.6 Ga (Rb-Sr), 2.05 Ga (U-Pb), 2.0-1.9 Ga (Ar-Ar), 1.7 Ga, 1.0Ga

(Ar-Ar), 0.5 Ga (U-Pb). Combining each of these results with petrography, structural and

geochemistry records it was possible to make link to a distinct tectonic episode that might took place

in the Campo Belo Metamorphic Complex as follow: 2.9-2.7 Ga event is correlated to the protolith

age, the 2.6 Ga is linked to the metamorphic age and might be associated to the deformational event

responsible for the dextral strike-slip Cláudio shear zone.

The 2.05 Ga correspond to high-grade metamorphic event as attested by U-Pb data for

charnockites. It is the peak of the prograde metamorphic event that achieves the granulite facies

metamorphism followed by charnockite formation. As a consequence, the ages at about 2.0-1.9Ga can

be correlated not only with the age of exhumation of the CBC but the age of the retrograde

metamorphic processes from this high-grade metamorphic event at about 2.05 Ga. The data suggests

that the CBC was largely and quickly exhumed after the 2.05Ga metamorphic event.

xxi

The ages at about 1.7 and 1.0Ga can be correlated to the emplacement and crystallization of

the two swarm dykes. The NW-SE structures into which these swarm dykes were emplaced can be

correlated with the Staterian continental rift scenario (1.8-1.6Ga).

The latest thermal evidence in the CBC is the emplacement of the ~1.0Ma gabbros, emplaced

in preexisting NW-SE structures and can be correlated to the Macaubas rift. The results displayed in

this study stand for the final stabilization of the CBC that happened in the Mesoproterozoic and not in

the Achaean.

The 0.5 Ga correspond to no more than the disturbance of the system attested in the U-Pb date.

The new data placed imperative information on the tectonic evolution in the Campo Belo Complex

which appears comparable in many ways to the others Archaean continental crust.

xxii

CAPÍTULO 1 CONSIDERAÇÕES GERAIS

1.1 INTRODUÇÃO

Esta tese apresenta os resultados da cartografia geológica e dos estudos petrográficos,

estruturais, geoquímicos e isotópicos realizados num fragmento da porção meridional do Cráton São

Francisco (Fig. 1a, b). Por outro lado, é a continuidade dos estudos do mestrado do autor (Oliveira

1999).

A área estudada compreende, principalmente, os litotipos do Complexo Metamórfico Campo

Belo e é balizada pelas cidades de Oliveira, Carmópolis de Minas, Cláudio e Itapecerica (Fig. 1.1b).

Subordinadamente afloram uma unidade supracrustal, que pode ser correlacionada ao Supergrupo Rio

das Velhas (Machado Filho et al. 1983), e um enxame de diques máficos. Este complexo insere-se no

contexto geotectônico do Cráton São Francisco Meridional, no estado de Minas Gerais, Brasil

e vários foram os estudos já desenvolvido no Complexo Metamórfico Campo Belo (Silva et al.

1978, Machado Filho et al. 1983, Teixeira 1993, Carneiro et al. 1996 a, b, 1997 a, b, c, 1998 a, b,

Carvalho Júnior et al. 1997, 1998, Fernandes et al. 1997, 1998, Corrêa da Costa et al. 1998, Oliveira et

al. 1998 a, b, 1999, Oliveira 1999, Oliveira & Carneiro 1999, Corrêa da Costa 1999).

Em termos geotectônicos a porção meridional do Cráton São Francisco (Fig. 1.1a) é um

segmento crustal de evolução policíclica, tectonicamente estável em relação aos cinturões móveis do

Ciclo Brasiliano (Alkmim et al. 1993). O acervo geocronológico da região pode ser encontrado em

Teixeira et al. (1996, 1998) que, também, propuseram um modelo de evolução crustal arqueana e

paleoproterozóica, caracterizado por sucessivas etapas de acresção/diferenciação, associados a

processos de retrabalhamento crustal posteriores.

As rochas gnáissicas do Complexo Metamórfico Campo Belo têm composição muito variável

(e.g. granítica, granodiorítica e tonalítica) e podem ser agrupadas em três unidades distintas: Gnaisses

Cláudio, Itapecerica e Candeias (Oliveira et al. 1998 a, b, 1999, Oliveira 1999, Oliveira & Carneiro

1999, 2001). Os gnaisses tipo Cláudio apresentam coloração cinza (localmente com mobilizados

róseos) e composição, predominante, granodiorítica. Os gnaisses tipo Itapecerica apresentam

coloração rósea e composição mais granítica. Já os gnaisses granulíticos tipo Candeias apresentam

coloração esverdeada e composição em geral granodiorítica à granítica. A unidade supracrustal é

composta por peridotito, hornblendito, clorita-hornblendito, anfibolito, granada-sillimanita-xisto,

granada-sillimanita-quartzito, filito grafitoso e formação ferrífera bandada. O enxame de diques

máficos, que descreve grandes lineamentos de direção NW/SE, é formado por gabronorito, gabro e

diabásio (Oliveira et al. 1998b).

Oliveira, 2004 Evolução tectônica de um.........

Figura 1.1 Painel (a) –Mapa Geológico da porção meridional do Cráton São Francisco (modificado de Campos Sales 2004). Lefenda: 1- Coberturas cratônicas Neoproterozoicas indivisas; 2- Supergrupo Espinhaço; 3- Grupos São João Del Rei/Andrelândia; 4—Grupo Dom Silvério; 5- Granitóides Paleoproterozoic; 6- Seqüências Greenstone; 7- Supergrupo Minas; 8- Granitóides Neoarqueanos; 9- Intrusões máficas Neoarqueanas e Mesoproterozoicas; 10- Suíte plutônica ultramáfica Neoarqueanas; 11- Supergrupo Rio das Velhas; 12- Complexos Metamórficos Arqueanos; 13- Falhas e fraturas (CSZ = Zona de Cisalhamento Cláudio; JBSZ = Zona de Cisalhamento Jeceaba-Bom Sucesso; 14- Traços axial dos Eixos de dobras; 15- Contatos litológicos; 16- Cidades. Painel (b) Mapa geológico simplificado da área estudada (modificado de Oliveira 1999, Oliveira & Carneiro 2001). Simbologia: 1) Unidade máfica fissural; 2) Unidade Supracrustal; 3) Unidade gnáissica Candeias; 4) Unidade gnáissica Itapecerica; 5) Unidade gnáissica Cláudio; 6) Contatos inferidos; 7) Foliação; 8) Zona de cisalhamento Cláudio, e 9) Afloramentos escolhidos: A – Pedreira de rocha ornamental Alemão; B – Pedreira de rocha ornamental Oliveira; C - Pedreira de rocha ornamental Lila; D – Pedreira de rocha ornamental

2

Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004

3

Itapecerica; E - Pedreira de rocha ornamental Marilan; F - Pedreira de rocha ornamental Kinawa; G - Pedreira de rocha ornamental Corumbá; H – Pedreira de rocha ornamental Fazenda Corumbá; I – Pedreira de rocha ornamental Carmópolis de Minas; J – Afloramento de Gabronorito; K e L – afloramentos de Gabros e M.- afloramento de anfibolito da Unidade Supracrustal

1.2 OBJETIVOS DA TESE

1 - determinar a relação cronoestratigráfica entre as rochas das unidades gnáissicas, seqüência

supracrustal e unidade máfica fissural na região estudada;

2 – determinar, através de estudos isotópicos, a pluralidade dos eventos deformacionais,

acrescionários, de migmatização e de magmatismo, que ocorreram durante o arqueano. Assim como, a

contribuição dos eventos posteriores (e.g. transamazônico e brasiliano) na evolução tectônica da área;

3 - associar os dados obtidos em campo (unidades litodêmicas e a cinemática dos corpos) e

laboratório (petrológicos, geocronológicos e geoquímicos) para modelar a evolução tectônica regional;

4 - comparar os dados obtidos na área com aqueles em terrenos de igual natureza, a nível

mundial, estabelecendo, assim, uma correlação evolutiva para o Craton São Francisco Meridional.

1.3 METODOLOGIA

Como o acervo petrotectônico da região é muito complexo e variado, para elevar o seu

conhecimento geológico, foi necessário executar um estudo amplo envolvendo geologia estrutural,

petrologia, geoquímica e geocronologia para tratar, com maior precisão, a sua evolução tectônica.

O mapeamento geológico, parte da petrografia e parte dos estudos cinemáticos foram

realizados por Oliveira (1999). Nos anos de 2000 e 2001 foram realizadas mais etapas de campo,

totalizando 60 dias. Nessas etapas foram visitados pontos previamente selecionados por Oliveira

(1999). Durante essas etapas foram realizados estudos mais detalhados da cinemática dos corpos, suas

relações no contexto regional e coletadas amostras para estudos geoquímicos e isotópicos.

Das amostras coletadas foi realizado um novo estudo petrográfico e, desse estudo, foi feita

uma seleção para geoquímica e geocronologia.

Ao final do ano de 2000 iniciou-se a preparação de um artigo com informações previamente

obtidas no mestrado juntamente com os dados coletados no doutorado. Ao término de 2001 o

doutorando obteve uma bolsa sanduíche e se transferiu para a Universidade de Queensland na

Austrália para obter os dados geoquímicos em rocha total (elementos maiores, traços e terras raras) e

geocronológicos (U-Pb em zircão, Sr-Nd em rocha total e Ar-Ar em anfibólio e biotita). O

detalhamento de cada uma das etapas está exposto a seguir.


4

1.3.1 Revisão bibliográfica 1:

Revisão dos estudos realizados no Complexo Metamórfico Campo Belo em termos regionais

(e.g. Silva et al. 1978, Machado Filho et al. 1983) e mesmo ao nível de detalhe (e.g. Teixeira 1993,

Teixeira & Silva 1993, Teixeira & Canzian 1994, Teixeira et al. 1996, 1997, 1999, Carneiro et al.

1996 a, b, 1997 a, b, c, 1998 a, b, Carvalho Júnior et al. 1997, 1998, Fernandes et al. 1997, 1998,

Pinese et. al. 1995, Corrêa da Costa et al. 1998, Oliveira et al. 1998 a, b, 1999, Oliveira 1999, Oliveira

& Carneiro 1999, Corrêa da Costa 1999, Campos et al. 2003).

1.3.2 Revisão bibliográfica 2:

Revisão dos estudos efetuados em terrenos granito-greenstone de igual natureza em outros

crátons com enfoque em dados geocronológicos e geoquímicos (e.g. Tankard 1982, Allen & Condie

1985, Goodwin 1991, MacDonough et al. 1991, Myers 1993, Sarkar et al. 1993, Miller et al. 1994,

Windley 1998, Friend et al. 1996, Mueller et al. 1996, Nutman et al. 1996, Tribuzio et al. 1996,

Moorbath et al. 1997, Collerson & Macdonald 1998, Hamilton 1998, Bebout et al. 1999, Becker et al.

1999, 2000, Bodorkos et al. 2002, Sandiford & MacLaren 2002, Sandiford et al. 2003).

1.3.3 Trabalho de Campo

Os trabalhos de campo tiveram como fundamento a caracterização mais detalhada das 5

unidades litodêmicas definidas por Oliveira et al. (1998 a, b, 1999), Oliveira & Carneiro (1999) e

Oliveira (1999). Nessa etapa, além dos estudos cinemáticos das estruturas, foram coletadas amostras

representativas para estudos petrográficos, geoquímicos e geocronológicos.

1.3.4 Análise petrográfica

As amostras representativas das unidades litodêmicas foram estudadas em microscópio óptico

com a finalidade de se observar texturas, estruturas e associações minerais diagnósticas dos eventos

ígneos e metamórficos regionais. Tais estudos foram de suma importância para definição das unidades

que foram alvo das análises geoquímicas e geocronológicas.

1.3.5 Preparação de amostras

Inicialmente as amostras foram preparadas no LOPAG (Laboratório de preparação de

amostras para geocronologia e geoquímica) do Departamento de Geologia da Universidade Federal de

Ouro Preto.

As amostras sofreram redução granulométrica para concentração dos pesados (geocronologia

U-Pb) e geoquímica (rocha total).


5

Posteriormente, as amostras foram submetidas a uma segunda fase de preparação que ocorreu

no Departamento de Geologia da Universidade de Queensland e no ACQUIRE (Advanced Centre for

Queensland University Isotope Research Excellent)/Austrália, conforme procedimentos descritos a

seguir.

1.3.6 Geoquímica:

No Departamento de Geologia da Universidade de Queensland/Austrália as amostras foram

pulverizadas em moinho de ágata e uma alíquota foi separada para o ataque químico.

As análises químicas em rocha total (elementos maiores, traços e elementos terras raras) foram

realizadas utilizando-se de um ICP-EOS (Inductively Coupled Plasma – Optical Emission

Spectroscopy) situado no laboratório do Department of Earth Sciences/University of Queensland e um

ICP-MS (Fisons PQ2 + Plasmaquad Inductively Coupled Plasma-Mass Spectrometer) situado no

ACQUIRE (Advanced Centre for Queensland University Isotope Research Excellent).

1.3.6.1 Metodologia ICP-OES

Do conjunto dos gnaisses estudados, 21 amostras foram selecionadas para as análises

geoquímicas de elemento maiores. As determinações químicas foram analisadas para 10 elementos

maiores (Al2O3, CaO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, SiO2, TiO2) pelo técnico do Laboratório

Mr. Michael Lawrence. O óxido de ferro foi determinado como ferro total (Fe2O3).

Foi utilizado 0,050 g de amostra em pó (secas por 1-2 horas em cadinhos de porcelana a

1050C) as quais foram colocadas, posteriormente, em cadinhos de grafita (primeiramente foi

necessário polir os cadinhos com uma colher). Em seguida, adicionou-se 0,200 g de LiBO2

(Metaborato de lítio) e, então, fez-se a homogeneização do material e levou-se ao forno, por 1 hora, a

uma temperatura de 10000C.

Os béqueres de teflon utilizados foram mantidos em uma vasilha contendo 10% de HNO3 para

limpeza por um período de 24 horas. Após isso, adicionou-se aos béqueres uma mistura de 25 ml de

uma solução de HNO3 a 10%, com 10ppm de lutécio (Lu), como padrão interno e, em seguida, a

amostra fundida (ao final de uma noite, nessa solução, as amostras devem ter uma aparência de flocos

de neve). Se a amostra não for completamente fundida é necessário adicionar 1ml de HF concentrado.

Após a dissolução completa das amostras foi adicionado 25 ml de água destilada, para que a

concentração final alcançasse a relação 50mg/50ml. As curvas de calibração foram obtidas pela

diluição dos padrões JB3 (basalto), JA3 (andesito) e JR2 (riolito). A perda ao fogo foi determinada


6

aquecendo-se aproximadamente 2 g de amostra a 10000C por cerca de 2 horas.

1.3.6.2 Metodologia ICP-MS

Foram selecionadas 25 amostras de gnaisses para as determinações químicas de elementos

traço e terras-raras (Li, Be, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Sn, Cs, Ba, La, Ce, Pr,

Nd, Sm, Eu, Tb, Gd, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Pb, Th, U). O procedimento analítico foi

orientado pelo responsável técnico do laboratório (Dr. Alan Greig) e seguiu as descrições de Eggins et

al. (1997).

Procedimentos:

Parte 1 - Os béqueres de teflon PTFE (7ml) foram deixados por 1 dia em uma vasilha de vidro

(1000ml) com água destilada e 3ml de detergente. Depois disso, eliminou-se a água e enxaguaram-se

os béqueres com água destilada (3 vezes) que foram colocados na vasilha de vidro (1000ml) e

completada com água destilada. Esse conjunto foi fervido por 1 hora em uma chapa quente dentro de

uma capela e, em seguida, os béqueres foram secos em uma capela de pressão positiva.

Parte 2 – Preparou-se uma solução com água destilada (e.g. 24ml), ácido clorídrico (HCl,

18ml) e ácido nítrico (HNO3, 6ml), que é a composição da aqua regia. Essa solução foi distribuída nos

béqueres de teflon até cobrir a sua base e, em seguida, colocou-se os béqueres dentro de jaquetas de

aço que foram levadas ao forno por 2h (temp. de 178 °C). Retiradas do forno foram resfriados por,

aproximadamente, 2h. Após esse período, os béqueres foram retirados das jaquetas de aço e levados

para uma capela de pressão positiva, onde foram abertos (é necessário precaução ao abrir os béqueres,

pois a pressão interna devido ao vácuo faz com que o material pule para fora). Depois de abertos, os

béqueres foram lavados com água destilada, repetiu-se esse procedimento por duas vezes. Após essa

lavagem, colocou-se 2 gotas de HNO3 nos béqueres e adicionou-se HF (ácido fluorídrico) até a sua

linha mediana (~4ml). Os béqueres assim tratados foram colocados novamente nas jaquetas de aço e

levados ao forno por 1 dia e meio. Após esse período, os béqueres foram lavados com água destilada

(2X) e ficaram limpos para uso.

Ataque químico

Em cada béquer, previamente limpo, adicionou-se 16 gotas de 8NQD de HNO3, pesou-se

aproximadamente 0,10000g de amostra e adicionou-a sobre o ácido e pingou-se mais 16 gotas do

mesmo ácido. Adicionou-se 2ml de HF e colocou as amostras para secarem em uma chapa quente

dentro de uma capela de pressão positiva por 1- 2 horas sem tampar os béqueres. Em seguida, foi

adiciononado-se 16 gotas de 15.8N de HNO3 e 3,5ml de HF e, então, se colocou os béqueres nas

jaquetas de aço que foram levados ao forno por 60h (180°C). Após esse período, abriu-se os béqueres


7

que foram secos por aproximadamente 2h. Então, se adicionou 4ml de HCL e, dentro das jaquetas, os

béqueres foram ao forno por mais 24h. Abertos e secos por aproximadamente 2horas, receberam ácido

nítrico concentrado (15,08 N), até cobrir a amostra e foram secos em chapa quente (repetir 1X). Com

pipetas, então foi adicionado 2 X 1000µl de ácido nítrico destilado e 2X 1000µl de H2O destilada.

Fechados e levados para uma capela com pressão positiva e a 80°C ficaram por 24hs. Após isso, os

béqueres, ainda fechados, foram agitados suavemente e girados de ponta a cabeça de modo a agrupar

todas as gotículas das paredes internas. O liquido foi transferido para um tubo de centrifuga e o béquer

foi ainda lavado com H2O destilada (2X), para recuperar o restante do material, e foi centrifugado por

20min. Após esse tempo, se restar resíduo sólido no fundo do tubo, esse material deverá ser

transferido para o béquer e tentar-se-á dissolvê-lo novamente. Se dissolvida, separa-se uma alíquota

para determinação dos elementos traços e terras raras e outra parte para geoquímica isotópica.

Esse procedimento de ataque químico seguiu as descrições de Eggins et al. (1997), exceto que

o elemento Tm não foi usado como padrão interno. Os padrões utilizados foram: BHVO-1 (amostra de

basalto proveniente do Observatório Vulcânico do Havaí - Hawaiian Volcano Observatory) e o W-2

(um diabásio dos laboratórios do United State Geological Society - U.S.G.S.). As concentrações para

W-2 foram derivadas parcialmente de sua relação com o primeiro padrão ou foi baseada em um acervo

de dados de padrões já publicados (A. Greig com. pessoal 2002). O molibdênio não foi analisado

porque as jaquetas de aço de teflon causam contaminação. Baseado em análises repetidas de digestões

múltiplas dos padrões (um total de 54) a reprodutibilidade, bem como o desvio padrão relativo da

maioria dos elementos, é de 1%, exceto para Be e Zn, os quais estão na média de 2-4% e o Sn o qual

apresenta desvio padrão de 15-20%.

1.3.7 Geocronologia:

Quatro métodos geocronológicos diferentes foram utilizados e estão descritos a seguir:

1.3.7.1 Rb-Sr

Uma alíquota da amostra, após ataque químico (conforme procedimento descrito na parte

geoquímica), foi levada para uma coluna de resina de troca catiônica para concentração do Sr

utilizando-se de diferentes concentrações de HCL (1N HCl, 2.5 N HCl, and 6N HCl), conforme

procedimentos descritos por Wendt & Collerson (1999).

As medidas da composição isotópica de estrôncio foram feitas em espectrômetro de massa VG

54-30 (Sector multi-collector mass spectrometer in static mode in the ACQUIRE laboratory at the

University of Queensland).


A reprodutibilidade para o método geocronológico do Sr do ACQUIRE é controlada pela

repetição de análises do padrão internacional EN1 e NBS-987 (carbonato de estrôncio), cujo valor é de

0.710251±20 (2σ) medido por um período de 7 anos.

A razão 87Rb/86Sr foi determinada utilizando-se da concentração em ppm do Rb e Sr obtidas

no Fisons PQ2+ Plasmaquad Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) conforme

metodologia descrita anteriormente. Os cálculos efetuados encontram-se na Tabela 1.1.

Os valores das razões 87Sr/86Sr foram normalizadas em função da relação 86Sr/88Sr igual a

0,1194 tendo sido utilizadas nos cálculos as constantes recomendadas por Steiger & Jaeger (1978).

Tabela 1.1 – Cálculo para determinação da razão 87Rb/86Sr.

A B C D E F G H I

Rb Moles Rb 87Rb Sr Moles Sr 86Sr Rb/Sr 87Rb/86Srppm ppm

A1/A4 B1*A3 F9 D1/E1 F1*D3 A1/D1 C1/G11 AO-1 211.79 2.4780 0.6898 131.34 87.6058 1.4992 0.1478 1.6125 4.666123 87Rb 0.2784 86Sr 0.0986

abundância

5 88/88 1 88 0.8256 87.9056 A5/A9 = 0.8131 F5*D5 = 71.47626 84/88 0.0067 84 0.0056 83.9134 A6/A9 = 0.0055 F6*D6 = 0.46057 86/88 0.1194 86 0.0986 85.9093 A7/A9 = 0.0971 F7*D7 = 8.34048 87/88 0.1037 87 0.0702 86.9089 A8/A9 = 0.0843 F8*D8 = 7.32869 SUM (A5:A8)= 1.2299 A9/A9 = 1.0000 SUM (H5:H8)= 87.6058

4

87/88 é o valor da amostra (e.g AO-1) medida no espectrômetro vezes o valor de 86/88

87.44245

Quantidade de isotopos

AO1 Sr - atomic WtAmostra

Peso atômico do Rb 85.4678 Peso atômico

do Sr

Cálculo para determinação da razão 87Rb/86Sr

1.3.7.2 Sm-Nd

Depois de coletado os REEs da coluna de Sr, o material foi seco e a ele adicionou-se uma

fraca concentração de HCL e esse concentrado foi levado para uma coluna de resina de troca catiônica

para separação do Nd. Diferentes concentrações de HCL (1N HCl e 2.5 N HCl) foram utilizadas para

obter o Nd.

A concentração isotópica do neodímio foi medida no espectrômetro de massa VG 54-30

(Sector multi-collector mass spectrometer in static mode in the ACQUIRE laboratory at the University

of Queensland).

A reprodutibilidade para Nd também é controlada pela repetição de análises do padrão

internacional La Jolla and Ames metal Nd padrão com valores 143Nd/144Nd = 0.511861±11 (2σ) e

0.511977±12(2σ). Na determinação da razão Sm-Nd foi seguido os mesmos passos para Rb-Sr.

Maiores detalhes para separação dos elementos Sr e Nd ver Wendt & Collerson (1999).

8


A razão 147Sm/144Nd foi determinada utilizando-se da concentração em ppm do Sm e Nd

obtidas no Fisons PQ2+ Plasmaquad Inductively Coupled Plasma-Mass Spectrometer (ICP-MS)

conforme metodologia descrita anteriormente. Os cálculos efetuados encontram-se na Tabela 1.2.

Tabela 1.2 – Cálculo para determinação da razão 147Sm/144Nd

A B C D E F G H

Sm Moles Sm 147Sm Nd Moles Nd 144Nd 147Sm/144Nd Sm/Ndppm ppm

A1/A4 B1*A3 D1/D4 E1*D3 C1/F1 A1/D11 AO-1 0.88 0.0059 0.0009 5.61 0.0389 0.0093 0.0954 0.157723 147Sm 0.15 144Nd 0.2384 At wt Sm 150.3656 At wt Nd 144.2379

Amostra

Cálculo para determinação da razão 147Sm/144Nd

Os erros experimentais usados para calcular as regressões lineares utilizando-se do Isoplot

foram de 0,5% para 87Rb/86Sr e 0.1% para 147Sm/144Nd baseados nos erros dos padrões medidos no

laboratório e 26ppm para 87Sr/86Sr e 143Nd/144Nd baseados nos erro externo baseado nos padrões NBS

SRM 987 e EN-1 medidos nos últimos 7 anos no ACQUIRE.

1.3.7.3 Ar-Ar

Sete amostras foram escolhidas para determinação Ar-Ar, as quais separou-se uma quantidade

de 20 cristais de biotitas e/ou anfibólios utilizando-se de uma lupa binocular. Esses cristais foram

lavados, primeiro em água destilada e depois em álcool absoluto para eliminar as impurezas

superficiais, impregnadas nos cristais que foram colocados para secar. Depois de secos, uma média de

10 cristais foi colocada num disco de alumínio juntamente com padrões de sanidina. Selou-se o disco e

o mesmo foi encaminhado para a irradiação no Oregon State University Triga Reator-USA (CLICIT

Facility) por um período de 14 horas, utilizando-se do “Fish Canyon sanidine neutron fluence

monitors”. Após essa irradiação, resguardando o período correto de resfriamento das amostras, deu-se

início às análises 40Ar/39Ar, pelo método de passo incremental de laser no Laboratório de

geocronologia - UQ-AGES (the University of Queensland Argon Geochronology in Earth Science

Laboratory).

Os procedimentos seguiram as descrições de Vasconcellos (1999a, b) e Vasconcelos et al.

2002

1.3.7.4 U-Pb

Os zircões concentrados inicialmente no LOPAG foram reconcentrados na Universidade de

Queensland, utilizando-se líquidos densos. Primeiro utilizou-se o bromofórmio para eliminar

impurezas (e.g. feldspatos e quartzo). Em seguida foi utilizado o iodeto de metileno para separar a

9


apatita de zircão e, por fim, utilizou-se o separador magnético Frantz para obter o concentrado final de

zircão.

A partir desse concentrado utilizou-se um microscópio binocular para catar os melhores

cristais de zircão que foram montados numa pastilha (mount) contendo em torno de 30-40 zircões das

5 amostras estudadas e um padrão (Fig. 1.2). A pastilha foi submetida a catodoluminescência e, então,

definiu-se os melhores cristais a serem datados. As razões isotópicas foram obtidas num equipamento

CAMECA IMS1270 Ion Microproble do Swedish Museum of Natural History, na Suécia. Para

maiores detalhes sobre os métodos analíticos ver Whitehouse & Russel (1997) e Whitehouse et al.

(1999).

Figura 1.2 – Mapa esquemático dos zircões das 5 amostras selecionadas (a numeração das amostras selecionadas está em negrito).

1.3.8 Tratamento dos dados:

Os programas utilizados para construção dos vários digramas, figuras e tabelas foram os seguintes:

• Excel 2000 para construção dos diagramas de geoquímica e diagramação das tabelas.

• Corel Draw 9 (para confecção de algumas das figuras)

• Freehand 8 (confecção das figuras Ar-Ar)

• Isoplot versão 3.0 (Aplicativo geocronológico para Microsoft Excel, Ludwig 1998).

10


11

• EndNote (programa de referências bibliográficas)

Para o cálculo dos parâmetros isotópicos de estrôncio e neodímio foram utilizados os

programas de Centro de Pesquisas Geocronológicas da USP.

1.3.9 Elaboração da Tese:

Os subseqüentes capítulos dessa tese são artigos publicados e submetidos ou a submeter.

Sendo assim, cerca de 90% dela foi elaborada em inglês. Especificamente, o capítulo 2, já publicado,

trata os dados petrográficos e estruturais (Oliveira, A.H. and Carneiro, M.A., 2001. Campo Belo

Metamorphic Complex: Tectonic evolution of an Archean sialic crust of the southern São Francisco

Craton in Minas Gerais (Brazil). Anais da Academia Brasileira de Ciências, 73(3): 397-415).

O capítulo 3 trata o comportamento geoquímico, com ênfase nas rochas charnockíticas e

enderbíticas. Esse artigo foi submetido à Lithos (Oliveira, A.H., Kamber, B.S., Collerson, K.D. and

Carneiro, M.A. Origin of charnockites and enderbites inferred from incompatible element loss during

prograde dehydration).

O capítulo 4 trata a geocronologia Rb-Sr e Sm-Nd. Esse artigo foi submetido aos Anais da

Academia Brasileira de Ciências (Oliveira, A.H., Carneiro, M.A., Kamber, B.S. and Collerson, K.D.

Recent advances among Sr and Nd concerning to the tectonic evolution of Campo Belo Metamorphic

Complex, southern portion of the São Francisco Craton, Brazil).

O capítulo 5 trata a geocronologia Ar-Ar. Esse artigo foi submetido ao Journal of South

American Earth Sciences (Oliveira, A.H., Vasconcelos, P.M. Carneiro, M.A and Carmo, I.O.

Implications for Transamazonian 40Ar/39Ar ages for the Campo Belo Metamorphic Complex, Southern

Sao Francisco Craton, Brazil).

O capítulo 6 trata a geocronologia U-Pb e o capítulo 7 das considerações finais. Pretende-se

submeter um artigo com esses dois capítulos ao Precambrian Research, mas isso só após a defesa

dessa tese...


12

CAPÍTULO 2 CAMPO BELO METAMORPHIC COMPLEX: TECTONIC

EVOLUTION OF AN ARCHAEAN SIALIC CRUST OF THE SOUTHERN SÃO FRANCISCO CRATON IN MINAS GERAIS

(BRAZIL)

2.1 ABSTRACT

Systematic geological studies performed in the study area allowed characterize six lithodemic

units: three gneissic, one amphibolitic, one supracrustal and one fissure mafic. The rock mineral

assemblage and the structural record of these lithodemic units indicate that the study area was affected

by five tectonothermal events. The structural pattern of the first and oldest event occurred under

granulite facies conditions and reveals an essentially sinistral kinematics. The second event, showing

dominant extensional characteristics, is related to the generation of an ensialic basin filled by the

volcano-sedimentary sequence of the supracrustal lithodemic unit. The third event, which is the most

expressive in the study region, is characterized by a vigorous regional migmatization process and by

the generation of the Claudio Shear Zone, presenting dextral kinematic movement. The four events is

represented by a fissure mafic magmatism (probably two different mafic dike swarm) and finally, the

fifth event is a regional metamorphic re-equilibration that reached the greenschist facies, closing the

main processes of the tectonic evolution of the Campo Belo Metamorphic Complex.

Key Words: Craton, Archean, tectonic evolution, lithodemic units, metamorphic complex.

2.2 INTRODUCTION

The São Francisco Craton is a vast Precambrian platform (Almeida 1977, Alkmim et al.

1993), which encompasses part of Minas Gerais and Bahia States (Fig. 2.1). Its southern portion (Fig.

2.1) presents significant expositions of Neoarchean granite-greenstone terranes; its tectonic evolution

started in the Mesoarchean (Teixeira et al. 1996, 1998, Carneiro et al. 1998a). From that period on the

first sialic crusts and supracrustal sequences were generated by means of successive

accretion/differentiation stages associated with crustal reworking processes (Teixeira 1993, Teixeira &

Silva 1993, Teixeira & Canzian 1994, Noce 1995, Carneiro et al. 1996 a, 1997 a, b, 1998 a, b, Pinese

1997, Teixeira et al. 1996, 1997, 1999).

The apex of these processes took place in the Neoarchean, during the Rio das Velhas

Tectonothermal Event (Carneiro et al. 1998a). Other events are also represented in this crustal

segment. They are: two events of tectonothermal nature in the Mesoarchean [3.2 and 2.9 Ga, (Teixeira

et al. 1996, 1998)]; a migmatization event [2.86 Ga., (Noce 1995)]; a fissure mafic magmatism event

[2.658 Ga., (Pinese 1997)] and a felsic magmatism event [2.65 Ga., (Noce 1995)]. Due to this

succession of events, the tectonic relationships between the generated (or reworked) units were


progressively masked. Therefore, a complex structural pattern was imprinted in the lithodemic units

that constitute the metamorphic complex, their supracrustal sequences and intrusive bodies that crop

out in the study area.

Divinópolis

CláudioCarmópolisde MinasItapecerica

Oliveira

Candeias Conselheiro Lafaiete

Ouro Preto

Barbacena

Campo Belo

Lavras

Studied Area

0 10 20 30Km

BRAZIL

Figure 2.1 Geologic map of southern São Francisco Craton (modified from Machado Filho et al. 1983). 1) Porto dos Mendes Granite; 2) São João del Rey Supergroup; 3) Minas Supergroup; 4) Formiga Granite; 5) Rio das Velhas Supergroup; 6) Divinópolis Metamorphic Complex, and 7) Barbacena Metamorphic Complex.

Recent research, involving regional (1:200,000) and detailed (1:10,000) mapping performed in

the Campo Belo, Oliveira, Itapecerica, Cláudio, Carmópolis de Minas regions has characterized new

lithodemic units and presented new facts to the tectonic evolution of the southern São Francisco

Craton (Carneiro et al. 1996a, b, 1997a, b, c, 1998a, b, Carvalho Júnior et al. 1997, 1998, Fernandes et

al. 1997, 1998, Fernandes & Carneiro 2000, Corrêa da Costa et al. 1998, Corrêa da Costa 1999,

Oliveira et al. 1998a, b, 1999, Oliveira 1999, Oliveira & Carneiro 1999).

In this work the results of one of these studies (Oliveira 1999) are presented, carried out in the

Cláudio, Itapecerica, São Francisco de Paula, Oliveira and Carmópolis de Minas region, involving

lithostructural mapping at the 1:200,000 scale (Figs. 2.1 and 2.2).

14


Monsenhor João Alexandre

Itapecerica60

70

3740

40

60

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85 4050

45 80

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251030

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8065

3545

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3015

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8055

3075

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40 40

45

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2536

0505

1 2 3 4 5 6 7 8

Carmoda

Mata

Oliveira

Carmópolisde M inas

Cláudio

São Franciscode Paula

0 2 4 6km

25 30

40

9

B

F

A

60

40

C

E

I

H

D

J

A

G

20°15´00´´

44°1

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44°3

0´00

´´44

°30´

00´´

20°45´00´´

N44

°15´

00´´

20°45´00´´

Figure 2.2 Geologic map of the study area (modified from Oliveira 1999), showing the different lithodemic units: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A - Fernão Dias road; B - Corumbá dimension stone quarry; C - Kinawa dimension stone quarry; D – Morro da antena; E - Vista Bela farm; F – Carmo da Mata dimension stones quarry; G – Marilan dimension stones quarry; H – Lila dimension stones quarry; I – Alemão dimension stones quarry; J – Oliveira dimension stones quarry.

2.3 GEOLOGIC CONTEXT

The sialic crust of the southern São Francisco Craton is constituted by gneisses, granitoids,

amphibolites, mafic and ultramafic rocks, schists and quartzites that were grouped by (Machado Filho

et al. 1983), in the Divinópolis and Barbacena metamorphic complexes (Fig. 2.1). Locally, remains of

the supracrustal sequences are found, correlated with the Rio das Velhas or Minas Supergroups.

The geographic distribution of the Divinópolis Metamorphic Complex, according to (Machado

Filho et al. 1983), occurs in the vicinity of Divinópolis, Itaúna, Formiga and Passa Tempo cities (Fig.

2.1). In the remaining region, rocks from the Barbacena Metamorphic Complex would predominate.

However, (Teixeira et al. 1996) consider these two complexes in the Campo Belo region (Fig. 2.1) as

a single unit, naming it Campo Belo Metamorphic Complex. According to (Machado Filho et al.

1983), the majority of the lithotypes listed above presents three principal deformation and/or fracturing

directions: NS; NW/SE and NE/SW.

15


16

The tectonic meaning of these directions according to (Machado Filho et al. 1983) is the

following: 1) NS direction, of more plastic character, would be responsible for the compression of

supracrustal sequences in a tight syncline; 2) the NW/SE structures displace the N/S structures,

slightly metamorphosed basic dikes were emplaced along this trend; 3) the last direction is related to

the generation of the Atlântico Mobile Belt, at the southeastern-eastern margin of the Southern São

Francisco Craton.

2.4 GEOLOGY OF THE AREA

Using satellite imagery and aeromagnetometric map interpretation and by means of regional

geologic sections (Oliveira 1999) it was possible to characterize six lithodemic units (Tab. 2.1 and Fig.

2.2). The older three of these units are gneissic, the fourth is amphibolitic (cannot be mapped at the

scale adopted), the fifth is a supracrustal sequence (of greenstone belt type), and the youngest is a

mafic dike swarm.

2.4.1 Gneissic Units

The petrographic distinction of the gneissic units (Fig. 2.2) was mainly based on their color

and composition. The Cláudio Unit rocks are gray and their composition is predominantly

granodioritic. Those from the Itapecerica Unit are light pink and their composition is predominantly

granitic. Finally, the Candeias Unit rocks are green and their composition is granitic to granodioritic.

The geologic contacts between the gneissic units are masked by the thick weathering mantle

and by the superposition of tectonothermal processes taking place since the Mesoarchean. However,

the gneissic units have distinct outcrop domains. As an example, the case of the Itapecerica Gneissic

Unit is given, its rocks cropping out in the surroundings of the city it borrows the name from and it is

characterized by intense migmatization, which imprinted an average mineralogic composition of

granitic nature. Such migmatization is more pronounced in the western portion of its geographic

occurrence.

Three pegmatite vein families are associated with the gneissic units. The first family is formed

by mobilizates concordant with the gneissic foliation and presents centimetric thicknesses and variable

(centimetric to metric) lengths. These mobilizates are essentially composed of feldspars and small

quantities of mica. The mobilizates are usually light pink and sometimes white.

The second pegmatite family is discordant from the gneissic foliation and is centimetric- to

metric-thick, with veins reaching some meters in length. It is in general coarse-grained and light pink,

formed by feldspar phenocrysts and less abundant centimetric mica flakes. The third family is

represented by pegmatite veins that crosscut either the gneissic foliation or the other two families.


These are centimetric-thick and metric-long veins. They are fine- to medium-grained and fill NS-

trending fracture planes.

Table 2.1 General characteristics of the rocks types of the study area and their probable ages and tectonothermal events.

Unit Lithology Rock-forming Minerals AgesTectono-thermal Events

quartzite Quartz, sillimanite and garnet

schist Sill imanite, quartz, mica and garnet

amphibolite Clinoamphibole, plagioclase and quartz

ultramaficOrthopyroxene,

olivine, clinoamphibole

Amphibolitic amphibolite Hornblende, plagioclase and quartz Probable age: 3,38-3,0 Ga1 (?)

E1, (E2?), E3, (E4?) and E5.

Candeias Gneiss

Gneiss of granodioritic to

granitic composition

Plagioclase, microcline, quartz and

hypersthene

Itapecerica Gneiss Gneiss of granitic composition

Microcline, plagioclase and quartz

Cláudio GneissGneiss of

granodioritic composition

Plagioclase, microcline and quartz

References: 1(Teixeira et al . 1996b), 1(Teixeira et al . 1998), 2(Carneiro et al . 1998b), 3(Pinese 1997).

Supracrustal (Rio das Velhas

Supergroup)

E3, (E4?) and E5

E4 and E5

gabbro

E1, (E2?), E3, E4 and

E5

Banded iron formation

Quartz and opaque minerals

Probable ages for the mafic-ultramafic magmatism,

followed by sedimentation (2.8 Ga2).

Probable age for the protolith (3.38-3.0 Ga1), and probable age for the migmatiza-tion

(2.75-2.72 Ga2).

Fissure Mafic

gabbronorite plagioclase, ortho- and clinopyroxenes Ages for the intrusions: 2.658

Ga3 and 1.875 Ga3.Plagioclase,

clinopyroxene

17


18

d can alter to biotite. Opaque minerals, zircon

and apatite are accessories, as well as rutile, tourmaline and allanite, which are rare. As secondary

mineral te occur.

n chlorite. Amphibole is rarer, fine-grained and alters to biotite. Common

accessories are zircon, apatite and opaque minerals. Sericite, chlorite, carbonate and epidote are

secondary minerals.

2.4.1.1 Cláudio Gneissic Unit

The rocks of this unit crop out in the eastern portion of the study area (Fig. 2.2) and present

granodioritic to dioritic, locally granitic, compositions. They are gray, fine- to medium-grained,

banded and migmatized. In some outcrops light pink pegmatitic mobilizates occur, giving a rosy tint to

the rock. Regionally, the color of these gneisses can vary, so that eastwards (Fig. 2.2), it is

predominantly gray, diminishing the percentage of light pink felsic mobilizates. The texture of the

Cláudio gneisses is predominantly granoblastic to granolepidoblastic. The crystals vary from

subidioblastic to idioblastic, medium- to fine-grained, being more rarely coarse-grained. Intergrowth

textures are common (e.g. perthite, myrmekite and sometimes antiperthite). In general, plagioclase is

the main constituent of these rocks (30-40%). The crystals are predominantly medium-grained and

those showing polysynthetic twinning vary from subhedral to euhedral. Quartz is in general the second

constituent in volume (20-30%). The microcline percentage is also variable, sometimes being

comparable to the other two constituents. However, it is more common to be subordinated (15-30%).

As varietal mineral, biotite (5-10%) shows light to dark or greenish brown pleochroism, altering to

chlorite. It has zircon inclusions and sometimes it is associated with opaque minerals. Amphibole

(hornblende) is rare and shows greenish pleochroism an

s chlorite, sericite, carbonate and epido

2.4.1.2 Itapecerica Gneissic Unit

The rocks from the Itapecerica Gneissic Unit crop out in the NW portion of the area and

present granitic composition are light pink, migmatized and show inequigranular to equigranular

textures and are medium- to fine-grained. In some outcrops closer to the contact with the Candeias

Gneissic Unit (Fig. 2.2), the rocks are greenish. In other localities the gneiss is grayish, as the outcrops

located east of Itapecerica city. Pegmatitic or amphibolitic dikes constituting boudins are common

features. The texture of the Itapecerica Gneissic Unit rocks is granoblastic to granolepidoblastic with

subidioblastic to idioblastic, fine- to medium-grained crystals. Microcline is the predominating

mineral (30-40%), plagioclase second in abundance (15-30%) and quartz is subordinated to feldspar

(±20%). Biotite (<10%) is a varietal mineral and presents dark brown to greenish pleochroism, locally

altering to pale-gree


19

k. Locally, biotite alters to chlorite.

Opaque minerals, zircon and apatite are the commonest accessories of the rock. Chlorite is the

commo rowing biotite and pyroxene.

, black and strongly

foliated. In most places, the amphibolites are found in the form of disrupted thin boudins, representing

disrupte

reach 10% of the volume and alters to amphibole.

Accessory minerals are zircon, opaque minerals and apatite. Besides biotite, secondary minerals are

chlorite sericite.

Ultramafic rocks (metaperidotite, chlorite-amphibole-schist and hornblendite), amphibolite,

2.4.1.3 Candeias Gneissic Unit

The gneisses of this unit crop out in the SW portion of the area (Fig. 2.2). In the most

deformed domains the rocks present granodioritic composition (opdalites). However, in the more

homogeneous domains the composition is granitic (charnockites). In general, these rocks are green,

slightly migmatized, deformed and medium- to coarse-grained. The mineralogic banding, in some

places, is difficult to be observed due to the great homogeneity of the bodies, except for the

hyperstene-biotite-gneiss and biotite-gneiss where the biotite planar orientation is more enhanced.

Close to the contact with the Itapecerica Gneissic Unit, light pink pegmatitic mobilizates are found.

The texture of the Candeias-type gneisses varies from granolepidoblastic to granoblastic and they are

subdioblastic to idioblastic, predominating granoblastic. Antiperthitic plagioclase (30-45%) is the

main constituent, quartz (20-25%) and the microcline content (15-20%) varies from sample to sample

but can reach up to 40%. Hyperstene, when present, is medium-grained, has low birefringence and

pleochroism varying from light pink to pale green. It alters to amphibole, biotite and chlorite. Biotite,

sometimes showing symplectitic intergrowth with quartz, presents light brown, sometimes greenish,

pleochroism. In general, this mineral defines the banding of the roc

nest secondary mineral, overg

2.4.2 Amphibolitic Unit

The rocks of this unit are fine-grained, melanocratic, dense, phaneritic

d and metamorphosed dikes emplaced in the rocks of the gneissic units.

The texture of the amphibolites varies from nematoblastic to granolepido-nematoblastic. The

crystals are fine- to medium-grained, strongly oriented and altered. Green hornblende (±50%),

sometimes altered to biotite, is a main mineral. Locally, biotite schist (or biotitite) results from

retrograde metamorphism and shearing. Plagioclase is the second constituent in volume (20-30%).

Quartz appears subordinated to the other minerals and its content varies from 5 to 10% of the rock

volume. Biotite, sometimes chloritized, is the product of amphibole transformation. Pale yellow

clinopyroxene, present in some rocks, can

and amphibole, epidote and

2.4.3 Supracrustal Unit


20

e-schist, garnet-sillimanite-quartzite and banded iron formation were characterized in

the study area.

ed by positive magnetic anomalies as can be observed in aeromagnetometric

maps (Oliveira 1999).

nd generally result from spinel transformation.

Chlorite is sometimes present as secondary mineral.

ine hornblende mass (±

70%). A small amount of opaque minerals represents the accessory minerals.

dominantly medium-grained granonematoblastic

hornblende crystals (± 95%) and opaque minerals.

and clinopyroxene occur occasionally. The main accessories are zircon,

opaque minerals and apatite.

garnet-sillimanit

Regionally, the Supracrustal Unit (Fig. 2.2) can be correlated with the Rio das Velhas

Supergroup, which crops out in the Quadrilátero Ferrífero in Minas Gerais. In some places, they are

strongly deformed, metamorphosed and sometimes cataclastic. In the field, the domain of the

ultramafic rocks is mark

The metaperidotite is a holocrystalline rock with inequigranular, anhedral to subhedral

crystals and grain-size varying from fine to coarse. It shows cumulatic and/or mesh texture. Enstatite

makes up 50% to 60% of the total volume of the rock. Anhedral clinopyroxene and olivine crystals

can reach up to 20% of the rock volume. The hornblende percentage varies from 20% to 50%. Spinel

can reach 10% of the rock volume and appears as granular crystals of high relief and color varying

from green to greenish-brown. Serpentine, rare in the majority of the thin sections, is found as fibro-

lamelar aggregates, resultant from the alteration of olivine and pyroxenes. The opaque minerals can

reach 5 to 10% of the rock. They are isotropic a

The chlorite-amphibole-schist is a greenish rock with porphyroblastic texture. Chlorite sums

up 30% of volume and appears as poikilitic porphyroblasts embedded in a f

The hornblendite is composed of pre

The amphibolite is a fine- to medium-grained rock, melanocratic, dense, phaneritic, black and

strongly foliated. The amphibolite is, in general, found in contact with ultramafic rocks, mainly in the

surroundings of Cláudio, in the supracrustal unit domain (Fig. 2.2). The main foliation trend is

NE/SW, accompanying the direction of the magnetic anomaly observed in the aeromagnetometric

maps (Oliveira 1999). The rock texture varies from nematoblastic to granonematoblastic. The crystals

are fine-to medium-grained. Green hornblende (± 40%), sometimes alters to biotite and chlorite.

Plagioclase (± 35%) is the second constituent in volume and alters to sericite and epidote, and quartz is

subordinated (± 10%). Biotite

The garnet-sillimanite-schist is a fine- to medium-grained, dark gray, strongly mylonitized

magnetic rock. The rock texture varies from granolepidoblastic to granonematoblastic. The crystals


21

minerals, alteration products after plagioclase. White

mica is also formed from sillimanite and biotite.

mica is the secondary mineral in the rock and it is the alteration product

after sillimanite and biotite.

and strongly

recrystallized. Magnetite contents vary from 30 to 40 % in terms of modal composition.

2.4.4 Fissure Mafic Unit

same lineament small diabase dikes can be observed as a dark, dense and fine-grained rock.

vary from subidioblastic to granoblastic. Quartz is the main constituent (± 40%). The crystals are

elongated (ribbons) and follow the rock foliation. Plagioclase (± 15%) is the second main constituent.

Sillimanite in variable proportions (up to 70%) presents yellow polarization color, is fine- to medium-

grained and the crystals are fibrous (more common) or prismatic. It alters to white mica as like biotite

(± 10%). Garnet (± 5%) is a light brown mineral with dark-gray polarization color, almost isotropic

with opaque minerals, quartz and white mica inclusions. Opaque minerals, zircon and rutile are the

commonest accessories in this rock. Rutile is translucent brown and can be associated with biotite.

White mica, epidote and carbonate are secondary

The garnet-sillimanite-quartzite is a fine-grained rock, pale greenish yellow to white,

mylonitized and with subvertical foliation. The texture varies from granolepidoblastic to

granonematoblastic and the grain-size varies from fine to medium. Quartz (± 70%) is the main rock

constituent. The crystals are elongated (ribbons) accompanying the foliation. Sillimanite (± 20%)

presents yellow polarization color, grain-size varying from fine to medium with fibrous crystals

(fibrolite) oriented according to the foliation. It alters to white mica. Garnet (± 3%) is a light brown

mineral and has dark gray polarization color. Rare biotite shows light to dark brown pleochroism, is

fine-grained, with zircon inclusions, and alters to white mica. Opaque minerals, zircon and rutile are

common accessories. White

Finally, the banded iron formation is a magnetic rock, of predominantly fine grain-size,

composed by magnetic minerals and quartz. This rock presents granoblastic texture and the contact

between grains is elongated due to deformation. Quartz (± 60%) is the main constituent of the rock,

with elongated crystals in the form of ribbons; it is fine- to medium-grained

This unit is represented by NW/SE-trending dikes (Fig. 2.2). These dikes have varied

thickness, from metric to dozens of meters and their length can reach dozens of kilometers. The dikes

are subvertical and commonly develop ramifications involving the gneissic massifs. Compositionally,

the dikes are composed of gabbronorite, gabbro and diabase. The gabbronorite and the gabbro are

medium- and coarse-grained, possibly representing variations of crustal emplacement during

magmatic crystallization. These rocks are in general dark-colored to greenish, and present massive

structure and phaneritic texture. However, when very fine-grained, they can be aphanitic. Along the


22

The gabbronorite shows ophitic, subophitic to intergranular textures, with subhedral to

euhedral crystals, and is medium- to fine-grained. Plagioclase is the most abundant mineral in volume

(40-60%). (Clino- and ortho-) pyroxene contents vary from 20 to 35% in volume. In general, augite

predominates over hyperstene and is predominantly medium-grained, presents variable polarization

color (from high to low), the crystals are poikilitic, altering to amphibole. Hyperstene is light pink,

pleochroic, from medium- to fine-grained. Hornblende with green pleochroism is the alteration

product of pyroxenes (uralitization), reaching up to 5% of the rock. Quartz appears in small quantities,

with clean, anhedral, fine-grained crystals. Biotite is rare and reddish, pleochroic. Zircon and opaque

minerals are accessory minerals. White mica, epidote, carbonate and chlorite are secondary minerals.

The gabbro presents ophitic, subophitic or intergranular textures. The medium- to fine-

grained crystals vary from subhedral to euhedral. Plagioclase is the most abundant mineral in volume,

varying from 30-50%. Augite contents, second component in volume, vary from 20 to 30%. The

crystals are poikilitic and alter to amphibole. Hornblende is the uralitization product after pyroxene,

being its content in volume variable between 5 and 15%. When present, microcline is rare, showing

cross-hatch twinning and is associated with myrmekite. Quartz appears in small quantities. Biotite is

also rare, reddish and pleochroic. Zircon, opaque minerals and apatite are accessory minerals. White

mica, epidote and chlorite are secondary minerals observed in some of the rocks.

2.5 METAMORPHISM

Three distinct and anachronic metamorphic peaks are registered in the rocks of the study area.

The first, of highest metamorphic grade related to the granulite facies, is present in the rocks of the

Gneissic Units and of the Amphibolitic Unit. The second, predominantly of the amphibolite facies, is

observed in the mafic-ultramafic and clastic-pelitic rocks of the Supracrustal Unit. The third is

characterized by regional retrograde metamorphic processes, which reached the greenschist facies and

cover the whole region.

The main paragenesis of the Cláudio, Itapecerica and Candeias Gneissic Units is constituted

by plagioclase ± quartz ± (antiperthitic) microcline ± amphibole ± hyperstene ± biotite. As secondary

paragenesis sericite, carbonate, chlorite and epidote occur. Antiperthite and hyperstene indicate that

these rocks underwent metamorphic conditions of the granulite facies. On the other hand, the

secondary paragenesis indicates greenschist-facies retrograde metamorphism.

The Amphibolitic Unit, enclosed in these gneissic units, also presents high-grade paragenesis,

with the presence of hyperstene, and retrograde metamorphic features of the greenschist facies,

characterized by the presence of chlorite.


23

However, Supracrustal Unit rocks show slightly different parageneses. Except for the

Supracrustal Unit ultramafic rocks, where green Al2O3-richer spinels would indicate metamorphic

conditions compatible with the amphibolite-granulite facies (Paktunç 1984), the mineral assemblages

of the schists (quartz ± plagioclase ± sillimanite ± garnet) and quartzites (quartz ± sillimanite ± garnet

± biotite) indicate that the metamorphism in these rocks reached at most the amphibolite facies.

Afterwards, as show the processes of sillimanite alteration to white mica and plagioclase

sericitization, these assemblages were re-equilibrated to greenschist facies conditions. However, if in

the Supracrustal Unit rocks metamorphism reached the amphibolite or even granulite facies, this was

note the case in the Fissure Mafic Unit rocks. These rocks have preserved igneous textures, showing

only metamorphic re-equilibration in the greenschist facies that, in fact, is common to all units

described in this paper.

So, two significant considerations can be expressed: a) the chronostratigraphic character of the

Fissure Mafic Unit that, due to its igneous textures and metamorphic parageneses at most of the

greenschist facies, indicates their crustal emplacement after the tectonothermal events of higher

metamorphic facies that affected the older units; b) the metamorphic facies variation between rocks of

the Gneissic, Amphibolitic and the Supracrustal Units. Two hypotheses can be put forward to explain

this matter. The first hypothesis would imply in the existence of two tectonothermal events. The first

one, of the granulite facies, would be previous to the generation of the Supracrustal Unit rocks and the

second one, locally reaching the granulite facies, would follow the generation of the Supracrustal Unit

rocks. The second hypothesis would favor the existence of a single event of the granulite facies that

occurred after the generation of the Supracrustal Unit rocks. Isotopic evidence supports the first

hypothesis, as attested (Teixeira et al. 1996, 1998, Carneiro et al. 1998b), which respectively defend a

metamorphic event of the granulite facies in the Mesoarchean and another of amphibolite facies in the

Neoarchean (Rio das Velhas Tectonothermal Event). Therefore, it is believed that the first possibility

is more realistic to explain the tectonic evolution of the region, according to the reasons exposed in the

following items.

2.6 STRUCTURAL ANALYSIS

The rocks of the study region present distinct structural features (Fig. 2.3, Fig. 2.4, Table 2.2).

For example, it is observed in the supracrustal unit rocks: 1) a mylonitic foliation (Sm) of

anastomosing character; 2) structures S/C; 3) S and Z intrafolial folds, tight and/or isoclinal sheath

mesofolds; 4) foliation sigmoids; 5) rotated quartz veins, and 6) tension gashes.The gneissic unit rocks

present, besides the structural features described above, the following ones: 7) a gneissic foliation (Sg)

of (local) mylonitic character and anastomosing pattern; 8) shear bands (Sb1 and Sb2); 9) pegmatitic


24

vein folds and rotated amphibolite boudins. Local normal and reverse folds and a regional mega-

structure here named Cláudio Shear Zone were characterized. The area also shows intense NW-SE-

trending crustal fracturing along which mafic dike swarms were emplaced (Fig. 2.2).

2.6.1 Foliations

The gneissic foliation (of mylonitic character) presents steep to low-angle dips. This foliation

is characterized by a fine compositional banding represented by the orientation of biotite, sericite and

amphibole. In general the foliation is anastomosing.

The (mylonitic) foliation Sm observed in the supracrustal unit rocks presents steep to low-

angle dips and is characterized by the orientation of the following minerals: pyroxene (in the

ultramafic rocks); amphibole (in the amphibolites); elongated quartz (ribbons ) and sillimanite (in the

quartzites); biotite, sericite and sillimanite (in the schists).

The structural style and strain of the foliation deformation indicate that they could have

originated from the same deformation phase. Sg and Sm foliations present inflections along the study

area so that the main trend is NE-SW (Figs. 2.3A and B). Locally, a change in the foliation direction is

observed (from NS to NW-SE). The NW-SE predominates on the NS (Fig. 2.3A). The lateral relation

between these directions can be seen in some key outcrops (points 9A, 9D, 9G, 9I; Fig. 2.2).

In these points the predominant foliation (NE-SW) is folded and its trajectory in folds inflects

to NS and NW-SE. The last trend represents one of the flanks of the fold of 2 to 3 meters of amplitude.

Thus the several trends characterized in the region as a whole reflect the principal foliation inflection

(NE/SW). In general the foliation dip is steep (70° to 80°), mainly along the Cláudio Shear Zone (Fig.

2.2). However outside the Cláudio Shear Zone domain medium to low-angle dips (50° a 20°) are

observed. The mineral lineation (lm) also presents a certain variation: from strike (Kinawa quarry,

point 9C, Fig. 2.2) to oblique (Corumbá quarry, point 9B; Fig 2.2) and dip (in the quartzites, point

9D). In these three cases, the foliation planes present NE-SW trends.

2.6.2 Shear bands

Shear bands Sb1 and Sb2 are subvertical and displace the foliation of the gneissic units, as seen

in horizontal and vertical planes of the quarries of the region. The foliation Sb1 is oriented according to

NW/SE and E-W trends (Figs. 2.3C and D) and the foliation planes are filled by pegmatitic

mobilizates. The NW/SE trend presents a dextral kinematic component whereas the E-W trend has a

sinistral component (seen in the horizontal plane). Foliation Sb2 is oriented along the ENE/WSW and

NW/SE trends (Figs. 2.3E and F). Sb2 does not present pegmatitic mobilizates in its planes. The

kinematics of the ENE/WSW foliation has a sinistral component and that of the NW/SE plane a


dextral component (seen in the horizontal plane). These two foliations form a pair R’ and X (anti-

Riedel and secondary, Fig. 2.4H). Shear bands Sb1 and Sb2 are different in the following aspects: the

first group presents pegmatitic mobilizates along its planes, lacking in the second group. Besides, the

kinematic trends of both groups are opposite (Figs. 2.3C; 2.3D; 2.3E and 2.3F).

0.5 % 1.0 % 1.5 % 2.1 % 2.6 % 3.1 % 3.6 % 4.1 %

1.2 % 2.4 %

3.7 %

4.9 %

6.1 %

4.5 % 9.1 %

13.6 % 18.2 %

22.7 % 27.3 % 31.8 %

5.9 %

11.8 %

17.6 % 23.5 % 29.4 % 35.3 %

41.2 %

11.1 %

22.2 %

33.3 %

44.4 %

A B

C D

E F

4.3 %

8.7 %

13.0 %

17.4 %

21.7 %

25

Figure 2.3 Stereographic diagrams representing the polar projections of the foliations of the study area: A) General foliation poles of the area (Sg and Sm), of NE/SW NS and NW/SE directions. Number of foliation measurements: 388; Maximum: 310/40, 156/40 and 200/37. B) Foliation poles Sg and Sm and mineral lineation of the Cláudio Shear Zone domain. Number of foliation measurements: 246; Maximum: 310/40 and 155/40. Number of lineation measurements: 21 C) Polo to foliation of Dextral Sb1 (Shear bands) of NW/SE direction. Number of foliation measurements: Maximum: 040/82 and the rosette represents the foliation direction. D) Polo to foliation of Sinistral Sb1 (Shear bands) of E-W direction. Number of foliation measurements: 22; Maximum: 130/82 and 344/83 and the rosette represents the plane direction. E) Polo to foliation of Sinistral Sb2 (Shear


bands) of ENE/WSW direction. Number of foliation measurements:17 ; Maximum: 218/88 and 040/85. F) Polo to foliation of Dextral Sb2 (Shear bands) of NW/SE direction. Number of foliation measurements: 9; Maximum: 170/83 and 350/85.

X

R'

Sb2

A B

D

F

H

S 150/70 C (130/75)

''Sb ''2

110 290Anf. Sb2

0 10cm

PH

PH

0 20cm

0 20cm

0 4mPV

E

60 250

Peg.Diab.

0 5m

PV

N

280 100

PH

C

Anf.

Peg.

''Sb ''=2 140/75

PH

0 20cm

PH

Sg=140/70

0 10cm

PH

Sg=150/63

CS

GT

Figure 2.4 Kinematic indicators (A-G) and schematic models explaining one of the event active in the study area (H). A) Structure S/C in gneiss of Cláudio Unit, with Event 1 sinistral movement; B) Sigmoid foliations of the Supracrustal Unit schists with dextral movements; C) Amphibolite boudin rotated clockwise and broken by shear zones (shear bands Sb2, X); D) Asymmetric folded pegmatite vein, Z-shaped, with dextral movement in the gneisses; E) (Brecciated) diabase dike in Cláudio Unit gneisses, displacing pegmatite vein and the gneissic foliation, denoting Event 3 Phase 2 normal movements; F) Shear bands (Sb2 , R’); G) Rotated amphibolite boudins, vertical plane view, with reverse movements; H) Schematic model representing Event 4, sinistral, indicating shear bands (Sb2), reverse faults and the intrusion of the Fissure Mafic Unit. Symbology: PH –Horizontal plane; PV – Vertical plane; Peg. – Pegmatite; Anf. - Amphibolite; Diab. - Diabase, and R’ and X (anti-Riedel and secondary).

2.6.3 Structures S/C

Structures S/C [S150/70 C(130/75), seen in the horizontal plane AC] can be observed in

restricted points in the area (points 9C and 9B; Fig 2.2). This foliation dimensions are centimetric and

the kinematics indicates sinistral movements (Figs 2.4A).

26


Table 2.2 Simplified table of the structural elements associated with the tectonothermal events

Tectono-thermal Events

Foliation

Kinematic indicators

Faults and Shear

Zones

Metamorphism

(M)

Associated

Rocks E5 M3

greenschist facies

E4

Foliation sigmoids, “shear bands” (Sb2, R’ and X)

Reverse fault

Fissure Mafic Unit

E3

Gneissic foliation (of mylonitic character or not) and Sm foliation (of the Supracrustal Unit)

“Z” folds, foliation sigmoids, amphibolite and rotated and displaced pegmatite veins, “shear bands” (Sb1)

Cláudio Shear Zone (ZCC) and normal fault

M2 Amphibolite facies

Migmatization and deformation of the previous units

E2

(?)

(?)

(?)

(?)

Mafic-ultramafic magmatism and sedimentation of the Supracrustal Unit

E1

Gneissic foliation (?), fold with double vergence and S/C

“S” and sheath folds, S/C structures

(?)

M1

Granulite facies

Formation of the protolith of the Gneissic and Amphibolitic Units

2.6.4 Folds

Folds are observed locally and can be described as: intrafolial, asymmetric and tight S and Z;

sheath mesofolds (rare) and tight and/or isoclinal (with flanks inverted or not). The most common

folds are S and Z (the latter being more prominent). The S and Z intrafolial folds are important in the

characterization of two of the phases of tectonic evolution of the study area.

2.6.5 Foliation Sigmoids

Foliation Sigmoids are observed both in schists and gneisses and characterize movements with

dextral directional components (seen in plane AC), where the foliation dip is steep and of high strain

[e.g. 130/80 (lm=225/40)]. Locally, sigmoids indicative of normal movements with an oblique

component are observed (seen in vertical plane AC).

27


28

2.6.6 Pegmatites

In the gneissic unit domain Z-folded pegmatitic mobilizates are observed indicating dextral

movement seen in plane AC. Locally appear veins denoting normal movements.

2.6.7 Quartz Veins

Both in the supracrustal unit rock and the gneissic unit domains, rotated centimetric quartz

veins are observed. These veins indicate dextral movement, seen in plane AC. Locally, veins

indicating reverse movement occurs.

2.6.8 Amphibolite Boudins

In the domains of the three gneissic units centimetric to metric amphibolite boudins occur

disrupted, strongly deformed and rotated, in general showing dextral movement. Locally there are

boudins indicating sinistral movement.

2.6.9 Faults

Faulting systems can be observed in several key outcrops (points 9A, 9C, 9D, 9G, 9H and 9J;

Fig. 2.2). At the sides of the Fernão Dias highway, a (NW/SE) fault plane is filled with volcanic

material that precedes or is coeval to the fault generation, appearing as a breccia. The fault plane cuts

the gneissic banding, is subvertical and displaces pegmatite veins, indicating normal movement (Fig.

2.4E). In the Montueira Ridge, close to Carmópolis de Minas, a fault with the same trend is observed,

yielding a steep slope. The reverse faulting of local nature does not present regional correlation of

major magnitude. The indicators for this movement are the (sinistral) Sb2 foliation, seen in the vertical

plane, and the foliation sigmoids seen in the gneisses, schists and quartzites.

Finally, the mega-structure that characterizes the Cláudio Shear Zone, firstly revealed by

aeromagnetometric images (Oliveira 1999), describes a roughly NE-SW alignment (Fig. 2.2). Along

the Cláudio Shear Zone are observed migmatized gneisses, ultramafites, amphibolites, quartzites and

schists showing high deformation strain. The foliation inside the mega-structure shows medium- to

high-angle dips and mylonitic character. Its fabric describes NE/SW (Fig. 2.3B) and subordinately NS

and NW/SE trajectories (Fig. 2.3A), characterizing regional foldings.

The general fabric is represented by “S” tectonites composed of pegmatitic mobilizates

concordant with the foliation. Other features presented by this mega-structure are: Z intrafolial folds,

foliation sigmoids, amphibolite boudins, pegmatite and rotated quartz veins with dextral movement.


29

The meaning of the variation of the different (strike, oblique or dip) mineral lineation trends, contained

in the foliation planes is still an open issue in the regional context.

2.7 TECTONIC EVOLUTION

It is known that the period between 3.6 and 2.5 Ga was responsible for the worldwide

formation of granite-greenstone terranes, with their apex around 3.0-2.6 Ga (e.g. Windley 1976,

Hamilton et al. 1979, Tankard 1982, Goodwin 1991, Myers 1993, Sarkar et al. 1993, Worden et al.

1995, Byerly et al. 1996, Friend et al. 1996, Kroner et al. 1996, Collerson & Macdonald 1998,

Hamilton 1998).

In the case of the southern São Francisco Craton, it was in the Neoarchean (from 2.78 to 2.70

Ga) that this crustal segment had its main accretion stage and crustal reworking (e.g. Noce 1995,

Machado et al. 1996, Carneiro et al. 1998a).

Considering that in the southwestern portion of the study area in the Campo Belo region the

gneissic units present zircon U-Pb ages of the order of 3.2 Ga. (Teixeira et al. 1996, 1998), it is

possible to infer that the beginning of the crustal evolution of the studied segment started in the

Mesoarchean.

Thus, on a primordial sialic crust formed by the protoliths of the gneissic and amphibolitic

units, a tectonothermal event took place, preserving some kinematic and petrologic records that

characterize the event 1.

2.7.1 Event 1

It is the oldest event or the event that was not totally obliterated by the successive events

imprinted in the rocks of the study region. Its deformational structures are foliations of S/C-type,

sinistral-rotated amphibolitic boudins and centimetric intrafolial S folds that are observed locally (Fig.

2.4). Systematically, the rocks that were affected by this event have relic parageneses of the granulite

facies. Later migmatization processes related to Event 3 developed on this fabric.

2.7.2 Event 2

Similarly to the Quadrilátero Ferrífero, where the Rio das Velhas Supergroup crops out, the

study area also presents remmants of the greenstone-belt type supracrustal sequences, with ultramafic

and clastic-pelitic rocks (Fig. 2.2). In the case of the Quadrilátero Ferrífero, consider that the

sedimentation and magmatism responsible for the formation of the Rio das Velhas Supergroup rocks

would be in operation around 2.8 Ga., in an ensialic-type basin (Carneiro et al. 1998b). This would be,


30

in principle, the situation of the study area where, during Event 2, of essentially extensional

characteristics, the ensialic basin would be installed for sedimentation and magmatism of the

Supracrustal Unit (or Rio das Velhas Supergroup).

2.7.3 Event 3

As said before, it was between 2.78 and 2.70 Ga that in the Southern São Francisco Craton the

main accretion and crustal reworking phase took place (e.g. Carneiro et al. 1998a, b, Machado et al.

1996, Noce 1995). Such evolution was related to the Rio das Velhas Tectonothermal Event. The peak

of this event occurred around 2.78 – 2.77 Ga (Machado & Carneiro 1992, Carneiro et al. 1998b), when

the zircons of the Alberto Flores Gneiss from the Bonfim Metamorphic Complex suffered

overgrowing, indicating isotopic re-equilibration under high amphibolite metamorphic facies

conditions. These metamorphic conditions have correspondence in the study region, where an ample

and vigorous migmatization process is observed, imposed on the rock fabric generated during Event 1.

In structural terms, the migmatitic rocks originated during this process present the same kinematic

elements of the Supracrustal Unit rocks.

Two important considerations derived from this fact: a) migmatization follows the

Supracrustal Unit sedimentation and magmatism; b) due to the tectonic intensity of this process, the

presence of high amphibolite or granulite facies parageneses, common in the Supracrustal Unit rocks

of the study region but lacking in the Quadrilátero Ferrífero Archean supracrustal rocks, would be

explained. In hierarchic terms, Event 3 can be divided in two phases. To Phase 1 would correspond the

migmatization process and the structural elements mainly found in the Cláudio Shear Zone (Fig. 2.2),

which are: asymmetric Z folds, foliation sigmoids, pegmatitic vein folds and rotated amphibolite

boudins indicating dextral movement.

Associated with the final phase of this dextral directional flow brittle-ductile shear zones are

observed, here represented by Sb1 foliations (shear bands, Figs. 2.3C and 2.3D), filled by pegmatitic

mobilizates. NW/SE- trending Sb1 presents a dextral component and the less conspicuous E/W-

trending Sb1 has a sinistral component. To Event 3, Phase 2 would correspond a crustal relaxation

phase with the generation of normal-component faults, observed in the Supracrustal Unit quartzites

that crop out south of Cláudio and in the gneissic rocks of the sides of the Fernão Dias highway, north

of Carmópolis de Minas.

The lineation contained by the foliation planes is steep (dip to slightly oblique) and the

kinematic indicators associated with this phase are rotated pegmatite veins and fault planes filled by

brecciated volcanic material (Fig. 2.4G). This phase is not expressive, does not obliterate the previous

one and its kinematics does not have regional correspondence.


31

2.7.4 Event 4

The initial phase of this event had a character flow brittle-ductile, here represented by Sb2

foliations was generated (shear bands, Figs. 2.3E and 2.3F) with forming a par R’ and X (R’ anti-

riedel and secondary, Fig. 2.4H) of NE/SW and NW/SE directions and high-angle dips. These

foliations crosscut the mylonitic gneissic foliation (points 9B, 9C, 9F, 9G and 9H; Fig 2.2). Seen on

the XZ plane, these foliations indicate reverse movements. With the final phase the emplacement of

the 2.658 Ga old, NW/SE-trending fissure mafic magmatism could be correlated (e.g. Pinese et al.

1995, Pinese 1997) and the 2.612 Ga granitic magmatism, interpreted as the final stage of the Archean

platform consolidation (Noce 1995). Regionally this event can be correlated to the Rio das Velhas

Event 2, with sinistral component (Endo 1997).

2.7.5 Event 5

The fifth event is a regional metamorphic re-equilibration to the greenschist facies, later than

the fissure mafic magmatism. Lacking more precise chronostratigraphic indicators to limit its

minimum age, it is suggested that Event 5 be of Transamazonian age or younger.

2.8 CONCLUSIONS

From the geologic results discussed above and geochronologic data of the southern São

Francisco Craton, the following conclusions can be formulated:

1) the protoliths of the gneisses from the three lithodemic units and the amphibolitic unit were

emplaced in the crust in the Mesoarchean and reworked in this period and mainly in the Neoarchean;

2) the volcano-sedimentary sequence of the supracrustal unit was deposited before Event 3,

which is the main migmatization event in the area;

3) the main deformational events in the area essentially occurred in the Neoarchean. Such

conclusion is based on Sm-Nd geochronologic data (Pinese 1997), obtained from dikes of

gabbronoritic composition that crystallized from magma 2.658 ± 44 Ma ago. Such dikes are not

deformed and only present its original magmatic paragenesis re-equilibrated to the greenschist facies.

Thus, as in the study area, similar dikes are found intrusive in the gneissic and supracrustal units that

have relic high-grade metamorphic parageneses, showing that the fissure mafic unit was placed in the

crust after the main Archean tectonothermal events of the Campo Belo Metamorphic Complex.


32

CAPÍTULO 3 ORIGIN OF CHARNOCKITES AND ENDERBITES INFERRED FROM

INCOMPATIBLE ELEMENT LOSS DURING PROGRADE DEHYDRATION.

3.1 ABSTRACT

Charnockites and enderbites of theCandeias Unit in the Campo Belo Metamorphic Complex,

in the southern part of the São Francisco Craton display unusual trace element patterns when

compared to typical Archaean granitoids of both upper and lower continental crust. Combination of

major and trace element geochemical evidence allowed detecting element losses caused by prograde

dehydration. Studied rocks from the Campo Belo Complex display relative depletion in some of the

most incompatible elements (i.e. Cs, Rb, U, Nb, Ta and W) in multi-element spider diagrams. Some

elements (i.e. Cs, Rb and U) have traditionally been considered mobile with fluids but we found that

others (i.e. Ta, W), which are often considered immobile, are also deficient in our samples. The

difference in averages of charnockites and enderbites studied and the average of Barberton Archaean

granitoids, upper continental crust (UCC) and lower crust (LC) are also well expressed by ratios of

some of the most incompatible elements. For example, the average of Nb/Ta in UCC is 12 and the

average in LC is 9.17, while our charnockites and enderbites yield 17.53 and 21.54, respectively

similar to many eclogites. Another unusual aspect of the rocks studied concerns the abundance of the

heat producing elements, which display the expected low U but higher K and Th concentrations than

UCC and lower crust. Therefore charnockites and enderbites are not important components of the

lower crust and metamorphic dehydration is not the reason for the low heat production in the LC.

Finally, characteristic loss of highly incompatible elements can be used as a fingerprint of

charnockites and enderbites that formed by metamorphic dehydration of granitoids.

Keywords: Campo Belo Complex, dehydration, incompatible elements, charnockites,

enderbites

3.2 INTRODUCTION

Orthopyroxene-bearing granites, granodiorites and tonalites are called charnockites, charno-

enderbites and enderbites, respectively (Holland 1900, Tilley 1936, Le Maitre et al. 1989, Frost et al.

2000). The term “charnockite series” was first used to describe a suite of rocks of granitic composition

containing orthopyroxene from Madras (Holland 1900). The original acid end-member of “charnockite

series” (i.e. hypersthene granite) was essentially defined as charnockite by Holland (1900). Tilley

(1936) defined the term enderbite when studying unusual rocks from Enderby Land (Antarctica),

containing hornblende or biotite, which is uncommon in the “charnockites”, described by Holland


34

(1900). “Charnockite series” occur from the Archaean to Phanerozoic, but are more common in

Archaean terrains. They have been studied in many places around the world including India (e.g.

Madras), Enderby Land-Antarctica (Napier Complex), Sri Lanka, Southern Africa (e.g. Limpopo

Belt), Brazil (e.g. Complexo Jequie), and Eastern Siberia. There is a profusion of scientific literature

covering the charno-enderbite theme yet no true consensus exists about the origin of at least some

charnockite occurrences including classic localities. Charnockites have at least three possible,

divergent origins. These are: (i) granitic rocks metamorphosed to the granulite facies (metamorphic)

(i.e. Friend 1981); (ii) granitoid rocks in which pyroxene crystallized directly from an anhydrous

magma (igneous) (i.e. Jordt-Evangelista 1996, Zhao et al. 1997, Percival & Mortensen 2002), and (iii)

granitoid rocks that recrystallised during CO2 flushing (arrested) (i.e. Harris & Bickle 1989, Santosh et

al. 1991a, b, Satish-Kumar & Santosh 1998). Not surprisingly then, there are many controversies and

discussions about the origin of many particular charnockite occurrences.

A good example of ongoing uncertainty is the origin of “arrested charnockite” in Chilka Lake

(India). Dobmeier & Raith (2000) postulated that it formed as a result of localized synkinematic fluid

migration and was genetically linked to the host leptynite in which it is enclosed. These authors also

proposed that the charnockites are completely unrelated to enderbite layers in the same outcrops. In a

comment, Bhattacharya & Sen (2002) claimed that many rocks interpreted by Dobmeier & Raith

(2000) as enderbite are, in reality, charnockites or charno-enderbites. Hence, Bhattacharya & Sen

(2002) proposed a common origin for charnockite and enderbite.

Friend (1981) claimed that the majority of charnockites crystallized from anhydrous magmas.

However, Bohlender et al. (1992) distinguished between two origins of charnockite in the southern

marginal zone of the Limpopo belt: one is magmatic with intrusive relationships and preserved

igneous textures; the other is metamorphic with preserved relict banding. Those authors based their

evidence on field relationships, petrography and bulk rock geochemistry. Frost et al. (2000) defined

charnockites (sensu lato) as rocks that have crystallized under low water activities and they further

claimed that such rocks are not restricted to a particular geologic time, a specific chemical

composition or a certain tectonic environment.

It is be beyond the scope of this study to summarise all conflicting views on charnockite and

enderbite petrogenesis. The few cases quoted here are sufficient to illustrate that since “charnockite

series” were first recognized by Holland (1900) these rocks have been a matter of continuos

controversy. In this study we will use geochemical data, based on interpreted fluid-mobility of

elements, to distinguish igneous from metamorphic “charnockite series”. To this end, we next discuss

recent geochemical evidence that we believe to be useful for identifying metamorphic dehydration.


35

3.3 BRIEF REVIEW OF RELEVANT GEOCHEMICAL RESEARCH

During metamorphic dehydration some major and trace elements are very mobile and tend to

be lost to escaping fluids. In a classic study of calcareous sediments from the Vassalboro Fromation,

Ferry (1983) showed the mobility of some major elements with increasing metamorphic grade. Ferry

(1983) discussed that during metamorphism, decarbonation and dehydration cause mass transport of

CO2, and H2O, which are accompanied by loss of K and Na. Much experimental work has since been

published in which effects of dehydration, trace element concentration, mineral/fluid partitioning and

fO2 was studied (e.g. Brenan et al. 1995a, b, 1998). The behaviour of major and trace elements and

REE during prograde and retrograde metamorphism has also been studied extensively in natural

metamorphic rocks (Allen & Condie 1985, Miller et al. 1994, Tribuzio et al. 1996, Bebout et al. 1999,

Becker et al. 1999, 2000). The combined observations from those studies have helped to improve

understanding of continental crust formation. Brenan et al. (1995a) studied the fluid/mineral partition

coefficients of some minerals from the upper mantle (e.g. garnet, amphibole, clinopyroxene and

olivine) to investigate the effects of fluids on trace element transport within the mantle. The most

important result obtained by experimental and empirical studies is that certain trace elements are

transported into the mantle more easily than similarly compatible chemical twins following slab

dehydration.

Brenan et al. (1995a) reported data for more lithophile behavior of B, Be and Li than could be

expected from relative incompatibility during progressive dehydration of oceanic crust. This explains

the overabundance of these elements in island arc lavas. In addition, the behavior of rare earth and

other trace elements from high-pressure rocks (e.g. eclogites) has been studied to better understand the

chemical inventory of continental crust as a whole (Miller et al. 1994, Brenan et al. 1995a, Tribuzio et

al. 1996, Bebout et al. 1999, Becker et al. 1999). Miller et al. (1994) and Brenan et al. (1995b)

proposed that the enrichment of Pb in continental crust is directly related to the transfer of Pb from

slabs in basalt-derived fluids during prograde metamorphism. An important principle established in

these studies is the use of trace element ratios between elements of similar compatibility in MORB-

melting (e.g. Pb/Ce or Pb/Nd). Tribuzio et al. (1996) presented results of REE redistribution during re-

equilibration of Fe-gabbros under blueschist and eclogite facies. They demonstrated that there is

incorporation of REE into lawsonite and titanite in blueshist facies and allanite-epidote in eclogite

facies. This shows the additional possibility of element transfer between metamorphic minerals.

Becker et al. (1999) proposed that there is preferential loss of some elements during subduction of

altered oceanic basalts (e.g. K, Ba, Cs, Rb, U, and Pb) but that other elements are largely immobile

(Th, Sr, Nd, Sm, Y, the HFSE, Cr, Co and Ni). As a result it was claimed that eclogites retain all Nb

despite extensive dehydration.


36

Allen & Condie (1985) examined in southern India the relationship between prograde and

retrograde gneiss-charnockite. They identified that there is a wide scatter of element abundances and

element ratios evidently caused by prograde and retrograde reactions. In the case of prograde

reactions, there is a small loss of Y, REE and Mg relative to Fe and gains of Ta, Pb and CO2. On the

other hand, retrogressed gneisses are enriched in Pb, Th, Hf, Rb, Zn relative to Co, Nb relative to Ta

and Hf relative to Ta. In both cases, the fluid phase contained a mixture of CO2, H2O and halogens.

In summary, study of metamorphic dehydration has demonstrated the preferential mobility of

certain elements, which are overabundant in continental crust. However, no systematic treatment,

using high-quality data, of dehydration-induced element depletion has been published for charno-

enderbites. In this paper we will show that despite the complexities of trace element transfer, it is

possible to identify systematic features that are clearly related to prograde dehydration.

A complication of any empirical study is that high-grade rocks are typically retrogressed (i.e.

re-hydrated) at least to some extent. Because it is unknown whether the retrograde fluids carried with

them fluid-mobile trace elements (such as Pb) the possibility of re-enrichment needs to be considered.

In view of this, we selected for this study extremely fresh samples, many of which were obtained in

quarries exposing unweathered fresh rock faces (Fig. 3.1a and b). Next we will describe the relevant

aspects of regional geology and sample characteristics.

3.4 SÃO FRANCISCO CRATON: RELEVANT GEOLOGIC RECORD

The crystalline basement of the São Francisco Craton (Fig. 3.2a) comprises Archaean medium

to high-grade metamorphic rocks, and granite-greenstone associations (Cordani et al. 2000). The

geological evolution of the Southern part of São Francisco Craton involved four metamorphic

complexes: Bação, Bonfim, Belo Horizonte and Campo Belo (Fig. 3.2a). They are composed

essentially of TTG gneisses, mafic intrusions and greenstone belts (e.g. Machado & Carneiro 1992,

Teixeira 1993, Teixeira et al. 1996, Carneiro et al. 1998a, b, Noce et al. 1998).

This study focused on rocks from Campo Belo Metamorphic Complex. Samples were

collected around the cities Oliveira, São Francisco de Paula, Itapecerica, Cláudio and Carmópolis de

Minas (Fig. 3.2b). Over the last 10 years, the general geological framework was established with

mapping, structural, geochemistry and geochronological studies (Machado Filho et al. 1983, Teixeira

& Figueiredo 1991, Teixeira 1993, Teixeira et al. 1996, Carneiro et al. 1998 a, b, Noce et al. 1998,

Teixeira et al. 1998, Correa da Costa 1999, Oliveira 1999, Fernandes & Carneiro 2000, Oliveira &

Carneiro 2001, Teixeira et al. 2000).


a

b

Figure 3.1 Panel (a) Photograph of massive charnockite with incipient foliation taken in Alemão dimension stone quarry (Candeias Unit). Panel (b) Photograph taken at Marilan dimension stone quarry (Itapecerica Unit) illustrating the quality of rocks collected from unweathered core. The rocks from this outcrop are pink to grey, strongly migmatized but of exceptional physical quality.

According to these authors, the region is formed predominantly by metamorphic rocks and

subordinately by igneous rocks, which jointly comprise geological records from the meso to neo-

Archaean. The dominant rock-type is migmatitic gneiss with schieren and stromatic structures and

mafic enclaves. After the first substantial sialic crust and supracrustal sequences were generated,

successive accretion and differentiation stages associated with crustal reworking affected this area

during the Archaean and Proterozoic (Teixeira et al. 1996).

37


38

Oliveira (1999) and Oliveira & Carneiro (2001) subdivided the studied area into a number of

lithological units (Cláudio, Itapecerica and Candeias Unit, Supracrustal Unit and Mafic Unit, Fig.

3.2b) whose features are summarised below.

3.5 SAMPLES SELECTION

For the present study, samples were selected from the Cláudio, Itapecerica and Candeias Units

(Oliveira 1999, Oliveira & Carneiro 2001). From each unit fresh samples were taken from different

outcrops and within outcrops care was taken to subsample visibly different gneiss varieties (Fig. 3.2b

(outcrops A-I)). Specifically, we chose 6 samples from Candeias unit (charnockites and enderbites

from 2 outcrops; A and B), 7 samples from Itapecerica Unit (migmatized gneisses and granitoid from

3 outcrops; C, D and E) and 9 samples from Claudio Unit (gneisses, migmatized gneisses, and

granitoid from 4 outcrops; F, G, H and I).

3.6 PETROGRAPHY FEATURES

The charnockites in the studied area crop out along Oliveira and São Francisco de Paula Town

(Fig. 3.2b). The rocks were collected at Oliveira stone quarry and Alemão stone quarry (Fig. 3.2b, A

and B). In most domains the rocks exhibit massive textures with incipient foliation. These rocks are

olive green, slightly to strongly migmatized, and medium- to coarse-grained. The mineralogical

banding is difficult to identify macroscopically giving a homogeneous aspect to the bodies. The

texture of the gneisses varies from granolepidoblastic to granoblastic. The grains are predominantly

medium-sized and show intergrowths of the perthite, myrmekite and antiperthite types. The variety

from the Oliveira outcrop (samples 7A and B, Table 3.1) is slightly to strongly deformed. The main

constituents from this group are antiperthitic plagioclase (30-45%), quartz (30%), microcline (10-

20%), biotite (<10%), amphibole (~1%), hypersthene (~1-3%), and the commonest accessories are

apatite, zircon, and an opaque mineral. In some thin sections hypersthene is absent. The charnockite

from Alemão quarry (samples 8A, B, C and D, Table 3.1) is only slightly deformed; massive in hand

specimen and with grains that vary from medium to coarse (Fig. 3.1b). Under the microscope,

microcline (40-50%) is the main constituent followed by plagioclase (25-30), quartz (20-25), biotite

(~5), hypersthene, and rare amphibole.

Accessories are represented by apatite, zircon, and opaques. Chlorite is sometimes found as a

secondary mineral, overgrowing biotite. In general, plagioclase is medium-grained, with subhedral to

anhedral crystals. Quartz is medium- to fine-grained, anhedral, with undulatory extinction and

generation of sub grains. Microcline shows perthite intergrowth and pericline twinning in subhedral to

anhedral grains. Hypersthene, when present, is medium-grained, has low birefringence and

pleochroism varying from light pink to pale green. In some samples it is altered to amphibole, biotite


and chlorite along the rims. Biotite, sometimes showing symplectitic intergrowth with quartz, has light

brown pleochroism. The main accessories are opaques, zircon and apatite.

Figure 3.2 – Panel (a) -Geological map of the southern São Francisco Craton (modified by Campos Sales 2004). Key: 1- Neoproterozoic undivided cratonic cover; 2- Mesoproterozoic Espinhaço Supergroup; 3- Mesoproterozoic(?) São João Del Rei/Andrelândia Groups; 4-- Mesoproterozoic (?), Dom Silvério Group; 5- Paleoproterozoic granitoids; 6- Paleoproterozoic indiscriminate greenstone-type sequences; 7- Paleoproterozoic

39


40

Minas Supergroup; 8- Neoarchean granitoids; 9- Neoarchean and Mesoproterozoic gabbroic and dioritic rocks (sills and dikes); 10- Neoarchean ultramafic plutonic suite; 11- Neoarchean Rio das Velhas Supergroup; 12- Archean metamorphic complexes partially reworked on Proterozoic time; 13- Faults and fractures (CSZ = Cláudio Shear Zone; JBSZ = Jeceaba-Bom Sucesso Shear Zone); 14- Fold axes; 15- Lithologic contacts; 16- Cities. Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001) showing the following features: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A – Alemão dimension stone quarry; B – Oliveira dimension stone quarry; C - Lila dimension stone quarry; D – Itapecerica dimension stone quarry; E - Marilan dimension stone quarry; F - Kinawa dimension stone quarry; G - Corumbá dimension stone quarry; H – Fazenda Corumbá dimension stone quarry and, I – Carmópolis de Minas dimension stone quarry.

The samples from Itapecerica Unit are pink and locally dark gneiss bodies. This unit is

strongly migmatized and the rocks are fine to medium grained (Fig. 3.1a). The rocks are

predominantly of granitic composition but can vary to granodioritic. The main constituents of these

rocks are microcline followed by quartz, plagioclase, biotite, apatite, zircon, opaques and epidote. The

samples from the dark parts (maybe the protolith) are granodioritic to tonalitic in composition with

abundant amphibole and some pyroxene.

The Claudio Unit samples are gneisses of granodioritic to tonalitic, locally granitic

compositions. They were chosen for comparison with charnockites. They are predominantly grey but

in some outcrops (e.g. Kinawa, Fig. 3.2b-F), they are locally pink due to the occurrence of pegmatite

fluids. At Fazenda Corumba stone quarry the rocks vary from dark, grey to white (granodioritic to

granitic). The rocks are fine- to medium-grained, slightly to strongly migmatized. In general,

plagioclase is the main constituent of these rocks followed by quartz, microcline, biotite, amphibole

(not common), zircon, apatite, titanite (not common), opaques, white-mica, chlorite and epidote. Some

samples show antiperthitic intergrowth. The main petrographic differences of the units are:

compositional (i.e. presence of hypersthene), grade of migmatization and color of the bodies (i.e.

green, pink, grey, dark and white).

3.7 ANALYTICAL METHODS

Whole rock samples where analysed for major and trace elements. Major element contents

were analysed by ICP-OES (Inductively coupled plasma optical emission spectrometry). These

analyses were carried out at Earth Science Department of the University of Queensland.

ICP-OES

Samples were analysed for all 10 major oxides at the Department of Earth Sciences,

University of Queensland by Mr Michael Lawrence. Major oxides were determined by a Lithium

Metaborate fusion of silicate rocks, followed by analysis using Inductively Coupled Plasma-Optical

Emission Spectroscopy (ICP-OES). Iron is determined as total ferric iron and expressed as Fe2O3T.


41

Procedure - 50mg of powdered rock sample was weighed into graphite crucibles and mixed

with 200mg of LiBO2. The samples were then fused for 60 mins at 1000°C. Fused samples were

transferred to 100mL Teflon beakers containing 25mL of 10% HNO3 solution containing a 10ppm Lu

internal standard. The samples were stirred, covered and allowed to sit until they were fully dissolved

(approximately 120-180 mins, although in some cases overnight). 25mls of deionised water was then

added to bring the final concentration of silicate rock to 50mg/50ml.

All 10 major oxides are determined directly from the solution using matched silicate reference

materials as calibrating standards. Calibration standards were prepared in the same manner. Accurate

calibration curves were achieved by fusing 25mg of the calibration standard to provide a 3-point

calibration curve. Where possible, the concentration of each element is determined at a number of

wavelengths to ensure that interferences are minimised. The calibration standards and another

reference standard were run as unknowns at then end of each batch for quality control purposes. The

Lu signal is monitored to allow for correction of instrument drift.

ICP-MS

Rare earth element (REE) and trace element abundances were analysed by ICP-MS

(Inductively coupled plasma mass spectrometry).

The trace elements were determined by ICPMS at the Advanced Centre for Queensland

University Isotopic Research Excellence (ACQUIRE). Sample preparation and analytical procedures

used were identical to those of Eggins et al. (1997) except that Tm was not used as an internal standard

and W-2, a U.S.G.S. diabase standard, was used as the calibration standard. Our preferred

concentrations for W-2 and the measured concentrations and relative standard deviations for AGV-1

(an average of 26 analyses of 9 digestions analysed over four years) are shown in Table 1.

Concentrations for W-2 were derived partly by analysing it relative to synthetic standards (Li, Cr, Ni,

Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th, and U) or are based on an assessment of published

standard data (A. Greig, pers. comm.). Results are reported in Table 3.1.


Table 3.1 Major (wt%), trace element (ppm) concentrations and selected trace element ratios of Campo Belo Metamorphic Complex.

42

034260

49

.7

.5

33

01

Gneiss UnitRock typeSample ID AO-07A AO-07B average AO-08A AO-08B AO-08C AO-08D average

SiO2 67.45 68.73 68.09 73.11 73.41 74.33 75.02 73.97TiO2 0.49 0.39 0.44 0.22 0.21 0.20 0.23 0.22Al2O3 14.71 15.64 15.17 13.46 13.36 13.44 13.34 13.40Fe2O3 4.16 3.08 3.62 1.74 1.69 1.48 1.66 1.64MnO 0.06 0.04 0.05 0.03 0.03 0.03 0.03 0.MgO 1.75 1.08 1.42 0.46 0.44 0.33 0.45 0.CaO 3.23 3.27 3.25 1.63 1.61 1.56 1.62 1.Na2O 4.23 4.69 4.46 3.46 3.35 3.29 3.35 3.36K2O 1.60 1.57 1.59 4.05 4.01 4.27 4.28 4.15P2O5 0.12 0.23 0.17 0.06 0.03 0.04 0.04 0.04

Cs 0.68 0.51 0.59 0.34 0.33 0.32 0.32 0.33Rb 58.06 49.68 53.87 96.15 96.70 99.37 98.64 97.71Ba 533 462 497 1083 1080 1122 1143 1107Th 6.02 8.60 7.31 25.31 27.16 24.67 20.87 24.50U 0.47 0.40 0.44 0.52 0.49 0.52 0.45 0.Nb 11.59 9.17 10.38 5.59 5.22 5.94 5.66 5.60Ta 0.78 0.41 0.60 0.23 0.24 0.29 0.27 0.26W 0.02 0.01 0.01 0.02 0.03 0.03 0.03 0.03La 28.03 36.15 32.09 53.90 44.81 52.41 46.60 49.43Ce 51.96 62.64 57.30 101.93 83.75 98.41 87.78 92.97Pr 5.69 6.31 6.00 10.72 8.80 10.24 9.17 9.73Pb 13.16 15.20 14.18 27.04 27.21 29.82 27.47 27.88Sr 377.28 405.84 391.56 163.55 160.54 157.90 155.97 159.49Nd 20.26 20.86 20.56 35.76 29.33 33.87 30.25 32.30Be 1.66 1.66 1.66 0.89 0.82 0.75 0.78 0.81Zr 131.38 166.29 148.84 114.02 101.61 143.93 125.39 121.24Hf 3.24 4.30 3.77 2.97 2.67 3.81 3.38 3.21Sm 3.83 3.10 3.47 5.78 4.76 5.52 4.86 5.23Eu 1.00 1.03 1.01 0.76 0.73 0.74 0.75 0.75Gd 3.45 2.36 2.91 4.50 3.72 4.14 3.56 3.98Tb 0.52 0.31 0.42 0.57 0.48 0.54 0.46 0.51Dy 2.92 1.58 2.25 2.79 2.39 2.57 2.22 2.49Ho 0.58 0.29 0.44 0.51 0.45 0.47 0.41 0.46Y 15.24 7.30 11.27 12.88 11.43 11.79 10.08 11.54Er 1.52 0.73 1.13 1.20 1.09 1.10 0.96 1.09Tm 0.22 0.10 0.16 0.15 0.14 0.15 0.13 0.14Yb 1.32 0.59 0.96 0.90 0.85 0.87 0.76 0.84Lu 0.19 0.09 0.14 0.13 0.13 0.13 0.12 0.13Li 22.26 22.85 22.55 18.62 18.00 17.12 17.69 17.86

Hf/W 200 487 301 125 102 117 129 118Th/U 12.8 21.3 16.8 49.1 55.2 47.6 46.9 49Nb/Ta 14.9 22.3 17.4 24.0 21.4 20.4 20.8 21La/W 1727 4094 2561 2266 1711 1611 1774 1818Cs/Rb 0.012 0.010 0.011 0.004 0.003 0.003 0.003 0.003Sr/Nd 18.6 19.5 19.0 4.6 5.5 4.7 5.2 4.9Ce/Pb 3.95 4.12 4.04 3.77 3.08 3.30 3.20 3.La/Yb 21.2 61.3 33.5 60.1 52.5 60.2 61.6 58.5Th/Cs 8.87 16.93 12.32 73.66 81.70 77.96 65.79 74.83Cs/Th 0.11 0.06 0.08 0.01 0.01 0.01 0.02 0.Eu/Eu* 0.82 1.10 0.94 0.43 0.51 0.45 0.53 0.48

Major element (WT%)

Trace element (ppm)

Selected trace element ratios

CandeiasEnderbite Charnockite


Gneiss UnitRock type Dark Mig.Sample ID AO-01 AO-03 AO-10 AO-02 average AO-09A AO-9B average AO-04

SiO2 71.92 70.68 72.31 71.83 71.69 72.01 72.01 69.46TiO2 0.14 0.26 0.16 0.23 0.20 0.17 0.17 0.50Al2O3 13.00 14.34 13.89 13.69 13.73 12.91 12.91 15.45Fe2O3 0.91 1.91 1.35 1.69 1.47 1.99 1.99 3.09MnO 0.01 0.03 0.03 0.03 0.03 0.04 0.04 0.04MgO 0.03 0.28 0.04 0.06 0.10 0.04 0.04 0.55CaO 0.49 3.19 1.17 1.25 1.52 0.77 0.77 2.72Na2O 2.53 3.84 3.78 3.03 3.29 3.30 3.30 4.85K2O 7.10 3.96 4.54 5.79 5.35 5.17 5.17 2.00P2O5 0.04 0.08 0.01 0.04 0.04 0.03 0.03 0.12

Cs 0.92 1.06 1.38 0.96 1.08 2.39 1.60 2.00 2.33Rb 211.79 137.34 200.38 187.25 184.19 304.37 247.24 275.81 134.34Ba 1133 646 693 746 804 278 238 258 810Th 3.77 29.26 29.66 41.12 25.95 51.72 35.02 43.37 18.56U 0.60 2.11 4.08 2.21 2.25 27.38 21.98 24.68 2.39Nb 5.53 12.66 17.39 8.48 11.01 37.27 26.64 31.96 7.72Ta 0.19 0.31 0.93 0.17 0.40 1.90 1.18 1.54 0.48W 0.01 0.01 0.08 0.01 0.03 0.07 0.04 0.05 0.10La 10.10 49.45 37.36 109.65 51.64 58.98 42.43 50.70 40.65Ce 17.24 94.61 72.66 216.28 100.20 127.38 91.86 109.62 79.04Pr 1.73 10.17 8.03 23.01 10.73 14.49 10.37 12.43 8.39Pb 36.40 29.16 29.82 39.57 33.74 39.99 33.84 36.91 27.28Sr 131.34 141.10 142.91 104.19 129.89 52.38 44.10 48.24 155.16Nd 5.61 34.78 28.16 75.75 36.07 50.11 35.67 42.89 27.61Be 0.45 1.65 2.11 0.88 1.27 2.58 1.80 2.19 2.06Zr 14.24 176.11 135.34 252.42 144.53 178.68 143.89 161.28 196.93Hf 0.46 4.82 4.40 7.58 4.31 6.07 4.91 5.49 6.27Sm 0.88 6.70 6.50 12.52 6.65 12.23 8.42 10.33 4.62Eu 0.76 0.87 0.65 0.88 0.79 0.41 0.33 0.37 0.70Gd 0.66 5.44 6.62 8.90 5.41 13.00 8.72 10.86 3.56Tb 0.09 0.72 1.18 1.10 0.78 2.41 1.65 2.03 0.48Dy 0.50 3.50 7.39 5.00 4.10 15.05 10.39 12.72 2.28Ho 0.11 0.61 1.59 0.81 0.78 3.16 2.20 2.68 0.39Y 2.81 14.60 49.08 18.91 21.35 92.09 61.79 76.94 10.16Er 0.32 1.40 4.64 1.75 2.03 8.97 6.23 7.60 0.91Tm 0.05 0.17 0.74 0.21 0.29 1.37 0.95 1.16 0.12Yb 0.32 0.97 4.74 1.20 1.81 8.26 5.73 6.99 0.74Lu 0.05 0.15 0.70 0.18 0.27 1.18 0.82 1.00 0.12Li 15.86 21.27 20.15 16.58 18.46 41.56 28.94 35.25 23.75

Hf/W 33 442 53 591 143 89 121 101 65Th/U 6.3 13.9 7.3 18.6 11.5 1.9 1.6 1.8 7.8Nb/Ta 28.6 41.5 18.7 49.7 27.5 19.6 22.6 20.7 16.1La/W 738 4531 448 8557 1710 867 1044 933 424Cs/Rb 0.004 0.008 0.007 0.005 0.006 0.008 0.006 0.007 0.017Sr/Nd 23.4 4.1 5.1 1.4 3.6 1.0 1.2 1.1 5.6Ce/Pb 0.47 3.24 2.44 5.47 2.97 3.19 2.71 2.97 2.90La/Yb 31.2 50.7 7.9 91.6 28.6 7.1 7.4 7.2 54.7Th/Cs 4.08 27.48 21.50 42.79 23.98 21.60 21.82 21.69 7.98Cs/Th 0.24 0.04 0.05 0.02 0.04 0.05 0.05 0.05 0.13Eu/Eu* 2.88 0.42 0.30 0.24 0.39 0.10 0.12 0.11 0.50

Itapecerica

Trace element (ppm)

Fine Migmatite

Table 1-continued


Major element (WT%)

Migmatite (pink + Grey)

43


Gneiss Unit ItapecericaRock type Dark GneissSample ID AO-06 AO-11 AO-15A AO-15B AO-17 AO-18 average

SiO2 61.45 70.98 73.16 73.61 72.58TiO2 1.01 0.36 0.25 0.18 0.26Al2O3 15.39 15.66 13.48 13.70 14.28Fe2O3 6.41 3.00 2.11 1.29 2.14MnO 0.11 0.05 0.04 0.02 0.04MgO 2.54 0.70 0.20 0.45CaO 3.43 3.40 1.19 1.16 1.92Na2O 4.18 4.64 2.99 3.27 3.64K2O 2.66 1.62 5.31 5.39 4.11P2O5 0.76 0.08 0.04 0.00 0.04

Cs 3.87 1.42 1.92 1.75 1.13 1.27 1.50Rb 240.35 62.30 176.96 188.79 107.08 152.42 137.51Ba 527 336 856 703 444 373 543Th 24.53 8.44 32.67 39.96 19.03 32.98 26.61U 6.02 1.09 4.78 5.25 2.45 6.60 4.03Nb 30.72 11.18 12.66 13.82 13.32 8.60 11.91Ta 2.61 0.50 0.51 0.43 0.94 0.18 0.51W 0.12 0.08 0.09 0.04 0.03 0.04 0.06La 75.71 27.25 61.08 74.34 57.59 29.90 50.03Ce 157.71 51.56 121.64 150.37 97.50 55.80 95.37Pr 18.57 5.75 13.24 16.35 9.89 6.49 10.35Pb 15.91 14.67 31.49 34.51 18.95 34.38 26.80Sr 347.78 230.42 132.67 121.70 208.17 66.45 151.88Nd 69.31 20.72 44.18 54.70 32.68 22.57 34.97Be 3.12 2.06 1.65 1.68 2.25 1.31 1.79Zr 209.89 257.71 213.25 229.29 333.04 153.50 237.36Hf 5.06 6.43 5.89 6.57 7.73 4.76 6.28Sm 13.18 4.42 8.11 9.95 5.41 4.57 6.49Eu 2.18 1.04 0.76 0.77 1.19 0.58 0.87Gd 10.82 4.34 6.12 7.44 4.37 3.71 5.20Tb 1.64 0.65 0.82 1.01 0.64 0.47 0.72Dy 9.14 3.46 3.90 4.86 3.35 2.14 3.54Ho 1.78 0.66 0.67 0.83 0.64 0.35 0.63Y 46.82 15.98 17.87 20.44 18.37 9.09 16.35Er 4.61 1.56 1.59 1.86 1.70 0.78 1.50Tm 0.65 0.20 0.20 0.23 0.24 0.09 0.19Yb 3.79 1.13 1.13 1.27 1.49 0.54 1.11Lu 0.52 0.16 0.17 0.19 0.22 0.09 0.17Li 63.59 28.29 17.41 19.37 28.36 15.74 21.84

Hf/W 41 77 62 148 300 133 110Th/U 4.1 7.8 6.8 7.6 7.8 5.0 6.6Nb/Ta 11.7 22.6 24.7 32.2 14.1 46.8 23.2La/W 611 325 644 1676 2237 835 879Cs/Rb 0.016 0.023 0.011 0.009 0.011 0.008 0.011Sr/Nd 5.0 11.1 3.0 2.2 6.4 2.9 4.3Ce/Pb 9.91 3.52 3.86 4.36 5.14 1.62 3.56La/Yb 20.0 24.2 54.0 58.8 38.7 54.9 45.1Th/Cs 6.33 5.92 16.98 22.82 16.80 25.94 17.74Cs/Th 0.16 0.17 0.06 0.04 0.06 0.04 0.06Eu/Eu* 0.54 0.71 0.31 0.26 0.72 0.41 0.44

Migmatite

Trace element (ppm)


Table 1-continuedCláudio

Major element (WT%)

44


45

02

63

87

28

1812

Gneiss UnitRock typeSample ID AO-20A AO-20B average AO-14 AO-16 average AO-12 AO-19 average

SiO2 72.05 72.05 64.72 68.68 66.70 66.75 73.44 70.09TiO2 0.40 0.40 1.15 0.61 0.88 0.05 0.03 0.04Al2O3 15.08 15.08 14.10 14.59 14.35 17.41 13.15 15.28Fe2O3 2.81 2.81 6.93 4.86 5.90 2.06 0.28 1.17MnO 0.06 0.06 0.10 0.07 0.08 0.03 0.01 0.MgO 0.68 0.68 1.50 1.00 1.25 -0.15 -0.15CaO 2.65 2.65 2.60 3.09 2.84 2.27 1.00 1.Na2O 4.56 4.56 3.86 4.32 4.09 4.11 2.83 3.47K2O 2.27 2.27 2.59 2.10 2.34 5.72 5.83 5.78P2O5 0.13 0.13 0.42 0.18 0.30 0.10 0.01 0.06

Cs 3.03 2.98 3.00 4.14 4.19 4.16 2.17 1.66 1.92Rb 84.89 93.28 89.08 214.28 147.94 181.11 119.78 162.83 141.31Ba 398 525 461 498 170 334 1399 970 1185Th 15.80 9.77 12.79 31.20 20.51 25.85 16.49 14.90 15.70U 1.78 3.54 2.66 8.13 3.54 5.83 5.91 1.84 3.Nb 12.03 12.57 12.30 40.40 49.45 44.92 2.61 2.44 2.52Ta 0.82 0.86 0.84 3.26 2.52 2.89 0.35 0.32 0.34W 0.05 0.05 0.05 0.23 0.06 0.15 0.12 0.08 0.10La 38.95 32.14 35.55 98.49 49.49 73.99 20.45 23.31 21.88Ce 80.29 65.37 72.83 193.19 101.43 147.31 39.44 46.48 42.96Pr 9.57 7.89 8.73 21.24 11.69 16.46 4.41 5.05 4.73Pb 17.79 18.71 18.25 19.64 18.17 18.91 35.17 32.13 33.65Sr 289.05 298.55 293.80 133.48 97.31 115.39 256.11 153.69 204.90Nd 37.19 30.60 33.90 73.02 42.25 57.63 16.02 17.02 16.52Be 2.05 1.92 1.99 4.27 3.09 3.68 1.60 1.43 1.51Zr 169.35 154.27 161.81 567.68 327.29 447.48 163.65 10.19 86.92Hf 4.64 4.14 4.39 13.25 9.08 11.17 4.48 0.37 2.43Sm 8.75 7.51 8.13 13.80 9.86 11.83 3.46 3.18 3.32Eu 1.20 1.28 1.24 1.38 0.82 1.10 1.12 0.72 0.92Gd 8.61 8.19 8.40 13.32 10.79 12.06 3.17 2.32 2.74Tb 1.27 1.42 1.34 2.20 1.86 2.03 0.43 0.31 0.37Dy 6.63 8.61 7.62 13.11 10.74 11.93 2.09 1.51 1.80Ho 1.17 1.72 1.45 2.70 2.06 2.38 0.38 0.25 0.32Y 28.87 43.52 36.20 76.51 53.21 64.86 9.57 6.69 8.13Er 2.72 4.41 3.57 7.44 5.10 6.27 0.88 0.62 0.75Tm 0.34 0.60 0.47 1.10 0.64 0.87 0.11 0.08 0.10Yb 1.87 3.19 2.53 6.62 3.33 4.97 0.67 0.51 0.59Lu 0.25 0.41 0.33 0.94 0.42 0.68 0.10 0.07 0.09Li 43.79 45.51 44.65 45.51 66.78 56.15 14.15 5.94 10.04

Hf/W 102 91 97 58 147 77 38 4 24Th/U 8.9 2.8 4.8 3.8 5.8 4.4 2.8 8.1 4.1Nb/Ta 14.7 14.6 14.6 12.4 19.7 15.6 7.5 7.5 7.5La/W 856 707 781 429 801 508 172 280 216Cs/Rb 0.036 0.032 0.034 0.019 0.028 0.023 0.018 0.010 0.014Sr/Nd 7.8 9.8 8.7 1.8 2.3 2.0 16.0 9.0 12.4Ce/Pb 4.51 3.49 3.99 9.84 5.58 7.79 1.12 1.45 1.La/Yb 20.8 10.1 14.0 14.9 14.9 14.9 30.5 45.8 37.1Th/Cs 5.22 3.28 4.26 7.53 4.90 6.21 7.59 8.95 8.Cs/Th 0.19 0.31 0.23 0.13 0.20 0.16 0.13 0.11 0.Eu/Eu* 0.41 0.50 0.45 0.31 0.24 0.28 1.01 0.77 0.90

Table 1-continuedCláudio


Dark gneiss GranitoidGray gneiss

Trace element (ppm)

Major element (WT%)


3.8 RESULTS

The dataset obtained in this study is given in Table 3.1. Based on the rock classification

diagram in Na2O x CaO x K2O space (Glikson 1979), the studied rocks classify as thondhjemites,

tonalites, granodiorites, adamellites and granites (Fig. 3.3a). The orthopyroxene-bearing rocks from

Candeias Unit fall into fields 1 and 4 (tonalite and granite composition) and thus classify as enderbite

and charnockite, respectively. The rocks from Itapecerica and Claudio Units, petrographicaly defined

as dark and grey gneisses, have chemistry typical of Archaean TTG (samples 4, 6, 11, 16, 17 and

20A). Sample 11 is a migmatite from Claudio Unit.

1 2

3

45

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10Yb

La/Y

b

0

1

2

3

4

5

6

7

8

9

10

60 65 70 75 80SiO2

MgO

+Fe

O+M

nO2

3

0

1

2

3

4

5

6

78

9

10

0 2 4 6 8%K2 O

Na O2 K O2

CaO

a b

c d

Na

O+K

O-C

aO2

2

Figure 3.3 (a) Chemical classification diagram in Na2O x CaO x K2O space (Glikson 1979) used to divide the studied rocks into: 1 – tonalities, 2 - granodiorites, 3 - adamellites, 4 – granites, and 5 – thondhjemites. (b) (MgO + Fe2O3 + MnO) vs SiO2 showing the expected trend from fractionation of ferromagnesian silicates and Fe-oxides. (c) (Na2O + K2O – CaO) vs K2O diagram shows compositional relationship for Archaean gneisses and granitoids, in which studied samples plot along the calc-alkaline trend. (d) Plot of La/Yb vs Yb comparing rocks from Candeias, Claudio and Itapecerica Units. The trend shows that studied rocks vary from high La/Yb and low Yb to low La/Yb and high Yb contents but the majority of our rocks (mainly charnockite series) are falling into the Archaean TTG field, characterized by HREE-depletion. Solid diamonds = Candeias Unit, solid square = Itapecerica, and open triagle = Claudio Unit.

46


47

3.8.1 Similarities and differences from our rocks compared with typical Archaean

granitois from Barberton normalized by N-MORB

The average of rocks from Candeias, Itapecerica and Cláudio Units in N-MORB normalized

spidergrams (Fig. 3.4 a, b and c), indicate that the three units display some similarities in terms of

fractionation. Otherwise, they record some interesting aspects in terms of concentration of several

elements that will be explained in detail for each graph, principally when they are compared with

typical Archaean granitoids from Barberton.

In Harker variation diagrams, the suites from the three units form moderately coherent trends

typical of differentiation of calc-alkaline series. In (MgO + FeO + MnO) versus SiO2 (Fig. 3.3b), an

excellent negative correlation is obtained attesting to the removal of ferromagnesian silicates

(pyroxene + amphibole) ± oxides. Successive fractionation of sodic plagioclase followed by K-

feldspar explains the strong trend in (K2O + Na2O – CaO) vs K2O space (Fig. 3.3c). Migmatite

samples tend to scatter most widely but in general the studied samples appear to have preserved their

major element coherence.

The similarity with other Archaean granitoids extends to REE systematics. The majority of

samples are strongly depleted in heavy REE (HREE) and some samples are also enriched in light REE

(LREE). These yields steep MORB-normalised REE patterns (Fig. 3.4 a, b and c) and La/Yb vs Yb

systematics characteristic of Archaean TTG (Fig. 3.3 d, Martin 1986). A few samples, particularly

some of the migmatites and grey gneisses, are not as depleted in HREE. In the case of the migmatites,

this may be the result of segregation of major REE hosts (mainly apatite; Ayres & Harris 1997) but in

the grey gneisses, high relative HREE abundance is a primary feature. There is variety in the relative

abundance of Eu (e.g. Fig. 3.4) which tightly correlates with the abundances of Al2O3 and Sr. This

strongly suggests that plagioclase removal has depleted the more potassic melts preferencially in Eu

(compared to Sm and Gd).

In multielement spider diagrams the studied rocks also show general similarity with typical

Archaean granitoids and upper continental crust. When elements are arranged in order of decreasing

MORB-melting incompatibility, all studied rocks display a strong negative slope. Some of the typical

continental anomalies (positive Pb and Li; negative Nb and Ta) are also well-developed. However, our

main thesis is that the studied rocks, despite all similarities with typicall TTG, show very remarkable

deviations that are not easily appreciated on chondrite or MORB-normalised plot.


Candeias

0.1

1

10

100

1000

N-M

OR

B no

rma

lised

Itapecerica

0.1

1

10

100

1000

N-M

OR

B n

orm

alis

ed

0.01

0.1

1

10

100

1000

N-M

OR

B n

orm

alis

ed

a b

c

Cs

Rb

Ba Th U Nb

Ta W La Ce

Pr Sr Nd

Zr Hf

Sm Eu Gd

Ti Tb Dy

Ho

Y Er Tm Yb Lu

Cs

Rb

Ba

Th

U Nb

Ta W La Ce

Pr

Sr

Nd

Zr

Hf

Sm

Eu

Gd

Ti

Tb

Dy

Ho

Y Er

Tm

Yb

Lu

Cláudio

Cs

Rb

Ba

Th

U Nb

Ta W La

Ce

Pr

Sr

Nd

Zr

Hf

Sm

Eu

Gd

Ti

Tb

Dy

Ho

Y Er

Tm

Yb

Lu

Figure 3.4 N-MORB-normalized trace element patterns of average gneisses from A – Candeias, B – Itapecerica, and C – Claudio Units. Elements arranged in order of decreasing incompatibility in MORB-melting (after Sun & McDonough 1989). Normalizing values were taken from Sun & MacDough (1989) except W where the average of Indian, Pacific and Atlantic MORB was calculated from data presented by Newson et al. (1996). All patterns of all rock types from the three units display strong fractionation of LREE over HREE and many other typical features of Archaean TTG gneisses. Enderbites (solid black diamonds) from Candeias unit have virtually identical patterns to charnockites s.s. (solid gray squares). No significant differences are found for the three gneiss types of the Itapecerica Unit (solid dark grey diamond represents pink migmatite, open square represents fine migmatite, and solid grey triangles is the dark gneiss, believed to be the protolith of the migmatites). In Claudio Unit, the four differentiated gneiss types have patterns similar in shape but there is a difference in total abundance of trace elements, particularly the HREE. The most depleted rocks are granitoids (solid grey circles), which are believed to have formed as a result of migmatization. The most enriched gneisses are grey migmatites (solid grey diamond) while grey gneisses (open diamonds) and dark gneisses (solid grey triangles) are intermediate. The similarity of all studied gneisses to typical Archaean granitoids is evident in panel (a) from comparison with average upper continental crust (open circles) of McLennan (2001) and average 3.0-3.2 Ga Barberton granitoids (solid dark triangle; Kleinhanns et al. in press). However, appreciable differences exist in some of the most incompatible elements (see Figs. 3.5 and 3.6).

3.8.2 Deviations of expected trace element abundances

Closer inspection of the multi-element spider diagrams on Fig. 3.5 reveals that the studied

rocks display irregular abundance of some of the most incompatible trace elements, when compared to

the patterns of typical upper continental crust or averages of Archaean granitoids. These features are

present in most of the studied rock types but best expressed by the charnockites and enderbites of the

48


49

Candeias unit (Figs. 3.5 and 3.6a). It can be seen that the Candeias gneisses are depleted in Cs, Rb,

and U by a factor of 10-50. While this could be attributed to mobility of these LILE during low T

processes (but we emphasise that these are unaltered, extremely fresh samples), the irregularities in W

and Ta are certainly not due to weathering or alteration. Indeed, both charnockites and enderbites show

negative slopes in their Nb-Ta tie lines, which imply superchondritic Nb/Ta ratios (higher than

MORB). This is a rare feature in terrestrial rocks. Equally, the W/La ratios of the Candeias gneisses

are more than a factor of 10 lower than in average Barberton highlands granitoids.

While these features are most evident in the orthopyroxene-bearing Candeias gneisses, many

lithologies from Itapecerica and Claudio share similar properties (Fig. 3.6b and c). General depletion

in W, U, and Ta is found in all Itapecerica gneisses, while Cs and Rb deficits are more variable. The

Claudio unit gneisses are all depleted in U and W but the behaviours of Ta, Rb and Cs are more akin

to typical TTG.

While it might be argued that these deviations are trivial, we point out that logarithmic

chondrite or MORB normalisation is not suitable to expose details of the most incompatible elements

at continental crust abundance. This point is illustrated on Fig. 3.5, where the identical data are

normalised to typical upper continental crust. In this normalisation, the deficiencies are readily

appreciated. The Candeias gneisses (enderbites and charnockites) show the typical depletion in HREE,

some irregularities in fluid-mobile elements like Pb and Be but very strong negative anomalies in W,

Ta, U, Rb and Cs.

Focussing on the most incompatible elements only (Fig. 3.6) it becomes obvious that average

granitoid gneisses from Barberton show a much smoother pattern with only a small anomaly in Cs.

These rocks do show a pronounced negative W anomaly, which we attribute to the uncertainty in

upper crustal W abundance (i.e. an artefact of normalisation). However, the important point is that the

Candeias gneisses are a factor of 10 times more depleted in W still. The depletions in Ta, U, Rb and

Cs are also easily exposed. Gneisses from Itapecerica show equally deep W and Cs deficits but are less

coherent in their depletion of Ta and U. No Rb-depletion is evident. The averages of Claudio unit

gneisses are apparently less depleted in these trace elements. However, individual samples can also

show comparable depletions, while other show slight enrichment (relative to the average) in Ta, U and

Rb.


Candeias

0.001

0.01

0.1

1

10

Cs

Rb

Ba

Th

U Nb Ta

W

La

Ce Pr

Pb Sr

Nd

Be Z r

Hf

Sm

Eu

Gd Tb

Dy

Ho Y

Er

Tm

Yb

L u

Li

Upp

er C

rust

nor

mal

ised

Figure 3.5 – Upper Continental Crust normalization of same data (average of Candeias Unit) as shows in Fig. 3.4. Normalizing values were taken from McLennan (2001). This demonstrates clearly that certain elements (see below) are underabundant in comparison with typical Archaean granitoids as can be seen in detail in Fig. 3.6. The fractionation of HREE is evident in Candeias Unit. No major differences are found between the three units with the exception of the fine gneiss from Itapecerica Unit (open square). In all units unusual deficiencies in the following elements are evident: Cs, U, Ta, and W while Be, Rb and maybe Sr are moderately depleted.

3.9 DISCUSSION

The combination of geochemical features that characterise charnockites and enderbites from

the Campo Belo Metamorphic Complex is unusual when compared to most published studies of

granitoid rocks (Figs. 3.4, 3.5, 3.6). However, we are not aware of a single published geochemical

study that reports the full suite of incompatible trace elements for orthopyroxene-bearing granitoids

and it remains to be seen whether the features discovered here are typical of charnockites and

enderbites in general. Regardless, at this stage the main issue is to evaluate the likely processes that

could have caused these features and to draw conclusions regarding the petrogenesis of the studied

rocks.

Two competing scenarios need to be tested: (i) either the relative depletion in some of the

most incompatible elements was caused by metamorphic processes, most likely by loss to a fluid that

escaped during prograde dehydration or, (ii) the peculiar trace element patterns were caused by

fractional crystallisation during a purely igneous history in water-undersaturated magma chambers.

The textural evidence from outcrop and microscopic study is not conclusive in this matter. The

charnockites do not show a pervasive deformation fabric (as shown in Fig. 3.1b) but this might simply

reflect post-deformational thermal annealing.

50


Candeias

0.001

0.01

0.1

1

10

Cs Rb Ba Th U Nb Ta W La Ce Pr Pb Sr

Upp

erC

rust

norm

alis

edItapecerica

0.01

0.1

1

10


Upp

erC

rust

Nor

mal

ised

a

Cs

Rb

U Ta

W

Sr

b

Cláudio

0.0

0.1

1.0

10.0


Upp

erC

rus

tnor

mal

ised

c

Figure 3.6 – Detail of Upper Continental Crust normalized average patterns of Candeias, Itapecerica, and Claudio Units. Panel (a) highlights depletion of Cs, Rb, U, Nb, Ta, W and Sr by comparison with average of typical Archaean granitoid rocks from Barberton (Kleinshanns et al. 2004). For instance, the average from Barberton shows small depletion Cs, W and possibly Sr. when compared with Itapecerica and Claudio Units some similarities and differences are evident (panels b and c). Fine migmatite from Itapecerica Unit is enriched in Th, U and Nb and it is depleted in Ba and Sr. The dark gneiss shows similar characteristics as Barberton granites while grey migmatites display the same trend as Candeias charnockites (panel c). Details from Claudio Unit show that granitoids are the most depleted. Dark gneiss is slightly enriched in U, Nb and Ta. Fine migmatite is enriched in Th and U. Migmatite and grey gneiss have similar trends as Candeias charnockite.

In the following, we will demonstrate that correlations exist between the extents of depletion

in incompatible elements (e.g. Cs, U, Ta), which seem unrelated to indicators of fractional

crystallisation. From these observations, we will develop our preferred model that dehydration-related

element loss during the prograde path of a high-grade metamorphic event superimposed these features

upon igneous protoliths.

When the depletion of U is expressed relative to Th (i.e. elevated Th/U ratios) we find a strong

positive correlation with the depletion of W relative to La (Fig. 3.7a). With the exception of

charnockites (but not enderbites) from the Candeias unit all samples indicate that W and U were lost

together. Because the relative incompatibility of W is poorly known we also expressed W-depletion

relative to Hf (Fig. 3.7b), which is significantly more compatible than La. Again a strong positive

correlation is found (omitting the high Th/U charnockites from the Candeias unit) pointing to coeval 51


52

loss of U and W. When the relative U-depletion is compared to that of Cs (in a Th/U vs. Th/Cs

diagram, Fig. 3.7c) all samples, including the Candeias unit charnockites, define a strong positive

linear correlation. An interesting feature is that the charnockites have very high Th/U and Th/Cs ratios,

indicating that the igneous protoliths were already enriched in Th. This explains why these samples

plot off the trends in the previous two diagrams. In Fig. 3.7d it can be seen that most rocks from the

three units display elevated Nb/Ta ratios, which are generally much higher than average continental

crust (Nb/Ta = 12; McLennan 2001). It is also evident that relative loss of Ta is generally related to

loss of W. While the correlation is not as strong as those between the previously studied ratios, certain

lithologies (e.g. the Candeias unit enderbites) show very strong positive colinearity. A very strong

positive correlation is again found when the relative losses of Cs and Rb are compared (Fig. 3.7e).

This indicates that the samples most depleted in the most incompatible element Cs (relative to Rb) are

also the most depleted in Rb, when compared to Th. In summary, the relative depletions in Cs, Rb, U,

Ta and W are systematic, with few exceptions, across all lithologies in three different tectonic units

that underwent the same metamorphic history. Could these correlated features be of magmatic origin

caused by fractional crystallization?

In Figure 3.8 various trace element systematics are shown that are generally expected to form

during fractional crystallisation. The studied sample suite shows the effects of crystallisation of

plagioclase, K-feldspar and amphibole/clinopyroxene. A very strong trend is found between the extent

of (negative) Eu anomaly and the Sr/Nd ratio (Fig. 3.8a) indicating that the samples most depleted in

Eu are also lacking Sr (relative to REE), which is best explained by prior removal of plagioclase. This

is corroborated by the observation that the Eu-depleted samples are also relatively poor in Al (Fig.

3.8b). This trend is caused largely by plagioclase (see e.g. the enderbite samples from Candeias unit),

while K-feldspar cause only small (negative) covariation as borne out by the position of the

charnockite datapoints. A well-defined positive correlation is found between indicators of alkalinity

and Pb content (Fig. 3.8c), which most likely shows that the incompatible Pb is incorporated into K-

feldspar during the latest stages of crystallisation. There is an increase in the Ce/Pb ratio with

increasing abundance of combined Fe, Mn and Mg oxides. This shows that Pb was concentrated into

the remaining melt as clinopyroxene and amphibole crystallised while Ce was preferentially

incorporated into the liquidus phases (probably including apatite). In summary, the majority of

samples show geochemical features that are expected for calc-alkaline fractionation trends.

The important observation, in our view, is that no significant correlation exists between the

various differentiation indicators (i.e. Eu/Eu*, Sr/Nd, Al2O3, Na2O+K2O-CaO, Ce/Pb and

MgO+Fe2O3+MnO) and the anomalous trace element ratios (Th/U, La/W, Hf/W, Th/Cs, Nb/Ta and

Cs/Rb). Namely, the average r2 among these ratios is (0.194; calculated by converting negative

correlations into positive r2). The only significant (i.e. r2>0.3) are found with the Cs/Rb and Th/Cs


53

ratios, while ratios not involving Cs have extremely low degree of correlation (typically r2<0.15). Thus

we rule out that the observed incompatible trace element depletions were caused by fractional

crystallization. Rather, we believe that these features were caused by selective element loss to a

metamorphic fluid phase. This is supported by the fact that many of the most dehydrated rocks

(enderbites and charnockites) are the most depleted in those elements.

The average ratios used in Fig. 3.7 (Th/U, La/W, Hf/W, Th/Cs, Nb/Ta, Cs/Rb and La/Yb)

highlight just how different the studied samples are from average upper continental crust (UCC). In

UCC, these ratios are 3.82, 15, 2.9, 2.33, 12, 0.04 and 13.64 respectively (McLennan 2001). By

comparison the averages for enderbites and charnockites are: 16.8 and 49.7, 2561 and 1818, 301 and

118, 12.3 and 74.8, 17.5 and 21.6, 0.011 and 0.0033 and 33.6 and 58.5 (Table 3.1). While it is

generally appreciated that Cs, Rb and U can be mobile during dehydration (or aqueous alteration), W

and certainly Ta are generally considered to be relatively immobile. However, it has already been

shown (Kamber & Collerson 2000) that Ta is significantly more fluid-mobile than its geochemical

twin Nb and the coherent depletion in Rb, Cs, U, Ta and W suggests that in the studied rocks, these

elements were lost to a fluid, whose chemistry favoured their incorporation, while other fluid-mobile

elements, such as Pb, were evidently less affected (Fig. 3.8c). It is unknown how widespread these

geochemical characteristics are among metamorphic rocks, largely because very few complete datasets

exist in the literature. In addition, the studied rocks are remarkably unaltered (compared to common

metamorphic rocks) and were therefore not affected by retrograde fluids, which can obscure the

prograde element depletion (Sorensen 1997). Regardless, from the present evidence at least two

implications can be drawn.

Charnockites and enderbites that formed by complete metamorphic dehydration of granitoids

can be expected to show characteristic and systematic depletions in certain highly incompatible trace

elements. The resulting chemistry should thus allow their distinction from primary igneous water-

undersaturated charnoenderbites. Berger et al. (1995) postulated that charnockites and enderbites from

the Northern Marginal Zone of the Limpopo Belt represent primary igneous rocks. Here we use their

Th and U data to test this hypothesis building on our observation that metamorphic charnockites are

strongly depleted in U. The average U content of chanockite and enderbite analysed (by isotope

dilution) by Berger et al. (1995) is 2.4 and 2.8 ppm, respectively. This is much higher than the

contents found in our study (0.49 and 0.44 ppm). Indeed, the average Th/U ratios determined by

Berger et al. (1995) are 5.02 and 4.16 (charnockite and enderbite, respectively). These are within error

of Archaean UCC (Kramers & Tolstikhin 1997) and indicate no preferential U-loss, supporting Berger

et al. (1995) hypothesis. Our study demonstrates the need for more complete and accurate trace

element data of metamorphic rocks.


54

With regard to the composition of the lower crust, the decoupling of the heat producing

elements K, Th and U observed in our rocks is of significance. It is clear, from estimates using

xenoliths and terrain models that the lower crust is on average neither charnoenderbitic in composition

nor of completely dehydrated nature. Many models for lower crust formation exist but they all agree

that the lower crust is less felsic than the upper crust (e.g. Rudnick & Fountain 1995). For example,

according to Rudnick (1992) basaltic underplating supplies heat to melt the lower crust, generating

granitoid liquids with negative Eu anomalies. The disparitity between upper and lower crust in bulk

composition is also reflected in the distribution of heat producing elements (chiefly K, Th and U).

McLennan (2001) estimated that concentrations of K, Th and U in the upper continental crust are 2.8

(weight percent), 10.7 (ppm) and 2.8 (ppm), while the lower crust has much lower concentrations of

0.53, 2, and 0.53, respectively. The lower crustal estimate, which is calculated from mass balancing

upper crustal and total crustal abundances, is quite robust because the total heat production in the crust

and the distribution of heat producing elements are accurately known for many continental areas (e.g.

Rudnick & Fountain 1995, Sandiford & MacLaren 2002, Sandiford et al. 2003). The heat producing

element abundance decreases strongly with increasing depth in the crust. By implication, the lower

crust is strongly depleted in heat producing elements compared to the upper crust, a prediction that has

been confirmed by chemical analyses of deep crustal sections and xenoliths derived from the deep

crust (e.g. Rudnick & Fountain 1995, Kramers and Tolstikhin 1997, Sandiford & MacLaren 2002).

Hence the question arises how lower continental crust ended up with its low inventory of K, Th and U.

As our enderbites and charnockites have K, Th and U concentrations of 1.32 and 3.45 (weight

percent); 7.31 and 24.5 (ppm), and 0.44 and 0.49 (ppm) the insight gained from our study is of

relevance for this issue. Three different types of models have been proposed for the lower crust:

(i) The lower crust is low in K, Th and U because it consists largely of gabbroic cumulates

rich in plagioclase (Kramers & Tolstikhin 1997). In this case, no mechanism for K, Th and U removal

is required. However, Murphy et al. (2003) found that lower crustal xenoliths did not have Pb-isotope

compositions that could be expected for low (Th&U)/Pb cumulates, but that those which are

unradiogenic required removal of Th and U long after formation of igneous protoliths.

(ii) The second hypothesis is that continental crust reorganises itself until it reaches a thermal

state compatible with heatflow across the crust/mantle boundary. A crustal section in which too much

heat is produced in the lower section will experience melting of the lower/middle crust. This happens

due to i) conductive dissipation following voluminous mid crustal plutonism, ii) accretion of material

with high radiogenic heat production at the base of the crust during continental collision or iii) because

of sediment-loading of high U, Th, K upper crustal rocks (e.g. Bodorkos et al. 2002, Sandiford &

MacLaren 2002, Sandiford et al. 2003). As the incompatible elements K, Th and U readily partition

into the melt, they are transported into upper crustal levels via granite plutons.


55

(iii) The third hypothesis, originally advocated by O’Nions et al. (1979) envisages

metamorphic K, Th and U loss. The relatively fluid-mobile elements (including the heat producing

nuclides) could be transported in aqueous fluids that escaped from the rocks during prograde

dehydration. However, this proposal, which was founded on Pb-isotope data of granulites, is not

supported by our observations from the Campo Belo granulites. Our enderbites, charnockites and

migmatites are very depleted in U only, while Th and K remained relatively immobile. Selective loss

of U also appears unlikley in view of the relatively high Th/U ratio of the upper crust (Hemming &

McLennan 2001).

The combined evidence thus favours crustal differentiation by melting of sections of lower

crust that contained too much K, Th and U. The last pulse of K-rich granitoids that marks final

stabilisation of many Archaean cratons is also observed in the Campo Belo terrains and the

metamorphic event experienced by our granulites may ultimately have been related to cratonisation

but it did not contribute significantly to the redistribution of heat producing elements.

3.10 SUMMARY

Unusually fresh, unaltered late Archaean enderbites, charnockites and migmatites from the

Campo Belo complex were used for a comprehensive geochemical study that has yielded the

following results and conclusions:

(i) All samples preserve many major element and trace element characteristics of the pre-

metamorphic protoliths, which are identified as typical TTG. Features include steep REE patterns,

high Sr/Y ratios, and general enrichment/depletion typical of calc-alkaline fractionation series.

However, all studied rocks but in particular charnockites and enderbites show very peculiar depletions

in some of the most incompatible elements.

(ii) Some of the depleted elements (Rb, Cs, U) are generally regarded as fluid mobile and their

low abundance in the studied rocks is thus compatible with metamorphic dehydration. There is

minimal retrograde or later low-grade overprint of the studied rocks and we attribute the loss of Cs, Rb

and U to escape of aqueous fluids during prograde metamorphism that culminated at granulite grade.

(iii) Depletions in Cs, Rb and U are, however, also correlated with depletions in elements (Ta,

W) that are not generally regarded as fluid-mobile. Depletion in Ta (but not Nb) has previously been

demonstrated for eclogites and must have been caused by the same processes that led to loss of U, Cs

and Rb. Hence, under certain circumstances Ta and W can also be fluid mobile.


Figure 3.7 Selected trace element rations of gneiss from Candeias (solid diamonds), Itapecerica (solid square) and Claudio (open triangle) unit: (a) Th/U vs La/W, (b) Th/U vs Hf/W, (c) Th/U vs Th/Cs, (d) Cs/Rb vs Cs/Th, and (e) Nb/Ta vs Hf/W. All diagrams show positive covariation, which reflect systematic depletion in certain elements as discussed in the text.

56


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15 20 25Sr/Nd

Eu/E

u*

0

2

4

6

8

10

12

0 10 20 30 40 50Na O+K O -CaO2 2

Pb

(ppm

)

0

2

4

6

8

10

12

0 2 4 6 8MgO + Fe O + MgO2 3

Ce/P

b

10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

12 13 14 15 16 17 18Al O2 3

Eu

/Eu

*

a b

c d

Figure 3.8 Selected relationship between major and trace element from Candeias (solid diamonds), Itapecerica (solid square) and Claudio (open triangle) samples: (a) Eu/Eu* vs Sr/Nd, (b) Eu/Eu* vs Al2O3, (c) Pb vs (Na2O + K2O – CaO), and (d) Ce/Pb vs (MgO + Fe2O3 + MnO). All diagrams are showing the expected trend from fractionation of plagioclase (a, b); plagioclase and K-feldspar (c); and ferromagnesian silicates and Fe-oxides (d).

(iv) These relative element depletions are powerful fingerprints for study and classification of

charnockites and enderbites. Our study demonstrates the need for comprehensive datasets. Only

limited literature data is available. Using our new criteria, we confirm an igneous origin for

charnockites and enderbites from the Limpopo Belt of southern Africa (i.e. lack of U depletion).

(v) Finally, because element loss during metamorphic dehydration is only effective at

depleting U but not Th and K, this process cannot be implied for the low heat production rate of the

lower crust. Hence, the lower crust must be inherently depleted in these elements or have lost the

combination of the three elements to granites by partial melting.

57


58

CAPÍTULO 4 RECENT ADVANCES AMONG Sr AND Nd CONCERNING TO THE

TECTONIC EVOLUTION OF CAMPO BELO METAMORPHIC COMPLEX, SOUTHERN PORTION OF THE SÃO FRANCISCO

CRATON, BRAZIL.

4.1 INTRODUCTION

Strontium and neodymium isotopic signatures can potentially provide imperative information

on the evolution of polymetamorphic Archaean terrains. However, they may be poorly correlated

when the Rb-Sr and Sm-Nd systems reflect strong disturbances that occurred during younger events,

long after the formation of the rocks (e.g. metamorphic dehydrations).

Previous Rb-Sr and Sm-Nd studies on Archaean rocks have documented large behavior

divergences in these isotopic systems when undergoing deformational and/or metamorphic events (e.g.

Cameron et al. 1981, Collerson 1983, Tobisch et al. 1994, Ayres & Harris 1997).

As reported by Cameron et al. (1981) “the Rb-Sr isochrones from polymetamorphic terrains

display a wide scatter as a consequence of”: 1) open-system behavior of the samples during later

metamorphism; 2) Variation in the initial 87Sr/86Sr owing to heterogeneity of the protoliths; and 3)

Inclusion of rocks of different primary ages.

Collerson et al. (1983) says: “Nd and Sr isotopic data from high-grade gneissic complexes

metamorphosed under granulite facies conditions may yield equivocal and potentially misleading

interpretations of isochrons, significance of model ages, and estimates of crustal residence time”.

Hickman (1984), studying sheared and unsheared gneisses from the Limpopo mobile belt in

South Africa, reported the behavior of the Rb-Sr systematic. He showed that the unsheared gneisses

preserved their properties and, in contrast, the sheared zones displayed an age of metamorphic age.

The composition of the sheared gneiss was reset to that of the younger granites.

Moorbath et al. (1986) stated that the Sm-Nd system has a relatively small degree of Sm-Nd

fractionation, whereas it is greater for Rb-Sr. This means that the former is more susceptible to errors

than Rb-Sr isochrones.

Moorbath et al. (1997) pointed out that the interpretation of the Sm-Nd system is valid if the

analyzed rock(s) has (have) not been subject to open-system behavior or Nd-isotope homogenization.

It is the scope of this study to display new Rb-Sr and Sm-Nd whole-rock data for the gneisses

(migmatized or not) and the amphibolite of the gneissic unit (Fig. 4.1b). In addition, we are going to


60

point out that the Rb-Sr and Sm-Nd can give reliable information on the metamorphic age and the

estimated age of rock formation, even if the Rb-Sr and Sm-Nd systems had been clearly affected by an

earlier event as attested by Oliveira et al. (submitted to Lithos). We are going to show the correlation

between strictly cogenetic rocks and a combination of “non-cogenetic” group of rocks. The relative

chronology between cogenetic and non-cogenetic rocks discussed in this paper is largely based on

observations made during fieldwork and geochemical behavior.

We are now going to review relevant published geochronological data for the area, whose

interpretation has certainly displayed a complex evolution for the southern São Francisco Craton

Basement (Fig. 4.1a).

4.2 GEOCHRONOLOGICAL OVERVIEW

The study area encompasses a sialic fragment of the Campo Belo Metamorphic Complex. In

regional terms, the area belongs to the southern portion of the São Francisco Craton, which is a

polycyclically evolved crustal segment, tectonically stable in relation to the Brasiliano Cycle mobile

belts (Alkmim et al. 1993).

The rocks of this region belong to the Divinópolis and Barbacena metamorphic complexes

(Machado Filho et al. 1983) that were later grouped in the Campo Belo Metamorphic Complex by

Teixeira et al. (1996). Besides the Complex, a mafic dike swarm and a supracrustal unit also crop out;

the latter, according to Machado Filho et al. (1983), can be correlated with the Rio das Velhas

Supergroup.

The geochronological record of the region can be found in Teixeira et al. (1996, 1998), who

also proposed an Archaean and Paleoproterozoic crustal evolution model characterized by successive

accretion/differentiation stages associated with subordinated crustal reworking processes.

The history of the Campo Belo Metamorphic Complex is compatible with the evolution of the

Bonfim and Belo Horizonte Metamorphic Complexes (Machado et al. 1992, Teixeira 1993, Teixeira &

Silva 1993, Teixeira et al. 1996, 1998, Pinese 1997, Carneiro et al. 1998a).


Figure 4.1 Panel (a) -Geological map of the southern São Francisco Craton (modified by Campos Sales 2004). Key: 1- Neoproterozoic undivided cratonic cover; 2- Mesoproterozoic Espinhaço Supergroup; 3- Mesoproterozoic(?) São João Del Rei/Andrelândia Groups; 4-- Mesoproterozoic (?), Dom Silvério Group; 5- Paleoproterozoic granitoids; 6- Paleoproterozoic indiscriminate greenstone-type sequences; 7- Paleoproterozoic Minas Supergroup; 8- Neoarchean granitoids; 9- Neoarchean and Mesoproterozoic gabbroic and dioritic rocks (sills and dikes); 10- Neoarchean ultramafic plutonic suite; 11- Neoarchean Rio das Velhas Supergroup; 12- Archean metamorphic complexes partially reworked on Proterozoic time; 13- Faults and fractures (CSZ = Cláudio Shear Zone; JBSZ = Jeceaba-Bom Sucesso Shear Zone); 14- Fold axes; 15- Lithologic contacts; 61


62

16- Cities. Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001) showing the following features: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A – Alemão dimension stone quarry; B – Oliveira dimension stone quarry; C - Lila dimension stone quarry; D – Itapecerica dimension stone quarry; E - Marilan dimension stone quarry; F - Kinawa dimension stone quarry; G - Corumbá dimension stone quarry; H – Fazenda Corumbá dimension stone quarry and, I – Carmópolis de Minas dimension stone quarry

Teixeira et al. (1998), dating zircons (ion probe U/Pb) from migmatites of the Campo Belo

Complex, defined a polyphase Archaean history. They proposed three major geological events in that

area. The first took place at 3,205±17 Ma and would mark the formation of a granitoid crust. A second

period of intrusions occurred at 3,047±25 Ma, and the third event is recorded at 2,839±17 Ma. The

youngest age was considered to be the crystallization age of the neosome (migmatization event,

Teixeira et al. 1998).

Machado & Carneiro (1992) and Carneiro et al. (1998a, b) defined the Rio das Velhas orogeny

(2,780¯2,700 Ma) that represents the most important event that affected the Campo Belo Complex.

According to these authors, mafic-ultramafic intrusions were emplaced between ca. 2.75 and 2.66 Ga

in association with distensional phases of the Rio das Velhas orogeny. Endo & Machado (1998) also

defined discontinuities or distensional phases at the margins of the Rio das Velhas Greenstone Belt

during this period.

According to Oliveira (1999), Oliveira & Carneiro (2001) and Oliveira et al. (1998a, b, 1999)

the study region can be further divided in five lithodemic units: three gneissic, one supracrustal and

one fissural mafic. The gneissic rocks of the Campo Belo Metamorphic Complex vary widely in

composition (e.g. granitic, granodioritic and tonalitic) and can be grouped in three distinct units:

Cláudio, Itapecerica and Candeias Gneisses.

Thus, as the petrologic-tectonic picture is very complex and varied, a comprehensive study

including structural geology, petrography, geochronology and geochemistry is necessary to deal with

its tectonic evolution in a more precise way, completing the geologic mapping, petrographic definition

and kinematic study by Oliveira (1999) and Oliveira & Carneiro (2001), contributing to a better

geological knowledge of the region.

To point out these advances, new geological data obtained from a cratonic fragment located

among the cities of Itapecerica, Cláudio, Monsenhor João Alexandre, Carmópolis de Minas, Oliveira

and São Francisco de Paula (Fig. 4.1) are presented for gneisses, granitoids, and amphibolite rocks

from the gneissic unit.


63

4.3 SAMPLE SELECTION AND ANALYTICAL PROCEDURES

During fieldwork, eight quarries were selected for further laboratory work (Fig. 4.1). From

each quarry a group of rocks was carefully sampled from fresh fronts (about 20kg for each group) for

Rb-Sr and Sm-Nd analyses.

Sr and Nd isotope measurements were carried out at the Advanced Centre for Queensland

University Isotopic Research Excellence (ACQUIRE), using a VG 54-30 Sector multi-collector mass

spectrometer in static mode. Procedures were identical as described by Wendt et al. (1999). The long-

term (7 years) reproducibility of the NBS SRM 987 Sr and La Jolla and Ames metal Nd standards are 87Sr/86Sr = 0.710251±20 (2σ) and 143Nd/144Nd = 0.511861±11 (2σ) and 0.511977±12(2σ),

respectively.

Rb and Sm ratios were obtained from trace element analyses. The trace elements were

determined by ICPMS at the Advanced Centre for Queensland University Isotopic Research

Excellence (ACQUIRE). Sample preparation and analytical procedures used were identical to those of

Eggins et al. (1997) except that Tm was not used as an internal standard and W-2, a U.S.G.S. diabase

standard, was used as the calibration standard. Our preferred concentrations for W-2 and the measured

concentrations and relative standard deviations for AGV-1 (an average of 26 analyses of 9 digestions

analysed over four years) are shown in Table 1. Concentrations for W-2 were derived partly by

analysing it relative to synthetic standards (Li, Cr, Ni, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th,

and U) or are based on an assessment of published standard data (A. Greig, pers. comm.).

4.4. RESULTS

The ages reported in this study were calculated using the ISOPLOT program (Ludwig 1998).

Experimental (2 sigma) errors used in the regression analyses were 0.5% for 87Rb/86Sr and 0.1% for 147Sm/144Nd (based on standard reproducibility) and 26ppm for 87Sr/86Sr and 143Nd/144Nd (external

error based on NBS SRM 987 and EN-1 measurements over 6 years) at ACQUIRE. The Rb-Sr and

Sm-Nd isotopic data presented are from: 5 gneisses from Marilan quarry; 1 gneiss from Lila quarry, 1

gneiss from Itapecerica quarry 2 enderbites from Oliveira quarry; 3 charnockites from Alemão quarry;

2 gneisses from Kinawa quarry; 2 gneisses from Corumbá quarry; 4 gneisses from Fazenda Corumbá

Quarry; 3 amphibolites from Kinawa quarry and 1 amphibolite from Corumbá quarry (Figs 4.2 and

4.3, Tables 1 and 2).

All gneisses are slightly to strongly migmatized and deformed. The amphibolites occur as

strongly deformed boudins. Some isochron diagrams were chosen to illustrate the comparison between

a strictly cogenetic group of rocks with a combination of “non-cogenetic” group of rocks.


Figure 4.2 Panel (a-h) – a) gneisses from Marilan quarry; b) gneiss from Lila quarry, c) gneiss from Itapecerica quarry; d) enderbites from Oliveira quarry; e) charnockites from Alemão quarry; f) gneisses from Kinawa quarry; g) gneisses from Corumbá quarry; h) gneisses from Fazenda Corumbá Quarry.

64


Figure 4.3 Panel (a-b) – a) amphibolites boudins from Kinawa quarry and b) amphibolite boudins from Corumbá quarry. The samples were collected in assorted fraction alond the boudin outcrop.

65


Table 4.1 Rb-Sr whole-rock analytical data for representative samples from the Campo Belo Metamorphic Complex.

.

Field number Gneiss Unit Rb Sr 87Rb/86Sr 87Sr/86Sr Error(ppm) (ppm) 2sigma

AO-1 211.79 131.34 4.6661 0.8686 5AO-2 187.25 104.19 5.2003 0.9034 4AO-3 137.34 141.10 2.8181 0.8137 5AO-4 134.34 155.16 2.5069 0.8017 5AO-6 240.35 347.78 2.0010 0.7728 5AO-9 304.37 52.38 16.8168 1.3260 4

AO-10 200.38 142.91 4.0594 0.8600 4

AO-7A 58.06 377.28 0.4456 0.7192 4AO-7B 49.68 405.84 0.3493 0.7176AO-8A 96.15 163.55 1.7021 0.7764 5AO-8B 96.70 160.54 1.7285 0.7772AO-8C 99.37 157.90 1.8063 0.7788

AO-11 62.30 230.42 0.7829 0.7369 5AO-12 119.78 256.11 1.3541 0.7529 5AO-14 214.28 133.48 4.6480 0.8748 4AO-15 176.96 132.67 3.8616 0.8484 4AO-16 147.94 97.31 4.4013 0.8770 4AO-17 107.08 208.17 1.4893 0.7612 5AO-18 152.42 66.45 6.6402 0.9555 5AO-19 162.83 153.69 3.0676 0.8204 5AO-20 84.89 289.05 0.8503 0.7363 5

AO-25A 10.40 126.09 0.2388 0.7112AO-25B 7.80 135.03 0.1673 0.7085AO-26 47.01 138.32 0.9841 0.7403AO-27 165.79 69.90 6.8660 0.9574AO-28 58.31 116.91 1.4441 0.7447

Itapecerica

Candeias

Cláudio

Cláudio amphibolites

Uncertainties reported for each ratio are 2 σ(mean)

66

Table 4.2 - Sm-Nd whole-rock analytical data for representative samples from the Campo Belo Metamorphic Complex. The ε(T1) was calculated fro 2.72Ga (supposed age for the formation of the rock) and ε (T2) was calculated for 2.62Ga (supposed age for the “metamorphic” event).

Field number Gneiss Unit Sm Nd 147Sm/144Nd 143Nd/144Nd Error ε(0) TDM (Ma) T (Ga) E (T)(ppm) (ppm) 2 sigma Rock formation

AO-1 0.88 5.61 0.0954 0.51067 20 -38 3088AO-2 12.52 75.75 0.1000 0.51080 10 -36 3042AO-3 6.70 34.78 0.1165 0.51114 10 -29 3023AO-4 4.62 27.61 0.1011 0.51085 8 -35 3006AO-9 12.23 50.11 0.1476 0.51161 9 -20 3388

AO-10 6.50 28.16 0.1396 0.51144 -23 3380

AO-7A 3.83 20.26 0.1143 0.51096 7 -33 3239 2.76 -3.6AO-7B 3.10 20.86 0.0898 0.51065 7 -39 2974AO-8A 5.78 35.76 0.0977 0.51074 7 -37 3061 2.05 -10.8AO-8B 4.76 29.33 0.0981 0.51072 6 -38 3109 2.05 -11.1AO-8C 5.52 33.87 0.0985 0.51069 6 -38 3148 2.05 -11.7

AO-11 4.42 20.72 0.1289 0.51116 7 -29 3460 2.74 -5.1AO-12 3.46 16.02 0.1306 0.51116 9 -29 3535AO-14 13.80 73.02 0.1142 0.51100 9 -32 3184 2.65 -4.1AO-15 8.11 44.18 0.1110 0.51093 10 -33 3183AO-16 9.86 42.25 0.1411 0.51148 9 -23 3366AO-18 4.57 22.57 0.1224 0.51118 8 -28 3160AO-19 3.18 17.02 0.1131 0.51100 7 -32 3141AO-20 8.75 37.19 0.1422 0.51155 9 -21 3258

AO-25A 5.10 18.35 0.1682 0.51211 -10 3214AO-26 6.21 26.22 0.1431 0.51159 8 -20 3191AO-27 1.55 6.26 0.1493 0.51165 10 -19 3373AO-28 5.43 20.07 0.1637 0.51195 12 -13 3437

CHUR - Goldstein et al. (1984) 0.51264

Itapecerica

Cláudio

Amphibolite Cláudio

Candeias

4.4.1 Outline of petrography features

The samples from Itapecerica Unit are pink and locally dark gneiss bodies (Fig. 4.3, panel a, b

and c). This unit is strongly migmatized and the rocks are fine to medium grained. The rocks are

predominantly of granitic composition but can vary to granodioritic. The main constituents of these

rocks are microcline followed by quartz, plagioclase, biotite, apatite, zircon, opaques and epidote. The

samples from the dark parts (maybe the protolith) are granodioritic to tonalitic in composition with

abundant amphibole and some pyroxene.

The Claudio Unit samples are gneisses of granodioritic to tonalitic, locally granitic

compositions. They are predominantly grey but in some outcrops (e.g. Corumbá, Fig. 4.2 e, sample

AO 15), they are locally pink due to the occurrence of pegmatite fluids, the same as seen in the

Kinawa quarry (not showed in the pictures).

At Fazenda Corumba stone quarry the rocks vary from dark, grey to white (granodioritic to

granitic). The rocks are fine- to medium-grained, slightly to strongly migmatized. In general,

plagioclase is the main constituent of these rocks followed by quartz, microcline, biotite, amphibole

(not common), zircon, apatite, titanite (not common), opaques, white-mica, chlorite and epidote. Some

samples show antiperthitic intergrowth. The main petrographic differences of the units are:

compositional (i.e. presence of hypersthene), grade of migmatization and color of the bodies (i.e. dark,

green, grey, and white).

The charnockites and enderbite in the studied area crop out along São Francisco de Paula

Town (charnockites) and Oliveira Town (enderbites), (Fig. 4.1b, A and B). The rocks were collected at

Alemão stone quarry and Oliveira stone quarry (Fig. 4.2 G and H). In most domains the rocks exhibit

massive textures with incipient foliation. These rocks are olive green, slightly to strongly migmatized,

and medium- to coarse-grained. The mineralogical banding is difficult to identify macroscopically

giving a homogeneous aspect to the bodies. The texture of the gneisses varies from granolepidoblastic

to granoblastic.

The charnockite from Alemão quarry (samples 8A, B, C and D, Table 4.1 and Figs 4.1A and

4.2 h) is only slightly deformed; massive in hand specimen and with grains that vary from medium to

coarse (Fig. 4.2h). Under the microscope, microcline is the main constituent followed by plagioclase,

quartz, biotite, hypersthene, and rare amphibole.

The variety from the Oliveira outcrop (samples 7A and B, Table 4.1 and 4.2, Figs 4.1b and

4.2g) is slightly to strongly deformed. The main constituents from this group are antiperthitic

plagioclase, quartz, microcline, biotite, amphibole, hypersthene, and the commonest accessories are

apatite, zircon, and an opaque mineral. In some thin sections hypersthene is absent.


4.4.2 Rb-Sr Sytematics

The isochron diagram displayed at Fig. 4.3a shows a combination of 3 migmatized, cogenetic

rocks from Marilan quarry (samples 2, 3 and 4, Itapecerica Unit, Fig. 4.1b-point E and Fig 4.2a). The

samples presented at Fig. 4.3a yield a model I isochron equivalent to an age of 2,609 ± 27 Ma and an

initial 87Sr/86Sr of 0.7072 ± 0.0012 (MSWD = 0.34). The analogous group of migmatized, cogenetic

gneisses (samples 16, 17 and 18, Cláudio Unit, Fig. 4.1b, point H) from Fazenda Corumbá quarry

displays a model I isochron (Fig. 4.3b) equivalent to an age of 2,603 ± 15 Ma and an initial 87Sr/86Sr of

0.70511 ± 0.00049 (MSWD = 0.85). When the sample 19 is integrated it exhibits a similar age

correlation (Fig. 4.3c). It shows a Model 3 Solution (±95%-conf.) on 4 points with an age of 2,623 ±

280 Ma and initial 87Sr/86Sr =0.706 ± 0.018 (MSWD = 70).

The regression lines displayed at Figs. 4.3a and 4.3b are not totally precise for the reason that

only three samples were used for each regression. However the results are representative in terms of a

regional scenario. For the Marilan group, samples 1 and 6 were not used, because they present

unrealistic initial ratios. Model ages for samples 2, 3 and 4 are coherent with the time of rock

formation (Table 4.1). Machado & Carneiro (1992) reported similar ages (about 2.75Ga) for zircons

from the Rio das Velhas orogeny (2,780¯2,700 Ma).

Model ages listed in table 4.1 for both Marilan and Fazenda Corumbá quarries are very

similar. The initial ratio for sample 16 may indicate the source for a crustal component melt. On the

other hand, samples 17, 18 and 19 display intermediate initial ratios that indicate a range of

intermediate melt sources.

Three-point isochrons are considered to be of uncertain reliability. We will then combine

cogenetic and “non-cogenetic” samples from different quarries, taking some from the same gneissic

unit and with similar composition. The combination of cogenetic and “non-cogenetic” samples from

Marilan (samples 2, 3, and 4), Lila (sample 9) and Itapecerica (sample 10) quarries, which belong to

the Itapecerica Unit and show approximately the same composition, display a robust regression

isochron, corresponding to an age of 2,571 +33/-65 Ma and an initial 87Sr/86Sr of 0.7079 +0.0038/-

0.00081 (Fig. 4.4a)

The same combination above, excluding sample 9 which yields high 87Rb/86Sr and 87Sr/86Sr

concentrations, display a Model 1 Solution (±95%-conf.) for 4 points, equivalent to an age of 2,603 ±

23 Ma and initial 87Sr/86Sr of 0.7074 ± 0.0011 (MSWD = 0.60, Probability = 0.55, Fig. 4.4b). The

robust isochron displayed in Fig. 4.4c is a combination of samples 2, 3, 4, 9 and 10 and one sample

from the Kinawa quarry (11). The results show an age equivalent to 2,593 +13/-67 Ma and initial 87Sr/86Sr of 0.7077 + 0.0022/-0.00061. 69


0.78

0.80

0.82

0.84

0.86

0.88

0.90

0.92

1.5 2.5 3.5 4.5 5.587Rb/86Sr

87Sr

/86S

r

Age = 2609 ± 27 MaInitial 87Sr/86Sr =0.7072 ± 0.0012

MSWD = 0.34

data-point error ellipses are 2σ

0.72

0.76

0.80

0.84

0.88

0.92

0.96

1.00

0 2 4 687Rb/86Sr

87Sr

/86Sr

Age = 2603 ± 15 MaInitial 87Sr/86Sr =0.70511 ± 0.00049

MSWD = 0.85


8

a b

Figure 4.3 Rb-Sr isochron diagram showing the nearby correlation between Marilan quarry and Fazenda Corumba quarry. The three point isochron are considering be of uncertain realiability.

The same group of samples displayed at fig. 4.4c, excluding sample 9, display Model 1

Solution (±95%-conf.) for 5 points, resulting an age of 2,603.5 ± 8.9 Ma and initial 87Sr/86Sr of 0.7074

± 0.00021 (MSWD = 0.40, probability=0,75, Fig. 4.4d). The robust regression line shown in Fig. 4.4e

is a combination of samples 2, 3, 4, 10 and 11 and migmatites from the Corumbá quarry (samples 14

and 15). This isochron exhibits an age of 2,556 +50/-87 Ma and an initial 87Sr/86Sr of 0.7079 +0.0038/-

0.0018. This group, excluding sample 9, shows a robust regression line of 2,597 +12/-270 Ma and

initial 87Sr/86Sr of 0.7075 +0.0078/-0.0022 (Fig. 4.4f).

The information is further constrained when rocks from fig. 4.4e are grouped with migmatites

from the Fazenda Corumbá quarry (samples 16, 17, 18 and 19). They display a robust regression for

11 points of 2,597 ± 56/-110 Ma with initial 87Sr/86Sr =0.7074 ± 0.0015/-0.0034 (4.4g). The

combination of all rocks, excluding samples 8 and 9, show similar information and displays a robust

regression isochron for 17 points corresponding to an age of 2,613 +75/-90 Ma and an initial 87Sr/86Sr

of 0.7043 +0.0031/-0.0021 (Fig. 4.4 h).

Model ages and initial ratios listed in table 4.1 are variable when analyzed independently, but

geologically possible.

70


0.7 8

0.8 0

0.8 2

0.8 4

0.8 6

0.8 8

0.9 0

0.9 2

1.5 2 .5 3.5 4.5 5.587Rb/86Sr

87S

r/86 Sr

0.6

0.8

1.0

1.2

1.4

0 4 8 12 16 2087Rb/86Sr

87S

r/86S

r

Age = 2571 + 33/-65 Ma

Initial 8 7Sr/86Sr =0.7079 ±+0.0038/-0.00081

Age = 2603 ± 23 Ma

Initial 87Sr/86 Sr =0.7074 ± 0.0011MSWD = 0.60, Probability = 0.55

0.6

0.8

1.0

1.2

1.4

0 4 8 12 16 2087Rb/86Sr

87Sr

/86Sr

Age = 2593 +13/-67 MaInitial 87Sr/86Sr =0.7077 +0.0022/-0.00061

a b

cdata-point error ellipses are 2σ

data-point error e llipses are 2σ data-point error ellipses are 2σ

0.70

0.74

0.78

0.82

0.86

0.90

0.94

0 2 4 687Rb/86Sr

87Sr

/86Sr

Age = 2603.5 ± 8.9 MaInitia l 8 7Sr/8 6Sr =0.70738 ± 0 .00021

MSWD = 0.40, probability = 0.75


d

0 .6

0 .8

1 .0

1 .2

1 .4

0 4 8 12 1 6 2087Rb/86Sr

87Sr

/86Sr

data-point error ell ipses are 2σ

e

0.70

0.74

0.78

0.82

0.86

0.90

0.94

0 2 4 687Rb/86Sr

87Sr

/86Sr


f

Age = 2597 +12/-270 MaInitial 87Sr/86Sr =0.7075 +0.0078/-0.00022

Age = 2556 +50/-87 MaInitia l 87Sr/86Sr =0.7079 +0.0038/-0.00018

0.65

0.75

0.85

0.95

0 2 4 6 887Rb/86Sr

87Sr

/86Sr

Age = 2597 +56/-110 M aIn itia l 87Sr/86Sr =0.7074 +0.0015/-0.0034


g

0 .6 5

0 .7 5

0 .8 5

0 .9 5

1 .0 5

0 2 4 6 887Rb/86Sr

87S

r/86 Sr

Age = 2613 +75/-90 MaInitial 87Sr/ 86Sr =0.7043 +0.0031/ -0.0021

data-point error ellipses are 2 σ

h

Figure 4.4 Rb-Sr isochron diagram showing in close proximity correlation between the three gneisses units.

In general the combinations of gneisses (migmatites) from all units display a similar evolution.

The majority of the samples yield Neoarchean ages or metamorphic ages and Mesoarchaean protoliths.

71


Most of the model ages support the Rio das Velhas orogeny postulated by Machado & Carneiro

(1992). The population of three cogenetic amphibolites from the Kinawa quarry shows an age of 2,695

± 130 Ma and an initial 87Sr/86Sr of 0.70189 ± 0.00075 (MSWD = 5.7) (Fig. 4.5a, point F, Cláudio

Unit). The same amphibolites from the Kinawa quarry, when grouped with one “non-cogenetic”

amphibolite from the Corumbá quarry, display a Model I isochron of 2566 ± 54 Ma with an initial 87Sr/86Sr of 0.7027 ± 0.0023 and MSWD of 72 (Fig. 4.5b, point G). The combination of these samples

with another amphibolite from the Carmópolis de Minas quarry gives an unrealistic result, as shown in

fig. 4.5c. Initial ratios for samples 25a, b and 26 from the Kinawa quarry indicate the source for a

mantle component melt. Nevertheless, the model ages are not conclusive. For samples 27 and 28, the

initial ratios are not convincing and display unreliable results. However the combination of these rocks

displays an interesting errochron, similar to the evolution from the gneisses.

0 .70

0 .71

0 .72

0 .73

0 .74

0 .75

0.0 0.2 0.4 0.6 0.8 1.0 1.287Rb/86Sr

87S

r/8 6S

r

Age

= 26 95 ± 130 M aInitial

87Sr/86Sr =0.70189 ± 0 .00075

MSWD

= 5.7

data-po int error e llipses are 2σ

0.65

0.75

0.85

0.95

1.05

0 2 4 6 887Rb/86Sr

87Sr

/86Sr

Age

= 2566 ± 54 MaIn itial

87Sr/86Sr =0.7027 ± 0.0023

MSWD

= 72

data-po int error ellipses are 2 σ

0.65

0.75

0.85

0.95

1.05

0 2 4 6 887Rb/86Sr

87S

r/86S

r

Age = 2569 +130/-1900 MaInitial 8 7Sr/ 86Sr =0.7023 ± 0.0082

data-point error ellipses are 2 σ

a b

c

Figure 4.5 Rb-Sr isochron diagram showing in close proximity correlation between the three gneisses units.

72


73

4.4.3 Sm-Nd Systematics

Sm-Nd “errochrons” for the same group of rocks analyzed for Rb-Sr will be presented,

excluding sample 17 (Sm-Nd data not available). Certain combinations of cogenetic and “non-

cogenetic” gneisses result in poor correlations and exhibit great scatter of MSWD values. Therefore,

apart from the high age errors, the results appear to be similar to the Rb-Sr results. The “errochron”

diagram presented at Fig. 4.6a reveals a group of 3 cogenetic rocks from the Marilan quarry (samples

2, 3 and 4). This result yields a Model III Solution errochron equivalent to an age of 3,014 ± 270Ma

and an initial 143Nd/144Nd of 0.50882 ± 0.00021 (MSWD = 2). The combination of rocks from

Marilan, Lila and Itapecerica quarries (samples 2, 3, 4, 9 and 10, Fig. 4.6b) shows a robust regression

“errochron” age of 2,539 +750/-580 Ma and an initial 143Nd/144Nd of 0.5091 + 0.00023/-0.0025.

The combination of samples 2, 3, 4, 9, 10, 11, 14 and 15 (Fig. 4.6c) displays a robust

regression “errochron” age of 2,539 +580/-640 Ma and an initial 143Nd/144Nd of 0.50914 + 0.00041/-

0.00052. Fig. 4.6d represents the set of rocks of Fig.4.6c including the migmatites from Fazenda

Corumbá (samples 16, 17, 18 and 19). The combination of these “non-cogenetic” rocks shows a robust

regression “errochron” age of 2,572 +250/-470 Ma and an initial 143Nd/144Nd of 0.5091 + 0.00024/-

0.00030. The same result is obtained when a migmatite sample from Carmópolis de Minas quarry

(sample 20, Fig. 4.6e) is included. The robust regression “errochron” displays an age of 2,593 +340/-

280 Ma and an initial 143Nd/144Nd of 0.50910 + 0.00024/-0.00030. Fig. 4.6f shows a combination of

almost all rocks from the gneissic unit (Table 4.2). The result displays a robust regression errochron

age of 2,593 +280/-490 Ma and an initial 143Nd/144Nd of 0.50907 + 0.00024/-0.00030.

4.5 DISCUSSION

The São Francisco Craton has a polyphase Archaean history (Teixeira et al. 1998, Table 4.3)

and, based on some aspects of its evolution, we are going to focus the interpretations of our results on

previous existing results.


0.5 107

0.5 108

0.5 109

0.5 110

0.5 111

0.5 112

0.5 113

0.096 0.100 0.1 04 0.108 0.112 0.116 0.1 20147Sm/144 Nd

143 N

d/14

4 Nd

Age = 3014 ± 290 MaInitial 143Nd/144Nd =0. 50882 ± 0.00021

MSWD = 2.0

data-point error e llipses are 2σ

0.5106

0.5108

0.5110

0.5112

0.5114

0.5116

0.5118

0.09 0.11 0.13 0.15147Sm/144Nd

143 N

d/14

4 Nd

Age = 2539 +750/-580 Ma

In itial 143Nd/14 4Nd =0.5091 ± 0.0014


0.5106

0.5108

0.5110

0.5112

0.5114

0.5116

0.5118

0.09 0 .11 0.13 0.15147Sm/144Nd

143 N

d/14

4Nd

Age = 2539 +580/-640 Ma

In itial 14 3Nd/144Nd =0.50914 ± 0.00046


0.510 6

0.510 8

0.5110

0.5112

0.5114

0.5116

0.5118

0.09 0 .11 0.1 3 0.15147Sm/14 4Nd

143 N

d/14

4Nd

Age = 2572 + 250/-470 Ma

Initial 14 3Nd/144Nd =0.50910 ± 0.00027


0.5106

0.5108

0.5110

0.5112

0.5114

0.5116

0.5118

0.09 0.11 0.13 0.15147 Sm/144Nd

143 N

d/14

4 Nd

Age = 2593 +340/-280 MaInitial 143Nd/144Nd =0.50908 ± 0.00027

data-point error e llipses are 2σ

0.5104

0.5106

0.5108

0.5110

0.5112

0.5114

0.5116

0.5118

0.08 0.10 0.12 0.1 4 0.16147Sm/144 Nd

14

3 Nd/

144 N

d

Age = 2593 +280/-490 Ma

Initia l 143Nd/144Nd =0.50907 ± 0.00029

data-point error ell ipses are 2σ

a b

c d

e f

Figure 4.6 Sm-Nd errochron diagram showing nearby correlation between the three gneisses units as demonstrated for Rb-Sr.

As reported by Machado & Carneiro (1992), Teixeira et al. (1996, 1998a), and Carneiro et al.

(1998 a, b), the period of 2,612-2,593 Ma is associated with a Neoarchaean pulse of granite plutonism,

representing the youngest generation of granites in the southern part of the São Francisco Craton.

An immediate consequence of these studies has some important implications in the

interpretation of our data.

1. What does the Rb-Sr and Sm-Nd method date in high-grade metamorphic terrains

means?

74


Table 4.3 – Summary of the Archaean tectonomagmatic events in the southern part of the São Francisco Craton in the light of U/Pb and Sm-Nd data. This table was summarized by Teixeira et al. 2000.

Age (Ma)

2,612-2,593 Late-tectonic granitoid intrusions after amalgama and final crust stabilization [1,2]Data compiled from Carneiro et al. 1998a; Teixeira et al. 1996, 1998a and Noce et al. 1998).[1] Campo Belo gneissic-migmatitic terrain, [2] Bonfim dome and [3] Belo Horizonte dome

Formation of the Bonfim TTG suite and the Rio das Velhas greentone belt (includingCongonhas and Caeté) - the Rio das Velhas orogeny [2]. Crustal reworking andemplacement of mafic-ultramafic bodies and gabbro-noritic dikes [1]. Granitoidintrusions in the entire terrain [1,2] and regional migmatization [2]. Progressive terrainassembly starting at 2,780 Ma ago.

2,778 - 2,698

[1] Crystallization of the neossome material in migmatites, as well as TTG crustreworking [2,3]

2860 ± 10 to 2,839 ± 17

3205 ± 25 [1] Emergence of early continental crust, compatible with TDM ages on the countryrocks, up to 3.25 Ga. Formation of the Piumhi greentone belt and correlatives.

Characteristics of the crustal evolution in the Southern São Francisco Craton

[1] Major generation of the Campo Belo crust and progresswive magmatic accretion.The gneisses have Pb isotopic signatures consistent with mantle-like single-stageevolution, yielding comparable TDM ages between 3.00 and 2.90 Ga. Origin iof theBelo Horizonte gneiss protholiths [3], as well as part of the Bonfim gneiss protholiths.Development of granulitic facies metamorphism.

3047± 25

2. Are our results reflecting the reactivation of earlier discontinuities, as recorded at the

margins of Rio das Velhas Greenstone Belt (according to Endo & Machado 1998)?

3. Can the Rb-Sr and Sm-Nd data presented here be seen as evidence for a strong

metamorphic event that affected the area around ~2,650-2,550 Ma? The period of ca. 2.6 Ga is

associated with the Neoarchaean granitic pulse. For the same period, Teixeira et al. (1996) reported the

isotopic resetting of the Rb-Sr and Pb/Pb systems in the Campo Belo Complex, and defined a regional

thermal overprint most likely related to low-temperature fluids.

4. Thus, could the ca. 2.6-2.59 Ga granitoids be products of the migmatisation event and

record the same history as the migmatisation period?

5. How many migmatization events occurred in the Campo Belo Metamorphic Complex?

Our results stress out a tectonothermal event at about 2.6Ga in the Campo Belo Metamorphic

Complex, even if most of the results are from combinations of cogenetic and “non-cogenetic” rocks.

Some of the isochrons were determined with only three points, which is not consistent statistically.

They were used as an indication of the behavior of some cogenetic samples.

75


76

Recently, Oliveira et al. (paper submitted to Lithos) reported that the rocks from the studied

area had lost some elements (i.e. Sr, Nb, and Ta) via fluid loss during the prograde metamorphic event.

This fact seems to be useful for our Rb-Sr and Sm-Nd interpretations once it implies that there were

losses of some trace and REE elements probably about 2.6Ga.

Some authors give some important elucidations about the behavior of the Sm-Nd and Rb-Sr

systems under high-grade metamorphic events. Numerous experimental studies have established that

Sm-Nd fractionation occurs during melt formation and their ratios are affected by crustal anatexis or

by late-stage fractional crystallization (i.e. Harrison & Watson 1983, Pimentel & Charley 1991,

Montel 1993).

Moorbath et al. (1997) identified in early-Archaean rocks suites affected by late

tectonothermal events the opening of the Sm-Nd system, leading to effective resetting accompanied by

complete or near complete Nd-isotope homogenization. Based on this information and taken with care,

it is fair to consider that the Sm-Nd isochrons can provide reliable information on the timing of the

tectonothermal processes (i.e. migmatization) that affected the studied area.

For Sato & Siga Junior (2000), the production of juvenile continental crust was larger in the

Proterozoic than in the Archaean. According to them: “there was a certain increase in the depletion

rate of Sm-Nd and Rb-Sr in the upper mantle at about 2.2Ga, as a reflex of the strong differentiation of

the upper mantle to continental crust”. As illustrated by Collerson (1983), the Rb-Sr systematics

obtained from a large population of multiply metamorphosed and deformed rocks (including

migmatization) has the ability to depict the age of the protolith (known from U/Pb zircon dating).

When smaller populations are used (e.g. neosomes, or biotite-rich paleosomes), the Rb-Sr method

often dates the time of migmatization and not of the protolith formation.

For the studied area, Teixeira (1993) interpreted the Rb-Sr ages of ca. 2.566±53 Ga for

enderbites in terms of reworking (complete isotope resetting), without a detailed explanation on how

the Sr homogenization could have occurred.

Using our new Rb-Sr and Sm-Nd data, including trace element geochemistry for a wide

variety of samples, we will further test this hypothesis. Large, homogenous, fresh samples were taken

from 8 different quarry fronts from Itapecerica, Cláudio and Oliveira Units.

The data presented here are, in general, compatible with the earlier results of Teixeira (1993),

but the isochrons for different rock types have distinct initial 87Sr/86Sr isotope ratios. A straightforward

interpretation of these isochrons, which define the time of protolith formation (and not of

migmatization), shows that the initial ratios correspond to a range of mantle- (amphibolites, 87Sr/86Sr =


0.7019), intermediate- (migmatites from Fazenda Corumbá quarry, 87Sr/86Sr = 0.7051), and crustal-

derived (Marilan quarry, 87Sr/86Sr = 0.7072) melt sources. Therefore, the best combination of cogenetic

and “non-cogenetic” samples displays initial ratios of the order of 0.707.

If the Rb-Sr and Sm-Nd regression lines (or robust regression lines) define the age of

migmatization, the maximum protolith age can be estimated from the difference between the initial Sr

and/or Nd-isotope ratios of the regression and of the coeval mantle. In order for complete resetting to

occur, it is necessary to homogenize Sr and/or Nd-isotopes at the quarry scale. An equilibration of this

magnitude would also seriously affect the Rb-Sr and Sm-Nd ratios of different parts of the quarry, as

already tested by many experimental studies (i.e. Collerson 1983; Hickman 1984, Tobisch et al. 1994,

Moorbath et al. 1997). Indeed, based on our geochemical data, we can point out that some elements

were lost during the metamorphic event that affected the Candeias, Cláudio and Itapecerica Units (Fig.

4.1).

In the Cs/Rb vs Cs/Th plot (Fig. 4.7) a strong positive trend is displayed, which requires

preferential element loss in the order Cs>Rb>Th. Hence, when compared with the Upper Continental

Crust average (MacLennan 2001), our rocks are depleted in Cs and Rb. The continental crust has a

Cs/Rb ratio of about 0.036 (MacDonough et al. 1991), whereas our rocks have lower ratios.

Apparently, Sr was not affected by element loss, as there is no obvious depletion in this element.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.00 0.10 0.20 0.30 0.40Cs/Th

Cs/

Rb

Figure 4.7 - Positive trend correlation between Cs;Rb vs Cs;Th from Cláudio (open triangle), Itapecerica (solid square) and Candeias (solid diamond) Units.

The important conclusion from the trace element investigation is that the present Rb-Sr ratio

for the quarry is certainly lower than the pre-migmatization ratio. Hence, when calculating the crustal

residence time prior to 2.57 Ga from the present-day Rb-Sr, we will obtain a maximum estimate.

For the late Archaean mantle, we assume an initial 87Sr/86Sr of 0.7014 and use the difference

between the initial values recorded by the isochrons to estimate the maximum protolith age. 77


For the Fazenda quarry we obtain a crustal prehistory of only ca. 70 Ma, while the samples

from Marilan allow estimating a prehistory of ca. 120 Ma. This means that the protoliths of these

migmatites formed between 2,670 and 2,730 Ma, the period which Machado & Carneiro (1992)

defined the Rio das Velhas orogeny (2,780¯2,700 Ma). According to Machado & Carneiro (1992) this

was the time when the study area was intruded by mafic magmas.

The neodymium isotope data display similar values at about 2.5 Ga but with larger errors

(table 2 and Fig. 4.4). The results by themselves are not conclusive statistically but in association with

the Rb-Sr method, they provide an interesting assumption. At least we can assume that around 2.6 Ga

the Rb-Sr and Sm-Nd systematics were disturbed.

The results of the Sm-Nd data allow us to put some constraints on the behavior of this system

for the high-grade metamorphic terrain. εNd values (chondritic uniform reservoir - CHUR) vary from

–20 to –38 and imply that at the time of “metamorphism”, Sm-Nd was disturbed. Or, alternatively, the

rocks were derived from different protoliths. A complex evolution is indicated by εNd values

calculated at 2.6 Ga, which vary from ~-2 to -5.3 for the migmatites, and -0.79 to -3.44 for the

amphibolites (table 4.2). This implies that a large amount of recycled older crust participated in the

early evolution of the Campo Belo Complex. All rocks show older model ages but with variable εNd

along their evolution (Fig. 4.8). It may indicate different rates of element losses during later processes.

T (Ga)

εNd

Amphibolites

Gneisses (migmatised)

Depleted Mantle

2.6 Ga

CHUR

20

10

0

-10

-20

-30

-40

-500 0.5 1 1.5 2 2.5 3.53 4 4.5

Figure 4.8 Nd versus time (DePaolo 1981) for the studied samples (see table 4.2 for reference)

78


79

Campos et al. (2003), studying zircons from mesosome and leucosome of migmatites from the

Passa Tempo Metamorphic Complex, southeast of the Campo Belo Metamorphic Complex, defined

the minimum age for the granulite-facies metamorphism at 2,622 ± 18Ma (mesosome) and 2,599 ±

45Ma (leucosome). This possibility clearly shows that our new data support the premise that the Rb-Sr

and Sm-Nd systematics are a result of disturbances during prograde metamorphic event.

4.6 SUMMARY

In this study, calculations based on a combination of cogenetic and “non-cogenetic” rocks led

us estimate the age of disturbances in the Rb-Sr and Sm-Nd systems. Samples from different gneissic

units, including the amphibolites (boudins), display a similar evolution.

From the isotopic results discussed above, it is understandable that the crustal evolution of the

Campo Belo Metamorphic Complex had some contributions around 2.6 Ga. The Sr/Nd systematic by

itself is not conclusive and may yield equivocal misleading interpretations. However, some remarkable

conclusions and inferences can be drawn:

1 – Based on geochemical evidence, 87Sr/86Sr initial rations, εNd (CHUR), fSm-Nd, it is clear that

the Rb-Sr and Sm-Nd systematics were substantially affected during 2.6 Ga;

2 – The model proposed by Teixeira et al. (1996) with Rb-Sr and Pb/Pb isotopic resetting

associated with “low-temperature fluids” is realistic but it is difficult to envisage the complete

resetting at the quarry scale.

3 – Recently, Campos et al. (2003), dating migmatite zircons, defined the minimum age for

the granulite-facies metamorphism in the southeast part of the São Francisco Craton. Such a model has

been favored for the studied rocks are hypersthene-bearing gneisses under granulite metamorphic

facies. From this it may have the same correlation to our Sr/Nd results and the same implications for

the evolution of the Campo Belo Metamorphic Complex.

4 - Alternatively, the studied rocks were contaminated by unradiogenic (mantle) Sr during the

2.6 Ga migmatization event, as recorded by the amphibolite isochron initial ratio.

For the next paper (work in progress) the combined data (trace element geochemistry, Sr-

isotopes, Nd isotopes, Ar-Ar ages and U/Pb zircon ages) will yield a complete picture, not only for the

timing, but also for the element mobility associated with the migmatization event(s).


80

CAPÍTULO 5

IMPLICATIONS FOR TRANSAMAZONIAN 40Ar/39Ar AGES FOR THE CAMPO BELO METAMORPHIC COMPLEX, SOUTHERN SAO

FRANCISCO CRATON, BRAZIL.

5.1 ABSTRACT

The gneisses, amphibolites, gabbronorite and gabbros from Cláudio, Supracrustal and Mafic

Fissural Units in the Campo Belo Metamorphic Complex display imperative information for the

evolution of the Transamazonian and post-Transamazonian event in the Archaean basement in the

southern part of the São Francisco Craton. By combining field, structural, petrographic and the 40Ar/39Ar data, the evolution of the Campo Belo Metamorphic Complex within the Transamazonian

and post-Transamazonian period is obvious. 40Ar/39Ar age plateaus of about 2.0-1.9 Ga have been

obtained for amphiboles and biotites from the (migmatized) Cláudio Unit gneisses and amphibolites

from the Cláudio and Supracrustal Units. These results indicate that during the Transamazonian

orogeny, the Campo Belo Metamorphic Complex reached at least the greenschist to amphibolite facies

once the closing temperature of amphiboles and biotites are 500°C and 300°C respectively. The

question that still remains is related to the deformational event, whether it is Archaean or

Transamazonian. Based on previous work in the nearby Quadrilátero Ferrífero and our fieldwork

information, we believe that there are great possibilities for the deformational event having taken place

some time about 2.2Ga, related to the Transamazonian orogeny but not in Archaean and the 2.0-1.9 Ga

representing the cooling ages of this event. The 40Ar/39Ar age plateaus of about 1.7Ga and 0.9Ga have

been obtained for biotites and amphiboles of undeformed and unmetamorphosed gabbronorites and

gabbros from the Mafic Fissural Unit. These results suggest that at 1.7Ga intense break up of the crust

(Staterian period, 1.8-1.6Ga, related to the continental rift scenario or Espinhaço rift) or a simple

reactivation of the pre-existent Archaean structures occurred. The 1.0-0.9Ga intervals can be

correlated with the reactivation of the Mesoproterozoic structures and might be correlated to the

Macaubas rift.

5.2 INTRODUCTION

The 40Ar/39Ar technique has been widely used with success for dating weathering,

sedimentary, metamorphic and tectonic processes. It has been used to infer the thermal histories of

many periods of the Earth’s early evolution (e.g. evolution of the Pilbara, São Francisco and Amazon

Cratons).


However, there is no widely known information on how the 40Ar/39Ar system behaved under

high-grade metamorphic conditions at the basement of the southern São Francisco Craton (Fig. 5.1a).

Figura 5.1 Panel (a) -Geological map of the southern São Francisco Craton (modified by Campos Sales 2004). Key: 1- Neoproterozoic undivided cratonic cover; 2- Mesoproterozoic Espinhaço Supergroup; 3- Mesoproterozoic(?) São João Del Rei/Andrelândia Groups; 4-- Mesoproterozoic (?), Dom Silvério Group; 5- Paleoproterozoic granitoids; 6- Paleoproterozoic indiscriminate greenstone-type sequences; 7- Paleoproterozoic Minas Supergroup; 8- Neoarchean granitoids; 9- Neoarchean and Mesoproterozoic gabbroic and dioritic rocks (sills and dikes); 10- Neoarchean ultramafic plutonic suite; 11- Neoarchean Rio das Velhas Supergroup; 12- Archean metamorphic complexes partially reworked on Proterozoic time; 13- Faults and fractures (CSZ = 82


83

Cláudio Shear Zone; JBSZ = Jeceaba-Bom Sucesso Shear Zone); 14- Fold axes; 15- Lithologic contacts; 16- Cities. Panel (b) Geological map of the study area (modified from Oliveira 1999, Oliveira & Carneiro 2001) showing the following features: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A – Kinawa dimension stone quarry; B - Corumbá dimension stone quarry; C – Amphibolite outcrop, D – Noritic Gabbro outcrop and, E e F – Gabbros outcrops.

Until now, the foremost aim of geochronological research has been to settle the timing of the

crust formation in the Campo Belo Metamorphic Complex (CBC), with limited emphasis on its

tectonometamorphic evolution. Competing models for the evolution of the southern São Francisco

Craton have been reported along the years, to build a tectonic evolution of the Archaean basement

(Teixeira 1985, 1993, Machado et al. 1992, Teixeira & Silva 1993, Teixeira et al. 1996, 1998, Pinese

1997, Carneiro et al. 1998a, b). Most of the works mainly draw attention on U-Pb, Rb-Sr, Sm-Nd and

K-Ar techniques. The U-Pb method has been more widely used and yields more accurate information,

whereas the 40Ar/39Ar has not been used with the same frequency and the results, up to date, have not

been as precise. The Campo Belo Metamorphic Complex (southern São Francisco Craton, Fig 1) has

been the subject of much debate, primarily when the migmatization and the deformational event(s),

and the mafic swarm dykes that emplaced the region in large scale are concerned. In order to consider

and investigate the evolution and the potential problems of the CBC, we are now going to deal with

the 40Ar/39Ar results for minerals (biotites and amphiboles) from gneisses, amphibolites and

gabbros/noritic gabbros. In particular, the aim of this study is to understand the timing of some of the

latest tectonothermal event(s) that occurred in the CBC (e.g. intrusion of NW swarm mafic dikes, Fig

1b).

5.3 PROBLEMS TO BE ADDRESSED

The southern part of the São Francisco Craton has been extensively studied, raising many

questions that result in a wide range of geochronological data (Teixeira 1985, 1993, Machado et al.

1992, Teixeira & Silva 1993, Teixeira et al. 1996, 1998, Pinese et al. 1995, Carneiro et al. 1998a, b).

Nonetheless, many questions remain unanswered. Based on field relationship, petrologic and structural

analyses, Oliveira & Carneiro (2001) recognized three gneissic units (Cláudio, Candeias and

Itapecerica), one supracrustal unit (ultramafics, mafics, garnet-sillimanite-schist, garnet-sillimanite-

quartzite and banded iron formation), and one fissural mafic unit (gabbronorite dykes) in the CBC

(Fig. 1). According to these authors, the gneissic units are strongly deformed (including

migmatization) and metamorphosed up to the granulite facies. Amphibolite boudins are found within

the three gneissic units. The supracrustal unit is strongly deformed and metamorphosed at least up to

the amphibolite facies. The fissural mafic unit is not deformed and not metamorphosed. The dykes

crosscut the gneisses and supracrustal units.


84

Based on this information and on some of the geochronological data available for the CBC,

Oliveira & Carneiro (2001) defined five tectonic events for the studied area, the most recent being

related to the positioning of some of the undeformed gabbronorite dykes (ca. 2,658Ma, Pinese 1997).

Subsequently, Oliveira & Carneiro (2001) defined the minimum age for the deformational event that

strongly affected the CBC.

However, there are some tricky situations when assessments are made using some of the

existing data (e.g. Pinese 1997), our field relationships and our new data, which will be shown in this

study. The Sm-Nd isochron age for gabbronorite dykes (ca. 2,658 Ma, Pinese 1997) was defined as the

time of the emplacement of this rock. This information together with other published data, our new 40Ar/39Ar data and field observations will be used to better understand the CBC evolution and to

answer the following questions:

• Is ~2,658 Ma the age of the swarm dyke emplacement?; Are the high-grade

metamorphic and/or deformational event(s) older than 2,658 Ma?; Is there more than one

migmatization event?; Which is the Transamazonian influence in the Archaean

basement of the São Francisco Craton?

Hence, our aim is to better evaluate the relation between our data and the existing data.

5.4 GEOLOGICAL SETTING

The Campo Belo Metamorphic Complex (CBC) records a polyphase crustal history ranging

from Mesoarchaean to Neoarchaean (Teixeira 1985, 1993, Machado et al. 1992, Teixeira & Silva

1993, Teixeira et al. 1996, 1998, Pinese 1997, Carneiro et al. 1998 a, b). The geochronological results

indicate that the evolution of this terrain was most intensive during the Rio das Velhas Orogeny at

2.78-2.7 Ga (Machado & Carneiro 1992). The only evidence for the Transamazonian Orogeny (2.16-

2.0 Ga) in the CBC are the K-Ar and 40Ar/39Ar data in the 2.0-1.7 Ga interval obtained for biotites and

amphiboles from some of the gneisses (Teixeira & Figueiredo 1991, Teixeira et al. 1996).

However, Archaean rocks from the nearby Quadrilátero Ferrífero show ample evidence of

Transamazonian deformation and metamorphism at 2.065-2.035 Ga (e.g. Machado et al. 1992, Alkmin

& Marshak 1998). The close proximity, lithological similarities, and possible analogous tectonic

histories of the two areas suggest that the CBC should also record stronger evidences for

Transamazonian processes.

We will present 40Ar/39Ar step-heating analyses for biotites and amphiboles from three

different units of the CBC: gneisses and amphibolites from the Cláudio Unit (Fig 1b - outcrop A and


85

B); amphibolites from the Supracrustal Unit (Fig 1b - outcrop C), and gabbros and a gabbronorite

from the Fissural Mafic Unit (Fig 1b-D, E and F).

5.5. SAMPLE SELECTION AND ANALYTICAL PROCEDURES

Samples were selected from the Cláudio Unit (2 gneisses and 1 amphibolite, from Kinawa and

Corumbá stone quarry), from the Supracrustal Unit (1 amphibolite), and from the Fissural Mafic Unit

(2 gabbros and 1 noritic gabbro). From each unit, fresh samples were collected to avoid weathering

interferences.

The samples were analyzed by the laser incremental heating 40Ar/39Ar method at the Argon

Geochronology in Earth Science Laboratory (UQ-AGES) of the University of Queensland.

Each rock sample weighed 10-15 kg. From each sample 100 grams of fragments containing

amphiboles and biotites were crushed, sieved to 0.2-2mm, and washed in absolute ethanol in an

ultrasonic bath. Approximately 10 single crystals of biotite and amphibole from each sample were

hand-picked under a binocular microscope. The minerals were loaded into Al-irradiation disks, as

illustrated in Vasconcelos et al. (2002), and irradiated, together with Fish Canyon sanidine neutron

fluence monitors, for 14 hours at the CLIC-Facility at the Oregon State University Triga reactor. After

a two-month cooling period, the samples were loaded into a copper disk, placed in an ultra-high

vacuum extraction line, and incrementally heated by an Ar ion laser beam. Each fraction of gas

released was cleaned by a cold trap operated at –140ºC, two C-40 SAES getter pumps, and analyzed

by a MAP-215-50 mass spectrometer. Details of the analytical procedures and correction factors are

presented in Vasconcelos (1999a, b) and Vasconcelos et al. (2002).

5.6 RESULTS

5.6.1 Petrograph Features

The gneisses that were chosen are dark gray (probably the protoliths) and gray (locally pink)

gneisses. The rocks are fine- to medium-grained, and strongly migmatized. The dark gray gneiss is

predominantly composed of plagioclase followed by quartz, microcline, biotite, amphibole (rare),

zircon, apatite, titanite (not common), opaques, white mica, chlorite, and epidote. Some samples show

antiperthitic intergrowths.

The gray gneiss is constituted by plagioclase and microcline, followed by quartz, biotite,

apatite, zircon, opaques and epidote. The main difference is that the dark gneiss has some amphibole

in its composition and is most likely the protolith. Both gneisses have granodioritic composition.


86

The amphibolite from the gneissic unit is fine-grained, melanocratic, dense, phaneritic, black

and strongly foliated. In most places, the amphibolites are found in the form of disrupted thin boudins,

representing disrupted and metamorphosed dykes emplaced in the gneissic units. Green hornblende,

sometimes altered to biotite, is the main constituent. Locally, biotite schist (or biotitite) results from

retrograde metamorphism and shearing. Plagioclase is the second constituent, followed by quartz and

sometimes chloritized biotite. Pale yellow clinopyroxene, present in some rocks, can reach 10% of the

volume and alters to amphibole. Accessory minerals are zircon, opaque minerals and apatite.

The amphibolite from the Supracrustal Unit is a fine- to medium-grained rock that is

melanocratic, dense, phaneritic, black and strongly foliated. In general, the amphibolite is found in

contact with ultramafic rocks, mainly in the surroundings of the Cláudio town (Fig. 5.1). Green

hornblende, sometimes altered to biotite and chlorite, is the main constituent, followed by plagioclase

and subordinated quartz (< 10%). The main accessories are zircon, opaque minerals and apatite.

The noritic gabbro and the gabbro are generally medium- and coarse-grained, possibly

representing variations of crustal emplacement during magmatic crystallization. These rocks are in

general dark-colored to greenish, and present massive structure and phaneritic texture. When very

fine-grained, they can be aphanitic.

The gabbronorite shows ophitic, subophitic to intergranular textures, with subhedral to

euhedral crystals, and is medium- to fine-grained. Plagioclase is the most abundant mineral followed

by clino- and orthopyroxene (augite and hyperstene). Hyperstene is light pink, pleochroic, medium- to

fine-grained. Hornblende with green pleochroism is the alteration product of pyroxenes (uralitization).

Quartz appears in small quantities as clean, anhedral, fine-grained crystals. Zircon and opaque

minerals are accessories. White mica, epidote, carbonate and chlorite are secondary minerals and

product of saussuritization.

The gabbro presents ophitic, subophitic or intergranular textures. Plagioclase is the most

abundant mineral. Augite is the second component in volume and hornblende is its uralitization

product. When present, microcline is rare, showing crosshatch twinning and is associated with

myrmekites. Quartz appears in small quantities. Biotite is also rare, reddish and pleochroic. Zircon,

opaque minerals and apatite are accessories. White mica, epidote and chlorite are secondary minerals

observed in some of the rocks.


87

Run ID# Sample 40Ar/39A

5.6.2 40Ar/39Ar Sytem

The results shown in this study correspond to biotites and amphiboles from migmatites and

amphibolite boudins within the Gneissic Unit, amphibolite from Supracrustal unit and gabbros and

noritic gabbro from the Fissural Mafic Unit. 40Ar/39Ar isotopic data for the three different CBC units

are shown in Table 1 in separated groups.

From the first group, three biotite single crystals from the Cláudio Unit gneisses were

analyzed (samples OPU 1436, Runs 2709-01/02/03, Fig 5.2 and OPU 1197, Runs 2708-01/02/03, Fig

5.3, Table 1).

Table 5.1 – Ar-Ar results for amphibolites and gabbros from the Campo Belo Metamorphic Complex.

r 37Ar/39Ar 36Ar/39Ar 40*Ar/39Ar %Rad Age ± (Ma) J Ar40 (nA) Ar40 Moles2180-01A OPU-1449 1747.838 3.413317 0.6653571 1555.228 88.8 3347.519 55.47862 0.003469 1.092725 9.46E-152180-01B OPU-1449 702.3066 5.344283 0.455155 570.3704 80.9 1969.075 32.50222 0.003469 0.8074775 7.00E-152180-01C OPU-1449 373.2233 4.913256 7.87E-02 351.554 93.9 1438.391 13.70003 0.003469 0.8779572 7.61E-152180-01D OPU-1449 387.5748 8.594833 2.28E-02 383.823 98.4 1527.159 7.685193 0.003469 1.841213 1.60E-142180-01E OPU-1449 396.5858 10.97339 3.68E-02 389.5734 97.5 1542.529 15.07735 0.003469 0.7594066 6.57E-152180-01F OPU-1449 457.3651 13.8996 3.23E-02 453.3518 98.2 1704.758 37.30393 0.003469 0.2634937 2.28E-152180-01G OPU-1449 485.8857 14.99253 5.75E-02 475.0664 96.7 1756.822 35.72347 0.003469 0.3027964 2.61E-152180-01H OPU-1449 561.7035 23.16973 3.58E-02 562.1008 98.4 1951.616 20.1375 0.003469 0.8053211 6.99E-152180-01I OPU-1449 629.0363 31.49738 0.1028571 614.7294 95.6 2059.95 52.7584 0.003469 0.2943828 2.54E-152180-01J OPU-1449 716.7209 61.16437 2.20E-03 753.2787 100.6 2317.468 31.40362 0.003469 0.5677816 4.91E-152180-01K OPU-1449 444.5306 55.87882 0.3020013 374.3751 80.9 1501.618 101.1494 0.003469 7.97E-02 6.95E-162180-01L OPU-1449 491.0943 23.63671 -0.1996837 561.2839 112.4 1949.882 71.70731 0.003469 0.1121377 9.77E-162180-02A OPU-1449 7802.523 55.62703 5.171618 6534.552 80.5 5708.352 658.4087 0.003469 0.3960506 3.42E-152180-02B OPU-1449 1377.848 0 0.99901 1082.64 78.6 2813.168 83.48084 0.003469 0.5656868 4.90E-152180-02C OPU-1449 477.4907 9.571774 5.71E-02 464.5012 96.6 1731.678 31.71706 0.003469 0.3450225 2.99E-152180-02D OPU-1449 478.6707 7.912679 4.68E-03 480.5828 99.8 1769.813 9.330807 0.003469 2.334085 2.03E-142180-02E OPU-1449 459.8378 6.366028 -2.72E-03 463.2119 100.3 1728.585 12.66193 0.003469 1.2817 1.11E-142180-02F OPU-1449 448.1851 16.35887 1.57E-02 449.9899 99.2 1696.561 38.2117 0.003469 0.2330477 2.00E-152180-02G OPU-1449 486.0663 12.54496 -2.57E-02 499.0556 101.8 1812.646 27.84972 0.003469 0.3732559 3.21E-152180-02H OPU-1449 601.5396 21.75173 -2.52E-02 620.1705 101.5 2070.788 31.96062 0.003469 0.4945986 4.27E-152180-02I OPU-1449 590.0297 27.88138 -7.86E-02 627.7667 104.3 2085.811 40.44253 0.003469 0.3171633 2.75E-152180-02J OPU-1449 631.8542 40.89171 3.34E-02 643.7113 98.9 2116.942 51.12788 0.003469 0.267714 2.32E-152180-02K OPU-1449 511.8784 46.31487 4.15E-02 520.2003 98.3 1860.458 66.19894 0.003469 0.1452477 1.26E-152180-02L OPU-1449 751.577 58.4231 -0.1631197 838.837 107 2459.892 123.7599 0.003469 0.1012284 8.81E-162180-03A OPU-1449 817.0952 35.17219 7.68E-02 817.3889 97.6 2425.231 401.0683 0.003469 3.66E-02 3.19E-162180-03B OPU-1449 981.1253 0 0.2590971 904.5613 92.2 2562.14 116.0211 0.003469 0.1949464 1.68E-152180-03C OPU-1449 476.7265 20.79379 0.0447432 472.0363 97.6 1749.647 38.50641 0.003469 0.2668675 2.31E-152180-03D OPU-1449 413.1431 11.27289 -5.31E-02 433.1374 104 1654.901 35.84023 0.003469 0.2210207 1.92E-152180-03E OPU-1449 436.9986 31.71816 4.54E-02 435.7823 97.5 1661.503 84.59877 0.003469 9.56E-02 8.38E-162180-03F OPU-1449 440.6527 40.15272 0.1593132 408.2474 90 1591.557 91.05933 0.003469 0.0792954 6.88E-162180-03G OPU-1449 789.0109 77.11564 0.1566265 791.7252 94.9 2382.862 130.8885 0.003469 0.1191047 1.04E-152180-03H OPU-1449 1280.738 135.3115 0.3312702 1319.055 93.2 3100.191 162.9218 0.003469 0.1770943 1.53E-152180-03I OPU-1449 763.0077 157.5188 0.3688003 749.4204 87.3 2310.773 184.138 0.003469 0.0747308 6.48E-162180-03J OPU-1449 349.509 162.737 0.9341264 97.31874 24.7 524.7626 485.6647 0.003469 2.05E-02 1.69E-162180-03K OPU-1449 815.3812 139.0875 0.4506273 768.3007 85 2343.302 231.2616 0.003469 5.90E-02 5.13E-162180-03L OPU-1449 727.5408 189.7613 0.5338377 674.7336 80.4 2176.016 289.3983 0.003469 4.20E-02 3.59E-16


Continued

88

Run ID# Sample 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40*Ar/39Ar %Rad Age ± (Ma) J Ar40 (nA) Ar40 Moles2181-01A OPU-1449 106.8412 0.9271115 3.64E-02 96.22137 90 519.6211 13.38902 0.003469 0.1751133 1.51E-152181-01B OPU-1449 264.7779 5.65E-02 2.87E-03 263.9453 99.7 1172.73 6.067581 0.003469 1.721638 1.49E-142181-01C OPU-1449 325.8661 0.443608 -2.99E-03 326.8855 100.3 1367.458 4.265551 0.003469 4.376956 3.79E-142181-01D OPU-1449 356.5383 0.2834306 -2.37E-03 357.3308 100.2 1454.607 5.788318 0.003469 4.249619 3.68E-142181-01E OPU-1449 350.8351 0.6484853 -1.67E-04 351.0947 100 1437.096 10.22687 0.003469 1.195985 1.04E-142181-01F OPU-1449 351.3958 9.65E-02 -2.48E-02 358.7486 102.1 1458.564 17.66338 0.003469 0.4859516 4.20E-152181-01G OPU-1449 377.8242 0 -3.05E-02 386.8334 102.4 1535.221 17.13318 0.003469 0.5280277 4.57E-152181-01H OPU-1449 309.8572 2.641925 -3.40E-02 320.6952 103.3 1349.212 39.65302 0.003469 0.1532382 1.34E-152181-01I OPU-1449 334.403 0.7657065 3.38E-02 324.6574 97 1360.912 20.93505 0.003469 0.3991233 3.44E-152181-01J OPU-1449 332.6162 0.4214739 -2.30E-02 339.5462 102.1 1404.212 19.16739 0.003469 0.4168005 3.61E-152181-01K OPU-1449 238.9554 0 -9.70E-04 239.2412 100.1 1090.162 9.766659 0.003469 0.6134046 5.31E-152181-01L OPU-1449 177.0969 0 -3.35E-02 186.9942 105.6 901.9965 20.79065 0.003469 0.1610508 1.40E-152181-02A OPU-1449 338.3415 0 4.82E-02 324.1108 95.8 1359.302 39.63087 0.003469 0.17568 1.52E-152181-02B OPU-1449 424.1656 0.1169202 -3.84E-03 425.3433 100.3 1635.303 7.017803 0.003469 2.965867 2.57E-142181-02C OPU-1449 406.7154 0.4190923 -4.23E-03 408.1188 100.3 1591.224 5.565933 0.003469 5.313475 4.60E-142181-02D OPU-1449 419.9988 0 -4.81E-03 421.4207 100.3 1625.36 6.862374 0.003469 3.653772 3.17E-142181-02E OPU-1449 397.5142 0 5.63E-03 395.8503 99.6 1559.158 19.14188 0.003469 0.5175781 4.48E-152181-02F OPU-1449 342.4591 22.44436 0.1823479 294.9865 84.8 1271.394 79.8735 0.003469 8.53E-02 7.41E-162181-02G OPU-1449 420.0019 0 -7.48E-02 442.1093 105.3 1677.199 46.29022 0.003469 0.1928405 1.66E-152181-02H OPU-1449 482.1943 0 -7.56E-02 504.5345 104.6 1825.157 32.50879 0.003469 0.3237331 2.80E-152181-02I OPU-1449 453.0268 0 -8.26E-03 455.4663 100.5 1709.894 37.55124 0.003469 0.264877 2.28E-152181-02J OPU-1449 454.5916 1.721516 -0.0169764 460.2995 101.1 1721.58 31.73915 0.003469 0.336589 2.91E-152181-02K OPU-1449 420.123 0 -3.43E-02 430.2443 102.4 1647.651 141.0108 0.003469 4.78E-02 4.09E-162181-02L OPU-1449 423.0378 0 -0.4817237 565.3863 133.6 1958.573 472.1283 0.003469 1.24E-02 9.78E-172181-02181-02181-02181-02181-02181-02181-02181-02181-02181-02181-02181-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-03E OPU-1448 333.837 4.453163 -0.4843536 478.8112 143 1765.651 167.8844 0.003469 2.46E-02 2.14E-162183-03F OPU-1448 229.0444 9.999579 -1.25E-02 235.1764 102 1076.206 37.62711 0.003469 9.24E-02 8.05E-162183-03G OPU-1448 274.7886 0 -3.78E-02 285.9463 104.1 1243.213 257.8928 0.003469 0.0143556 1.22E-162183-03H OPU-1448 360.1054 40.43517 -0.5230469 532.9917 143.8 1888.778 211.9883 0.003469 2.13E-02 1.84E-162183-03I OPU-1448 322.2749 87.87887 0.1704026 297.1601 86.5 1278.105 283.7079 0.003469 1.83E-02 1.57E-162183-03J OPU-1448 333.9741 203.4607 0.1153953 368.5338 94.5 1485.644 326.4127 0.003469 0.01682 1.42E-162183-03K OPU-1448 412.8615 160.2661 0.2019222 412.2032 88.6 1601.774 232.0209 0.003469 3.09E-02 2.67E-162183-03L OPU-1448 694.9224 352.0703 -0.4346609 1131.208 122.4 2875.976 372.0831 0.003469 2.87E-02 2.48E-16

3A OPU-1449 212.9778 0 0.2628963 135.2912 63.5 694.2147 11.64202 0.003469 0.578805 5.03E-153B OPU-1449 313.628 0.2834591 2.07E-02 307.6077 98.1 1310.017 6.976649 0.003469 1.675673 1.45E-143C OPU-1449 339.3865 0.3168235 4.01E-03 338.3012 99.7 1400.63 6.27521 0.003469 3.475123 3.00E-143D OPU-1449 321.9626 3.25E-02 -3.16E-03 322.9057 100.3 1355.749 7.096637 0.003469 2.986041 2.59E-143E OPU-1449 296.5725 0.4888675 1.19E-02 293.182 98.8 1265.804 8.317242 0.003469 1.484219 1.28E-143F OPU-1449 289.6907 0 2.32E-03 289.0029 99.8 1252.791 16.73579 0.003469 0.4669849 4.02E-153G OPU-1449 374.0562 0 -2.69E-02 382.0166 102.1 1522.303 48.07734 0.003469 0.1625992 1.40E-153H OPU-1449 254.1665 2.185524 0.1484874 210.7825 82.8 990.1091 73.78488 0.003469 6.43E-02 5.56E-163I OPU-1449 366.5403 1.331553 2.91E-03 366.1273 99.8 1479.022 18.18604 0.003469 0.492826 4.26E-153J OPU-1449 353.2706 0 -1.88E-02 358.8209 101.6 1458.766 25.97333 0.003469 0.3139652 2.72E-153K OPU-1449 344.9902 0 -0.0236825 351.9876 102 1439.614 26.10149 0.003469 0.2845822 2.45E-153L OPU-1449 237.2128 0.4619912 3.26E-02 227.686 96 1050.203 179.2703 0.003469 2.19E-02 1.81E-161A OPU-1448 -14989.3 0 3.481819 -16018.2 106.9 1.00E-20 2797.274 0.003469 0.1315681 1.14E-151B OPU-1448 2591.687 6.326802 0.1119839 2570.537 98.7 4139.04 250.7975 0.003469 0.2866706 2.49E-151C OPU-1448 432.8749 2.146638 4.36E-02 420.7805 97.1 1623.731 62.0424 0.003469 0.1386549 1.20E-151D OPU-1448 317.5836 6.345264 -0.012582 323.2397 101.3 1356.734 58.6752 0.003469 0.1021599 8.94E-161E OPU-1448 201.0267 1.254422 -5.39E-02 217.239 108 1013.3 30.35419 0.003469 0.1174413 1.02E-151F OPU-1448 200.1956 6.936418 0.0416263 189.36 94.1 910.9547 21.40816 0.003469 0.1909983 1.65E-151G OPU-1448 412.4518 16.30751 5.82E-02 401.1179 96.1 1572.996 79.58252 0.003469 9.26E-02 8.13E-161H OPU-1448 456.693 18.86632 -9.60E-02 493.0852 106.5 1798.913 177.345 0.003469 4.05E-02 3.52E-161I OPU-1448 693.4927 13.71642 -0.1303426 740.2261 105.7 2294.717 165.0871 0.003469 7.02E-02 6.13E-161J OPU-1448 598.0215 0 0.3879206 483.3902 80.8 1776.389 426.0099 0.003469 2.77E-02 2.40E-161K OPU-1448 1354.688 19.56403 -5.30E-02 1391.048 101.3 3179.24 163.6344 0.003469 0.1940719 1.68E-151L OPU-1448 1067.166 0.2260272 0.2204755 1002.192 93.9 2704.067 133.4107 0.003469 0.1727848 1.50E-152A OPU-1448 501.9196 13.24415 0.4696964 367.5841 72.6 1483.034 436.8021 0.003469 2.46E-02 2.14E-162B OPU-1448 1237.954 0 0.9406496 959.9914 77.5 2644.084 684.3466 0.003469 4.86E-02 4.24E-162C OPU-1448 635.9517 0 -1.257028 1007.403 158.4 2711.337 414.5803 0.003469 2.18E-02 1.90E-162D OPU-1448 223.7789 0 -0.5544353 387.6138 173.2 1537.306 192.8202 0.003469 1.49E-02 1.30E-162E OPU-1448 223.0071 17.9459 -8.36E-02 252.3011 111.7 1134.282 68.39537 0.003469 4.86E-02 4.22E-162F OPU-1448 203.5284 5.657342 -3.60E-03 205.8535 100.7 972.2019 13.83351 0.003469 0.2903119 2.50E-152G OPU-1448 194.8299 0.020586 -0.0905931 221.6041 113.7 1028.811 25.56305 0.003469 0.1261844 1.10E-152H OPU-1448 244.0153 23.0683 -4.46E-02 263.2652 106.1 1170.507 50.01251 0.003469 7.08E-02 6.11E-162I OPU-1448 274.392 39.69995 1.86E-02 279.819 99.1 1223.858 177.5125 0.003469 2.38E-02 1.98E-162J OPU-1448 242.3187 0 -0.3746994 353.0415 145.7 1442.581 256.5719 0.003469 1.22E-02 9.40E-172K OPU-1448 202.2819 12.89667 0.2181246 140.1034 68.6 714.5959 134.2296 0.003469 2.76E-02 2.30E-162L OPU-1448 241.6169 15.80829 -0.2603228 323.3736 132.3 1357.129 94.74907 0.003469 3.36E-02 2.83E-163A OPU-1448 1051.271 0 -1.822158 1589.718 151.2 3380.959 357.3832 0.003469 3.98E-02 3.47E-163B OPU-1448 2126.553 0 -1.976382 2710.573 127.5 4225.314 424.8829 0.003469 0.0739194 6.48E-163C OPU-1448 687.4936 0 -2.053751 1294.376 188.3 3072.276 387.3412 0.003469 2.26E-02 1.98E-163D OPU-1448 1314.005 0 -5.115503 2825.635 215 4293.24 1505.941 0.003469 1.07E-02 9.25E-17


continued Run ID# Sample 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40*Ar/39Ar %Rad Age ± (Ma) J Ar40 (nA) Ar40 Moles2184-01A OPU-1446 817.942 0 0.1117804 784.9101 96 2371.442 214.2008 0.003469 0.061539 52184-01B OPU-1446 1593.002 0 1.043347 1284.692 80.6 3061.204 369.3299 0.003469 9.61E-02 82184-01C OPU-1446 1780.376 47.09152 0.6985208 1631.732 88.6 3420.875 318.2798 0.003469 0.1227217 12184-01D OPU-1446 815.1066 89.11835 0.301074 782.1631 89.9 2366.818 524.2797 0.003469 2.67E-02 22184-01E OPU-1446 295.6558 56.66198 0.3580562 202.3411 65.7 959.332 231.6686 0.003469 2.32E-02 22184-01F OPU-1446 232.7525 6.166049 2.39E-03 233.5391 99.9 1070.554 16.46174 0.003469 0.3093892 22184-01G OPU-1446 195.7881 8.115561 6.23E-03 195.6989 99.4 934.7404 7.060019 0.003469 0.7245493 62184-01H OPU-1446 216.0508 8.948283 -9.25E-02 245.6265 113 1111.868 28.07577 0.003469 0.142517 12184-01I OPU-1446 220.8952 44.23801 0.1810237 176.3449 77.3 861.1125 48.20153 0.003469 0.0817516 72184-01J OPU-1446 249.346 109.8593 0.3065722 181.3491 67.1 880.4397 98.7686 0.003469 4.76E-02 42184-01K OPU-1446 390.5477 226.8535 -2.84E-02 495.8109 106.7 1805.196 67.12064 0.003469 0.1033536 82184-01L OPU-1446 974.2678 747.1708 0.4067008 1924.876 93.6 3677.214 351.847 0.003469 5.25E-02 42184-02A OPU-1446 1936.25 0 -5.42E-02 1952.276 100.8 3699.413 211.4438 0.003469 0.1977057 12184-02B OPU-1446 2140.737 0 0.1665368 2091.524 97.7 3808.189 392.7798 0.003469 0.1304559 12184-02C OPU-1446 950.1853 54.08004 -0.2099347 1056.669 107 2778.663 234.2353 0.003469 7.27E-02 62184-02D OPU-1446 283.7227 24.32528 0.3277985 192.0439 66.5 921.0638 148.8412 0.003469 3.62E-02 32184-02E OPU-1446 241.303 0 -3.01E-03 242.1907 100.4 1100.22 40.37247 0.003469 0.110881 92184-02F OPU-1446 200.4554 6.175158 1.96E-03 201.2307 100 955.2444 5.428173 0.003469 0.9983303 82184-02G OPU-1446 194.83 7.261765 -7.13E-03 198.5174 101.4 945.2166 7.067428 0.003469 0.7988963 62184-02H OPU-1446 180.2689 17.77555 0.0392063 172.2245 94.3 845.0417 39.47394 0.003469 7.51E-02 62184-02I OPU-1446 198.907 12.95873 -1.71E-02 206.858 103 975.866 18.73667 0.003469 0.2169482 12184-02J OPU-1446 184.0721 53.44958 -1.08E-02 198.9082 104 946.6642 31.98316 0.003469 8.97E-02 72184-02K OPU-1446 195.3922 209.7168 -6.93E-03 250.8326 109.4 1129.375 101.4957 0.003469 0.0321899 22184-02L OPU-1446 140.3121 726.8198 0.4212003 148.5654 51.7 749.8862 438.6124 0.003469 1.13E-02 92184-03A OPU-1446 573.8021 18.30331 -6.44E-02 602.0048 103.6 2034.347 126.4224 0.003469 7.10E-02 62184-03B OPU-1446 439.2595 0 6.96E-02 418.6917 95.3 1618.409 65.17932 0.003469 0.1343139 12184-03C OPU-1446 214.8636 6.460583 0.1801415 162.8758 75.5 808.0394 44.20745 0.003469 0.1093635 92184-03D OPU-1446 150.8062 10.00895 -2.55E-02 160.2518 105.5 797.5157 39.67867 0.003469 6.17E-02 52184-03E OPU-1446 149.9444 10.77722 7.92E-04 151.7013 100.4 762.791 32.70909 0.003469 7.84E-02 62184-03F OPU-1446 181.8389 5.857783 7.01E-03 180.9705 99.1 878.9846 8.091221 0.003469 0.5593799 42184-03G OPU-1446 181.6922 5.317775 2.62E-02 175.0287 96 855.9942 7.745055 0.003469 0.6295535 52184-03H OPU-1446 166.2782 2.092875 2.44E-02 159.4601 95.8 794.3284 28.16837 0.003469 0.1153242 12184-03I OPU-1446 128.2674 0.8417313 1.96E-03 127.8279 99.6 662.1426 6.30289 0.003469 0.4785461 42184-03J OPU-1446 138.9073 3.30032 -1.73E-02 144.6089 103.9 733.4719 17.29961 0.003469 0.1499647 12184-03K OPU-1446 140.3522 1.787206 -9.10E-03 143.3597 102 728.2581 15.06858 0.003469 0.1777856 12184-03L OPU-1446 248.6851 49.68697 2.47E-02 254.1668 98.6 1140.498 277.9777 0.003469 1.61E-02 12194-01A OPU-1201 281.1536 0 9.59E-02 252.8265 89.9 1136.035 41.15543 0.003469 0.1270595 12194-01B OPU-1201 534.9059 9.62E-02 1.21E-02 531.3865 99.3 1885.249 7.454326 0.003469 3.114593 22194-01C OPU-1201 560.8096 0.2179675 -4.15E-03 562.1383 100.2 1951.695 6.810146 0.003469 6.170714 52194-01D OPU-1201 572.2424 0.100056 -4.77E-03 573.6996 100.2 1976.057 6.893978 0.003469 8.650077 72194-01E OPU-1201 567.0492 0.3675642 -3.25E-03 568.1837 100.2 1964.475 5.511804 0.003469 9.663971 82194-01F OPU-1201 578.8595 6.85E-02 -9.92E-04 579.1852 100.1 1987.502 4.324224 0.003469 31.87206 22194-01G OPU-1201 536.6616 0.796846 1.21E-02 533.4534 99.3 1889.792 12.87829 0.003469 2.100658 12194-01H OPU-1201 574.3566 0.3754547 -3.63E-03 575.6092 100.2 1980.049 5.499069 0.003469 11.34757 92194-01I OPU-1201 568.4706 0.2751079 -8.82E-03 571.2073 100.5 1970.833 8.059273 0.003469 4.279093 32194-01J OPU-1201 588.0862 1.424331 -1.42E-03 589.2057 100.1 2008.222 16.61049 0.003469 1.361636 12194-01K OPU-1201 529.1992 0 0.1325582 490.0274 92.6 1791.839 77.56839 0.003469 0.2121937 12194-01L OPU-1201 504.768 13.83029 0.1332461 471.0584 92.4 1747.325 120.9061 0.003469 0.1014526 82194-02A OPU-1201 343.3807 0 0.3869409 229.0388 66.7 1054.927 42.19444 0.003469 0.2334845 22194-02B OPU-1201 565.1915 0.1869084 2.55E-03 564.5262 99.9 1956.754 6.600706 0.003469 5.082433 42194-02C OPU-1201 572.8978 0.4233072 -6.03E-03 574.8829 100.3 1978.531 7.102812 0.003469 7.737461 62194-02D OPU-1201 576.4192 0.2775048 -4.45E-03 577.8671 100.2 1984.758 6.113786 0.003469 8.693728 72194-02E OPU-1201 575.4453 0.1319339 -4.46E-03 576.825 100.2 1982.586 5.028979 0.003469 9.178036 72194-02F OPU-1201 572.7666 9.04E-02 -4.40E-03 574.1089 100.2 1976.913 6.847305 0.003469 8.359644 72194-02G OPU-1201 578.7531 0.354073 -3.30E-03 579.8992 100.2 1988.985 5.458435 0.003469 12.04834 12194-02H OPU-1201 584.0718 9.09E-02 -1.95E-03 584.6927 100.1 1998.919 4.714739 0.003469 20.43789 12194-02I OPU-1201 586.6818 0.4402634 -4.63E-03 588.2648 100.2 2006.286 5.828993 0.003469 11.21447 92194-02J OPU-1201 585.1809 0.3802012 -1.06E-02 588.494 100.5 2006.758 9.691809 0.003469 4.049575 32194-02K OPU-1201 580.2636 0.3789895 -2.46E-02 587.7247 101.3 2005.175 19.39525 0.003469 1.181823 12194-02L OPU-1201 601.5041 0 -1.42E-02 605.7036 100.7 2041.827 20.931 0.003469 1.096475 92194-03A OPU-1201 196.8448 0 -7.29E-02 218.3739 110.9 1017.345 38.75059 0.003469 0.104178 92194-03B OPU-1201 550.3241 0.2464153 -4.48E-03 551.7612 100.2 1929.546 6.0049 0.003469 7.223352 62194-03C OPU-1201 567.0687 0 -6.03E-03 568.8507 100.3 1965.879 5.646704 0.003469 9.456384 82194-03D OPU-1201 571.4658 7.03E-02 -4.49E-03 572.8264 100.2 1974.228 5.592074 0.003469 11.37252 92194-03E OPU-1201 566.6289 0.2883143 -3.04E-03 567.6629 100.2 1963.378 5.33504 0.003469 11.30298 92194-03F OPU-1201 568.3456 0 -4.93E-03 569.8009 100.3 1967.878 6.140637 0.003469 10.02246 82194-03G OPU-1201 569.3312 7.52E-02 -4.16E-03 570.597 100.2 1969.551 4.447071 0.003469 16.70033 12194-03H OPU-1201 573.3402 0.2214342 -3.05E-03 574.3464 100.2 1977.41 5.13146 0.003469 18.72183 12194-03I OPU-1201 547.2694 0.6229975 1.83E-03 547.0178 99.9 1919.33 16.41572 0.003469 1.22425 12194-03J OPU-1201 587.5007 0 -7.65E-03 589.7593 100.4 2009.36 20.02703 0.003469 1.087149 92194-03K OPU-1201 604.2464 2.719483 -2.20E-02 612.1199 101.1 2054.729 49.53078 0.003469 0.3005671 22194-03L OPU-1201 631.7556 0 -0.1617863 679.5626 107.6 2185.04 78.74072 0.003469 0.1728907 1

.39E-16

.33E-16

.06E-15

.31E-16

.03E-16

.68E-15

.27E-15

.23E-15

.06E-16

.15E-16

.95E-16

.52E-16

.71E-15

.13E-15

.29E-16

.12E-16

.59E-16

.65E-15

.92E-15

.57E-16

.88E-15

.71E-16

.76E-16

.61E-17

.21E-16

.16E-15

.45E-16

.32E-16

.80E-16

.84E-15

.44E-15

.00E-15

.13E-15

.30E-15

.53E-15

.32E-16

.10E-15

.70E-14

.34E-14

.50E-14

.36E-14

.76E-13

.82E-14

.82E-14

.72E-14

.18E-14

.84E-15

.95E-16

.01E-15

.40E-14

.70E-14

.52E-14

.95E-14

.24E-14

.04E-13

.77E-13

.69E-14

.51E-14

.02E-14

.49E-15

.11E-16

.26E-14

.19E-14

.86E-14

.77E-14

.68E-14

.45E-13

.62E-13

.06E-14

.41E-15

.60E-15

.50E-15

89


continued

90

Run ID# Sample 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40*Ar/39Ar %Rad Age ± (Ma) J Ar40 (nA) Ar40 Moles2195-01A2195-01B2195-01C2195-01D2195-01E2195-01F2195-01G2195-01H2195-01I2195-01J2195-01K2195-01L2195-02A2195-02B2195-02C2195-02D2195-02E2195-02F2195-02G2195-02H2195-02I2195-02J2195-02K2195-02L2195-03A2195-03B2195-03C2195-03D2195-03E2195-03F2195-03G2195-03H2195-03I2195-03J2195-03K2195-03L2197-01A2197-01B2197-01C2197-01D2197-01E2197-01F2197-01G2197-01H2197-01I2197-01J2197-01K2197-01L2197-02A2197-02B2197-02C2197-02D2197-02E2197-02F2197-02G2197-02H2197-02I2197-02J2197-02K2197-02L2197-03A2197-03B2197-03C2197-03D2197-03E2197-03F2197-03G2197-03H2197-03I2197-03J OPU-1447 597.3459 0 -0.175215 649.1212 108.7 2127.384 128.9442 0.003469 7.14E-02 6.24E-162197-03K OPU-1447 596.6439 0 0.4018914 477.8842 80.1 1763.47 391.6108 0.003469 2.73E-02 2.34E-162197-03L OPU-1447 690.8331 10.24771 -0.1548909 742.7606 106.7 2299.157 63.43309 0.003469 0.2084706 1.80E-15

OPU-1201 -5328.76 725.993 19.23759 -22411.1 205.6 1.00E-20 5553.918 0.003469 1.91E-02 1.65E-16OPU-1201 43850.93 0 -55.02038 60109.45 137.1 9642.463 10037.95 0.003469 8.46E-02 7.28E-16OPU-1201 962.2963 2.425494 -0.8438889 1213.927 125.9 2978.158 214.6449 0.003469 8.61E-02 7.50E-16OPU-1201 569.3311 56.6344 -0.4703434 742.3322 125.2 2298.407 157.9089 0.003469 6.35E-02 5.50E-16OPU-1201 711.1115 37.10556 -0.2494512 808.8471 110.8 2411.239 125.2695 0.003469 0.12258 1.06E-15OPU-1201 623.163 4.223846 -1.74E-02 630.5072 100.9 2091.2 11.01023 0.003469 2.882039 2.49E-14OPU-1201 596.6716 4.715508 -4.10E-03 600.2427 100.3 2030.773 5.480608 0.003469 13.18604 1.14E-13OPU-1201 582.7369 4.76492 -6.34E-03 586.9515 100.4 2003.582 5.857551 0.003469 8.519933 7.36E-14OPU-1201 582.5627 5.518765 -1.76E-03 585.7874 100.2 2001.18 17.05419 0.003469 1.284807 1.11E-14OPU-1201 613.9022 6.052143 -1.23E-02 620.6393 100.7 2071.719 17.35033 0.003469 1.11639 9.68E-15OPU-1201 648.8312 8.297447 -7.98E-03 655.6652 100.5 2139.935 26.84655 0.003469 0.6630435 5.74E-15OPU-1201 623.4151 1.3654 -0.1208442 659.8646 105.7 2147.943 49.3133 0.003469 0.2859618 2.47E-15OPU-1201 -1923.32 58.0513 2.288506 -2705.62 134.9 1.00E-20 829.4608 0.003469 4.03E-02 3.51E-16OPU-1201 -11665.4 51.18307 9.417664 -14984.2 123.8 1.00E-20 3832.591 0.003469 0.0609974 5.30E-16OPU-1201 810.7805 24.46477 -1.037533 1138.895 138 2885.719 263.0095 0.003469 5.52E-02 4.79E-16OPU-1201 936.7759 33.40083 -0.2124923 1026.305 107 2737.467 200.3528 0.003469 0.1088833 9.42E-16OPU-1201 642.5646 5.842078 -9.72E-02 674.5142 104.5 2175.605 34.55242 0.003469 0.583611 5.06E-15OPU-1201 581.0839 4.575725 -1.27E-02 587.094 100.7 2003.875 9.630903 0.003469 5.479017 4.75E-14OPU-1201 583.0057 5.571941 -2.40E-04 585.8085 100.1 2001.224 13.34458 0.003469 1.650195 1.43E-14OPU-1201 664.1889 0 -0.3029191 753.7007 113.5 2318.199 134.1489 0.003469 0.112248 9.77E-16OPU-1201 576.9186 0 -1.27E-02 580.6563 100.6 1990.558 52.17223 0.003469 0.2669014 2.31E-15OPU-1201 703.1042 0 -6.98E-02 723.7422 102.9 2265.568 77.98853 0.003469 0.2338408 2.02E-15OPU-1201 726.3989 0 -0.1651388 775.1967 106.7 2355.039 63.3992 0.003469 0.2716582 2.35E-15OPU-1201 637.4862 0 -0.6917983 841.9118 132.1 2464.807 203.9507 0.003469 5.28E-02 4.55E-16OPU-1201 -2391.24 0 -5.976665 -625.138 26.1 1.00E-20 62646.13 0.003469 7.21E-03 6.06E-17OPU-1201 3753.459 0 -5.468123 5369.289 143 5370.482 2918.77 0.003469 2.01E-02 1.74E-16OPU-1201 1332.556 0 -1.579539 1799.309 135 3571.839 541.2145 0.003469 3.79E-02 3.27E-16OPU-1201 31949.27 1303.24 -73.33874 651033.2 168.1 13932.65 29342.96 0.003469 1.80E-02 1.57E-16OPU-1201 706.4421 0 -2.53E-04 706.5161 100 2234.595 201.1554 0.003469 0.0614913 5.32E-16OPU-1201 588.9595 4.931784 -1.91E-02 597.0615 101 2024.302 9.389779 0.003469 2.449361 2.12E-14OPU-1201 585.7548 4.480125 -2.11E-03 588.5847 100.2 2006.945 8.071585 0.003469 4.19276 3.62E-14OPU-1201 588.5042 5.372116 -2.83E-03 591.9984 100.2 2013.955 11.676 0.003469 2.308088 2.00E-14OPU-1201 587.9821 2.991231 0.0059618 587.6904 99.7 2005.104 65.11457 0.003469 0.1885169 1.63E-15OPU-1201 577.3757 0 -6.55E-02 596.7195 103.4 2023.605 99.41925 0.003469 0.114819 1.00E-15OPU-1201 608.6054 3.028832 -1.39E-02 614.2638 100.7 2059.02 24.61705 0.003469 0.6842729 5.91E-15OPU-1201 664.8311 3.083371 -6.27E-02 685.084 102.8 2195.303 63.15903 0.003469 0.2300299 1.98E-15OPU-1447 -159.007 0 0.1291249 -197.164 124 -2078.1 3680.576 0.003469 3.56E-03 2.53E-17OPU-1447 5082.311 214.1152 -2.426878 6848.475 114.4 5789.505 2122.276 0.003469 4.46E-02 3.92E-16OPU-1447 -3164.32 558.51 11.06965 -10533.5 202 1.00E-20 3741.209 0.003469 1.45E-02 1.27E-16OPU-1447 142452.1 0 -301.2719 231477.9 162.5 12068.55 251196 0.003469 9.95E-03 8.73E-17OPU-1447 -444.21 212.6103 4.472642 -2057.21 393.8 1.00E-20 1359.998 0.003469 7.65E-03 6.69E-17OPU-1447 -361.648 88.02013 2.357926 -1121.02 290.8 1.00E-20 861.9825 0.003469 1.11E-02 9.76E-17OPU-1447 758.6059 9.488237 -6.77E-02 784.5869 102.7 2370.899 72.71104 0.003469 0.2362239 2.04E-15OPU-1447 595.9044 12.23734 -1.92E-02 607.7738 101.1 2046 10.81844 0.003469 2.293041 1.99E-14OPU-1447 589.2591 12.55377 -1.28E-03 595.8831 100.2 2021.899 8.212396 0.003469 5.660081 4.90E-14OPU-1447 571.2375 11.82533 2.08E-03 576.3431 100.1 1981.581 7.984511 0.003469 3.013153 2.60E-14OPU-1447 585.8601 9.207178 -1.61E-02 595.1824 100.9 2020.469 22.45288 0.003469 0.948452 8.24E-15OPU-1447 660.3475 0 -0.2368203 730.3271 110.6 2277.269 119.3234 0.003469 0.1366989 1.19E-15OPU-1447 -970.164 48.79684 3.696507 -2131.91 212.2 1.00E-20 897.2115 0.003469 1.92E-02 1.63E-16OPU-1447 -640.908 0 0.9942626 -934.713 145.8 1.00E-20 2045.651 0.003469 8.00E-03 7.00E-17OPU-1447 776.3328 0 -7.805427 3082.835 397.1 4436.418 3202.18 0.003469 4.86E-03 4.49E-17OPU-1447 595.5114 15.06713 -1.44024 1033.237 171.7 2746.955 586.2128 0.003469 1.32E-02 1.15E-16OPU-1447 1206.048 0 -1.590609 1676.072 139 3462.065 502.5746 0.003469 3.59E-02 3.13E-16OPU-1447 615.0359 12.18084 -2.65E-02 629.2064 101.4 2088.644 20.78431 0.003469 0.8972141 7.78E-15OPU-1447 588.3161 13.18823 -5.03E-04 595.0173 100.2 2020.131 13.62359 0.003469 1.366829 1.18E-14OPU-1447 594.0254 13.69345 8.01E-03 598.4963 99.8 2027.223 18.57128 0.003469 0.913064 7.91E-15OPU-1447 382.0926 16.08467 0.4460657 254.4151 65.8 1141.324 202.7553 0.003469 4.09E-02 3.51E-16OPU-1447 378.5005 41.14386 0.2348748 321.6198 82.5 1351.949 246.2478 0.003469 2.85E-02 2.44E-16OPU-1447 513.6598 4.256368 3.24E-02 505.9281 98.2 1828.325 135.862 0.003469 6.48E-02 5.65E-16OPU-1447 716.144 20.47163 0.1287131 689.6448 94.9 2203.736 183.2776 0.003469 7.52E-02 6.54E-16OPU-1447 -5877.21 166.899 6.521143 -8828.52 132.6 1.00E-20 1793.013 0.003469 4.82E-02 4.21E-16OPU-1447 1651.524 15.8452 -2.727981 2486.616 148.9 4085.288 749.4877 0.003469 2.68E-02 2.32E-16OPU-1447 1074.102 0 -1.195211 1427.286 132.9 3217.756 310.939 0.003469 4.89E-02 4.24E-16OPU-1447 665.4 17.95465 -4.90E-02 689.9936 102.4 2204.38 43.55493 0.003469 0.2800759 2.42E-15OPU-1447 586.7699 12.56349 -2.65E-02 600.9017 101.5 2032.11 15.94522 0.003469 1.168782 1.01E-14OPU-1447 594.2444 12.35462 -1.54E-03 600.8903 100.2 2032.087 10.16749 0.003469 3.602395 3.12E-14OPU-1447 567.1243 12.24068 -1.13E-02 576.3905 100.8 1981.68 11.97652 0.003469 1.580963 1.37E-14OPU-1447 567.5506 8.349698 -4.27E-03 572.8292 100.3 1974.234 30.94914 0.003469 0.4473234 3.86E-15OPU-1447 497.0765 16.65657 0.0808818 480.1043 95.5 1768.69 128.0252 0.003469 6.53E-02 5.73E-16


91

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Cumulative %39Ar Released

Figure 5.2 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles from samples OPU 1436 (Fig. 5.1b-point A)


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Cumulative %39Ar Released Figure 5.3 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles from samples OPU 1197 (Fig. 5.1b-point B).

92


Fig. 5.2 displays 40Ar/39Ar isotopic data for three biotite single crystals from the dark gneiss of

the Corumbá quarry (Fig 1b-A). It shows an inhomogeneous cumulative % of 39Ar release, but well-

correlated integrated ages of 1,920±2Ma, 1,864±2Ma, and 1,838±3Ma. The analogous information

was obtained for the three-biotite single crystals from grey gneiss of Kinawa quarry, which yields an

integrated age of 1,937±4Ma, 1,898±3Ma, and 1,886±3Ma. The substantial information is that both

samples display well-defined steps with similar integrated ages. In addition, the samples show similar

spectral behaviors with loss of radiogenic argon by diffusion. The Ca/K rations show a similar

disturbance as well, probably related to impurities. Both samples are from different quarries but of

similar tectonic evolution, as observed by Oliveira & Carneiro (2001), Oliveira et al. (submitted to

Lithos). From the same Gneissic Unit, three biotite single crystals (Runs 2194-01/02/03, Fig 5.4) and

three-hornblende single crystals (Runs 2195-01/02/03, Fig 5.5) from amphibolites boudins were

analyzed. The three biotite single crystals (Fig. 5.4) yields similar, well-defined plateaus and

integrated ages differing not more than 10Ma. The Run 2194-01 displays a precise plateau of

1,975±10Ma. The same is screening for the Run 2194-02 that are evidence for two plateaus of

1987±4Ma and 1999±5Ma. The Run 2194-03 illustrates another accurate plateau of 1,970±3Ma.

With exception of Run 2194-02, the Ca/K rations are very low and the plateaus from the three

single crystals are homogeneous.

The three-amphibole single crystals (Fig 5.5) show a good link with the biotite single crystals

displayed in Fig. 5.4. Runs 2195-01, 02, and 03 exhibit ages of 2,027±14 Ma, 2,003±8 Ma and

2,016±7 Ma respectively (Fig 5.5). As expected, the ages are older than those for the biotites. It is

related to the closing of the temperature system.

Assuming total equilibration at about 1.9-2.0Ga, the plateaus can be fit in a broadly similar

evolution that may be correlated with the coolest prograde episode, discussed later on.

The second group consists of three biotite single crystals from the amphibolite from the

Supracrustal Unit (Runs 2197-01/02/03, Fig 5.6). The analyses of these three amphibole single crystals

(Runs 2197-01, 02 and 03, Fig 6) yield precise plateaus, despite a younger, but not significant

disturbance (Fig 5.6-2197-02 and 03). The plateaus occur at 2,031±13 Ma, 2,040±20 Ma and 2,032±9

Ma (Fig 5.6).

Dykes represent the third distinctive group of rocks. They are undeformed and

unmetamorphosed and they can be subdivided into two generations: gabbronorite and gabbros. These

rocks display alteration, such as saussuritization and uralitization, as attested by Oliveira & Carneiro

(2001).

93


A

B C D E F G H I J KL

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1975 ± 10 Ma


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1999 ± 5 Ma 1987 ± 4 Ma

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Figure 5.4 - Plot 40Ar/39Ar apparent age vs quarry (Fig 5.1b, point A)

Cumulative %39Ar released

94

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egrated Age = 1965 ± 2 Ma

1970 ± 3 Ma

cumulative argon released for biotites of amphibolites from Corumbá


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Figure 5.5 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of amphibolites from Corumbá quarry (Fig 5.1b, point A)

95



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Figure 5.6 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of amphibolites from Supracrustal Unit (Fig 1b, point C).

96


This group belongs to the Fissural Mafic Unit of Oliveira & Carneiro (2001). Three amphibole

crystals (Runs 2180-01/02/03, Fig 5.7), and three biotite single crystals (Runs 2181-01/02/03, Fig 5.8)

were taken from the gabbronorite

40Ar/39Ar isotopic data for Runs 2180-01, 02 and 03 yield good plateaus, but not of the same

high quality as for the previous two units. The amphibole single crystals exhibit plateaus at 1,530±9

Ma, 1,752±15 Ma and 1,690±40 Ma. The three biotite single crystals of Runs 2181-01/02/03 yield

three disturbed plateaus at 1,332±3Ma, 1,611±13Ma and 1,340±30Ma respectively (Fig 5.8).

Most likely, the high thermal gradient produced by the intrusion of dykes in the “cold gneissic

unit” caused alterations such as saussuritization, uralitization and chloritization, as attested by Oliveira

& Carneiro (2001). Having this in mind, the age variation for amphiboles and biotites from the same

rock is attributed to contamination by another mineral phase. This is clearly shown by Ca/K rations

illustrated in Figs. 5.7 and 5.8, lacking uniform behavior mainly at the end of the spectrum.

Two more gabbros, representing the “second” dyke swarm generation, were analyzed. Three

amphibole crystals from each gabbro were chosen and analyzed (Runs 2183-01/02/03, Fig 5.9 and

Runs 2184-01/02/03, Fig. 5.10). The first one shows good plateaus at 940±50 Ma, 990±30 Ma and

1080± 40Ma. The second one (Fig. 5.10) also shows good correlations and displays accurate plateaus

at 960±50Ma, 952±7Ma and 864±14Ma.

The gabbros give an idea of the behavior of two dissimilar intrusions that occurred in different

periods of the Earth’s evolution, but share analogous parameters (hot intrusions in contact with cold

host rocks). This may explain the disturbance at the beginning and at the end of the spectrum.

More details will be discussed on the next section.

5.7 DISCUSSION

The combined 40Ar/39Ar are interpreted as part of two distinct events that affected the southern

part of São Francisco Craton. The first one evidences the tectonothermal event that affected the

Gneissic Unit and the Supracrustal Unit. The second one is related to the emplacement of mafic dykes.

The 40Ar/39Ar results for both gneisses from the Cláudio Unit give an integrated age (Figs. 5.2

and 5.3). Note that both gray and dark-gray gneisses indicate ages of about 1,900Ma, and the

uncertainties from one step to another are very low. The variation from one step to another can be

attributed to alteration processes (as chloritization surrounding biotite).

97


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1530 ± 9 Ma


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Figure 5.7 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbronorite from fissural mafic Unit (Fig 5.1b – point D).



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Figure 5.8 Plot 40Ar/39Ar apparent age vs cumulative argon released for biotites of gabbronorite from fissural mafic Unit (Fig 5.1b – point D).

99


940 ± 50 Ma


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Figure 5.9 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbro from fissural mafic Unit (Fig 5.1b – point E).

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940 ± 20 Ma


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Figure 5.10 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbro from fissural mafic Unit (Fig 5.1b – point F).

101


The radiometric data for amphibolites from the Cláudio and the Supracrustal Units show good

correlation, with very low errors. In addition, the plateaus are very smooth lines with no disturbances

along the spectrum (Figs 5.4 and 5.5). The same behavior is attested by the amphiboles of the

Supracrustal Unit with a small disturbance at the end of the spectrum (Fig 5.6-2197-02 and 03). The

accuracy of the obtained data is compatible with the plateau definition, which comprises at least three

contiguous steps (e.g. Berger & York 1981, McDougall & Harrison 1988).

As defined by Oliveira & Carneiro (2001) the Gneissic Unit (Cláudio, Itapecerica and

Candeias) was affected by a high-grade metamorphic event (up to the granulite facies), and the

Supracrustal Unit by at least an upper amphibolite facies event. Both units experienced a strong

deformational event, which Oliveira & Carneiro (2001) characterized as the Cláudio shear zone (Fig

5.1), the most evident structure in the studied area. Recently, Oliveira et al. (2003, submitted to Anais

da Academia Brasileira de Ciências and commented in chapter 4) characterized the age of 2.6 Ga as of

a migmatization event. The adjacent structures are folds; shear bands, S/C foliations, and rotated

boudins. Recently, Oliveira et al. (2003, in preparation and commented in the following chapter)

recognized a granulite facies event at about 2.1 Ga. Hence, it remains unclear whether this event can

be seen as a part of a second prograde event followed by deformation related to the Cláudio shear zone

or it is related to an isolated charnockitic intrusion, as better explained in the following chapter.

From the exposed above, reliable information exists on the times that the migmatization and

deformational events occurred in the Campo Belo Metamorphic Complex (CBC). It is possible that

more than one deformational and migmatization events affected the CBC.

In the literature, CBC records indicate TTG crust reworking (migmatization event) at ca.

2,860 Ma (Teixeira et al. 1996) and at ca. 2,600 Ma (Oliveira et al. 2003). In contrast, there remains

one question: Is the deformational event that affected the gneissic unit and supracrustal unit late

Archaean or Proterozoic, or both?

Field, structural, petrographic and the new geochronological data for the gneisses and the two

amphibolite generations help make some considerations:

1 – the isotope concordance of samples from two dissimilar units suggests how significant the 40Ar/39Ar data is for tracing the evolution of the high-grade metamorphic terrains,

2 – are these ages related to the Transamazonian metamorphic event (Transamazonian

orogeny), followed by a deformational event, as attested in the Quadrilátero Ferrífero (Machado et al.

1992, Teixeira 1993)?

102


103

3 – do these data only represent uplift and cooling at the time of the tectonomagmatic

evolution of the Mineira Belt (Teixeira 1992, Teixeira et al. 1997)?

4 – do these data show the low-grade metamorphic history that overprinted all rocks of the

Campo Belo Metamorphic Complex, as recognized by Oliveira & Carneiro (2001) in the studied area

and south of it, around the Campo Belo town (e.g. Carneiro et al. 1988 a, b, Fernandes & Carneiro

2000, Campos et al. 2003)?

Based on our study, we believe that the data presented here are correlated to the evolution of

the Mineira Belt (Teixeira et al. 1997). They represent not only continent exhumation, but also the

response to the NW-verging contraction proposed by Alkmin & Marshak (1998). These authors

correlated the 2.5-2.0 Ga intervals with various stages of the Paleoproterozoic Wilson Cycle.

From the considerations above, three hypotheses arises:

(i) Taking into account that the argon block temperature occurs around 500ºC for amphibole

and 300ºC for biotite (Teixeira at al. 1997), it is plausible to consider that the 40Ar/39Ar age for our

rocks underwent temperatures of at least upper greenschist to amphibolite facies;

(ii) The Supracrustal Unit of the studied area is a small remnant of the Mineira Belt, as

defined by Teixeira et al. (1997), around the Lavras Town;

(iii) The tectono-deformational event attested by Oliveira & Carneiro (2001) in CBC can be

correlated with the second stage of the Mineira Belt evolution, resulting in the development of a

dome-and-keel structure, as described by Alkmin & Marshak (1998) for the Quadrilátero Ferrífero.

The following results presented in our work correspond to two generations of mafic dykes that

crosscut the crystalline crust of the Campo Belo Complex (Fig 5.1). As previously discussed, the

gabbros and gabbronorite are neither deformed nor metamorphosed. The 40Ar/39Ar mineral plateau

ages indicate that the dykes are post-Archaean and they were emplaced between two different periods

of the CBC evolution: the gabbronorite between 1,752±15 and 1,530±9 Ma, and the gabbros between

1080±40 and 864±14 Ma (Figs. 5.7, 5.8, 5.9 and 5.10).

The plateaus in Fig 5.7 for the gabbronorite hornblendes show some disturbance along the

steps. This disturbance can be interpreted as alteration or inclusion of another mineral phase in the

hornblendes. The biotites (Fig. 5.8) show the same disturbed spectrum, with ages varying from

1,632±13 and 1,332±3 Ma. This can be explained by chloritization, as seen in the thin-section.

Another explanation is hydrothermalism. As discussed previously, intense saussuritization affected the

gabbronorite and gabbros.


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The gabbro amphiboles show either plateaus with some disturbance, as for the sample shown

in Fig. 5.9, or good plateaus, as for the sample of Fig. 5.10. The latter shows good agreement between

plateau and integrated ages.

Two important geochronological records exist for the NW-SE dykes of the southern São

Francisco Craton: one is related to the emplacement of the basic gabbronorite (Sm-Nd isochron age of

2,697± 65 Ma; Pinese et al. 1995, Pinese 1997), and the other, to the emplacement of tholeiitic dykes

at ca. 1.9 Ga (Pinese et al. 1995).

Our results are geologically significant and provide imperative information for the evolution

of the southern São Francisco Craton. Some hypotheses arise when comparing our data with previous

data for the same area:

(i) The NW-SE structures into which the mafic dykes were emplaced can be correlated

with the Staterian continental rift scenario (1.8-1.6Ga, Brito Neves et al. 1995). A similar episode, the

Uruguayan dyke swarm, occurred in the Rio de la Plata Craton between 1,727-1,700Ma (Teixeira et al.

1999). Another record, at ca. 1.7Ga, is related to the Espinhaço rift, affecting the São Francisco

Craton;

(ii) The emplacement of the noritic gabbros and gabbros cannot be earlier than ~2.0Ga. This

assertion is based on geochronological records for the gneisses and amphibolites from the Gneissic

and Supracrustal Units (Figs 5.2, 5.3, 5.4 and 5.5), which are strongly deformed and metamorphosed.

Hence, the NW-SE dykes are neither deformed or metamorphosed, and crosscut the gneissic and

supracrustal units, as be evidenced from field relations (Oliveira & Carneiro 2001);

(iii) The emplacement of the mafic dykes can be correlated with the extensional orogenic

phase, conditioned to the NW-SE, N-NE extensional fractures and E-W Riedel crustal fractures (Endo

& Machado 1998);

(iv) The latest thermal evidence in the CBC is the emplacement of the ~960Ma gabbros,

emplaced in preexisting NW-SE structures.

5.8 SUMMARY

Our 40Ar/39Ar results depict two of the latest events of the CBC evolution: the first one shows

that the Transamazonian orogeny intensely affected the Campo Belo Metamorphic Complex, and the

second one evidences the intense breaking-up of the sialic crust after post-Transamazonian orogeny.


105

From the considerations above, the gneisses, amphibolites, gabbronorite and gabbros of the

Campo Belo Metamorphic Complex have undergone wide disturbances since the earliest event that

affected the surrounding areas. The new data-set presented in our study yielded the following results

and conclusions:

(i) All gneisses and amphibolites from both units record the same peculiar closing temperature

at about 2.0Ga. It can be correlated with the age of exhumation and retrograde metamorphic processes

from the high-grade metamorphic event at 2.05 Ga, as attested by U-Pb data for charnockites (Oliveira

et al. 2003, in preparation).

(ii) The noritic gabbro was emplaced at about 1.7 Ga.

(iii) The two gabbros display the same age at about 1.0 Ga.

(iv) The Sm-Nd age for the undeformed gabbronorite of ca. 2,676 Ma presented by Pinese

(1997) is uncertain, once our 40Ar/39Ar results for deformed and metamorphosed gneisses and

amphiboles fall about 2.0 Ga and these rocks are crosscut by the noritic gabbro and gabbros. The latter

are post-Transamazonian, rather than Archaean.

(iv) Finally, our study demonstrates the need for comprehensive Ar-Ar datasets. Only limited 40Ar/39Ar data for high-grade metamorphic terrains is available in the literature.


106

CAPÍTULO 6 GEOCRONOLOGIA U-Pb

6.1 INTRODUÇÃO

Conforme apresentado no Capítulo 2 trata-se, a região em estudo, de uma crosta siálica

polideformada com eventos metamórficos que vão da fácies granulito/anfibolito superior à fácies

anfibolito inferior/xisto verde, além de variados eventos magmáticos intrusivos. Adicionalmente, esse

substrato siálico apresenta-se variavelmente migmatizado.

Em termos geocronológicos, os resultados Rb-Sr e Sm-Nd (em rocha total) apresentados no

Capítulo 4 mostraram que as rochas das Unidades Gnáissicas (Cláudio, Candeias e Itapecerica)

sofreram distúrbio isotópico por volta de 2,6 Ga.

Já os resultados geocronológicos Ar-Ar (Capítulo 5), obtidos a partir de anfibólios e biotitas

dessas unidades gnáissicas; de boudins de anfibolitos inseridos nessas unidades; e de anfibolitos da

Unidade Supracrustal mostraram um resfriamento situado entre 2,0-1,9 Ga.

Por sua vez, os resultados Ar-Ar obtidos a partir de anfibólios e biotita extraídos dos gabros e

gabronoritos da Unidade Máfica Fissural indicaram duas épocas distintas de resfriamento: a primeira

situada por volta de 1,7 Ga e a segunda situada por volta de 1,0 Ga. Como essas rochas exibem

características ígneas, não estão deformadas, tem apenas um metamorfismo incipiente de fácies xisto

verde, tais idades são, também, idades de cristalização. E, conseqüentemente, o reequilíbrio para fácies

xisto verde pode ser posterior a 1,0 Ga.

Apesar desse avanço, em termos da contribuição geocronológica apresentada nesta Tese, para

compreensão da evolução tectônica regional, algumas questões fundamentais continuam sem resposta.

Por exemplo: quais seriam as relações temporais entre os eventos de migmatização e granulitização?

seria, por acaso, o distúrbio isotópico manifestado pelos sistemas Rb-Sr e Sm-Nd o reflexo da

granulitização ou migmatização?

Para tentar responder essas questões, foram selecionados quatro afloramentos das unidades

gnáissicas e cinco rochas. A escolha dos afloramentos, e das amostras especificamente, foi

determinada pelas seguintes razões: a) determinar a idade de cristalização dessas rochas; b) determinar

a idade dos eventos de migmatização e granulitização.

As amostras selecionadas para esse estudo têm a seguinte proveniência (Figuras 6.1, 6.2): a

primeira é um charnockito da pedreira Alemão (amostra AH 08); a segunda é um enderbito da pedreira

Oliveira (amostra AH 07), a terceira é um gnaisse migmatítico de composição tonatítica da pedreira


108

Kinawa (amostra AH 11); a quarta é um gnaisse migmatítico de composição granodiorítica (amostra

AH 14) e, a quinta, um gnaisse, também migmatítico, de composição granítica (amostra AH 15).

Figura 6.1 – Mapa geológico modificado por Oliveira & Carneiro (2001), mostrando as diferentes unidades na área mapeada. Simbologia: 1 – Unidade Máfica Fissural; 2 – Unidade Supracrustal; 3 – Unidade Gnáissica Candeias; 4 - Unidade Gnáissica Itapecerica; 5 - Unidade Gnáissica Cláudio; 6 – Contato inferido; 7 – Foliação; 8 – Zona de Cisalhamento Cláudio e, 9 – Pontos dos zircões estudados.

6.2 RESULTADOS U-Pb

A preparação das amostras está descrita no Capítulo 1. Para datação dos zircões utilizou-se do

equipamento Ion Microprobe CAMECA IMS270 do Museu de História Natural da Suécia em

Estocolmo. Para calibração da razão Pb/U foi usado o padrão 915000 com idade de 1065 ± 0,3 Ma

(1σ). As concentrações de Pb e U são de 15 e 80ppm respectivamente (Wiedenbeck et al. 1995). Os

procedimentos analíticos em detalhe podem ser obtidos em Whitehouse & Russel (1997) e

Whitehouse et al. (1999).

De cada um das amostras escolhidas, para datação U-Pb, selecionou-se, em média, 35 zircões.

Esses zircões foram submetidos a catodoluminescência para seleção dos melhores grãos. Os resultados

analíticos são apresentados na Tabela 6.1.

Amostras 207Pb/206Pb 206Pb/238U 208Pb/232ThAH08-2-1 110 120 50 0,93 1,1 0,2 0.1264 ± 0.0012 0.3033 ± 0.0043 0.0934 ± 0.0029 2049 ± 17 1707 ± 21 1656 ± 25 1805 ± 54

0.1026 ± 0.0031 2046 ± 12 1911 ± 23 1885 ± 27 1974 ± 57AH08-18-1 170 180 84 1 1,1 0,08 0.1262 ± 0.0009 0.3450 ± 0.0048AH08-18-1@1 140 120 61 0,81 0,83 0,45 0.1229 ± 0.0009 0.3049 ± 0.0043AH08-6-2 130 80 53 0,52 0,62 0,37 0.1252 ± 0.0010 0.3152 ± 0.0044AH08-14-1 310 290 140 0,89 0,95 0,11 0.1246 ± 0.0005 0.3333 ± 0.0047AH08-14-2 150 67 61 0,43 0,46 0,1 0.1262 ± 0.0008 0.3334 ± 0.0047AH08-5-1 84 73 38 0,84 0,87 0,22 0.1244 ± 0.0012 0.3261 ± 0.0047AH08-5-2 110 110 48 0,87 0,93 0,24 0.1242 ± 0.0015 0.2921 ± 0.0042AH07-1-1 780 480 220 0,39 0,62 8,88 0.1446 ± 0.0017 0.2062 ± 0.0029AH07-7-1 1300 790 440 0,38 0,62 2,98 0.1722 ± 0.0007 0.2502 ± 0.0035AH07-11-1x 710 300 290 0,43 0,42 6,8 0.1711 ± 0.0031 0.2969 ± 0.0042AH07-11-2 1300 53 260 0,49 0,042 16,18 0.0969 ± 0.0018 0.1542 ± 0.0022AH07-14-2 1300 310 250 0,17 0,23 10,92 0.1184 ± 0.0013 0.1501 ± 0.0021AH11-2-1 190 350 120 1,4 1,8 0,78 0.1823 ± 0.0013 0.3506 ± 0.0052AH11-2-2 930 150 160 0,073 0,16 3,35 0.1135 ± 0.0014 0.1453 ± 0.0023AH11-2-3 390 51 120 0,083 0,13 1,26 0.1393 ± 0.0008 0.2711 ± 0.0038AH11-4-1 450 260 230 0,4 0,58 0,31 0.1808 ± 0.0008 0.3920 ± 0.0055AH11-19-1 470 260 300 0,45 0,55 0,15 0.1874 ± 0.0006 0.4787 ± 0.0067AH11-19-2 520 130 250 0,17 0,24 0,36 0.1758 ± 0.0014 0.3892 ± 0.0055AH14-1-1 460 530 210 0,81 1,2 0,38 0.1638 ± 0.0011 0.3014 ± 0.0045AH14-1-2 410 130 190 0,26 0,31 0,15 0.1744 ± 0.0007 0.3718 ± 0.0052AH14-6-2 280 260 170 0,81 0,9 1,11 0.1759 ± 0.0010 0.4090 ± 0.0057AH14-7-1 330 330 160 0,67 1 2,67 0.1759 ± 0.0009 0.3278 ± 0.0047AH14-7-2x 730 360 150 0,22 0,49 1,97 0.1427 ± 0.0008 0.1602 ± 0.0023AH14-12-1 100 79 48 0,57 0,79 0,84 0.1815 ± 0.0023 0.3363 ± 0.0047AH14-12-2 820 82 130 0,096 0,1 2,05 0.1195 ± 0.0014 0.1350 ± 0.0019AH15-1-1 65 37 42 0,58 0,57 0,69 0.1999 ± 0.0018 0.4719 ± 0.0067AH15-1-2 1200 78 190 0,36 0,066 6,25 0.0873 ± 0.0023 0.1257 ± 0.0018AH15-11-1 540 160 190 0,19 0,31 0,84 0.1594 ± 0.0012 0.2852 ± 0.0040

0.1018 ± 0.0031 1999 ± 14 1716 ± 21 1674 ± 25 1960 ± 570.0892 ± 0.0028 2032 ± 13 1766 ± 22 1724 ± 25 1726 ± 510.0994 ± 0.0028 2023 ± 7 1854 ± 23 1825 ± 27 1915 ± 520.0990 ± 0.0030 2045 ± 11 1855 ± 23 1822 ± 26 1907 ± 550.1021 ± 0.0031 2020 ± 17 1819 ± 23 1786 ± 27 1964 ± 570.0984 ± 0.0034 2017 ± 22 1652 ± 21 1601 ± 24 1896 ± 630.0760 ± 0.0026 2284 ± 20 1209 ± 16 1107 ± 33 1481 ± 490.0830 ± 0.0024 2579 ± 7 1440 ± 18 1285 ± 33 1611 ± 440.1406 ± 0.0087 2568 ± 30 1676 ± 21 1520 ± 46 2660 ± 1540.9260 ± 0.0318 1564 ± 35 925 ± 12 891 ± 30 13248 ± 3370.0758 ± 0.0032 1932 ± 20 902 ± 12 844 ± 25 1477 ± 600.1060 ± 0.0031 2674 ± 12 1938 ± 25 1762 ± 40 2037 ± 560.0440 ± 0.0033 1856 ± 21 874 ± 13 824 ± 17 871 ± 640.0728 ± 0.0037 2219 ± 10 1546 ± 19 1456 ± 26 1421 ± 700.0961 ± 0.0028 2660 ± 7 2132 ± 26 1978 ± 40 1854 ± 510.1191 ± 0.0034 2719 ± 5 2521 ± 29 2428 ± 44 2274 ± 610.0989 ± 0.0031 2613 ± 13 2119 ± 26 1980 ± 38 1906 ± 570.0916 ± 0.0029 2495 ± 11 1698 ± 22 1560 ± 32 1771 ± 540.1144 ± 0.0033 2600 ± 7 2038 ± 25 1893 ± 37 2189 ± 590.1237 ± 0.0037 2614 ± 9 2210 ± 26 2083 ± 40 2357 ± 670.0899 ± 0.0026 2615 ± 8 1828 ± 23 1664 ± 40 1741 ± 490.0538 ± 0.0016 2260 ± 9 958 ± 13 870 ± 19 1059 ± 310.1021 ± 0.0033 2667 ± 21 1869 ± 23 1692 ± 39 1965 ± 610.0973 ± 0.0053 1948 ± 20 817 ± 11 761 ± 14 1877 ± 980.1535 ± 0.0055 2826 ± 15 2492 ± 29 2335 ± 51 2887 ± 960.3807 ± 0.0173 1367 ± 51 763 ± 10 741 ± 14 6520 ± 2550.0814 ± 0.0038 2450 ± 13 1618 ± 20 1487 ± 30 1583 ± 71

valores corrigidos idade 207 corrigida

idade 206Pb/238U

idade 208Pb/232Th

idade 207Pb/206Pb

Th-U (medido) f206 (%)U

(ppm)Th

(ppm)Pb

(ppm)Th-U

(calculado)

Table 6.1 – Resultados analíticos

Figura 6.2 – Fotografias das amostras onde foram coletados os zircões datados: a) Gnaisse da pedreira Kinawa; b,c) Gnaisse da pedreira Corumbá; d) Gnaisse da pedreira Oliveira e, e) Pedreira de rocha ornamental Alemão.

Os cristais de zircão dessa rocha apresentam coloração castanha-avermelhada translúcida e são

de uma mesma população. Após o seu imageamento por catodoluminescência, três cristais de zircão

foram selecionados para datações U-Pb obtidas a partir de seis spots (Tabela 1, Figura 6.3). Em termos

morfológicos (Figura 6.3), os cristais escolhidos são prismáticos com faces e arestas um pouco

arredondadas. O zoneamento interno desses zircões é simples (não complexo) e, também, sugere um

zoneamento de derivação magmática com algumas bordas de sobrecrescimento metamórfico (Figura

6.3).

Trata-se de um gnaisse migmatítico de composição tonalítica que pertence à Unidade

gnáissica Cláudio e foi coletado na pedreira de rocha ornamental Kinawa (Figura 6.1C). É uma rocha

fortemente migmatizada e deformada e, de coloração cinza mostrando, localmente, inúmeros

mobilizados félsicos róseo e branco (Figura 6.2a).

6.2.1 Amostra AH 11


Figura 6.3 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH11. Os zircões pertencem a um gnaisse migmatítico da pedreira Kinawa (Figura 6.1 C)

O conjunto das razões U/Pb dessas análises, quando tratados no diagrama Concórdia (Figura

6.4), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2749 ± 6

Ma e, para o intercepto inferior, uma idade de U-Pb de 593 ± 14 Ma. O MSWD dessa discordia é de

0,89 e ela representa o melhor ajuste para o conjunto das razões obtidas e foi traçada a partir do

alinhamento das três razões destacadas em negrito da Figura 6.4. Todas as análises refletem perda de

Pb (ou ganho de U), agumas das quais muito acentuado. Um dos pontos que determinam o melhor

alinhamento encontra-se próximo ao intercepto superior e, um outro encontra-se próximo ao extremo

inferior (Figura 6.4).

A idade encontrada pelo intercepto superior é interpretada como época de cristalização do

protólito dessa rocha. A idade encontrada pelo intercepto inferior mostra distúrbio isotópico no

neoproterozóico.

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Figura 6.4 – Diagrama Concórdia U-Pb para os zircões da mostra AH 11 da Unidade Gnáissica Cláudio.

6.2.2 Amostra AH 14

Trata-se de um gnaisse migmatítico de composição granodiorítica e pertence à Unidade

gnáissica Cláudio e foi coletado na pedreira de rocha ornamental Corumbá (Figura 6.1D). É uma rocha

fortemente migmatizada e deformada e, de coloração cinza escuro, localmente, apresenta mobilizados

félsicos branco (Figura 6.2b).


de uma mesma população. Após o seu imageamento por catodoluminescência, quatro cristais de zircão

foram selecionados para datações U-Pb obtidas a partir de sete spots (Tabela 1, Figura 6.5). Em termos

morfológicos (Figura 6.5), os cristais escolhidos são prismáticos com faces e arestas um pouco

arredondados. O zoneamento interno desses zircões é simples (não complexo), excetuando-se o cristal

d que mostra pelo menos duas zonas bem distintas. Já o cristal a mostra inúmeras inclusões de outra

fase mineral. Entretanto todos os cristais escolhidos também sugerem um zoneamento de derivação

magmática com algumas bordas de sobrecrescimento metamórfico (Figura 6.5).

112


Figura 6.5 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH14. Os zircões pertencem a um gnaisse migmatítico da pedreira Corumbá (Figura 6.1 D)

O conjunto das razões U/Pb dessas análises, quando tratada no diagrama Concórdia (Figura

6.6), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2658 ± 7,2

Ma e, para o intercepto inferior, uma idade de U-Pb de 385 ± 12 Ma. O MSWD dessa discordia é de

1.1 e ela representa o melhor ajuste para o conjunto das razões obtidas e foi traçada a partir do

alinhamento das três razões destacadas em negrito da Figura 6.6. Todas as análises refletem perda de

Pb (ou ganho de U). Um dos pontos que determinam o melhor alinhamento encontra-se próximo ao

intercepto superior e, um outro se encontra próximo ao intercepto inferior (Figura 6.6).

A idade encontrada pelo intercepto superior é interpretada como época do protólito dessa

rocha. A idade encontrada pelo intercepto inferior mostra distúrbio isotópico no paleozóico.

Entretanto, as idades aparentes 207Pb/206Pb exibidas na Figura 6.9 exibem resultados, que

variam de 2614 ± 9 Ma, 2260 ± 9 Ma e 1948 ± 20 Ma que sãocorrelacionáveis a pelo menos 3

períodos já registrados na porção meridional do Cráton São Francisco.

113


Figura 6.6 – Diagrama Concórdia U-Pb para os zircões da amostra AH 14 da Unidade Gnáissica Cláudio.

6.2.3 Amostra AH 15

Trata-se de um gnaisse migmatítico de composição granítica. Essa rocha é pertencente a

Unidade gnáissica Cláudio e foi coletada na pedreira Corumbá (Figuras 6.1D). Essa rocha se

assemelha às rochas pertencentes à Unidade Gnáissica Itapecerica, principalmente, em termos de

coloração. É uma rocha fortemente migmatizada e deformada e, de coloração rósea (Figura 6.2c).


de uma mesma população. Após o seu imageamento por catodoluminescência, apenas dois cristais de

zircão foram selecionados para datações U-Pb obtidas a partir de três spots (Tabela 1, Figura 6.7). Em

termos morfológicos (Figura 6.7), os cristais escolhidos são prismáticos com faces e arestas um pouco

arredondados. O zoneamento interno desses zircões é simples (não complexo). Os cristais escolhidos

também sugerem um zoneamento de derivação magmática com algumas bordas de sobrecrescimento

metamórfico (Figura 6.7).

114


Figura 6.7 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH14. Os zircões pertencem a um gnaisse migmatítico da pedreira Corumbá (Figura 6.1 D)


6.8a), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2818 ±

1200 Ma e, para o intercepto inferior, uma idade de U-Pb de 688 ± 1200 Ma que são idades sem

significado geológicos. O MSWD dessa discordia é de 150. A reta foi traçada a partir do alinhamento

das três razões obtidas da Figura 6.8a. Um outro diagrama Concórdia (Figura 6.8b) fornecem uma

discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2910 +24/-23 Ma e, para o

intercepto inferior, uma idade de U-Pb de 911 +38/-37 Ma.

Por outro lado, os resultados individuais mostram para o ponto 1 do zircão a, uma idade

aparente de 2826 ± 15 Ma que pode estar próxima a idade do protólito dessa rochas. Esse pode

representar um cristal herdado. Já o ponto 2 desse mesmo zircão sugere um sobrecrescimento com

idade de 1367 ± 51 Ma que não tem um significado geológico, ao não ser de perda de chumbo. O

zircão (b) mostra uma idade de 2450 ± 13 Ma.

Foi construído um diagrama Concórdia (Figura 6.8c) com o conjunto das razões U/Pb obtidas

nos zircões das rochas da Figura 6.2b e 6.2c que são pertencentes ao mesmo domínio e foram

submetidas ao mesmo evento de deformação e migmatização. O conjunto dessas razões fornece uma

discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2715 ± 86 [±87] Ma e, para o

intercepto inferior, uma idade de U-Pb de 503 ± 153 Ma. O MSWD dessa discordia é de 153.

115


A idade encontrada pelo intercepto superior para essa combinção é interpretada como época de

cristalização desse migmatítico. A idade encontrada pelo intercepto inferior mostra distúrbio isotópico

no neoproterozóico.

Figura 6.8 – Diagramas Concórdia U-Pb para os zircões da amostra AH 15 da Unidade Gnáissica Cláudio (a, b) e para as amostras AH 14 e AH 15 da Unidade Gnáissica Cláudio.

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6.2.4 Amostra AH 07

Trata-se de um hiperstênio-gnaisse de composição tonalítica ou enderbítica que pertence à

Unidade gnáissica Candeias e foi coletado na pedreira de rocha ornamental Oliveira (Figura 6.1B). É

uma rocha esverdeada e, apesar de pertencer à mesma unidade que a amostra anterior (AH 08)

apresenta-se fortemente migmatizada e deformada (Figura 6.2d). Os cristais de zircão dessa rocha

apresentam coloração castanha-avermelhada translúcida e são de uma mesma população. Após o seu

imageamento por catodoluminescência, quatro cristais de zircão foram selecionados para datações U-

Pb obtidas a partir de cinco spots (Tabela 1, Figura 6.9). Em termos morfológicos (Figura 6.9), os

cristais escolhidos são prismáticos com bi-terminações bem desenvolvidas e apresentam zoneamentos

similares. Não foi possível determinar, com clareza, zonas de sobrecrescimento (Figura 6.9). O zircão

d apresenta características de um cristal prismático multifacetado. O zoneamento interno desses

zircões é simples e sugere a derivação de um zoneamento magmático.

Figura 6.9 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH07. Todos os cristais são de um enderbito da pedreira Oliveira (Figura 6.1 B)


6.10), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2765

+39/-38 Ma e, para o intercepto inferior, uma idade de U-Pb de 693 +21/ -21 Ma. O MSWD dessa

discordia é de 3.7 e ela representa o melhor ajuste para o conjunto das razões obtidas e foi traçada a

partir do alinhamento das três razões destacadas em negrito da Figura 6.10. Todas as análises refletem

perda de Pb (ou ganho de U) e elas estão posicionadas nas imediações do intercepto inferior.

A idade encontrada pelo intercepto superior é interpretada como época do protólito dessa

rocha. A idade encontrada pelo intercepto inferior mostra distúrbio isotópico no neoproterozóico.

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As idades aparentes 207Pb/206Pb exibidas na Figura 6.9 variam de 2579 ± 7 Ma, 2284 ± 20 Ma

e, 1932 ± 20 Ma. Esses resultados serão discutidos no próximo capítulo.

Figura 6.10 – Diagrama Concórdia U-Pb para os zircões da amostra AH 07 da Unidade Gnáissica Candeias.

6.2.5 Amostra AH 08

Trata-se de um charnockito de coloração esverdeada (Figura 6.2e) da Unidade Gnáissica

Candeias, coletado na pedreira de rocha ornamental Alemão (Figura 6.1A). Os zircões desse

charnoquito apresentam coloração castanha-avermelhada translúcida e são de uma mesma população.

Após o seu imageamento por catodoluminescência cinco cristais de zircão foram selecionados para

datações U-Pb, obtidas a partir de nove pontos (spots). Os resultados estão listados na Tabela 1. Em

termos morfológicos (Figura 6.11), os cristais escolhidos são prismáticos, com bi-terminações bem

desenvolvidas à exceção do cristal d. Os cristais apresentam zoneamento simples (não complexo) mas

os cristais d e e apresentam zoneamento de borda.

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Figura 6.11 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH08 e pertencentes ao charnockito da pedreira Alemão (Figura 6.1 A)

As idades aparentes 207Pb/206Pb exibidas na Figura 6.11, independentes de terem sido obtidas

nas bordas ou nas posições centrais dos cristais analisados, exibem uns padrões muito coerentes de

resultados, que variam de 2049 ± 17 Ma a 1999 ± 14 Ma.


6.12), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2066

+24/-18 Ma e, para o intercepto inferior, uma idade de U-Pb de 531 +150/-160 Ma. O MSWD dessa

discordia é de 0,93 e ela representa o melhor ajuste para o conjunto das razões obtidas e foi traçada a

partir do alinhamento das quatro razões destacadas em negrito na Figura 6.12. É importante salientar

que, apesar de todas as análises refletirem perda de Pb (ou ganho de U), elas estão posicionadas nas

imediações do intercepto superior.

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A idade encontrada pelo intercepto superior é, então, interpretada como época de cristalização

do charnockito. Mas, a idade encontrada pelo intercepto inferior apresenta uma margem de erro

excessiva e pode não ter significado aparente.

Figura 6.12 – Diagrama Concórdia U-Pb para os zircões da amostra AH 08 da Unidade Gnáissica Candeias.

6.3 SUMÁRIO

Os resultados mais evidentes que os resultados U-Pb mostraram são os seguintes:

1 – O protólito das rochas gnáissicas foi formado no intervalo de 3,0 e 2,75 Ga; 2 – O segundo

evento metamórfico ocorreu por volta de 2,05Ga. Esse evento atingiu a fácies granulito e foi caracterizado

através de datação U-Pb em zircões dos charnockitos; 3 - Um evento metamórfico, de menor intensidade, foi o

responsável pelo distúrbio das rochas durante o Evento Brasiliano, por volta de 0,5Ga. 4 – As rochas mostraram

perdas acentuadas de Pb (ganho de U) desde o Arqueano até o Paleozóico inferior. Discussões mais detalhadas

serão apresentadas no próximo capítulo.

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CAPÍTULO 7

CONSIDERAÇÕES FINAIS

7.1 GENERALIDADES

O Cráton São Francisco pode ser subdividido em duas porções sendo uma meridional e uma

setentrional. O acervo geocronológico permite subdividi-las em Províncias arqueanas e proterozóicas.

Na porção meridional do Craton São Francisco, de acordo com Teixeira et al. (2000), as

grandes etapas de formação de rochas estão situadas à volta de 3,2 Ga; 3,0 Ga; 2,8 Ga; 2,6-2,5 Ga; 2,2-

1,9 Ga e 2,0-1,7 Ga. Dessas, destacam-se as etapas de migmatização (2,8 Ga), granitogênese (2,6-2,5

Ga), formação do Cinturão Mineiro (2,20-1,90 Ga), soerguimento e resfriamento crustal (2,0–1,7Ga).

O cinturão Mineiro (Teixeira & Figueiredo 1991), relacionada à Orogenia Transamazônica, é

uma faixa móvel paleoproterozóica, parcialmente instalada sobre rochas arqueanas, intrudida por

granitóides com idades variando de 2,2-2,1 Ga (Teixeira et al. 1997, Noce et al. 1998, Valença et al.

2000, Ávila 2000). Esse cinturão está localizado nas bordas sul e sudeste do Craton São Francisco

(Figura.1.1), e caminha em direção ao norte.

Os pulsos metamórficos desse cinturão, de acordo com Ávila (2000), estão situados a volta de

2,13-2,12 Ga e 2,06-2,03. Idades dessa ordem de grandeza, 2,06-2,03 Ga foram encontradas por

Machado et al. (1989) nas rochas do Complexo Metamórfico Bação, que seria o substrato arqueano

retrabalhado na geração do Cinturão Mineiro.

Em linhas gerais, a porção setentrional do Craton São Francisco é constituída por uma crosta

siálica (e.g. charnockitos, enderbitos, gnaisses e migmatitos) e por seqüências vulcano-sedimentares.

Em termos geocronológicos, essas rochas podem ser enquadradas em duas províncias: a)

província de idade arqueana (3,0-2,7 Ga), onde se situa, por exemplo, o Complexo Jequié; b) província

de idade proterozóica inferior (2,25-1,9 Ga), onde se situam, por exemplo, os cinturões Itabuna e

Correntina-Guanabi à idades proterozóicas média a superior (compilados por Teixeira 1992)

No caso da província arqueana, Silva et al. (2002) confirmou a presença de um domínio

mesoarqueano, situado a volta de 3,0 Ga, caracterizado por intensa atividade tectônica. Além disso,

identificou um período de acresção crustal, situado entre 2,87 – 2,63 Ga, quando foram gerados os

protólitos dos ortognaisses granulíticos do domínio Itabuna – Salvador – Curaçá.


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Quanto à província de idade proterozóica inferior, o magmatismo pré-colisional do cinturão

Bahia Oriental foi datado em 2,20 - 2,13 Ga e o magmatismo sin-colisional em 2,09 Ga. Por sua vez, o

pico metamórfico de fácies granulito, relacionado a essa colisão, foi datado em 2,08 – 2,05 Ga (Silva

et al. 2002). Na província de idade neoproterozóica, Silva et al. (2002) reconheceu distúrbios

isotópicos situados à volta de 0,7 Ga (e.g. idades aparentes Pb-Pb e intercepto inferior U-Pb), nos

charnoquitos de Ihéus.

7.2 QUESTÕES ACERCA DA EVOLUÇÃO TECTÔNICA DO CRATON SÃO

FRANCISCO MERIDIONAL

Apesar do avanço, no conhecimento geológico da porção meridional do Craton São Francisco,

ocorrido nos últimos anos, algumas questões ainda continuam em aberto. Dentre elas destacamos as

seguintes:

1 Quantos eventos de migmatização tiveram lugar nessa região? Sabe-se que um deles pode

estar situado à volta de 2,86 Ga (Noce 1995, Teixeira et al. 1996)

2 Seria o período de 2,6 Ga o responsável pelo evento de migmatização, seguido de

deformação, sob fácies metamórfica granulito? Ou esse período marcaria apenas um distúrbio

isotópico, em um evento de menor intensidade, não necessariamente sob fácies granulito e sim,

reativação desse complexo com intrusões de corpos graníticos como já relatado anteriormente (Noce

1995, Teixeira et al. 2000) situados no Evento Rio das Velhas III de Endo (1997)?

3 Quantas etapas de formação de rochas tiveram lugar na evolução tectônica do Cinturão

Mineiro? Sabe-se que corpos granitóides foram colocados por volta de 2,2-2,1 Ga e que esse cinturão

apresenta retrometamorfismo para a Fácies Xisto-Verde

4 As rochas do Complexo Metamórfico Campo Belo teriam sido afetadas pelo Evento

Deformacional Transamazônico, como registrado nas rochas do Quadrilátero Ferrífero (e.g. Alkmim

& Marshak 1998)?

5 Estaria à geração da Zona de Cisalhamento Cláudio relacionada à primeira fase do Evento

Transamazônico, de componente N/S destral transpressivo/transtrativo (Endo 1997), registrado nas

rochas do Quadrilátero Ferrífero e adjacências?

6 Seria o período de 2,0-1,8 Ga a época de exumação do Complexo Metamórfico Campo

Belo?


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7 Seriam realmente arqueanos os diques de gabronoritos indeformados, de direção NW-SE,

conforme datação isocrônica Sm-Nd (Pinese 1997)?

Baseando-se nas questões, acima mencionadas, e nos dados apresentados nesta tese, traçar-se-

á, no próximo item, um painel evolutivo para o Complexo Metamórfico Campo Belo.

7.3 DISCUSSÃO

Oliveira (1999) e Oliveira & Carneiro (2001) subdividiram o Complexo Metamórfico Campo

Belo em seis unidades litodêmicas, sendo três delas gnáissicas, uma anfibolítica, uma supracrustal

(vulcâno-sedimentar) e uma máfica fissural.

De acordo com esses autores, essas unidades foram geradas no decorrer de cinco eventos

tectonotermais. O primeiro evento está relacionado ao período de formação do protólito das unidades

gnáissicas. O segundo está relacionado a um evento extensional com posterior deposição da unidade

vulcâno-sedimentar. O terceiro evento, de alto grau metamórfico e considerado o mais importante, foi

o responsável pelo rigoroso processo de migmatização que afetou a região. O mesmo evento foi o

responsável pela geração da Zona de Cisalhamento Cláudio com cinemática transpressiva destral. Em

termos geocronológicos, esse evento deveria ser anterior a 2658 Ma, pois essa foi a idade encontrada

por Pinese (1997), para os diques de gabronorito indeformados da região de Lavras. O quarto evento

foi caracterizado como fraturamento crustal e colocação da unidade máfica fissural que corta as

unidades arqueanas. Esses diques são indeformados, apresentam grau metamórfico incipiente e são

correlatos aos diques estudados por Pinese (1997). O quinto evento foi o retrometamorfismo regional

para xisto verde.

Com os novos dados geoquímicos apresentados nesta tese, o padrão geoquímico das rochas

félsicas do Complexo Metamórfico Campo Belo, principalmente em se tratando dos gnaisses de fácies

granulito (enderbitos) e charnoquitos, mostra particularidades distintas se comparados com

metagranitóides arqueanos típicos ou com a média das crostas superior e inferior e crosta total.

Observam-se também uma grande mobilidade para alguns dos elementos traços incompatíveis (e.g.

Cs, Rb, U, Nb, Ta e W). Essa mobilidade seria causada por desidratação devido ao aumento de

temperatura, atingindo a fácies granulito.

Os elementos Nb e Ta, por exemplo, são considerados imóveis no decorrer dos processos de

desidratação crustal por aumento de temperatura e perda de fluídos. Entretanto, as rochas do

Complexo Metamórfico Campo Belo são empobrecidas nesses elementos e as razões Nb/Ta são

altíssimas sendo comparadas com alguns eclogitos (ver Capítulo 3). Disso conclui-se que os

enderbitos e charnockitos, em questão, não são formados, exclusivamente, de componentes oriundos


124

da crosta inferior, como mencionado por vários autores (e.g. Harris & Bickle 1989, Santosh et al.

1991a, b, Rudnick & Fountain 1995, Kramers & Tolstikhin 1997, Satish-Kumar & Santosh 1998).

Mesmo porque, se assim fosse, as rochas estudadas deveriam ser empobrecidas nos heat production

elements (U, K e Th) o que não é o caso, a excessão do U. Sendo assim, possivelmente, formação dos

enderbitos e charnockitos ocorreu a partir da desidratação de granitóides, durante um processo

metamórfico de alto grau.

Os resultados U-Pb dos zircões das rochas gnáissicas (enderbitos inclusive), mostraram idades

para o intercepto superior situadas entre 2,9-2,7 Ga. Todavia, as idades aparentes 207Pb-206Pb, obtidas

em diferentes posições nos cristais de zircão, mostraram distúrbios isotópicos situados em 2,6 Ga, 2,2

Ga e 1,9 Ga. Isso sugere, a princípio, que se tratam da época de geração dos protólitos dos gnaisses e

de eventos tectônicos posteriores, superimpostos às rochas da região em estudo.

Idades situadas entre 2,60-2,55 Ga foram obtidas pelos métodos Rb-Sr e Sm-Nd nessas rochas,

por meio de isócronas e errócronas, numa combinação de rochas cogenéticas e não cogenéticas. No

entanto, todas as amostras tinham as mesmas características: eram deformadas e migmatizadas. Idade

semelhante foi encontrada por Teixeira (1985) no enderbito da Pedreira Oliveira, que é o mesmo

enderbito estudado nesse trabalho. Trata de uma idade isocrônica Rb-Sr de 2566 ± 53 Ma (Ri= 0,706).

Uma idade U-Pb dessa mesma ordem foi encontrada por Campos et al. (2003) em zircões do

leucossoma e mesossoma de um migmatito do Complexo Metamórfico Passa Tempo (sudeste da área

estudada). Esse conjunto de características está apontando, então, para um evento de migmatização do

substrato gnáissico situado por volta de 2,60-2,55 Ga. Então, aparentemente, sendo um do(s) evento(s)

de migmatização.

Aparentemente, então, o evento de migmatização antecede o evento de fácies granulito, pois o

resultado mais conciso e relevante foi obtido no charnockito da Pedreira Alemão. O conjunto das

razões U/Pb dessa rocha, quando tratada no diagrama Concórdia, forneceram uma discórdia que

apresenta, para o intercepto superior, uma idade U-Pb de 2066 +24/-18 Ma. Adicionalmente, as idades

aparentes 207Pb-206Pb mostram um padrão muito coerente e situado à volta de 2049 a 1999 Ma,

independentemente, se forem obtidas nas bordas ou porções mais centrais dos cristais. As idades Ar-

Ar, em anfibólios e biotitas dos gnaisses e anfibolitos, situadas entre 2027 - 1950 Ma, corroboram essa

interpretação, marcando o final de um evento de alto grau que teve lugar ao término do

Paleoproterozóico, uma vez que, a temperatura de fechamento do sistema Ar em anfibólio é em torno

de 500oC.

Idades U-Pb semelhantes a essa, em rochas similares, foram obtidas por Silva et al. (2002),

para a porção setentrional do Cráton São Francisco (cinturões Itabuna e Salvador-Curaçá e no Bloco

Jequié). Tais idades foram interpretadas como retrabalhamento crustal, com recristalização


125

metamórfica sob altas temperaturas e pressões, no decorrer da fase colisional do Cinturão Bahia

Oriental.

Situação semelhante também foi encontrada por Kamber et al. (1995 a, b) para o cinturão

Limpopo, na África do Sul. Esses autores caracterizaram uma sutura proterozóica de 2,2-2,0 Ga,

gerada em condições metamórficas de alto grau (fácies granulito), que foi o maior evento tectôno-

metamórfico que atingiu a região. As técnicas utilizadas por eles foram Pb-Pb e Sm-Nd (em granadas)

e Ar-Ar (em anfibólios).

Já os resultados Ar-Ar, principalmente em anfibólios, dos gabronoritos e gabros, mostraram

que eles se posicionaram na crosta em dois períodos distintos. O primeiro período ocorreu por volta de

1,7 Ga e pode ser correlacionado ao Rifte Espinhaço (ver Capítulo 5). O segundo período ocorreu por

volta de 1,0 Ga e podem ser correlacionadas ao Rifte Macaúbas. Sendo assim, fica evidente que os

diques de gabronorito, anteriormente datados por Pinese (1997), não são neoarqueanos, mas do final

do Paleoproterozóico – início Mesoproterozóico e final do Mesoproterozóico.

O último evento termal que afetou a área foi aquele responsável pelo distúrbio isotópico

identificado nas rochas estudadas. Isso fica evidente através dos interceptos inferiores nos diagramas

Concórdia que mostraram de idades U-Pb correlacionáveis ao Evento Brasiliano. As rochas félsicas, à

exceção do charnockito, mostraram perdas intensas de chumbo desde o Arqueano, como pode ser visto

no diagrama das Figuras 7.1 e 7.2.

Figura 7.1 – Diagrama U versus razão 207Pb/206Pb mostrando a perda de Pb com relativo ganho de U.


126

Figura 7.2 – Diagrama das razões 207Pb/235U versus 206Pb/238U mostrando o distúrbio isotópico a que foi submetidas as rochas estudadas do Arqueano ao Paleozóico.

7.4 SUMÁRIO

A evolução tectônica da região pode ser sumarizada em sete eventos. No primeiro evento foram formados os protólitos das rochas gnáissicas estudadas, no intervalo de tempo situado entre 2,9 e 2,75 Ga. O segundo evento, ocorrido por volta de 2,6 Ga e caracterizado como um distúrbio isotópico nos sistemas Rb-Sr e Sm-Nd, estaria relacionado a um possível evento de migmatização regional. O terceiro evento ocorreu por volta de 2,05 Ga, sob condições metamórficas de fácies granulito, foi responsável pela formação de charnockitos a partir da desidratação de granitóides. O quarto evento ocorreu por volta de 2,0 Ga, e trata-se do resfriamento crustal ocorrido logo após o evento da fácies granulito. O quinto evento está caracterizado pelo magmatismo máfico fissural, ocorrido por volta de 1,7 Ga, e pelo intenso fraturamento da crosta continental, ou mesmo reativação de falhas pré-existentes, de direção NW/SE. O sexto evento, ocorrido por volta de 1,0 Ga foi o responsável pela colocação dos gabros que aproveitaram as falhas pré-existentes de direção NW/SE e, finalmente, o sétimo evento, foi o responsável pelo distúrbio isotópico presente nas rochas da região estudada. Tal evento, situado a volta de 0,5 Ga pode ser correlacionado a Orogenia Brasiliana.

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