u–pb geochronology of the southern bras´ılia belt (se...

Precambrian Research 130 (2004) 27–55

U–Pb geochronology of the southern Brasılia belt (SE-Brazil):sedimentary provenance, Neoproterozoic orogeny and

assembly of West Gondwana

Claudio M. Valerianoa,∗, Nuno Machadob,c, Antonio Simonettib,Claudia S. Valladaresa, Hildor J. Seerd, Luiz Sergio A. Simõese

a TEKTOS–Geotectonics Study Group, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier 524/4006-A,Rio de Janeiro, RJ 20559-900, Brazil

b Centre de recherche en géochimie et géodynamique (GEOTOP-UQAM-McGill), Université du Québec àMontréal, Montréal, Que., Canada

c Département des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal, CP 8888,Succ. Centre-Ville, Montréal, Que., Canada H3C 3P8

d Centro Federal de Educação Tecnológica de Minas Gerais, Av. Amazonas 807, Araxá, MG, Brazile Departamento de Petrologia e Metalogenia, IGCE, Universidade Estadual Paulista, Caixa Postal 178,

CEP13506-900, Rio Claro, SP, Brazil

Received 13 February 2003; accepted 20 October 2003

Abstract

The Brasılia belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closingrelated to the formation of West Gondwana. This study reports new U–Pb data from the southern sector of the belt in orderto provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successionsaround the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamationof West Gondwana.

Ages of detrital zircons (by ID–TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive marginof the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá andPassos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and AndrelandiaGroup) and the autochthonous or Cratonic Domain (Andrelandia Group). The patterns of provenance ages for these units areuniform and are characterised as follows: Archean–Paleoproterozoic ages (3.4–3.3, 3.1–2.7, and 2.5–2.4 Ga); Paleoproterozoicages attributed to the Transamazonian event (2.3–1.9 Ga, with a peak at ca. 2.15 Ga) and to the ca. 1.75 Ga Espinhaço rifting of theSão Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoicsources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to theindividualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in theAraxá Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation inthe marginal basin.

Detrital zircons from the autochthonous and parautochthonous Andrelandia Group, deposited on the southern margin of the SãoFrancisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6–1.2 Ga source

∗ Corresponding author. Tel.:+55-21-2254-6675; fax:+55-21-2254-6675.E-mail addresses:[email protected] (C.M. Valeriano), [email protected] (N. Machado), [email protected] (A. Simonetti),

[email protected] (H.J. Seer).

0301-9268/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.precamres.2003.10.014

28 C.M. Valeriano et al. / Precambrian Research 130 (2004) 27–55

rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly coveredby the Neoproterozoic epicontinental deposits of the Bambuı Group, or in the outer paleo-continental margin, buried under theallochthonous units of the Brasılia belt.

Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event thatstarted with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (andGoiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicatedby monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasılia belt.

Whilst continent–continent collision was proceeding on the western margin of the São Francisco Craton along the southernBrasılia belt, eastward subduction in the East was generating the 634–599 Ma Rio Negro magmatic arc which collided with theeastern São Francisco margin at 595–560 Ma, much later than in the Brasılia belt. Thus, the tectonic effects of the Ribeira beltreached the southernmost sector of the Brasılia belt creating a zone of superposition. The thermal front of this event affected theproximal Andrelandia Group at ca. 588 Ma, as indicated by monazite age.

The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545–500 Ma in the Paraguay beltand ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collisionbetween the Paraná block and Goiás terrane with the São Francisco Craton along the Brasılia belt preceded the accretion of theAmazonian craton by 50–100 million years.© 2003 Elsevier B.V. All rights reserved.

Keywords:West Gondwana; U–Pb geochronology; Detrital zircon; TIMS; LA-ICPMS; Neoproterozoic

1. Introduction and objectives

The West Gondwana supercontinent was formedby the aggregation of Archean–Paleoproterozoic con-tinental blocks along Neoproterozoic mobile belts(Unrug, 1996). To unravel the diachronic historyof this major event in the Earth history, addressingthe detailed timing of ocean formation and closureis required for each orogenic belt. The Brasılia beltoccupies a key position in West Gondwana becauseof its central location (Fig. 1), and thus recordsthe interaction between the Paraná, Goiás and SãoFrancisco continental blocks (Strieder and Suita,1999; Pimentel et al., 1999, 2000). The formationof this continental nucleus was followed, during lateNeoproterozoic–Ordovician, by development of theParaguay and Araguaia belts in the West, and bythe Ribeira–Araçuaı belt in the East (Almeida et al.,2000). Later accretions took place during the Paleo-zoic along the western margin successively contribut-ing to form West Gondwana (Ramos, 1988).

The timing of passive margin build up along thesouthwestern border of the São Francisco Craton andof ocean closure—Brazilide ocean ofDalziel (1997)and Goianides ofWeil et al. (1998), however, are stillpoorly known and preclude detailed paleogeographicreconstructions. The purpose of this paper is to con-tribute to better define the formation of West Gond-

wana by studying a sector of the southern Brasıliabelt. Thus, age limits for the deposition of the passivemargin successions (Araxá, Canastra and Andrelandiagroups) and for orogenesis are presented. Dating de-trital zircons from metasedimentary rocks has proveda particularly useful approach for constraining sedi-mentation ages (i.e. younger than the youngest dateddetrital zircon) and provenance. New U–Pb data wereobtained by Isotope Dilution–Thermal IonisationMass Spectrometry (ID–TIMS) and by Laser AblationMulti Collection Inductively Coupled Plasma MassSpectrometry (LA-MC-ICPMS). The results yielda Neoproterozoic age for the sedimentation of theAraxá, Canastra and Andrelandia groups and revealan unexpected variety of Mesoproterozoic sources,still undetected in the São Francisco Craton.

2. Geological setting

2.1. The Tocantins Structural Province

The Tocantins Structural Province resulted fromthe interaction of three major paleocontinental blocks,whose present remnants are identified as the Ama-zonian, São Francisco and Paraná cratons (Striederand Suita, 1999; Pimentel et al., 2000). The latter iscompletely covered by the Paraná basin, with rather

C.M. Valeriano et al. / Precambrian Research 130 (2004) 27–55 29

Fig. 1. Simplified tectonic map of the southern Brasılia belt (adapted fromValeriano et al., 2000). (a) Tectonic domains mentioned in thiswork and sample location (in part). Black arrows: direction of tectonic transport; (b) the Tocantins Structural Province, comprising theBrasılia, Araguaia and Paraguay belts, in the context of West Gondwana (adapted fromAlmeida et al., 2000). AM, Amazonian Craton;PR, Parnaıba Block; SL-WA, São Luis-West African Craton; SF-Com, São Francisco-Congo Craton; PP, Parana block.

speculative outlines, and may be connected to the Riode la Plata craton (Almeida et al., 2000).

The Tocantins Structural Province comprises threemain branches: the Araguaia, the Paraguay and theBrasılia belts. The first two display tectonic vergencetoward the eastern and southeastern margins of theAmazon Craton, respectively, whilst the Brasılia beltdisplays opposite vergence towards the São Francisco

Craton. The southernmost Brasılia belt is truncated bythe NE–SW structural fabric of the Ribeira belt whichextends along the Atlantic coast.

2.2. The Bras´ılia belt

The Brasılia belt has two segments (Fig. 1):the northern one trends NE and displays dextral


transpressive kinematics (Fonseca et al., 1995), whilstthe southern one trends SE and displays sinistralcompression tectonics (Valeriano et al., 2000). Thesouthern Brasılia belt is divided into the followingtectonic domains, from east to west: (a) the CratonicDomain comprises the Archean–Paleoproterozoicrocks of the São Francisco Craton, which is mostlycovered by autochthonous and para-autochthonouspelitic–carbonatic rocks of the NeoproterozoicBambuı Group and older metasedimentary succes-sions (Tiradentes and Lenheiro Sequences, Carandaıand Andrelandia groups). (b) The External Domainis composed of thrust systems of predominantlythick passive margin metasedimentary successions inlow-metamorphic grade (Canastra, Ibiá and Paranoágroups) and subordinate Archean to Paleoproterozoicbasement slivers. (c) Metamorphic nappes composedof distal passive margin and slope deposits (Araxá andAndrelandia groups) in intermediate to high-pressuremetamorphic facies form the Internal Domain. TheAraxá Group locally contains ophiolitic melanges,interpreted as remnants of subducted oceanic basin(Strieder and Nilson, 1992). (d) The Goiás Massifmakes up a microcontinent of Archean and Paleopro-terozoic basement composed of granite–greenstoneand migmatite-gneissic terrains and varied anoro-genic Mesoproterozoic rock assemblages. Theseinclude the Juscelandia Sequence and large lay-ered mafic–ultramafic complexes (Niquelandia, CanaBrava and Hidrolina complexes) in granulite facies.The age of granulite metamorphism, ca. 780 Ma(Ferreira Filho et al., 1994), is the oldest recordof an orogenic episode in the Tocantins StructuralProvince. (e) Finally, the Goiás magmatic arc con-sists of metavolcano-sedimentary and metaplutonicrocks of island arc affinity that started to develop at

Table 1The three segments of the Southern Brasılia belt and names of lithostratigraphic units and tectonic elements

Tectonic Domain Internal Domain External Domain Cratonic Domain

Segment Thrust sheets Stratigraphic units

Northern Araxa nappe Araxa Group Ibia Group Canastra Group Bambuı GroupFurnas Passos Nappe Araxa Group Canastra Group; granite–

greenstone basement(Piumhi Massif)

Bambuı Group; Autochthonousbasement

Southern Guaxupe nappe;Luminarias nappe

Andrelandia Group Andrelandia Group;Allochthonous basement

Andrelandia Group (top); CarandaıGroup Lenheiro/Tiradentessequences Autochthonous basement

ca. 930 Ma as consumption of oceanic lithosphereinitiated within the Goianides ocean (Pimentel et al.,1997) that formerly separated the Amazonian and SãoFrancisco paleocontinents.

2.3. The southern Bras´ılia belt

The southern Brasılia belt comprises three synfor-mal allochthonous segments whose differential tec-tonic transport onto the foreland was accommodatedby WNW-trending lateral ramps reactivated at a latertime as sinistral strike–slip fault zones. For conve-nience of description, the synformal segments arenamed northern, Furnas and southern (Fig. 1). Withineach of these the Internal, External and Cratonic tec-tonic domains are well recognizable. However, asshown inTable 1, each segment has its own lithos-tratigraphic nomenclature, partly due to uncertaintiesof correlation.

2.3.1. Cratonic DomainThe exposed basement in the southern part

of São Francisco Craton consists of Archeangranite–greenstone terrains, in part reworked duringthe Transamazonian Orogeny (ca. 2.2–2.0 Ga), whichalso generated juvenile gneiss–migmatite–granitoidcomplexes and metavolcano-sedimentary successions.Late to post-orogenic granitoid magmatism is ex-tensive and has been dated between 1.9 and 1.8 Ga(Teixeira et al., 2000). Statherian (1.6–1.8 Ga) taphro-genesis in the São Francisco Craton is marked by1750–1705 Ma magmatism in the Espinhaço conti-nental rift system (Machado et al., 1989).

The ages of sedimentation of the Neoprotero-zoic supracrustal successions south and west of theSão Francisco Craton are poorly constrained by Nd


model ages between 1.0 and 1.15 Ga on presumablysyn-sedimentary amphibolite intercalations within theAndrelandia Group (Trouw et al., 1993).

To the north, the anchimetamorphic pelitic and car-bonatic rocks of the Bambuı Group, representing apost-glacial epicontinental marine environment, covervast areas of the São Francisco Craton.

2.3.2. External DomainThis domain is structurally sandwiched between

the Internal and Cratonic Domains. It is segmentedby WNW-trending transcurrent fault zones andcomprises a series of thrust systems of imbricatedpsammite-dominated proximal passive margin suc-cessions which, in the south, also contain basementthrust sheets. The whole domain displays greenschistfacies metamorphism. The three sectors of the Exter-nal Domain considered here are located to the east ofthe Araxá, Passos and Luminárias nappes (Fig. 1) and,for simplicity, are here referred to as the northern,Furnas and southern segments, respectively.

The northern segment comprises two main thrustsheets represented by the Ibiá (top) and Canastragroups, usually interpreted as proximal platform de-posits. The Ibiá Group is predominantly composedof fine-grained rythmic chloritic phyllites and calc-schists, and the Canastra Group comprises quartziticmetarenites and pelitic phyllite, where the increasingfrequency of quartzites to the top suggest a regressivesequence.

The Furnas segment displays a complex imbrica-tion of metarenites and metapelites of the Serra daBoa Esperança Sequence (Canastra Group) with thrustsheets of basement rocks represented by fragments ofan Archean granite–greenstone lithologic association.The largest of these granite–greenstone thrust sheets,south of the Piumhi locality (Fig. 2), contains predom-inantly komatiites and komatiitic to tholeiitic basalts,with locally preserved pillow and spinifex structure(Schrank, 1982). This basal volcano-sedimentary unitis intrude by younger calc-alkaline granitoid rockswhich are dated in this work (seeSection 6).

The southern segment of the External Domainsouth of the São Francisco Craton is represented bySE-transported thrust sheets of platformal quartzite–phyllite association, belonging to the AndrelandiaGroup, with slivers of the basement granite–green-stone association. The Andrelandia Group, which oc-

curs in the Internal, External and Cratonic domains, isinterpreted as a passive margin succession developedalong the southern border of the São Francisco Cra-ton (Paciullo et al., 2000). Metamorphism developedunder barrovian pressure regime.

2.3.3. Internal DomainThe Internal Domain comprises three synformal

nappes separated by WNW-trending lateral ramps:from north to south they are the Araxá, Passos andLuminárias nappes. The basal portion of the AraxáNappe contains predominantly amphibolites of tholei-itic E-MORB composition and minor ultramaficschists. Metapelitic schists and metachert predominateto the top with subordinate intercalations of quartziticmetapsamites. This lithological association was inter-preted bySeer (1999)as representative of ocean floorwith deep marine sedimentary facies. To the north,pelitic schists and metachert with abundant metabasicand ultramafic disrupted lenses were characterisedas ophiolitic mélange within the upper portions theAraxá Nappe (Strieder and Nilson, 1992).

The Araxá Group in the Passos Nappe is composedof metasediments of shelf to slope facies, wherenine lithostratigraphic units (Units A–I,Fig. 2) canbe individualised (Valeriano, 1992; Simões, 1995).The basal portion of the nappe contains a regres-sive sequence with carbonatic metapelites containingmarble lenses, followed by a gradual increase inthe number and thickness of quartzite intercalations(Unit A), culminating in a conspicuous and continu-ous layer of laminated micaceous quartzite of 50 min thickness (Unit B, Furnas Quartzite). The overly-ing muscovite–metapelitic layer (Unit C) grades to adistinct paragneiss layer (Unit D) which display rela-tively more intercalations of metabasic lenses of con-tinental geochemical character (Valeriano and Simões,1997). The upper units (E–I) are composed mainly ofmetapelitic schists with intercalations of paragneissand abundant calc-silicate rocks and amphibolitelenses. Major and trace element compositions of thesemetabasic rocks indicate they represent continentaland mid-ocean ridge basalts. The distribution of theanalysed samples shows that the MORB-type rockstend to predominate to the top. However, the presenceof continental basalts throughout the pile indicatesthat lithospheric thinning associated with the sedi-mentation of the Araxá Group in the area sampled


Fig. 2. (a) Simplified tectonic map of the Furnas segment of southern Brasılia belt. Adapted fromValeriano et al. (2000); (b) Lithostratigraphiccolumn of the Passos Nappe. Numbers refer to analysed samples.


by the Passos Nappe was not extensive enough togenerate oceanic crust (Valeriano and Simões, 1997).

Thrusting and exhumation of the Passos Nappewas associated with the main (D2) deformation char-acterized by the development of recumbent foldingand penetrative foliation. The rapid exhumation ofthis nappe permitted the preservation of an invertedmetamorphic gradient of medium to high-pressure,ranging from the greenschist facies (biotite zone) upto the amphibolite-granulite facies transition (Simões,1995). Low grade retrogressive reactions are as-sociated with late-D2 nappe exhumation (Simões,1995).

To the south, the Luminárias Nappe displayslithology similar to that of the basal Passos Nappe.Metamorphism ranges from greenschist facies at thebase, to high-pressure granulite facies in the upperstructural units (Campos Neto and Caby, 1999). Thissegment of the belt is more complete since the Lu-minárias Nappe is overlain by the Socorro-GuaxupéNappe, which is composed of low pressure granuliticrocks.

NeodymiumTDM ages of ca. 2.2 Ga for the CanastraGroup fine-grained metasedimentary rocks were inter-preted as indicating provenance from the São Fran-cisco Craton. Metasedimentary rocks from the Ibiáand Araxá groups display a bimodalTDM age pattern,with peaks between 2.3–1.9 and 1.4–1.0 Ga. The lat-ter group of ages was interpreted as indicating mixedprovenance from the cratonic sources with juvenileones, such as the the Neoproterozoic juvenile Goiásmagmatic arc (Pimentel et al., 2001).

The collisional stage of the orogen resulted innumerous small intrusions of syn-tectonic biotite–muscovite leucogranites that usually contain xeno-liths of the country rocks. Sm–Nd mineral isochronsyielded metamorphic ages of 637 Ma (two-pointisochron) and 596±32 Ma for the Araxá Group (Seer,1999).

3. Analytical procedures

Samples were crushed and pulverized with stan-dard equipment under clean conditions and the heavyminerals concentrated by panning at LOPAG (Univer-sidade Federal de Ouro Preto, Brazil) and at LGPA(Universidade do Estado do Rio de Janeiro, Brazil).

Manual panning dispenses the use of a Wilfley tableand minimises contamination. The heavy mineral con-centrate was passed through a Frantz magnetic sep-arator to extract monazite, titanite, rutile and zirconwhich was further separated into four magnetic andtwo diamagnetic fractions, the latter being preferredfor hand picking. As much as possible, only the min-eral grains free of alteration, inclusions and fractureswere selected for analysis. The minerals were analysedby ID–TIMS. Zircon grains larger than ca. 80ım fromsome samples were also analysed by LA-MC-ICPMS.Analyses were performed in the GEOTOP-UQAM atMontreal, Canada.

3.1. ID–TIMS

The analysed zircon fractions were abraded (Krogh,1982) between∼72 and 120 h to reduce grain sizeby ca. 30%, which experience has shown to be nec-essary to eliminate most or all of the recent leadloss in Brazilian zircons and render them concordantor subconcordant. This requirement severely limitsthe amount of the usable grains because if these aresmaller than 75–100�m, the abraded grains are toosmall to be manipulated or sample weight becomestoo low to yield reasonably precise analysis. Depend-ing on the grain size, the abraded grains may stillcontain peripheral zones where the U–Pb system re-mained open. Air pressure for abrasion was regulatedin order to keep the zircon crystals intact during theprocedure. Once the grains are broken, further abra-sion removes the periphery but also the inside of thecrystal where the U–Pb system may have remainedclosed.

Mineral dissolution, chemical extraction of U andPb and mass spectrometric analysis were carried outfollowing the procedures described inMachado et al.(1996a,b). Total procedural blanks average 10 pg Pband 2 pg U for zircon analyses and 15 pg Pb and 5pg U for titanite, rutile and monazite analyses. Theuncertainties in isotopic ratios presented in this work(Table 2) were calculated with an error propagationprogram, which takes into consideration the analyticalprecision of the measured isotopic ratio. Regressionswere calculated and concordia diagrams plotted usingthe Isoplot-Ex Version 2 (Ludwig, 2000). Errors arerepresented at the 1σ level but all ages are quoted atthe 95% confidence interval.

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Table 2U–Pb results by Isotope Dilution–Thermal Ionisation Mass Spectrometry

Sample Concentrations Atomic ratios Ages

Number Min.a Weight (�g) U(ppm)b

Pbrad.(ppm)b

Pbcom.(pg)c

206Pb/204Pbd

208Pb/206Pbf

206Pb/238Uf

±% (1σ) 207Pb/235Uf

±% (1σ) 207Pb/206Pbf

±% (1σ) 206Pb/238U

207Pb/235U

207Pb/206Pb

Disc. %

Internal Domain

Araxa Nappe–Serra Velha Granite486-1 M 5 6833 2072 30 7519 2.305 0.1042 0.20 0.876 0.21 0.06099 0.06 639 639 639 0.0486-2 Z(9) 2 580 72 9 1018 0.069 0.1274 0.22 1.173 0.36 0.06675 0.27 773 788 830 7.3486-4 Z(6) 3 424 43 7 1255 0.077 0.1038 0.23 0.875 0.59 0.06116 0.54 636 638 645 1.4

Passos Nappe–Araxa Group, Furnas Quartzite—Unit B1032-1 SZ 1 683 383 28 737 0.238 0.4647 0.27 10.400 0.29 0.16230 0.16 2460 2471 2480 0.91032-2 SZ <1 477e 115e 49 156 0.131 0.2302 0.53 2.726 2.30 0.08588 2.04 1336 1336 1335 0.01032-3 SZ 2 379 157 6 2975 0.099 0.3913 0.17 7.201 0.19 0.13348 0.06 2129 2137 2144 0.91032-4 SZ 2 144 59 27 244 0.258 0.3457 0.44 5.602 1.00 0.11754 0.89 1914 1916 1919 0.31032-5 SZ 4 423 169 41 997 0.102 0.3770 0.16 6.674 0.19 0.12839 0.08 2062 2069 2076 0.81032-6 SZ 3 333 176 26 951 0.475 0.3832 0.17 6.912 0.19 0.13081 0.06 2091 2100 2109 1.01032-7 SZ 2 749 297 317 127 0.096 0.3770 0.78 6.625 0.97 0.12744 0.55 2062 2063 2063 0.01032-8 M 6 675 4487 24 4166 18.257 0.3957 0.16 7.171 0.24 0.13145 0.14 2149 2133 2117 −1.8

Passos Nappe–Araxa Group, garnet–plagioclase–biotite schist—Unit E1131-1 M 6 7984 1607 49 6108 1.298 0.0988 0.16 0.822 0.18 0.06031 0.05 607 609 615 1.21131-2 M 5 5456 1307 48 3582 1.737 0.0990 0.17 0.827 0.19 0.06057 0.08 609 612 624 2.51131-3 SZ 3 60 7 7 204 0.135 0.1206 0.57 1.090 3.01 0.06609 2.74 734 753 809 9.9

Passos Nappe–Araxa Group, micaceous quartzite—Unit E1040-1 SZ 4 74 18 8 550 0.161 0.2298 0.33 2.704 0.55 0.08532 0.43 1334 1330 1323 −0.91040-2 SZ 3 361 70 9 1634 0.033 0.2039 0.20 2.226 0.23 0.07921 0.14 1196 1189 1177 −1.71040-3 SZ 8 121 26 11 1198 0.116 0.2048 0.31 2.301 0.32 0.08150 0.12 1201 1213 1233 2.91040-4 SZ 4 21 15 85 50 0.247 0.5713 1.37 16.654 2.22 0.21141 1.27 2913 2915 2916 0.11040-5 SZ 6 194 43 13 1392 0.205 0.1994 0.23 2.159 0.29 0.07853 0.18 1172 1168 1160 −1.11040-6 SZ 2 231 55 5 1374 0.141 0.2240 0.41 2.599 0.43 0.08417 0.27 1303 1300 1296 −0.61040-7 SZ 3 131 24 12 400 0.153 0.1759 0.44 2.048 0.87 0.08442 0.75 1045 1131 1302 21.4

Passos Nappe–Araxa Group, micaceous quartzite—Unit E1041-1 SZ 5 201 86 11 2308 0.157 0.3836 1.03 7.090 1.03 0.13404 0.11 2093 2123 2152 3.2

Passos Nappe–Araxa Group, amphibolite—Unit G1038-2 R(50) 42 1 0.1 64 20 0.544 0.1603 5.13 1.579 12.82 0.07147 9.32 958 962 971 1.41038-3 R(14) 183 0.3 0.03 29 30 0.130 0.0968 6.00 0.800 27.01 0.05992 22.29 595 597 601 0.9

Passos Nappe–Araxa Group, micaceous quartzite—Unit G1039-2 M 2 8159 2668 13 7556 2.832 0.0974 0.17 0.816 0.28 0.06075 0.21 597 604 630 5.5

Passos Nappe–Araxa Group, leucosome—Unit G1081-1 M <1 3591e 1807e 84 280 4.832 0.0986 0.37 0.819 2.34 0.06019 2.19 606 607 610 0.71081-4 M 3 1430 981 24 1173 6.663 0.1025 0.17 0.859 0.30 0.06077 0.2 629 630 631 0.31081-2 Z(70) <1 104e 12e 16 4944 0.123 0.1159 0.16 1.017 0.18 0.06366 0.06 707 712 730 3.41081-3 Z(54) 5 2041 228 18 3947 0.104 0.1115 0.16 0.968 0.18 0.06294 0.07 681 687 706 3.71081-5 Z(28) 2 1449 161 19 1099 0.110 0.1103 0.15 0.948 0.24 0.06235 0.17 674 677 686 1.81081-6 Z(45) 2 2694 320 18 2176 0.128 0.1160 0.15 1.018 0.19 0.06365 0.1 707 713 730 3.3

Passos Nappe–Araxa Group, orthoquartzite—Unit H1042-1 SZ 15 599 245 12 15470 0.314 0.3300 0.18 5.777 0.19 0.12698 0.04 1838 1943 2057 12.21042-7 SZ 11 260 77 10 4782 0.181 0.2628 0.19 4.309 0.23 0.11890 0.16 1504 1695 1940 25.11042-8 SZ 12 726 233 16 10071 0.158 0.2911 0.19 4.933 0.21 0.12290 0.05 1647 1808 1999 19.91042-9 SZ 4 419 69 18 1012 0.028 0.1688 0.21 2.421 0.24 0.10399 0.09 1006 1249 1697 43.9

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1042-11 SZ 2 62 11 5 309 0.014 0.1910 4.78 2.901 5.22 0.11017 1.87 1127 1382 1802 40.81042-3 M 9 10562 2045 24 25562 1.178 0.1001 0.17 0.835 0.19 0.06050 0.05 615 617 622 1.11042-5 M 8 1209 1571 18 3349 14.203 0.0982 0.24 0.813 0.46 0.06001 0.35 604 604 604 0.0

Passos Nappe–Araxa Group, amphibolite—Unit H1036-1 R(60) 46 3 0.4 127 23 0.346 0.0963 2.16 0.794 21.76 0.05978 20.04 593 593 596 0.5

External Domain—Furnas segment

Felsic metavolcanic unit (Costas schist)1130-1 Z(5) 5 178 35 11 1010 0.153 0.1872 0.18 2.333 0.19 0.09041 0.09 1106 1222 1434 24.91130-2 Z(9) 5 417 51 14 1218 0.084 0.1232 0.21 1.195 0.23 0.07031 0.13 749 798 938 21.31130-6 Z(12) 7 269 50 5 4753 0.095 0.1833 0.18 2.333 0.19 0.09228 0.08 1085 1222 1473 28.61130-7 Z(17) <1 1761e 276e 13 1342 0.110 0.1530 0.21 1.727 0.25 0.08188 0.14 918 1019 1243 28.1

Serra da Boa Esperança Sequence: quartzite1044-1 SZ 17 247 99 9 11278 0.115 0.3735 0.18 6.631 0.19 0.12877 0.06 2045.9 2064 2081 2.01044-3 SZ 7 408 148 15 4093 0.104 0.3445 0.20 5.668 0.22 0.11931 0.09 1908.5 1926 1946 2.21044-4 SZ 7 107 29 6 1890 0.173 0.2504 0.17 3.139 0.20 0.09092 0.09 1440.6 1442 1445 0.3

Serra da Boa Esperança Sequence: quartzite1046-3 SZ 3 206 51 10 946 0.126 0.2348 0.28 2.868 0.58 0.08860 0.51 1359.4 1374 1396 2.91046-4 SZ 2 322 113 11 1278 0.084 0.3407 0.22 5.611 0.27 0.11944 0.13 1890.1 1918 1948 3.4

Serra da Boa Esperança Sequence: metarkose1128-1 SZ 3 137 44 7 1058 0.217 0.2852 0.49 4.168 0.63 0.10601 0.45 1617 1668 1732 7.51128-3 SZ 3 488 175 13 2476 0.052 0.3545 0.49 6.182 0.51 0.12648 0.15 1956 2002 2050 5.3

Archean basement thrust sheet (Taquari granite)71-1 T(50) 217 49 48 984 380 0.895 0.5375 0.36 16.693 0.37 0.22526 0.06 2772.8 2917 3019 10.0

Archean basement thrust sheet–Hornblende orthogneiss1129-1 Z(6) 11 94 57 71 497 0.131 0.5269 0.24 15.684 0.26 0.21589 0.06 2728 2858 2950 9.21129-2 Z(7) 9 52 25 19 709 0.127 0.4312 0.28 10.650 0.34 0.17915 0.13 2311 2493 2645 15.01129-3 Z(6) 4 166 104 15 1533 0.124 0.5413 0.23 16.065 0.24 0.21526 0.06 2788.8 2881 2946 6.61129-4 Z(10) 9 112 69 7 4923 0.125 0.5330 0.32 15.855 0.32 0.21576 0.05 2753.9 2868 2949 8.1

Serra da Boa Esperança Sequence (Serra da Mamona unit)—metaconglomerate1084-1 SZ 10 28 19 5 1769 0.307 0.5363 0.25 14.707 0.28 0.19890 0.09 2768 2796 2817 2.21084-2 SZ 4 173 102 5 4056 0.165 0.4971 0.03 15.034 0.28 0.21937 0.08 2601 2817 2976 15.3

External Domain—Southern segment

Andrelandia Group quartzitesITA3-2 M 16 475 646 34 1343 15.625 0.0939 0.15 0.772 0.18 0.05957 0.08 579 581 588 1.7ITA2-1 M 9 1550 5518 184 1513 12.093 0.3108 0.15 5.352 0.17 0.12490 0.05 1744.4 1877 2027 15.9

a The number within parentheses indicates number of analysed grains: SZ: single zircon; Z: zircon population; M: single monazite; T: titanite population; R: rutile population.b Concentrations are known to 20% for weights below 20�g.c Total common Pb present in analysis corrected for Pb in spike.d Measured ratio, corrected for fractionation only.e Sample weight below the sensitivity of the microbalance (below 1�g); therefore, listed concentrations are minimum values.f Ratios corrected for spike, fractionation, blank and initial common Pb. Errors quoted are in percentage at the 1σ confidence level. Maximum total blanks for zircon analyses are 15 pg for Pb and 2 pg for U. Isotopic

composition of initial common Pb was calculated using the two-stage model ofStacey and Kramers (1975).


3.2. LA-MC-ICPMS

The selected grains together with fragments ofin-house standard zircon (UQ-Z8) were mounted onepoxy known to be devoid of Pb and U from pre-vious analyses and manually polished on alumina(Al2O3)-lapping film of progressively smaller grainsize from 9 to 0.3�m. The grain mount was succes-sively ultrasonicated in distilled water, washed withsub-boiling HCl 6.2N and with sub-boiling H2O andleft to dry under a class 100 clean hood. The resultsreported in this work were obtained with an excimerlaser coupled to a multicollector ICPMS system. Thelaser system consists of a 193 nm Lambda PhysikCompex 102-ArF laser delivering a maximum of200 mJ per pulse of 25 ns duration. The beam deliverysystem is designed by Merchantek-New Wave Re-search and closely resembles that described byHornet al. (2000). Zircon was analysed using fluencesof 10 ± 2 J cm−2 at 5 Hz laser frequency and beamdiameters between 35 and 80�m.

The laser ablation system is coupled to a MicromassIsoprobe, a multicollector mass spectrometer with anICP source and an hexapole collision cell. Data wasacquired in static, multicollection mode using sixFaraday collectors, in the only configuration possibleto encompass the large mass spread between204Pband 238U (∼14%). Before ablation data is collected,the collector positions and gains are verified by as-pirating a solution containing NIST 981Pb and U500standards. Each analysis consists of a 50 s on-peakbaseline measurement prior to the start of ablation,followed by two half-mass unit baseline measure-ments after ablation had commenced after which ablock of fifty 1 s integrations is acquired. The on-peakbaseline measurement compensates for204Hg presentin the Ar gas. Although204Pb was not measured dur-ing the analyses presented here, during more recentwork we obtained206Pb/204Pb values never lowerthan 2000 and generally higher than 5000. Therefore,no common lead corrections were applied.

After the laser frequency and energy and the beamdiameter have been chosen, the in-house standardUQ-Z8 is ablated and the nebuliser (carrier) gas (Ar)flow rate adjusted to obtain a mean206Pb/238U asclose as possible to that of the standard. The UQ-Z8in-house standard is a zircon megacrystal from thesame rock as UQ-Z1 reported on previously (Machado

and Gauthier, 1996), which was dated by ID–TIMSat 1143± 1 Ma. The207Pb/206Pb and the238U/206Pbvalues obtained for the standard during this workare precise to 0.1 and 1.3%, respectively. All anal-yses were corrected for U fractionation relative to238U/235U of 137.88. Age calculations and plottingwere done with Isoplot-Ex Version 2 (Ludwig, 2000).The precision of the isotopic ratios is reported andplotted as standard error of the mean at the 1σ levelbut all ages are quoted at the 95% confidence interval(Table 3). Due to the poor precision of the measured207Pb/235U values obtained for the youngest zircons(<1 Ga) ablated in an Ar atmosphere, the results arepresented on238U/206Pb vs.207Pb/206Pb concordiadiagrams (Tera and Wasserburg, 1972). The analyti-cal results are displayed inTable 2(ID–TIMS) andTable 3(LA-MC-ICPMS).

4. Results

The results are presented in tectonic organisationfrom the Internal to the Cratonic Domain, followingtheir geographical distribution from north to south.Each analysed sample and pertaining results are de-scribed separately, with TIMS data preceding thoseobtained by LA-MC-ICPMS.

5. Internal domain

5.1. Araxá nappe

5.1.1. Serra Velha GraniteThe Serra Velha Granite (sample 486,Table 2)

is one of a series of syn-tectonic granitoids em-placed into metasedimentary rocks and intercalatedortho-amphibolites of the Araxá Group. They are typi-cal biotite–muscovite-bearing collisional granites withgeochemical characteristics indicating derivation fromcrustal melting (Seer, 1999). Quartz, orthoclase, mi-crocline, plagioclase, white mica (phengite) and biotiteare the main constituents, with accessory garnet, tour-maline, monazite, apatite and zircon. Secondary whitemica, biotite, albite and chlorite grew under green-schist facies metamorphic conditions during the tec-tonic transport of the Araxá Nappe. The emplacementof the granitic bodies was controlled by gently dipping


Table 3U–Pb results by LA-MC-ICPMS

Group number 238U/206Pba ±% (1σ) 207Pb/206Pbb ±% (1σ) Ages (Ma) Disc. (%)

238U/206Pb 207Pb/206Pb ± Ma (2σ)

Internal DomainPassos Nappe–Araxa Group, Furnas Quartzite; Unit B; sample 1032

1 4.836 0.7 0.094 0.2 1212 1506 4 19.62 3.598 1.9 0.105 0.3 1581 1716 5 7.93 4.415 6.5 0.082 1.9 1316 1221 51 −7.84 5.482 1.2 0.079 0.5 1080 1167 9 7.55 2.858 5.9 0.122 0.5 1934 1990 9 2.86 4.923 4.8 0.076 2.2 1192 1088 45 −9.67 4.518 1.3 0.088 0.3 1289 1371 6 6.08 5.905 1.3 0.075 1.1 1009 1060 21 4.89 2.801 22.5 0.107 1.5 1968 1752 28 −12.3

10 4.652 2.6 0.076 0.9 1255 1085 18 −15.711 5.428 2.9 0.083 0.9 1090 1257 18 13.312 3.170 2.5 0.129 0.3 1768 2078 6 14.913 2.898 1.2 0.127 0.1 1911 2057 2 7.114 4.207 0.9 0.093 0.5 1375 1494 10 8.015 3.004 0.9 0.127 0.2 1852 2053 3 9.816 2.125 1.7 0.186 0.3 2486 2702 6 8.017 5.135 3.9 0.093 2.3 1147 1490 45 23.018 3.579 2.8 0.108 0.5 1588 1763 10 9.919 5.829 4.3 0.073 1.9 1021 1011 40 −0.920 4.629 1.2 0.087 0.3 1261 1350 5 6.621 4.959 4.3 0.082 1.2 1184 1246 24 5.022 3.199 3.5 0.104 0.3 1754 1701 6 −3.123 4.598 0.9 0.085 0.2 1269 1310 3 3.224 6.092 1.0 0.075 0.4 980 1062 9 7.825 4.598 0.9 0.085 0.2 1269 1310 3 3.226 2.410 1.0 0.155 0.1 2237 2403 2 6.927 2.490 2.3 0.129 0.2 2177 2089 4 −4.228 3.805 1.2 0.093 0.6 1504 1484 12 −1.429 4.043 1.6 0.088 0.9 1425 1383 17 −3.030 3.560 2.2 0.093 0.8 1596 1478 16 −8.022 3.599 1.8 0.117 0.3 1581 1914 5 17.423 3.079 3.3 0.107 0.5 1813 1757 9 −3.224 3.741 1.3 0.109 0.2 1527 1779 4 14.125 7.519 1.9 0.070 1.1 805 919 23 12.527 5.900 2.0 0.079 0.6 1009 1177 12 14.328 4.919 2.5 0.088 1.4 1193 1389 27 14.129 5.441 6.0 0.161 0.7 1088 2463 12 55.830 3.577 1.4 0.097 0.3 1589 1566 5 −1.5

Passos Nappe–Araxa Group, micaceous quartzite; Unit E; sample 10412 2.688 2.7 0.133 0.5 2039 2140 8 4.73 2.543 0.9 0.131 0.1 2138 2105 2 −1.54 2.848 2.5 0.126 0.3 1940 2041 5 4.95 2.879 1.3 0.128 0.3 1922 2076 6 7.46 2.726 1.3 0.132 0.2 2015 2126 4 5.27 6.167 1.1 0.074 0.6 969 1039 12 6.78 7.723 4.0 0.139 0.6 785 2218 10 64.69 4.375 4.5 0.130 1.6 1327 2094 28 36.6

10 3.631 4.7 0.130 1.3 1568 2092 22 25.011 3.995 2.8 0.144 1.1 1440 2259 14 36.3


Table 3 (Continued)


238U/206Pb 207Pb/206Pb ± Ma (2σ)

12 5.520 5.2 0.161 3.1 1073 2465 53 56.513 5.029 3.9 0.134 0.8 1169 2151 14 45.714 3.150 0.8 0.130 0.2 1777 2100 3 15.415 3.242 3.4 0.140 1.2 1733 2229 21 22.216 3.102 1.5 0.135 0.3 1801 2168 5 16.917 3.536 1.4 0.134 0.2 1606 2148 4 25.318 4.609 3.4 0.132 0.6 1266 2128 10 40.519 3.424 1.0 0.130 0.2 1652 2102 4 21.420 3.578 2.0 0.137 0.5 1589 2191 8 27.521 4.032 1.9 0.132 0.4 1428 2128 7 32.9

Passos Nappe–Araxa Group, quartzite; Unit H; sample 104212 5.115 8.9 0.105 3.0 1151 1734 74 33.613 7.605 2.1 0.100 0.8 796 1618 15 50.814 4.887 1.8 0.126 0.4 1200 2041 6 41.215 3.519 1.2 0.131 0.2 1612 2105 4 23.416 7.249 2.0 0.089 1.8 833 1393 35 40.217 7.991 2.6 0.091 2.3 760 1447 44 47.518 3.801 1.7 0.124 0.3 1506 2015 4 25.319 4.160 1.7 0.112 0.4 1389 1831 8 24.220 2.869 0.8 0.132 0.1 1928 2126 2 9.321 3.326 0.6 0.128 0.2 1694 2075 4 18.322 3.870 2.6 0.131 0.7 1482 2111 13 29.823 3.323 1.1 0.129 0.2 1696 2087 3 18.724 2.927 1.0 0.129 0.2 1895 2080 4 8.925 6.406 3.3 0.109 1.1 935 1788 21 47.726 2.842 1.1 0.137 0.2 1943 2183 3 11.027 2.599 1.2 0.154 0.2 2098 2395 3 12.428 2.986 2.6 0.130 0.4 1862 2097 6 11.229 2.899 1.9 0.134 0.2 1910 2152 4 11.230 3.130 1.2 0.195 0.1 1787 2784 2 35.831 3.165 3.2 0.128 0.5 1770 2068 8 14.433 3.144 1.2 0.127 0.2 1780 2050 3 13.134 3.031 1.4 0.133 0.3 1838 2132 5 13.835 3.244 1.9 0.133 0.4 1732 2141 7 19.136 3.199 0.7 0.121 0.1 1753 1977 2 11.337 7.577 5.7 0.071 2.5 799 943 52 15.238 2.411 2.2 0.127 0.2 2236 2051 3 −9.039 2.715 3.3 0.131 0.6 2021 2114 11 4.440 2.325 3.5 0.133 0.1 2306 2142 2 −7.741 2.919 2.0 0.126 0.6 1899 2045 11 7.142 2.669 4.4 0.130 0.9 2051 2097 16 2.243 2.472 4.5 0.130 0.2 2190 2099 4 −4.3

External Domain—Northern segment

Canastra Group, quartzite; sample HS411 2.133 1.5 0.141 0.1 2478 2237 2 −10.82 3.036 2.2 0.124 0.3 1835 2016 5 8.93 2.222 1.8 0.133 0.1 2395 2135 1 −12.24 3.435 1.8 0.112 0.3 1647 1827 6 9.95 2.963 1.0 0.128 0.2 1874 2069 3 9.46 5.262 1.1 0.083 0.4 1122 1265 8 11.37 3.814 2.9 0.107 0.6 1501 1742 11 13.9


Table 3 (Continued)


238U/206Pb 207Pb/206Pb ± Ma (2σ)

8 3.289 2.3 0.122 0.3 1711 1992 6 14.19 2.741 0.8 0.173 0.2 2005 2583 3 22.4

10 4.013 2.8 0.084 1.0 1434 1283 19 −11.811 2.507 3.2 0.172 0.4 2164 2581 6 16.212 5.154 0.8 0.096 0.4 1143 1544 7 25.613 2.813 5.9 0.128 0.9 1961 2073 17 5.414 1.790 3.6 0.206 0.2 2862 2875 3 0.515 5.859 3.5 0.079 2.3 1016 1180 45 13.917 5.782 2.0 0.080 1.2 1028 1197 23 14.118 4.205 5.0 0.091 2.9 1375 1451 56 5.219 4.983 1.8 0.081 0.4 1179 1226 8 5.620 3.552 7.2 0.090 1.7 1599 1430 41 −12.1

External Domain—Furnas segment

Serra da Boa Esperança Sequence; quartzite; sample 10445 3.119 1.0 0.116 0.1 1793 1899 2 5.66 2.314 2.8 0.187 0.3 2315 2715 4 14.77 4.883 1.3 0.089 0.4 1201 1397 7 14.08 4.702 1.8 0.087 0.6 1243 1370 11 9.39 3.128 1.6 0.122 0.2 1788 1978 4 9.6

10 3.411 4.3 0.120 0.6 1657 1962 10 15.511 3.831 1.6 0.114 0.6 1495 1858 11 19.512 3.222 2.7 0.122 0.5 1742 1988 9 12.313 2.431 3.1 0.148 0.9 2221 2324 15 4.414 3.714 3.2 0.113 0.4 1537 1841 8 16.515 3.138 1.4 0.119 0.3 1783 1944 5 8.316 6.879 2.2 0.086 2.0 875 1331 39 34.317 4.794 2.0 0.086 0.7 1221 1338 13 8.718 3.047 4.5 0.144 0.9 1830 2278 15 19.719 6.354 1.9 0.075 0.8 942 1072 15 12.120 6.712 5.7 0.072 3.4 895 989 70 9.521 4.177 7.9 0.086 4.1 1384 1346 81 −2.822 2.954 4.5 0.124 0.7 1879 2012 12 6.623 4.514 5.4 0.095 3.6 1290 1520 69 15.225 2.346 6.9 0.185 0.6 2289 2694 11 15.026 2.121 6.0 0.193 0.5 2490 2768 8 10.027 4.732 2.0 0.089 0.7 1236 1410 14 12.428 3.353 9.9 0.106 1.5 1682 1728 28 2.729 2.052 5.6 0.189 0.4 2559 2736 6 6.5

Serra da Boa Esperança Sequence; meta-arkose; sample 11284 4.439 0.9 0.092 0.4 1310 1469 8 10.95 2.469 1.0 0.132 0.2 2192 2123 3 −3.36 3.208 0.9 0.110 0.2 1749 1798 4 2.77 2.781 1.9 0.128 0.4 1980 2074 7 4.58 2.052 0.8 0.189 0.1 2559 2731 1 6.39 4.812 0.9 0.082 0.4 1217 1234 7 1.3

10 4.333 2.8 0.090 1.4 1339 1433 26 6.611 5.216 2.1 0.089 1.4 1131 1408 28 19.712 6.015 2.6 0.076 1.6 991 1085 32 8.713 4.984 1.4 0.087 0.8 1179 1360 16 13.314 5.175 2.2 0.087 1.4 1139 1364 28 16.515 5.110 2.2 0.092 1.7 1152 1463 32 21.3


Table 3 (Continued)


238U/206Pb 207Pb/206Pb ± Ma (2σ)

16 4.636 1.7 0.096 0.6 1259 1543 12 18.417 3.812 1.6 0.101 0.7 1502 1651 14 9.018 4.747 4.1 0.119 4.1 1232 1944 74 36.619 7.127 2.4 0.097 3.8 846 1558 72 45.720 3.015 3.7 0.129 1.3 1847 2088 22 11.621 4.942 3.0 0.098 1.6 1188 1582 30 24.922 2.792 1.8 0.126 0.3 1973 2041 6 3.323 2.986 1.1 0.134 0.3 1862 2150 4 13.424 2.966 1.1 0.122 0.2 1873 1985 4 5.725 3.147 1.8 0.123 0.4 1779 2005 8 11.326 4.437 1.6 0.088 0.6 1310 1372 12 4.527 4.518 1.1 0.089 0.4 1289 1398 7 7.828 3.165 2.4 0.138 0.3 1770 2200 6 19.6

Serra da Boa Esperança Sequence (Serra da Mamona unit); metaconglomerate; sample 10843 1.871 2.1 0.200 0.1 2760 2823 2 2.24 1.998 1.2 0.205 0.2 2616 2865 3 8.75 1.976 1.8 0.200 0.1 2640 2824 2 6.56 1.890 1.6 0.201 0.1 2738 2838 1 3.57 2.093 4.8 0.184 0.4 2518 2685 6 6.28 1.885 2.4 0.218 0.1 2744 2968 2 7.59 3.208 5.3 0.138 0.9 1749 2197 16 20.4

10 1.845 2.8 0.208 0.2 2792 2888 2 3.311 2.284 3.6 0.188 0.6 2341 2728 10 14.212 2.023 6.7 0.222 0.3 2590 2998 4 13.613 1.954 3.4 0.205 0.2 2664 2863 3 6.914 2.296 2.1 0.209 0.2 2330 2900 3 19.715 1.667 1.8 0.231 0.1 3029 3060 2 1.016 1.989 3.4 0.231 0.4 2626 3060 7 14.217 1.949 1.1 0.217 0.1 2670 2961 1 9.818 2.327 5.4 0.205 1.6 2305 2866 26 19.619 2.071 2.9 0.219 0.2 2540 2973 3 14.620 2.542 3.3 0.194 0.4 2139 2775 7 22.921 2.069 1.6 0.217 0.1 2542 2955 2 14.022 1.756 1.2 0.210 0.1 2906 2906 2 0.023 1.996 7.9 0.206 0.8 2618 2875 14 8.924 1.980 1.6 0.199 0.7 2635 2983 1 11.625 2.291 5.3 0.192 0.5 2335 2763 8 15.526 2.517 6.2 0.189 0.7 2157 2735 11 21.1

Andrelandia Group; quartzite; sample ITA11 4.836 0.7 0.094 0.2 1212 1506 4 19.62 3.598 1.9 0.105 0.3 1581 1716 5 7.93 4.415 6.5 0.082 1.9 1316 1221 51 −7.84 5.482 1.2 0.079 0.5 1080 1167 9 7.55 2.858 5.9 0.122 0.5 1934 1990 9 2.86 4.923 4.8 0.076 2.2 1192 1088 45 −9.67 4.518 1.3 0.088 0.3 1289 1371 6 6.08 5.905 1.3 0.075 1.1 1009 1060 21 4.89 2.801 22.5 0.107 1.5 1968 1752 28 −12.3

10 4.652 2.6 0.076 0.9 1255 1085 18 −15.711 5.428 2.9 0.083 0.9 1090 1257 18 13.312 3.170 2.5 0.129 0.3 1768 2078 6 14.9


Table 3 (Continued)


238U/206Pb 207Pb/206Pb ± Ma (2σ)

13 2.898 1.2 0.127 0.1 1911 2057 2 7.114 4.207 0.9 0.093 0.5 1375 1494 10 8.015 3.004 0.9 0.127 0.2 1852 2053 3 9.816 2.125 1.7 0.186 0.3 2486 2702 6 8.017 5.135 3.9 0.093 2.3 1147 1490 45 23.018 3.579 2.8 0.108 0.5 1588 1763 10 9.919 5.829 4.3 0.073 1.9 1021 1011 40 −0.920 4.629 1.2 0.087 0.3 1261 1350 5 6.621 4.959 4.3 0.082 1.2 1184 1246 24 5.022 3.199 3.5 0.104 0.3 1754 1701 6 −3.123 4.598 0.9 0.085 0.2 1269 1310 3 3.224 6.092 1.0 0.075 0.4 980 1062 9 7.825 4.598 0.9 0.085 0.2 1269 1310 3 3.226 2.410 1.0 0.155 0.1 2237 2403 2 6.927 2.490 2.3 0.129 0.2 2177 2089 4 −4.228 3.805 1.2 0.093 0.6 1504 1484 12 −1.429 4.043 1.6 0.088 0.9 1425 1383 17 −3.030 3.560 2.2 0.093 0.8 1596 1478 16 −8.0

Andrelandia Group; quartzite; sample ITA33 5.608 3.7 0.070 1.7 1058 935 35 −13.24 4.333 1.0 0.090 0.8 1339 1415 15 5.45 2.831 3.2 0.128 0.3 1950 2065 6 5.66 3.502 2.5 0.106 0.4 1619 1722 7 6.07 3.559 0.8 0.108 0.1 1596 1768 2 9.78 2.765 0.7 0.125 0.2 1990 2032 3 2.19 2.106 1.9 0.162 0.4 2504 2478 7 −1.1

10 4.168 1.4 0.094 0.5 1386 1515 9 8.511 2.151 1.1 0.178 0.1 2462 2633 1 6.512 3.116 5.6 0.127 3.1 1794 2057 55 12.813 3.091 0.9 0.116 0.2 1807 1897 4 4.814 2.775 2.8 0.130 0.3 1984 2093 6 5.215 3.997 1.9 0.097 1.4 1439 1566 26 8.116 2.983 1.9 0.122 0.2 1864 1977 3 5.717 1.819 4.3 0.193 1.4 2824 2771 23 −1.918 3.006 1.5 0.135 0.2 1851 2162 3 14.419 3.500 4.4 0.097 2.1 1620 1570 40 −3.220 4.038 2.1 0.090 0.8 1426 1417 16 −0.721 4.713 1.1 0.082 0.5 1241 1233 10 −0.622 3.637 3.3 0.111 0.5 1566 1815 10 13.723 2.848 1.8 0.115 0.3 1940 1887 5 −2.824 2.665 2.0 0.134 0.2 2054 2151 4 4.525 3.963 1.2 0.089 0.5 1450 1407 10 −3.126 2.545 2.5 0.122 0.4 2137 1986 7 −7.627 4.374 4.2 0.096 1.8 1327 1552 34 14.529 4.238 2.8 0.091 0.4 1366 1438 8 5.030 4.959 1.4 0.081 0.6 1184 1226 12 3.431 4.058 1.7 0.088 0.4 1420 1379 7 −3.032 3.703 2.5 0.120 0.3 1541 1952 6 21.1

a Values corrected for U fractionation relative to238U/235U = 137.88b Measured ratios.


Fig. 3. Concordia diagram for the Serra Velha Granite (sample486), Araxa Nappe.

penetrative shear zones formed during the main com-pressive deformation phase (D2), the late stages ofwhich resulted in the transport of the Araxá Nappe.

Zircons extracted from the Serra Velha Granite aresmall (<75�m), euhedral, vary in shape from equantto narrow, elongated, needle-like prisms and are gen-erally cracked and contain inclusions. A group of sixneedle-shaped zircons (analysis 486-4) is concordantat 639± 1 Ma and a sub-idiomorphic monazite grain(analysis 486-1) yielded an identical within error ageof 645+11/−12 Ma (Table 2, Fig. 3); a concordia age(Ludwig, 1998) for both analyses is 637± 1 Ma. Thisis the best estimate for the age of crystallisation of theSerra Velha Granite, and hence for the syn-collisionalemplacement of the Araxá Nappe. A minimum age of830 Ma was obtained for another fraction of nine sim-ilar zircons (486-2) indicating the presence of inheri-tance. A discordia between this analysis and 637 Mapoints to 1031 Ma, the probable age of inheritance.Four other analyses (not shown) were carried out oninclusion-bearing and fractured zircons but high lev-els of common Pb rendered them useless for precisedating.

5.2. Passos Nappe

Nine samples from the Araxá Group in this nappewere collected from the lower to the upper structurallevels (named units A to I,Fig. 2). The lower level was

affected by greenschist facies metamorphism whilstthe intermediate and the upper levels are at amphibo-lite and granulite facies, respectively. Results will bepresented from the lower to the upper levels of thenappe.

5.2.1. Unit B—Furnas QuartziteThe Furnas Quartzite is a 50 m thick layer of white,

fine-grained, laminated and micaceous metarenite,which forms the highest ridges of the Passos Nappe.Millimetric dark laminae in this unit contain anaccumulation of heavy minerals (placers) and arerepresented by sample 1032. Except for subordinatesmall euhedral grains, detrital zircons from this sam-ple are conspicuously well rounded and pitted andare devoid of metamorphic overgrowth. A total of 24detrital grains were analysed, 8 by ID–TIMS and 16by LA-MC-ICPMS.

Six of the zircons dated by ID–TIMS (Table 2,Fig. 4a) yielded207Pb/206Pb ages between 2144 and1919 Ma and were most likely derived from rocksformed or metamorphosed during the TransamazonianOrogeny. Another one yielded an age of 2480 Ma thesignificance of which will be discussed in the fol-lowing. The largest of the euhedral zircons yieldeda concordant age of 1335 Ma with a relatively largeerror in the207Pb/206Pb value, the 1336+ 6/−7 Ma206Pb/238U age being more reliable. Concordant zir-cons of Mesoproterozoic age were also found in sam-ples described in the following and are a significantfinding. A rounded and pitted monazite grain, obvi-ously detrital (1032-8M), yielded a207Pb/206Pb ageof 2117 Ma (−1.8% discordant). This result indicatesthat low grade metamorphism, (biotite zone of thegreenschist facies) at the base of the Passos Nappe didnot disturb the U–Pb system of detrital monazite.

Thirteen zircons dated by LA-MC-ICPMS cluster inthe 2.2–2.0 Ga (Table 3, Fig. 4b) interval and are alsotaken to be derived from units related to the Transama-zonian Orogeny. One zircon yielded a minimum ageof 2388 Ma and two others yielded minimum agesof 3362 and 3381 Ma. Detrital zircons with ages inthe 2.4–2.5 Ga interval were also found in the MinasSupergroup (Quadrilátero Ferrıfero area;Machadoet al., 1996a,b) but an igneous or metamorphic eventof this age is not known in the southern São FranciscoCraton. It could also be argued that zircons of this ageare Archean and were affected by Transamazonian


Fig. 4. Concordia diagrams for detrital zircon and monazite fromthe Furnas Quartzite (sample 1032, Unit B), Araxa Group, Pas-sos Nappe. (a) ID–TIMS results in normal concordia diagram(Wetherill, 1956); (b) LA-MC-ICPMS results in inverted concor-dia diagram (Tera and Wasserburg, 1972).

metamorphism at ca. 2.01 Ga (Machado et al., 1992).However, the fact that some of them are concor-dant or close to concordia (e.g. 1032-1) and above a2.8–2.0 Ga discordia, rather suggests that they wereformed at ca. 2.4–2.5 Ga. Therefore, it can only bestated that an unknown source of this age is mostlikely present in the São Francisco Craton.

The two Archean zircons (ages greater than 3381and 3362 Ma) are older than the oldest documentedrock in the southern São Francisco Craton, ananorthositic layer in a layered intrusion in the Pi-umhi greenstone belt (3116+ 10/−7 Ma; Machadoand Schrank, 1989). They are in the range of agesof zircon from detrital rocks of the Rio das VelhasSupergroup and the Minas Supergroup (QuadriláteroFerrıfero area;Machado et al., 1996a,b) in the south-ern sector of the craton, and for units of the São

Fig. 5. Concordia diagram pertaining to Unit E garnet–plagio-clase–biotite schist (sample 1131), Araxa Group, Passos Nappe.

Francisco Craton in the state of Bahia (Pinto et al.,1998; Nutman and Cordani, 1993; Martin et al., 1997;Leal et al., 2003).

5.2.2. Unit E—garnet–plagioclase–biotite schistSample 1131 is representative of this unit which

is in lower amphibolite facies. A euhedral monazitecrystal yielded a minimum age of 615 Ma (1.2% dis-cordant), whilst a rounded and frosted one yieldeda minimum age of 624 Ma and is more discordant(2.5%;Table 2, Fig. 5). Given the morphological char-acteristics of the monazite grains, the 615 Ma age isinterpreted as the approximate age of metamorphism.The age of the rounded monazite is taken to indicatethat the mineral is older and was affected by the ca.615 Ma metamorphism. These data indicate that theU–Pb system in monazite may be open in amphibolitefacies metamorphism, at ca. 764◦C (Simões, 1995) inagreement withHeaman and Parrish (1991)estimate.

A single prismatic detrital zircon has low U contents(60 ppm,Table 2), yields a206Pb/238U age of 734±4 Ma and a concordia age of 730± 3 Ma.

5.2.3. Unit E—micaceous quartziteSample 1040 was collected from a coarse-grained

garnet–biotite–muscovite quartzite in amphibolitefacies. The zircons extracted from this sample arerounded and pitted but smooth and display incipientpyramidal outgrowths. This is the lowest level ofthe Passos Nappe where zircon overgrowths are ob-served. Those features indicate that the metamorphic


Fig. 6. Concordia diagrams pertaining to detrital zircon from UnitE quartzite (sample 1040), Araxa Group, Passos Nappe.

conditions affecting the quartzite were compatiblewith the generation of new zircon.

Seven detrital zircons were analysed by ID–TIMS(Table 2, Fig. 6a). One yielded a concordant age of2916+21/−20 Ma, the large error being caused prob-ably by high common Pb. The other six yielded agesbetween 1323+ 8/−9 Ma and 1160 Ma, one of whichis concordant at 1296+ 6/−5 Ma.

Twenty-one zircons were analysed by LA-MC-ICPMS (Table 3, Fig. 6b) which yielded207Pb/206Pbages between 2463 and 907 Ma. Nine are concor-dant at 1924± 11 Ma, 1806± 6 Ma, 1794± 11 Ma,1756±11 Ma, 1756±9 Ma, 1565±5 Ma, 1506±8 Ma,1224±9 Ma and 907+69/−72 Ma. It is observed thatthe variety of ages obtained by this method is greaterthan that obtained by ID–TIMS. This is attributed inpart to the greater number of grains analysed and inpart to the fact that the largest grains were selected

for LA-MC-ICPMS analysis and the smaller ones forID–TIMS.

Those zircons dated between 2053 and 1863 Maare inferred to be from Transamazonian sources, andthose dated 1756 Ma are most likely derived fromanorogenic igneous rocks related to the Espinhaço riftsystem of the São Francisco Craton (Machado et al.,1989). The sources of zircons with Mesoproterozoicages ranging from 1565 to 1224 Ma are intriguingsince no rocks of these ages are known in the São Fran-cisco Craton or in the surrounding areas. However,these ages are significant as they indicate for the firsttime the presence of Mesoproterozoic magmatism,possibly of anorogenic character. Although it could beargued that some of these zircons are older and wereaffected by Neoproterozoic metamorphism, the factthat some grains are concordant (e.g. 1565, 1506 and1223 Ma) supports the inference that they are derivedfrom rocks of these ages. The youngest concordantzircon, dated at ca. 907 Ma provides the first reliablemaximum age of sedimentation for the Araxá Group.

5.2.4. Unit E—micaceous quartziteSample 1041 is from a quartz-schist (impure

metapsammite) intercalation within predominantgarnet–biotite–muscovite–metapelitic schists of UnitE, in the intermediate portion of the Passos Nappe.Zircon grains from this sample are also typicallydetrital—rounded and pitted and also display incipientpyramidal outgrowths. In addition, whilst in the sam-ples from lower grade rocks coloured zircons werecommon, in this sample all zircons are colourlessto pale yellow, suggesting that metamorphic heatingdiscoloured them, as has been observed in other areas(Machado, unpublished).

A colourless, small zircon was analysed byID–TIMS (Table 2, Fig. 7a) and yielded a minimumage of 2152 Ma (3.2% discordant). Most of the 20grains analysed by LA-MC-ICPMS (Table 3, Fig. 7b)yielded ages clustering in the 2259–2041 Ma intervaland are taken to be derived from rocks formed or meta-morphosed during the Transamazonian Orogeny. Theyoungest grain yielded a minimum age of 1039 Ma(6.7% discordant).

5.2.5. Unit G—amphiboliteRutile was separated from one of several amphi-

bolite layers intercalated in the predominantly detrital


Fig. 7. Concordia diagrams pertaining to detrital zircon from UnitE quartzite (sample 1041), Araxa Group, Passos Nappe.

Araxá Group. It is therefore inferred that mafic mag-matism was contemporaneous with sedimentation. Afraction of acicular, bright yellow rutile crystals ex-tracted from sample 1038 yielded a206Pb/238U con-cordant age 958+46/−45 Ma and another one of short,prismatic brown crystals an age of 595+ 35/−34 Ma(1038-2, 1038-3;Table 2, Fig. 8). Both fractions arevery poor in U (1 and 0.3 ppm) and yield relativelyhigh errors. The younger age is identical to the ageof a rutile fraction from another amphibolite interca-lation further up in the section (see in the following).

The older age could indicate that the acicular rutilewas part of the primary igneous assemblage in whichcase it may constrain the age of deposition of the AraxáGroup. It is in the 1.0–0.8 Ga range for ages of maficmagmatism in and around the São Francisco Craton(e.g.Chaves et al., 1997; Pedrosa-Soares et al., 1998;Machado et al., 1989) the significance of which willbe discussed in the following (seeSection 7).

Fig. 8. Concordia diagram for rutile from Unit G amphibolite(sample 1036) and Unit H amphibolite (sample 1038), AraxaGroup, Passos Nappe.

The ca. 595 Ma age for the short-prismatic rutilefraction suggests that this type of rutile grew during theretrometamorphism associated with the exhumation ofthe Passos Nappe. It falls in the 600–580 Ma range ofthe youngest K–Ar determinations on mica from thePassos Nappe (Valeriano et al., 2000).

It is also noteworthy the presence of two types ofrutile with different ages. This may indicate that meta-morphic grade was not high enough or that durationof the metamorphism was too short-lived to reset theolder rutile. The possibility should also be consideredthat the older rutile may have been included in otherminerals and shielded from metamorphic reactions.

5.2.6. Unit G—quartziteSample 1039 represents a coarse-grained quartzite

intercalated in predominant metapelitic schists of UnitG. One small, rounded and frosted monazite grain wasanalysed by ID–TIMS (1039-2,Table 2, Fig. 9b) andyielded a minimum age of 630 Ma (5.5% discordant).

5.2.7. Unit G—leucosome in Araxá GroupSample 1081 is from one of many sigmoidal leu-

cosome lenses within paragneiss and feldspathicgarnet–muscovite–biotite schist from the upper por-tions of the Araxá Group. These lenses are centimetricto decimetric in size and are composed of K-feldspar,quartz, oligoclase, garnet and corundum. They aresyn-tectonic to the main foliation (S2), with indica-tion of top to southeast tectonic transport given bysigmoidal trails (Fig. 9a). The D2 deformation phase


Fig. 9. (a) Photograph of sigmoidal leucosome veins (sample 1081)in Unit H schists, Araxa Group, Passos Nappe; (b) concordiadiagram pertaining to zircon and monazite from sample 1081.

is related with thrusting and exhumation of the PassosNappe, when anatexis of metasediments took place inrocks of appropriate compositions.

Two monazite crystals were analysed by ID–TIMS,one euhedral, the other rounded. The former (1081-4,Table 2, Fig. 9b) yielded a concordant age of 631±4 Ma, and the other (1081-1) a206Pb/238U age of606± 2 Ma. These ages are similar to other ages al-ready mentioned for monazite and zircon and are con-sidered to be related to two events of mineral growth(seeSection 7).

Four fractions of tiny, sub-equant to equant zirconare all discordant and yielded minimum ages between730 (two analyses) and 686 Ma (Table 2). Assum-ing that the discordance is caused mainly by the ca.630 Ma metamorphic event, the probable ages of in-heritance can be estimated by discordia lines betweenthe 631 Ma monazite and these fractions. They point to

upper intercepts of 914, 863 and 794 Ma, and are sim-ilar to the ages of the youngest detrital zircons foundin the metasediments.

5.2.8. Unit H—orthoquartzite at the top of PassosNappe

Sample 1042 is from a coarse-grained, well re-crystallized orthoquartzite intercalation within parag-neisses of Unit H, in the upper portions of the PassosNappe. Metamorphic conditions attained the amphi-bolite to granulite facies transition. The predominantzircon morphology is represented by grains with wellpreserved rounded, presumably detrital cores and dis-tinct euhedral overgrowths. Small equant, euhedral,multifaceted and colourless crystals typical of gran-ulite facies rocks are less abundant. Grains containingtwo or three cores surrounded by new zircon (synneu-sis) are also observed.

Five small equant grains were selected for ID–TIMSanalysis (Table 2). In spite of extended abrasion, allfive analyses are discordant (12–44%) and yield min-imum ages between 2057 and 1697 Ma (Fig. 10a).Cores were not observed when selecting these crystalsunder a binocular microscope but cathodolumines-cence imaging of several crystals of this type carriedout after the isotopic analysis, revealed the presenceof cores and may explain the observed pattern ofdiscordance. Even if it is postulated that the coresare most likely of detrital origin, it is worth to notethat the three most concordant analyses define a re-liable discordia with intercepts of 2123± 5 Ma and656± 16 Ma. This is compatible with the possibilitythat these zircons are derived from a single source ofTransamazonian age which underwent Neoproterozoicmetamorphism.

Two euhedral monazite crystals were analysed byID–TIMS: one is sub-concordant at 622+ 1/−2 Maand the other is concordant at 604+ 7/−8 Ma(Fig. 10a). These ages help constrain the metamorphichistory of the Passos Nappe and will be discussed inthe following.

The detrital cores of 31 zircons were analysed byLA-MC-ICPMS and with the exception of an Archeangrain they scatter along a discordia band between ca.2.2 and 0.6 Ga (Fig. 10b). Similarly to the zirconsanalysed by TIMS, this indicates derivation from aPaleoproterozoic source and strong Pb loss due toNeoproterozoic high grade metamorphism.


Fig. 10. Concordia diagrams pertaining to zircon and monazitefrom Unit H quartzite (sample 1042), Araxa Group, Passos Nappe.

5.2.9. Unit H—amphiboliteAn amphibolitic intercalation in the Araxá Group

similar to that sampled at lower metamorphic grade(Unit G, sample 1038) yielded orange, short prismsof rutile with low U contents and a206Pb/238U age of593+12/−13 Ma (Table 2, Fig. 8). This age is similarto the ca. 595 Ma age found for rutile of sample 1038and is interpreted as dating the retrometamorphismassociated with the exhumation of the Passos Nappe.

6. External Domain

6.1. Northern segment

The area east of the Araxá Nappe is in the classicalregion where the Canastra Group was originally de-fined byBarbosa (1954). A fine-grained orthoquartzite

Fig. 11. Concordia diagram for detrital zircon from Canastra Groupquartzite (sample HS41), External Domain.

with millimetric dark layers of heavy minerals, meta-morphosed under greenschist facies conditions, wascollected from the upper portion of the Canastra Group(sample HS41). Colourless to dark pink-purple zirconsare sub-rounded to well rounded, pitted and devoid ofmetamorphic overgrowth.

The ages of 20 grains analysed by LA-MC-ICPMS(Table 3, Fig. 11) range between 2875± 3 Maand 1226± 8 Ma. Archean and Paleoproterozoicsources have already been referred to above for theAraxá Group metasediments. The 1536–1180 Masub-concordant ages aare similar to those found in theAraxá Group and confirms the previous inference forthe occurrence of Mesoproterozoic sources in the SãoFrancisco Craton. The youngest zircon (1226± 8 Ma)provides an upper limit for the sedimentation of theCanastra Group.

6.2. Furnas segment

The External Domain to the east of the PassosNappe consists of an intensely imbricated thrust sys-tem (Ilicınea-Piumhi thrust system,Valeriano et al.,1995) comprising from bottom to top sheets of felsicmetavolcanic rocks (Costas schist), metasedimen-tary sequences (Serra da Boa Esperança Sequence;Valeriano, 1992), Archean granite–greenstone base-ment (Piumhi greenstone belt,Schrank, 1982) andchromitite-bearing serpentinites, which are in turnoverlain by an upward-coarsening metasedimentarysequence (Serra da Mamona unit). The age of themetasedimentary sequences is poorly constrained byca. 600 Ma K–Ar ages on mica (Valeriano et al.,


2000). All rock units are in the chlorite zone of thegreenschist facies. Results are presented from thelower to the upper structural levels.

6.2.1. Felsic metavolcanic unit (Costas schist)This unit forms a tectonic lens located along the

basal thrust of the External Domain, sandwichedbetween the autochthonous Bambuı Group and theSerra da Boa Esperança metasedimentary sequence.It is a homogeneous felsic metavolcanic rock (sample1130) composed of quartz, plagioclase, muscovite,epidote and chlorite. Zircons extracted from this sam-ple belong to a single type characterized by euhedral,colourless to pale yellow prisms with aspect ratioranging from 1:2 to 1:4. Three of four analyses de-fine a discordia with intercepts at 1721± 9 Ma and655± 4 Ma (Table 2, Fig. 12). One analysis falls be-low the discordia possibly because of insufficient airabrasion. The upper intercept age agrees with previousages for felsic magmatism related to the Statheriantaphrogenetic events such as those documented forthe Espinhaço Supergroup (Machado et al., 1989) andAraı Group (Pimentel et al., 1991). The 655± 4 Maage is inferred to date a metamorphic event, and is re-markably coincident with a K–Ar age of 659± 8 Mathat was obtained from coarse muscovite from thesame outcrop (Valeriano et al., 2000).

6.2.2. Metasedimentary sequence (Serra da BoaEsperança)

The basal thrust sheet of this sequence comprisesgrey, coarse-grained micaceous metarenite from which

Fig. 12. Concordia diagram for zircon from felsic metavolcanicrock (Costas schist, sample 1130), External Domain.

Fig. 13. Concordia diagrams pertaining to detrital zircon fromquartzite from Serra da Boa Esperança Sequence (sample 1044),External Domain.

two samples were collected (1044 and 1046). Zirconsextracted from the first sample are colourless or exhibitdifferent shades of pink and display typically detritalmorphological features. Two analyses by ID–TIMSyielded minimum ages of 2081 and 1946 Ma (both 2%discordant), a third one is concordant at 1445± 2 Ma(Table 2, Fig. 13a). Twenty four analyses by LA-MC-ICPMS cluster in the following age groups (Table 3,Fig. 13b). An Archean group (207Pb/206Pb agesof 2768–2694 Ma); three Paleoproterozoic groups(2324–2278 and 2012–1841 Ma and concordant anal-ysis at 1728± 28 Ma); a Mesoproterozoic group withsix grains between 1397 and 1331 Ma and a groupof Neoproterozoic grains the two most concordantyielding 207Pb/206Pb ages of 107 and 989 Ma.

Sample 1046 was collected from another out-crop of the same lithology and tectonic position as


sample 1044. A pink and a colourless rounded detri-tal zircons were analysed by ID–TIMS and yieldedminimum ages of 1948 and 1396 Ma (Table 2,Fig. 13a).

Sample 1128 is a meta-arkose that belongs to aunit of tectonic mélange formed along the sole thrustof the External Domain (near the locality of SantoHilário), where the basal thrusts sheets of the Serrada Boa Esperança metapsammites are thrusted overthe autochthonous Bambuı Group. This unit is het-erogeneously deformed, with intensely sheared gran-ite and quartzite fragments within a groundmass ofmetapelite or feldspathic metarenite. These character-istics suggest syn-orogenic sedimentation related tothe exhumation of the external allochthons, followedby their erosion and incorporation of the debris alongthe sole thrust.

The results of two single-grain analyses by ID–TIMS (Table 2, Fig. 13a) and of 25 by LA-MC-ICPMS(Table 3, Fig. 14) point to different proportions ofthe same age groups as was the case for the pre-vious samples. One of the grains is concordant at1234± 7 Ma and the youngest is 1086 Ma old (8.7%discordant).

The Archean and Paleoproterozoic provenance agesare similar to those found for the Araxá Group andcan be attributed to the sources previously mentioned.It is worth noting the 1.2–1.4 Ga old zircons were alsofound in the Araxá Group and further substantiatethe occurrence of a magmatic (or metamorphic) eventin that age range. The 1086 and 989 Ma old zircons

Fig. 14. Concordia diagram for detrital zircon from meta-arkosefrom Serra da Boa Esperança Sequence (sample 1128), ExternalDomain.

bracket the upper limit for the sedimentation of theSerra da Boa Esperança Sequence.

6.2.3. Archean basementThe Piumhi thrust sheet is a fragment of a typi-

cal Archean granite–greenstone association (Schrank,1982; Machado and Schrank, 1989). Two units weresampled: a coarse-grained alkali-granite (Taquarigranite, sample 71-1) that outcrops in the north ofthe thrust sheet and a hornblende-bearing orthogneiss(sample 1129) with granodioritic composition occur-ring in the south (Fig. 2). The granite displays intrusivecontact relationships with the Piumhi mafic–ultramaficvolcanic sequence. In the least deformed and meta-morphosed samples the primary mineralogy can be ob-served to include K-feldspar, quartz, biotite (altered tochlorite) and plagioclase. Fluorite, zircon and titaniteare found as accessory minerals. An ID–TIMS analy-sis of brown titanite (sample 71-1T) is 10% discordantwith a 207Pb/206Pb age of 3019 Ma (Table 2, Fig. 15).

Zircon extracted from the hornblende-orthogneissis pink, euhedral and vary in shape from subequant to2:1 prisms. Three analyses (Table 2, Fig. 15) define adiscordia with an upper intercept 2935±13 Ma whichis interpreted as the crystallisation age of the protolith.This age is younger than the 3116 Ma age for a maficlayered intrusion (Machado and Schrank, 1989), andindicates that Archean magmatic activity in the Piumhi

Fig. 15. Concordia diagram for zircon from a hornblende gneiss(sample 1129) and for titanite from the Taquari granite (sample71), both samples from the Piumhi granite–greenstone thrust sheet,External Domain.


granite–greenstone belt lasted for at least 280 millionyears, considerably longer than previously thought.

6.2.4. Serra da Mamona unitThe uppermost thrust sheet of the External Do-

main is separated from those below by a tectoniclens composed of chromitite-bearing serpentinitesand talc-schists. It is represented the Serra da Ma-mona Unit, an upward-coarsening metasedimentaryunit (Serra da Mamona unit;Valeriano, 1992) withbasal intercalations of banded iron formation (BIF)within carbonaceous metapelite which grade upwardsto quartzitic metarenite and overlying metaconglom-erates. Characteristically rounded quartz pebbles ofthe latter indicate the degree of sedimentary maturityof this unit. Abundant metapelite and BIF intraclastsare also present.

Detrital zircons extracted from a metaconglomer-ate layer (sample 1084) are typically pink-coloured,rounded and pitted. Two ID–TIMS analyses (Table 2,Fig. 13a) yielded minimum ages of 2976 Ma (15% dis-cordant) and 2817 Ma (2.2% discordant). The ages of24 zircons dated by LA-MC-ICPMS (Table 3, Fig. 16)are between 3060± 7 Ma and 2685 Ma, including an-other concordant zircon at 2906±2 Ma. The exceptionis a grain with a minimum age of 2197 Ma (20% dis-cordant) with a probable age between 2.2 and 2.3 Ga(depending on whether it was affected or not by Neo-proterozoic metamorphism at 0.6 Ga) indicating thatdeposition of the Serra da Mamona unit could be Pa-leoproterozoic or younger.

Fig. 16. Concordia diagram for detrital zircon from metaconglom-erate from Serra da Boa Esperança Sequence (sample 1084), Ex-ternal Domain.

6.3. Southern segment

The southern segment of the External Domain con-stitutes a transition zone between the autochthonoussouthernmost São Francisco Craton and the thinskinned thrust system of the Brasılia belt (Fig. 1). It iscomposed of both autochthonous and allochthonousunits of the predominantly metasedimentary An-drelandia Group and its Archean–Paleoproterozoicbasement. The age of sedimentation of the An-drelandia Group is limited by the Paleoproterozoic ageof its basement and by a ca. 567 Ma discordia lower in-tercept (Söllner and Trouw, 1997). Samples ITA-2 andITA-3 are from a thin thrust sheet of the AndrelandiaGroup whilst ITA-1 is from the underlying intenselyfolded but autochthonous Andrelandia Group. Thethree samples are orthoquartzites representing typicalunits of this group and are in lower amphibolite facies.

The morphological features of the zircons extractedfrom this sample indicate that they are typical detri-tal grains. Thirty zircon analyses by LA-MC-ICPMS(sample ITA-1,Table 3, Fig. 17a), yielded ages cluster-ing in the same Archean and Paleoproterozoic groupsalready mentioned. A series of concordant zircons inthe 1484–1246 Ma range was also found. Given thatthe unit sampled is autochthonous and that zirconsshown no overgrowth, these ages irrefutably docu-ment the presence of Mesoproterozoic sources in theSão Francisco Craton. In addition, zircons with agesin the 1.0–1.1 Ga range, including a concordant oneat 1011+ 39/−40 Ma, are the best estimate for theupper limit for the sedimentation of the AndrelandiaGroup. Zircons from the other sample from the al-lochthonous Andrelandia Group (ITA-3) analysed bythe same method yielded a similar pattern of ages(Table 3, Fig. 17b).

Two rounded monazite grains with rough surfacewere analysed from the same klippe of AndrelandiaGroup quartzites. One yielded a minimum age of588 Ma (1.7% discordant, ITA3-2) and the other2027 Ma (16% discordant, ITA2-1,Table 2, Fig. 17c).A discordia line constructed with the two grains yieldsan upper intercept at 2104 Ma and indicates that thelatter is probably detrital and was affected by Neo-proterozoic metamorphism. It is noted that in spiteof the growth of new monazite in sample ITA3, thedetrital zircons analysed do not show Pb loss relatedto the Neoproterozoic metamorphism.


Fig. 17. Concordia diagrams pertaining to Andrelandia Groupquartzites. (a) Detrital zircon from autochthonous domain quartzite(sample ITA1); (b) detrital zircon from allochthonous domainquartzite (sample ITA3); (c) monazite from allochthonous domainquartzites (samples ITA2 and ITA3), External Domain.

The data for this sample thus bracket the sedi-mentation of the Andrelandia Group in the Neopro-terozoic between the age of the youngest detritalzircon—ca. 1011 Ma—and the age of metamorphismat ca. 588 Ma.

7. Discussion and conclusions

7.1. Provenance ages and implications

The Araxá, Canastra and Andrelandia groups areconsensually interpreted as part of the sedimentarysuccession of the Neoproterozoic passive marginbordering the western and southern São FranciscoCraton (Dardenne, 2000; Trouw et al., 2000). Be-cause of the lack of constraints on the age of sed-imentation and reasonable stratigraphic correlation,the Serra da Boa Esperança Sequence was individu-alised (Valeriano et al., 2000). A common feature ofthe clastic rocks from these units is the similarity ofzircon provenance ages. As already mentioned, theArchean and Paleoproterozoic ages are compatiblewith erosion of the São Francisco Craton. Particu-larly interesting are the Mesoproterozoic provenanceages between ca. 1.2 and 1.6 Ga, with a cluster at ca.1.3 Ga (Fig. 18). It is worth noting that among zir-cons with ages in this range, several are concordantto sub-concordant, indicating that these ages are notdue to the effect of Neoproterozoic metamorphism onArchean–Paleoproterozoic grains. Therefore, one ofthe main conclusions of this work is that Mesopro-terozoic sources must be present in the São FranciscoCraton that have yet to be identified. These sourcesmay be located underneath the Bambuı Group, whichcovers most of the southern São Francisco Craton.The occurrence of Mesoproterozoic anorogenic mag-matism is compatible with pre-Bambuı rift structuresrecognised by seismic methods (Braun et al., 1993;

Fig. 18. Histogram depicting the207Pb/206Pb ages for detritalzircons from clastic rocks from the southern Brasılia belt. Onlyconcordant ages are plotted.


Teixeira et al., 1993). It is also possible that thesources are located at the western continental margin,which is presently overridden by the external–internalallochthons. The latter would imply that the lo-cus of the Neoproterozoic continental break-upoccurred preferentially along Mesoproterozoic riftsystems.

The ages of the youngest detrital zircons from theAraxá, and Andrelandia groups and the Serra da BoaEsperança Sequence lie between 1.0 and 0.9 Ga andprovide the upper limit for the age of sedimenta-tion. This is compatible with the possibility that theca. 958 Ma rutile age for an amphibolite from theAraxá Group dates syn-sedimentary mafic magma-tism. The lower limits are the metamorphic ages ofca. 630 Ma for the Araxá Group and of 588 Ma forthe Andrelandia Group. In addition, a Neoproterozoicage for the Serra da Boa Esperança Sequence sup-ports the idea to consider this unit into the proximalsuccessions that surround the São Francisco Craton.The age limits for the sedimentation of the Canas-tra Group remain poorly defined and it can only bestated that it must be younger than the youngest zir-con dated at 1266 Ma. The absence of metamorphicminerals also preclude the establishment of a reliablelower limit, although the coherence of the deforma-tion with that of the Araxá Group is compatible witha contemporary age of metamorphism.

The provenance ages presented here agree in partwith Nd TDM data for fine-grained metasediments ofthe Araxá Group (Pimentel et al., 2001): both data setsindicate a Paleoproterozoic provenance for some sam-ples. Other samples yieldTDM ages in the 1.0–1.4 Garange and were interpreted as a combination of Pale-oproterozoic sources from the São Francisco Cratonand of ca. 0.9 Ga sources from the Goiás magmaticarc. However, 1.2–1.3 Ga concordant detrital zirconsfound in some samples from the Araxá Group reportedhere document the presence of crustal sources of thisage in the São Francisco Craton. Therefore, the con-tribution from Goiás arc rocks is not required to ex-plain the Nd isotopic results for the Araxá Group inthe studied area. This may indicate that the concept ofAraxá Group, which is mapped as one vast single unitin the Brasilia belt, may be misleading and the unitmight need further subdivision.

In addition, whilst NdTDM ages for samples fromthe Canastra Group yield only Paleoproterozoic prove-

nance ages (Pimentel et al., 2001), our data for a sam-ple from this group yield Archean to Mesoprotero-zoic ages. This reveals a non-uniform distribution ofthe Mesoproterozoic sources in the continental area,which is compatible with localised magmatism alongrift structures.

7.2. Metamorphic stages

Ages of metamorphism were obtained on zircon,monazite and rutile and were also defined by discordialower intercepts. The peak of the main metamorphicevent, coeval with the main phase of deformation (D2),is marked by the crystallisation of syntectonic granite(Serra Velha) in the Araxá Nappe at 637± 1 Ma andby partial melting at 631± 4 Ma in the Passos Nappe.These ages together with those obtained for granulitefacies metamorphism in the Socorro-Guaxupé Nappefurther south in the belt—629–643 Ma (U–Pb andSm–Nd isochron; cited inCampos Neto and Caby,2000)—lead to the conclusion that the collision be-tween the Paraná block with the São Francisco Cratonoccurred in the 630–640 Ma interval. This age, to-gether with the age of mafic magmatism in the AraxáGroup (ca. 958 Ma) suggests that sedimentation ofthis group could have lasted 100–200 million years.

Older ages of 656± 16 Ma and 655± 4 Ma werefound for rocks from both the Internal and Externaldomains and must date an older metamorphic eventpossibly related to an early metamorphic phase.

Early collisional events such as the above-mentionedone (ca. 655 Ma), and an older one indicated by thegranitoid magmatism from the Marata Sequence,dated at 794± 10 Ma (Pimentel et al., 1999), canlead to the speculation that compressional events,such as terrane accretions, could have taken placeduring the time-span of sedimentation of the AraxáGroup as it is presently defined. If true, importantunmapped disconformities must be present and willeventually lead to further stratigraphic redefinitionsof this widespread unit.

Younger monazite dated at 606± 2 Ma and 604+7/−8 Ma indicate a second period of monazite growth,tentatively attributed to retrometamorphism during ex-humation of the Passos Nappe. Confirmation of theage of this event will require a more detailed study oftextural relationships of parageneses involving mon-azite. Intermediate monazite ages are discordant and


may be due to partial resetting of the U–Pb systemduring the nappe exhumation.

Although rutile ages of ca. 594 Ma are identicalwithin error to the 604–606 Ma monazite ages, theymay be interpreted as cooling ages as they are inthe range of the youngest K–Ar ages on white mica(600–567 Ma) for metasediments from the PassosNappe and the Furnas segment of the External Do-main (Valeriano et al., 2000). The 588 Ma minimumage for the monazite (1.7% discordant) from the An-drelandia Group south of the São Francisco Craton,may be attributed either to final stages of the ther-mal evolution of the Brasılia belt, or to the influenceof the main metamorphism (M1) of the Ribeira belt(590–565 Ma,Machado et al., 1996a,b). The latteris coherent with the fact that the area south of theSão Francisco Craton (Fig. 1) is a zone of tectonicsuperposition of the Ribeira belt on the southernmostBrasılia belt (Trouw et al., 2000).

7.3. Geodynamic implications

The presented data provide the timing of severalgeologic events that allow important to the detailedreconstruction of the history of West Gondwana as-sembly.

The evidence presented for Neoproterozoic sedi-mentation is taken to represent a major event of con-tinental rifting that led to the individualisation of theSão Francisco paleocontinent. Formation of the pas-sive margin was underway by at least 1.0–0.9 Ga.

Westward subduction of the distal sectors of the pas-sive margin (Araxá and Andrelandia groups) under theParaná block-Goiás terrane is dated by peak medium-to high-pressure metamorphism at 0.63–0.64 Ga. Con-tinuing collision led to the exhumation and coolingof the metamorphic nappes (Araxá, Passos and Guax-upé) at ca. 605 Ma, which mark the final stages oftectonometamorphic activity in the southern Brasıliabelt.

Whilst continent–continent collision was proceed-ing on the western margin of the São Francisco Cratonalong the southern Brasılia belt, eastward subductionin the east was generating the 634–599 Ma Rio Negromagmatic arc (Tupinambá et al., 2000). Peak meta-morphic conditions in the Ribeira belt were attainedwhen the Rio Negro arc collided with the eastern SãoFrancisco margin at 595–560 Ma (Machado et al.,

1996a,b) and reached the southernmost sector of theBrasılia belt creating a zone of superposition. Thethermal front of this event affected the proximalAndrelandia Group at ca. 588 Ma.

The participation of the Amazonian craton in the as-sembly of Western Gondwana occurred at 545–500 Main the Paraguay belt (Trompette et al., 1998) and ca.500 Ma in the Araguaia belt (Moura and Gaudette,1993, seeFig. 1b). This together with the results pre-sented in this work, lead to the conclusion that thesouthern Brasılia belt records a relatively early stageof West Gondwana assembly. The belt resulted fromthe collision between the Paraná block-Goiás terranewith the São Francisco Craton and precedes the ac-cretion of the Amazonian craton by 50–100 millionyears.

Acknowledgements

C. Valeriano acknowledges financial support fromCNPq (proc. 471931/01-2) and UERJ/SR-1 and apostdoctoral grant from CAPES (BEX-1032/99-2).The Micromass Isoprobe instrument and the Lambda-Physik-Merchantek-New Wave laser system installedat the UQAM were financed through Natural Sci-ences and Engineering Research Council of Canada(NSERC) and contributions from FCAR (Québec)and Fondation UQAM. The laboratory is maintainedin part with a NSERC MFA grant. A NSERC grantto N. Machado partly financed this work. Laboratorysupport from R. Lapointe is thankfully acknowledged.Marcio Pimentel and Wilson Teixeira provided com-ments on a previous version of the manuscript thatled to significant improvements.

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