1: Larrovere et al. 2017: Ciencias de la Tierra y Recursos Naturales del NOA. Relatorio del XX Congreso Geológico Argentino, San Miguel de Tucumán; 2: Dahlquist et al. 2008: Geologica Acta, Vol 6, Number 4; 3: Alasino et al. 2017: Ciencias de la Tierra y Recursos Naturales del NOA. Relatorio del XX Congreso Geológico Argentino, San Miguel de Tucumán; Veksler & Thomas, 2002: American Mineralogist, Vol. 87; 5: Pichavant, 1981: Contr. Min. Petrol., Vol. 76; 6: Gualda et al. 2012: J Petrol., Vol. 53; 7: Nabelek et al. 2012: EPSL, Vol. 317-318; 8: Boehnke et al. 2013: Chemical Geology, Vol. 351; 0 Pass-chier & Trouw, 1996: Microtectonics, Springer Verlag, Berlin; Stipp et al. 2002: J Struct Geol, Vol. 24.

1California Institute of Technology, Pasadena, USA2University of Southern California, Los Angeles, USA3CRILAR-CONICET/INGeReN-UNLaR, Anillaco, Argentina4California State University Fullerton, Fullerton, USA

Strain localization mechanisms (or lack thereof) in a ~ 10 km wide, syn- to post-magmatic mylonite zone in the Famatinian arc

B. Ratschbacher1,2, T. Cawood2, A. Lusk2, M. Larrovere3, C. Rick2, P. Alasino3, S. Paterson2, V. Memeti4

Motivation Geologic Map and Structural Data

Cross section

Famatinian orogeny

Geochemistry of arc rocks

E D

omai

nFa

ult z

one

W D

omai

n

Temperature-time evolution

Acknowledgements/References

Sierras Pampeanas shear zones. 0 . 0

7565-75

41

39

4242

steep

50

60

6658

63

7850

85

70

65

steep80

75

48

55

4050

52-85

steep

7857

42

43

25

20

60

58

80

90

30-65

25-50

26

40

40-60shallow

?

???

shallow

shallow-30

I-type granitoids

S-type granitoids

Cuesta de Randolfo shear zone

Solid-state foliation 25

Solid-state lineation

Shear zone (if absent equals to symbol size)

40

Late Neoproterozoic to Ordovician igneous and sedimentary host rocksFamatinian intrusive and extrusive rocksLate Devonian/ Early Carboniferous intrusive rocks

TS3.1/GMPV8.10

E Domain

W Domain

Fault zone

E Domain

W Domain

Fault zone

1

2

2

1 482.5 ± 4.9 Ma

482.5 ± 2.1 Ma459.6 ± 4.1 Ma

metaluminous

peraluminous

50

60

70

80

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

SiO2

ASI

MetavolcanicsTwo mica, grt-graniteKfsp-rich graniteBt-rich granite

Famatinian arc rocks

Summary

Numerous mechanisms have been proposed to localize strain in crustal rocks (e.g. grain size reduction). In contrast, mechanisms also exist that could act against strain localization, leading to unusually wide shear zones, (e.g. reaction hardening, expulsion of water). Several of these processes may be active in a single shear zone, as a function of temperature, composition and time. In hot orogens (e.g. the South American Cordillera), the presence of large amounts of magmatism influences crustal rheology and strain localization processes, which can lead to wide shear zones such as the Cuesta de Randolfo mylonite zone (CRMZ) studied here. We use structural mapping, microscopy, U-Pb geochronology and MELTS modeling to under-stand the structural evolution of the CRMZ and the influence of magmatism on shear zone development in the Famatinian orogeny, Argentina.

Late Devonian-

Early Carboniferousmagmatism

?

??

CarboniferousDevonianSilurianOrdovicianCambrianEdiacarian

Fam. magm.Fam. metamorphism

Age (Ma)560 520 400 360580 540 380 340 320500 480 460 440 420 300

Fam. tectonism

Ductile shear zonesAge range based on protolith age

Age range of deformationbased on geochronology

Age range of deformationbasd on field relations

~490 to 455 Ma2,3 subduction along Gondwana margin led to:-Famatinian arc (red box) → I- & S-type calc-alkaline intrusive and extrusive rocks, emplaced at all crustal levels.-High-T metamorphism (red box).-Syn-magmatic crustal shortening → ductile shear zones in mid- to deep-crust.

-Field evidence & geochronology indicates that shear zones start to develop during magmatism and continued until the end of the Silurian/Early Devonian.

-Late Devonian/Early Carboniferous magmatism (orange box) crosscuts the ductile shear zones and marks the end of Famatinian deformation (green box).

Figure 2: Geologic map of the Sierras Pampeanas, NW Argentina, exposing rocks of the Famatinian orogeny.

- The orientation of solid-state foliation and lineation are indicated for major shear zones.

- Note the division of Famatinian arc rocks into two magmatic belts: S-type granitoids in the east and I-type granit-oids in the west.

Figure 3: SiO2 versus aluminium saturation index (ASI) plot of whole rock geochemical data from the Famatinian arc3. - Arc rocks comprise both metalumnious (dominantly in the S-type igneous belt) and peralumnious (dominantly in the I-type igneous belt) compositions.

- All the rocks from the CRMZ are peraluminous.

Field pictures Photomicrographs Deformation mechanisms & temperature

Figure 4: Geologic map of the CRMZ.

- The CRMZ is divided into three compositionally, structurally and temporally distinct zones: the eastern domain, the western domain and the fault zone separating them.

n = 73

n = 30

n = 141

Figure 5: Stereoplots of foliation data from the different domains in the CRMZ.

Zone

4000

30001 km E DomainW Domain

Fault zone[m]

Figure 6: Cross section A-B through the central part of the CRMZ. Shown are foliation traces and sense of shear as determined in the field. Numbers indicate the emplacement order inferred from U-Pb zircon ages and field relations

A B

The authors thankfully mention undergraduate students from USC, Cal State Fullerton, National University of La Rioja and National University of Salta for helping to map this shear zone and their work in sample and data collection and interpretation during two Maymester field seasons. We further thank CRILAR for their hospitality and use of facilities.

12 333

Figure 7: Dike of the two mica, tourmaline-bearing granite (pink unit) intrudes the older biotite-pla-gioclase granite (blue unit). Strongly localized, ul-tramylonite zones are associated with dikes and veins originating from the younger two mica granite.

Figure 8: Mylonitic fault zone, placing the eastern domain (deeper section) on top of the western domain (shallower section).

E Domain

W Domain

Figure 9: Tourmaline pockets in the two mica gran-ite (pink unit). This unit is rich in tourmaline and thus boron, which can significantly lower its solidus tem-perature4,5.

Figure 10: The east domain is dominated by the oldest intrusive unit consisting of biotite- and plagioclase-rich granite (blue unit) and dikes of the two-mica, kfeldspar-rich granite (pink unit). Microstructures show evidence for intense plag sericitization, minor kfs BLG recrystallization, qtz SGR and brittle fracturing of kfs.

Figure 12: The west domain consist of two major units: the two mica, epidote- and garnet-bearing and tourmaline-rich granite (pink unit) and a biotite-poor but kfeldspar-rich granite (orange unit). Microtextures show evidence for submagmatic deformation, BLG recrystllization of kfs, plag SGR and qtz SGR and BLG recrystallization.

100%

85%

20%

solidus

900

800

700

600

500

400

300460.4 460.2 460.0 459.8 459.6 459.4 459.2 459.0

Age (Ma)

Tem

pera

ture

in °

C

Melt %

Low

er s

olid

us d

ue to

bor

on

Rel

ease

of f

luid

s

Magma emplacement

zircon crystallization

Fsp SGR

Qtz SGR

Qtz BLG

Stage 1 Stage 2 Stage 3 Stage 4

Stage 1: Emplacement of two mica granite; strain taken up by melt in W domain, and is distributed across the entire W domain. Bt-plag granite in E domain is below its solidus, behaves rigidly, and takes up little/no strain.

Stage 2: Protracted cooling of two mica granite; strain taken up by melt and possi-bly feldspar SGR in the W domain, and remains distributed across entire domain.

Stage 3: Strain taken up by qtz SGR and minor feldspar BLG in both domains. Strain distributed across W domain but localized in E domain, where dikes of two mica granite intruding the bt-plag granite form weak zones that localize strain. The release of fluids from these dikes causes seritization in bt-plag granite, and rheo-logical contrasts across contacts provide loci for localization.

Stage 4: Strain taken up by qtz BLG and micaceous shear bands (sericitized plag) along the fault zone (major contact between the two different granites), and along earlier-formed narrow shear zones in the E domain. Strain highly localized. Figure 16: Plot showing age (Ma) versus Temperature (°C) of the two mica granite. Constructed using rhyolite-MELTS modeling5 with sample B7 as a starting composition (two mica granite) and a 2D conductive cooling model6 combined with zircon saturation temperature calculations8 and U-Pb age to convert to absolute ages.

900 800 700 400500 300600 200Temperature in °C

magmatic/submagmaticKfs/Plag SGR

Kfs BLG

Kfs brittle fractureQtz SGR

900 800 700 400500 300600 200Temperature in °C

Kfs BLGQtz SGR

Qtz BLGKfs brittle fracture

900 800 700 400500 300600 200Temperature in °C

Kfs BLGQtz SGR

Kfs brittle fracture

Plag sericization

Intrusion of bt-plag granite:

Stage 1 & 2: intrusion of two-mica granite, distributed hypersolidus shortening

A

B

Figure 1: Plot showing the temporal evolution of the Famatinian orogeny(modified after 1).

host

Temporal history

Stage 3: subsolidus shortening, distributed in two-mica granite, localized in bt-plag granite.

MetavolcanicsTwo mica, kfs-rich graniteBt-poor, kfs-rich graniteBt-plag-rich graniteBasalt

Figure 11: The fault zone is characterized by the contact between the two-mica, kfeldspar-rich granite (pink unit) and the biotite- and plagioclase-rich granite (blue unit) comprising a mylonite zone. Microtextures show Kfs BLG recrystallization and intense qtz SGR and BLG recrystalliza-tion as well as kfs brittle fracturing.

500 μm 500 μm

100 μm 100 μm

500 μm 500 μm

Figure 13: Plot showing the temperature range of microstructure formation for the biotite- and plagioclase-rich granite (blue unit). Estimates of temperature range are from 9,10.

Figure 14: Plot showing the temperature range of microstructure formation for the fault zone. Estimates of temperature range are from 9,10.

Figure 15: Plot showing the temperature range of microstructure formation for the two-mica, kfeldspar-rich granite (pink unit). Esti-mates of temperature range are from 9,10.

- Strain localization (or the lack thereof) in the CRMZ is a function of (a) the temporal emplacement of intru-sive units and (b) their composition:

a) The presence of melt causes strain localization into the youngest unit (two-mica granite), although strain remains distributed across this unit.

b) In the bt-plag granite, fluid-induced sericitization leads to localized reaction weakening. This, together with rheological contrasts at contacts, leads to the for-mation of narrow high-strain zones.

Stage 4: localized strain alongfault zone & ultramylonites in bt-plag granite.

Qtz

Qtz

SGR & BLG recrystallization

Qtz

Qtz SGR & BLG recrystallization

Kfs

brittle fracture

Plag

Sericitization Qtz

Kfs

BLG recrystallization

submagmatic deformation?

Qtz

Kfs

Kfs

Qtz

Plag

Plag

Mica

SGR

1: Larrovere et al. 2017: Ciencias de la Tierra y Recursos Naturales del NOA. Relatorio del XX Congreso Geológico Argentino, San Miguel de Tucumán; 2: Dahlquist et al. 2008: Geologica Acta, Vol 6, Number 4; 3: Alasino et al. 2017: Ciencias de la Tierra y Recursos Naturales del NOA. Relatorio del XX Con-greso Geológico Argentino, San Miguel de Tucumán; Veksler & Thomas, 2002: American Mineralogist, Vol. 87; 5: Pichavant, 1981: Contr. Min. Petrol., Vol. 76; 6: Gualda et al. 2012: J Petrol., Vol. 53; 7: Nabelek et al. 2012: EPSL, Vol. 317-318; 8: Boehnke et al. 2013: Chemical Geology, Vol. 351; 0 Passchier & Trouw, 1996: Microtectonics, Springer Verlag, Berlin; Stipp et al. 2002: J Struct Geol, Vol. 24.