A previously unrecognised major orogenic front in Argentina1Monash University, School of Geosciences, 3800, Clayton, Victoria, Australia (Melanie.Finch@monash.edu; Roberto.Weinberg@monash.edu)

2Universidad Nacional de Salta, Buenos Aires 177, 4400 Salta, Argentina

Melanie Finch1, Maria Gabriela Fuentes2, Pavlína Hasalová1, Raul Becchio2, Nicholas Hunter1, and Roberto Weinberg1

RecrystallisedQtz+Fsp+Bt

RecrystallisedQtz+Fsp+Bt

RecrystallisedQtz+Fsp+Bt

RecrystallisedQtz+Fsp

Kfs(ii)

Pl(i)

Pl(i)

Pl

Pl

(iii)recrystallised

Pl

Pl

Kfs(ii)

Pl

Pl

Kfs

Kfs

Kfs

Qtz

Qtz

Qtz

Qtz

Fig. 12. Qtz ribbons wrapping around feldspar porphyro-clasts in protomylonites in (a) XPL and (b) PPL. Feld-spars show a variety of deformation mechanisms includ-

ing brittle fracture (i), free grain rotation (ii), and partial to complete dynamic recrystallisation (iii and yellow arrows). Qtz ribbons (orange arrows), formed through high temperature grain boundary migration, are occasionally isoclinally folded as a result of wrapping around the por-phyroclasts. When a feldspar porphyroclast wrapped in a quartz ribbon recrystallises and is sheared the result is a fine-grained mixture of feld-spar and quartz - this process destroys the compositional layering in the mylonite and connectivity of the phases resulting in a more homog-enously mixed matrix. Section parallel to stretching lineation.

1 cm

Brittle overprint of ultramylonites

Fig. 11. Breccia of silicified granite.

Fig. 9. Pegmatite clasts disaggregated through ductile shearing during mylonitisation faulted during the late brittle event.

Fig. 10. Pseudotachylyte in mylonitic gran-ite.

Fig. 4. Progressive disaggregation of pegmatite dykes (top) forms disconnected dykelets (bottom) and eventually discrete porphyro-clasts (middle). Rock face is vertical and parallel to stretch lineation.

Fig. 6. Asymmetric folds in Opx-Grt leucogranite in a mylonitic Grt+Crd+Opx+Sil migmatite. Top-to-SW thrusting (rock face is vertical and parallel to stretching lineation).

Opx+Grt leucosome

Fig. 7. Top-to-SW shear in mylonitic Opx migmatite (rock face is parallel to stretching lineation.

The El Pichao shear zone

Kfs

(i)Kfs

(i)Kfs

(iii)recrystallised

Kfs

Kfs

Qtz

recry

stallis

ed

Qtz+Kfs

Qtz

2 mm

Recrystallised Qtz+Kfs+Bt

Recrystallised Qtz+Kfs+Bt

1010

010

00S

ampl

e/ R

EE

cho

ndrit

e

REE Chondrite (Boyton, 1984)

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

SQ74

SQ86

SQ75

0.01

0.1

110

Sam

ple/

Ave

rage

cru

st

Average crust (Weaver & Tarney, 1984)

Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y Tm Yb

a) b)

Fig. 16. a) Chondrite normalised REE abun-dances and b) average crust normalised incom-patible element abundances of mylonites, gran-ites, migmatites, and metasedimentary rocks. REE patterns indicate that the mylonites are en-riched in REEs compared to granites proximal to the shear zone. Samples of the protomylonite, mylonite, and ultramylonite (green lines) are identical to each other and coincide with the field in grey corresponding to the composition of the

regional metasedimentary rocks (the Puncoviscana Formation). These results indicate that ultramyloniti-sation was not caused by mass loss or the infiltration of a fluid - that is, the El Pichao shear zone is a prod-uct of closed system shearing.

Geochemistry: closed system shearing

Cafayate granite

El Pichao mylonites (SQ30A-protomylonite; SQ80-ultramylonite; SQ77a-mylonite)

Mu+bi schist(SQ24-El Pichao; SQ33- Colalao del Valle)

Granites close to Sierra de Quilmes

Puncoviscana Formation near Sierra de Quilmes (punco1)

Puncoviscana Formation samples from Sierras Pampeanas (n = 157)

Sierra de Quilmes granites(SQ74 SQ75 San Pedro-Cafayate granite; SQ86 Tolombon tonalite)

Pun

covi

scan

aFo

rmat

ion

sam

ples

G

rani

ticsa

mpl

es

2 mm

El Pichao shear zone: key points

Opx+Grt leucosome

Mylonitic migmatite

Fig. 8. Geological map of El Pichao shear zone showing thrusting of granulite facies migmatites onto amphibolite facies schists. Waypoints are marked by black circles. The main shear plane dips to the NE with a down-dip stretching lineation (stereonets). All stereographic projections are lower hemi-sphere equal-area, the mean plane (x) indicated with a great circle and mean pole with a gray circle.

2 cm

C'C'

CC

SS

Pegmatite dyke

Disaggregated pegmatite dykeDisaggregated pegmatite dyke

Pegmatite dyke

ProtomyloniteProtomylonite

Fig. 1. Palinspastic schematic representation of the Gondwa-nan continents during the Terra Australis orogeny. El Pichao shear zone formed during the Pampean (555–515 Ma) and Famatinian orogenies (~490–350 Ma). Modified from Schwartz et al (2008).

Fig. 15. Feldspar δ - clast in ultramylonite showing top-to-SW shear (xpl). Tails of delta clast are progressively disaggregated and recrystallised to form porphyroclasts. Large delta clast shows brittle fracture along twinning plane, recrystallisation at margins, and ro-tation. Section parallel to stretching lineation

Within the ultramylonitic core of the shear zone there is an 150 m-thick band of faulted breccia and pseudotachylyte, marking a period of brittle deformation that post-dated ductile thrusting and mylonitisation.

Fig. 2. The Sierras Pam-peanas mobile belt and the Sierra de Quilmes. Location of the studied El Pichao shear zone shaded in grey in the inset and shown in detail in Fig. 8.

?

?

?

?

?

?

?

?

?

Nooutcrop

37

60

58

40

59

25

4044

26

4136

4634

41

5348

4545

605960

453045

4750

6164

38

14

45

50

43

38

72

65

48

24

34

78

3937

23

42

3232

5288

29

48

38

80

4136

4546

60

4436

2438

2125

4630

5731

23

4520

20

2214

4544

3328

24

Foliation

lineation

x = 091/39n = 46

x = 073/41n = 36

x = 092/39n = 28

Stretch

Felsic volcaniclastic rock

OpxGrt-CrdGrt Grt-Crd-Opx-Sil

Granite MigmatiteUltramylonite

Mylonite

ProtomyloniteGranite Migmatite

Granite MigmatiteOrthogneiss

Granite

Peritectic minerals in Tolombóncomplex migmatites

Breccia and pseudotachylyte

Grt-pelite

SchistMyloniteUltramylonite

Boundary between Opx-present and Opx-absent migmatites

Ultramylonitic core

Managua river

Anchillo river

Tolombón complex

500 metresN

Agua del Sapo complex

Tolombón complex

Tolombón complex79

41

29

No outcrop

Stretchlineation

High strain zone

3.5 km thick

Foliation

454546

49x = 083/41n = 73

Ultramylonitic core

1 km thick

The El Pichao shear zone (PSZ) is part of a system of thrust shear zones of the Sierras Pampeanas which outcrop discontinuously in NW Argentina (Fig. 2). This system is inter-preted as the major orogenic front of the Pampean and Famatinian orogenies at the western margin of Gondwana (Fig. 1).

The PSZ contains a high strain zone >3.5 km thick and a 1 km thick ultramylonitic core that overprints a granitic protolith (Fig. 8).

Ultramylonitic shear zones of this thickness are very rare. Other shear zones of comparable thickness include the Tres Arboles shear zone of the Sierras Pampeanas (15 km thick; Fig. 2; Whitmeyer & Simpson, 2003), the Grease River shear zone of the western Canadian shield (<1 km thick; Dumond et al., 2008), the Main Central Thrust of the Himalaya (~650 m thick; Srivastava & Srivastava, 2010), and the shear zones related to the Pan-African orog-eny in NW Africa (3 – 400 m thick; e.g., Arthaud et al., 2008; Ferkous & Leblanc, 1995).

The thick mylonites of the western Canadian shield have been previously reported to be a result of high temperature recrystallisation of feldspar porphyroclasts (Hanmer et al 1995).

Feldspar porphyroclasts of the PSZ mylonites show three main behaviours: (a) syn-shearing brittle fracture, (b) dynamic recrystallisation, or (c) grain rotation (Figs. 12, 15). This indicates that feldspar was a hard phase and therefore ultramylonitisation was not attributable purely to high temperature recrystallisation. This implies very high strain rates (γ >100; Norris and Cooper, 2003) and large displacements (>100 km), comparable to that of major shear zones globally.

Fig. 5. δ- (top) and σ- porphyroclasts in ultra-mylonite resulting from prolonged shearing of pegmatite dykelets. View is parallel to stretching lineation.

N 100 km

ARG

ENTI

NA

La Rioja

Tucumán

Tres Arbolesshear zone

CH

ILE

64° W68° W

28º S

High-grade

Bandedschist

Puncoviscana Formation

Qui

lmes

Fiam

balá

Am

bato

San Luis

Aconquija

San Juan

Cafayate

Altautina

Jujuy

Cafayate

Qui

lmes

Córdoba

Anca

sti

Calchaquies

Cumbres

SaltaSalta

32º S

Argentina

Chi

le500 km

Argentina

Chi

le

ParaguayBoliviaBolivia Paraguay

UruguayUruguay

The Sierras Pampeanas mobile belt

El Tigreshear zone

La Chilcashear zoneLa Chilca

shear zone

Precordillera exotic

terrane

Cordillera frontal

Low-grade

Colalaodel Valle

Tolombon

Cafayate

San Antonio

Santa Maria

Ruinas de Quilmes

26º 20'

26º 40'

26º 00'

66º 00'66º 20'

10 km

Tolombón Complex

Agua del SapoComplex

N

EL PICHAOSHEAR ZONE

El Divisidero

OvejeriaAnchillo

The Sierra de Quilmes

Quilmes

Managua

Fig. 13. Mica-, and feldspar- fish, and garnet with Fsp strain shadows indi-cating top-to-SW thrusting (section parallel to stretching lineation; PPL).

100 µm

Qtz ribbons

Qtz ribbons Kfs-fish

Bt

Sil

100 µm

CrdBt

CrdBt

GrtGrt

NENE

NENENENE

NENE

SWSW

Antarctica

India

Australia

West Gondwana

Pampean and Famatinian

orogens

Paleo-Pacific Ocean

East Gondwana

Terra Australis orogen

South America

Africa

NENE

NENE

NENE

NENE

SWSW

SWSW

SWSW

SWSW

SWSW

SWSWSWSW

Opx leucosomeOpx leucosome

Ultramylonitic coreFoliation

1 mm

Fig. 14. Crd-fish partially re-placed by Bt and Sil, indicating top-to-SW shear and Qtz ribbon showing subgrain rotation recrysta l l i sa t ion (white arrows; sec-tion parallel to stretching lineation; XPL).

Microstructures of the El Pichao shear zone

Outcrop structures of the El Pichao shear zone

Fig. 3. Ultramylonite grading into proto-mylonite with asymmetric dykelets indicating top-to-SW shear. Pegmatite dykes are sheared into discontinuous lenses, and the feldspar porphyro-clast percentage is ~20% in mylonites and >50% in protomylonites. Rock face is vertical and paral-lel to the stretching lineation.

References: Arthaud, et al (2008) in Pankhurst, R., et al eds., Geol. Soc. Lon., v. 294, p. 49-67; Boynton (1984) in Henderson, P. ed, Rare Earth Element Geochemistry, 63-114; Dumond et al (2008) Chem. Geol., v. 254, p. 175-196; Ferkous & Leblanc (1995) Min. Dep., v. 30, p. 211-224.; Hanmer et al (1995) J. Str. Geol., v. 17, no. 4, p. 493-507; Norris & Cooper (2003) J. Str. Geol. v. 25, no. 12, p. 2141-2157; Schwartz et al (2008) J. Geol., v. 116, no. 1, p. 39-61; Srivastava and Srivastava (2010) J. Geol. Soc. India, v. 75, p. 152-159; Weaver & Tarney (1984) Nature, v. 310, p. 575-577; Whitmeyer & Simpson (2003) J. Str. Geol., v. 25, no. 6, p. 909-922.

ProtomyloniteProtomylonite

MyloniteMylonite

UltramyloniteUltramyloniteSWSW NENENENESWSW

El Pichao shear zone of western Gondwana

ProtomyloniteProtomylonite

UltramyloniteUltramylonite

UltramyloniteUltramylonite

(ii)Kfs

a)a) b)b)