A previously unrecognised major orogenic front in previously unrecognised major orogenic front ......

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  • 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, Pavlna Hasalov1, 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 (555515 Ma) and Famatinian orogenies (~490350 Ma). Modified from Schwartz et al (2008).

    Fig. 15. Feldspar - 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 Tolombncomplex migmatites

    Breccia and pseudotachylyte

    Grt-pelite

    SchistMyloniteUltramylonite

    Boundary between Opx-present and Opx-absent migmatites

    Ultramylonitic core

    Managua river

    Anchillo river

    Tolombn complex

    500 metresN

    Agua del Sapo complex

    Tolombn complex

    Tolombn complex79

    41

    29

    No outcrop

    Stretchlineation

    High stra

    in zone

    3.5 km

    thick

    Foliation

    454546

    49x = 083/41n = 73

    Ultramylo

    nitic cor

    e

    1 km thic

    k

    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 (100; Norris and Cooper, 2003) and large displacements (>100 km), comparable to that of major shear zones globally.

    Fig. 5. - (top) and - 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

    Tucumn

    Tres Arbolesshear zone

    CH

    ILE

    64 W68 W

    28 S

    High-grade

    Bandedschist

    Puncoviscana Formation

    Qui

    lmes

    Fiam

    bal

    Am

    bato

    San Luis

    Aconquija

    San Juan

    Cafayate

    Altautina

    Jujuy

    Cafayate

    Qui

    lmes

    Crdoba

    Anca

    sti

    Calchaquies

    Cumbres

    SaltaSalta

    32 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

    26 20'

    26 40'

    26 00'

    66 00'66 20'

    10 km

    Tolombn 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-ca