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
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.
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).
Fig. 7. Top-to-SW shear in mylonitic Opx migmatite (rock face is parallel to stretching lineation.
The El Pichao shear zone
REE Chondrite (Boyton, 1984)
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
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
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
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)
El Pichao shear zone: key points
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.
Disaggregated pegmatite dykeDisaggregated pegmatite dyke
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.
x = 091/39n = 46
x = 073/41n = 36
x = 092/39n = 28
Felsic volcaniclastic rock
Peritectic minerals in Tolombncomplex migmatites
Breccia and pseudotachylyte
Boundary between Opx-present and Opx-absent migmatites
Agua del Sapo complex
49x = 083/41n = 73
1 km thic
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
Tres Arbolesshear zone
64 W68 W
The Sierras Pampeanas mobile belt
El Tigreshear zone
La Chilcashear zoneLa Chilca
Ruinas de Quilmes
66 00'66 20'
Agua del SapoComplex
EL PICHAOSHEAR ZONE
The Sierra de Quilmes
Fig. 13. Mica-, and feldspar- fish, and garnet with Fsp strain shadows indi-ca