A previously unrecognised major orogenic front in Argentina 1 Monash University, School of Geosciences, 3800, Clayton, Victoria, Australia ([email protected]; [email protected]) 2 Universidad Nacional de Salta, Buenos Aires 177, 4400 Salta, Argentina Melanie Finch 1 , Maria Gabriela Fuentes 2 , Pavlína Hasalová 1 , Raul Becchio 2 , Nicholas Hunter 1 , and Roberto Weinberg 1 Recrystallised Qtz+Fsp+Bt Recrystallised Qtz+Fsp+Bt Recrystallised Qtz+Fsp+Bt Recrystallised Qtz+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 recrystallised Qtz+Kfs Qtz 2 mm Recrystallised Qtz+Kfs+Bt Recrystallised Qtz+Kfs+Bt 10 100 1000 Sample/ REE chondrite 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 1 10 Sample/ Average crust 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) Puncoviscana Formation samples Granitic samples 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' C C S S Pegmatite dyke Disaggregated pegmatite dyke Disaggregated pegmatite dyke Pegmatite dyke Protomylonite Protomylonite 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. ? ? ? ? ? ? ? ? ? No outcrop 37 60 58 40 59 25 40 44 26 41 36 46 34 41 53 48 45 45 60 59 60 45 30 45 47 50 61 64 38 14 45 50 43 38 72 65 48 24 34 78 39 37 23 42 32 32 52 88 29 48 38 80 41 36 45 46 60 44 36 24 38 21 25 46 30 57 31 23 45 20 20 22 14 45 44 33 28 24 Foliation lineation x = 091/39 n = 46 x = 073/41 n = 36 x = 092/39 n = 28 Stretch Felsic volcaniclastic rock Opx Grt-Crd Grt Grt-Crd-Opx-Sil Granite Migmatite Ultramylonite Mylonite Protomylonite Granite Migmatite Granite Migmatite Orthogneiss Granite Peritectic minerals in Tolombón complex migmatites Breccia and pseudotachylyte Grt-pelite Schist Mylonite Ultramylonite Boundary between Opx- present and Opx-absent migmatites Ultramylonitic core Managua river Anchillo river Tolombón complex 500 metres N Agua del Sapo complex Tolombón complex Tolombón complex 79 41 29 No outcrop Stretch lineation High strain zone 3.5 km thick Foliation 45 45 46 49 x = 083/41 n = 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 ARGENTINA La Rioja Tucumán Tres Arboles shear zone CHILE 64° W 68° W 28º S High-grade Banded schist Puncoviscana Formation Quilmes Fiambalá Ambato San Luis Aconquija San Juan Cafayate Altautina Jujuy Cafayate Quilmes Córdoba Ancasti Calchaquies Cumbres Salta Salta 32º S Argentina Chile 500 km Argentina Chile Paraguay Bolivia Bolivia Paraguay Uruguay Uruguay The Sierras Pampeanas mobile belt El Tigre shear zone La Chilca shear zone La Chilca shear zone Precordillera exotic terrane Cordillera frontal Low-grade Colalao del 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 Sapo Complex N EL PICHAO SHEAR ZONE El Divisidero Ovejeria Anchillo 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 Crd Bt Crd Bt Grt Grt NE NE NE NE NE NE NE NE SW SW Antarctica India Australia West Gondwana Pampean and Famatinian orogens Paleo-Pacific Ocean East Gondwana Terra Australis orogen South America Africa NE NE NE NE NE NE NE NE SW SW SW SW SW SW SW SW SW SW SW SW SW SW Opx leucosome Opx leucosome Ultramylonitic core Foliation 1 mm Fig. 14. Crd- fish partially re- placed by Bt and Sil, indicating top- to-SW shear and Qtz ribbon showing subgrain rotation recrystallisation (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. Protomylonite Protomylonite Mylonite Mylonite Ultramylonite Ultramylonite SW SW NE NE NE NE SW SW El Pichao shear zone of western Gondwana Protomylonite Protomylonite Ultramylonite Ultramylonite Ultramylonite Ultramylonite (ii) Kfs a) a) b) b)