Permian nappe tectonics and high-grade metamorphism related to Gondwanide Orogeny in northern Patagonia terrane.

GONZÁLEZ, PABLO DIEGO; ORIOLO, SEBASTIÁN; SCHULZ, BERNHARD; NAIPAUER, MAXIMILIANO; MARCOS, PAULO; SATO, ANA MARÍA; VARELA, RICARDO; KRAUSE, JOACHIM; CÁBANA, MARÍA CECILIA

Londres

Encuentro; Metamorphic Studies Group Research in Progress Meeting; 2021

The Mineralogical Society

The focus of this contribution is to characterize a sequential order of deformational structures and associated metamorphic conditions of the igneous-metamorphic basement rocks from northern Patagonia terrane (41°28?30?S-65°40?30?W). Based on field mapping, mesoscopic structures are analysed to evaluate progressive and polyphasic deformation, developed under regional high-grade metamorphism.Three continuous deformational phases are recognized. During D1 phase, the formation of overturned tight F1 folds is associated with penetrative axial plane foliation S1. It exhibits P-T conditions close to the baric peak, and likely represents the progressive metamorphic stage associated with syn-kinematic growth of garnet-kyanite porphyroblasts in meta-pelites. D2 phase is recorded as either shear bands S2 transposition or mylonitic foliation, coeval with polymorphic transformation of kyanite to sillimanite owing to thermal peak. A nappe system related to kilometerscale El Jagüelito ductile shear zone characterizes the large-scale D2 structure of the investigated area. D3 phase is distinguished by refolding of the former fabric and formation of either kink-bands or crenulation cleavage S3 of the S1-S2 foliation planes, under retrogressive metamorphic conditions. The nappe system exhibit a top-to-the S/SW tectonic transport, and the El Jagüelito shear zone served as a right lateral ramp in the exhumation process among D2-D3 events.Petrographic analyses of metamorphic mineral assemblages and reaction textures combined with conventional geothermobarometric estimates and a pseudosection records the pressure-temperature conditions of metamorphism among D1-D3 events. In situ electron probe Th‐U‐Pb monazite dating of 272, 260, and 251 Ma constraints the main tectono-metamorphic event to Late Permian Gondwanide Orogeny. New results combined with existing geological and geochronological constraints allow inferring a clockwise P-T-D-time evolutionary path within the kyanite-sillimanite stability fields. Comprehensive regional comparison of the large-scale structural framework is indicative of an ongoing oblique subduction beneath the proto-Andean margin of Gondwana in the NE direction. This continental subduction and exhumation lasted at least 20 million years, nearly the entire Guadalupian-Lopingian series, producing transpression and metamorphic conditions of amphibolite-to-granulite facies, in medium- to high-pressure regimes, although the studied area occur in a back-arc to intracontinental settings. Geotectonic implications also includes a long-lived tectono-metamorphic history of the El Jagüelito shear zone, which was formed during Early Paleozoic and successively reactivated during Permian and Jurassic times.