A > 4 km wide ductile shear zone in the Cuesta de Randolfo: Late-stage strain localization in the Famatinian orogeny, Argentina

RATSCHBACHER, BARBARA; DEGRANDE, GEOFFREY; LARROVERE, MARIANO A.; PATERSON, SCOTT R.; ALASINO, PABLO H.

Ontario

Congreso; Cordilleran Section - GSA 112th Annual Meeting; 2016

Orogen-scale NW-SE trending ductile shear zones affecting Ordovician Famatinian and older rocks have been identified in thecentral and northwestern region of Argentina. In the Cuesta de Randolfo area, southern Puna of Argentina, we documented in detaila > 4 km wide shear zone. Calc-alkaline, peraluminous, Famatinian arc related coarse-grained biotite ganodiorite, white mica- andtourmaline-rich granite and locally pelitic metasediments form the protoliths to the mylonitic shear zone. Weak to moderatemagmatic fabrics are increasingly overprinted by NW-striking, moderate temperature subsolidus fabrics over a short distance whatis mapped as the edge of the shear zone. Solid-state foliations form a fan-like structure and are in most parts more intensivelydeveloped than steeply plunging mineral lineations, which also show a spread in trends from ~ 85 to 220°. Preliminary thin sectionobservations show that mica and quartz are intensively recrystallized, while feldspar deformation varies along the width of the shearzone from brittle cracking to minor subgrain formation. We thus infer temperatures of > 350 to