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

RATSCHBACHER, B.; DEGRANDE, G.; LARROVERE M.A.; PATERSON, S.; ALASINO, P.

Ontario

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

Orogen-scale NW-SE trending ductile shear zones affecting Ordovician Famatinianand older rocks have been identified in the central and northwestern region ofArgentina. In the Cuesta de Randolfo area, southern Puna of Argentina, wedocumented in detail a > 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 moderate magmatic fabrics are increasinglyoverprinted by NW-striking, moderate temperature subsolidus fabrics over a shortdistance what is mapped as the edge of the shear zone. Solid-state foliations form a fan-like structure and are in most parts more intensively developed than steeply plunging mineral lineations, which also show a spread in trends from ~ 85 to 220°. Preliminary thin section observations show that mica and quartz are intensively recrystallized, while feldspar deformation varies along the width of the shear zone from brittle cracking to minor subgrain formation. We thus infer temperatures of 350-550 °C during ductile deformation. The presence of chlorite in veins and compositional banding further indicate fluid flow during retrograde metamorphic conditions. Kinematics in the field show in parts reverse motion, while in thin sections (e.g. porphyroclast-matrix relationship and shear bands), kinematics are variable with pure shear being dominant. Field and thin section work so far indicates that the shear fabrics define a positive flower structure or double-vergent ductile shear zone showing no significant strike-slip motion but recording localized compressional deformation during the closing of the Famatinian orogeny.