THE ≥6-KM CUESTA DE RANDOLFO MYLONITE ZONE IN ORDOVICIAN FAMATINIAN PERALUMINOUS GRANITES, NW ARGENTINA: STRAIN-LOCALIZATION AS A FUNCTION OF PROTOLITH COMPOSITION

RATSCHBACHER, BARBARA C.; CAWOOD, TARRYN; LARROVERE, MARIANO A.; ALASINO, PABLO H.; LUSK, ALEXANDER D.; MEMETI, VALBONE

San luis

Congreso; XVIII Reunión de Tectónica; 2021

Universidad Nacional de San Luis

The Cuesta de Randolfo mylonite zone (CRMZ) in the southern Puna of Argentina comprises ≥6-kmof moderately to steeply dipping proto- to ultramylonites developed in peraluminous intrusive rocks ofthe Famatinian arc. From youngest to oldest, these include a tourmaline-bearing granite, K-feldsparbiotite granite, and biotite-plagioclase granodiorite. Based on whole rock chemistry, the former isidentified as a distinct, more evolved and particularly silica-rich intrusive phase. LA-ICP-MS U-Pb zirconages indicate that the intrusive units crystallized during two periods of Famatinian arc magmatism (∼484and ∼471 Ma). Distinctive solid-state deformation microstructures and temperatures characterize fourstructural zones, revealing strain localization governed by protolith mineralogy during cooling. Earlydeformation by quartz subgrain-rotation recrystallization and minor K-feldspar bulging at ∼500-450 °Cresulted in moderate strain distributed across the width of the youngest, quartz-rich intrusive unit (thetourmaline-bearing granite), forming the Western Distributed Zone. With cooling, strain localized intoreverse-sense shear zones active at 450?400 °C, forming the Western and Eastern Shear Zones along thecontacts between the youngest and the older intrusive units. Lastly, narrow ultramylonites developedalong the contacts between all intrusive units and along country-rock rafts, in particular in the EasternDomain. Deformation at ∼400?280 °C was driven by plagioclase-reaction weakening in the feldspar-richolder intrusive units. Progressive CRMZ deformation therefore transitioned from broadly- andhomogenously-distributed at moderate temperatures?enabled by the abundance and weakness ofquartz of the youngest units?to localized at low temperatures, driven by reaction softening ofplagioclase in the older units. We conclude that the composition of intrusive units (silicic andperaluminous) has a first order control on strain localization. The CRMZ is part of a larger N-S strikingnetwork of wide ductile shear zones deforming peraluminous Famatinian?aged igneous rocks. Theirsimilar composition likely caused the development of wide shear zones in the eastern part of theFamatinian orogen.