The ≥6-KM cuesta de Randolfo Mylonite zone in ordovician Famatinian peraluminous granites, NW Argentina: Strain-localization as a function of protolith composition

Abstract

The Cuesta de Randolfo mylonite zone (CRMZ) in the southern Puna of Argentina comprises 26 km of moderately to steeply dipping proto- to ultramylonites developed in peraluminous intrusive rocks of the Famatinian arc. From youngest to oldest, these include a tourmaline-bearing granite, K-feldspar-biotite granite, and biotite-plagioclase granodiorite. Based on whole rock chemistry, the former is identified as a distinct, more evolved and particularly silica-rich intrusive phase. LA-ICP-MS U-Pb zircon ages indicate that the intrusive units crystallized during two periods of Famatinian arc magmatism (~484 and ~471 Ma). Distinctive solid-state deformation microstructures and temperatures characterize four structural zones, revealing strain localization governed by protolith mineralogy during cooling. Early deformation by quartz subgrain-rotation recrystallization and minor K-feldspar bulging at ~500–450 °C resulted in moderate strain distributed across the width of the youngest, quartz-rich intrusive unit (the tourmaline-bearing granite), forming the Western Distributed Zone. With cooling, strain localized into reverse-sense shear zones active at 450–400 °C, forming the Western and Eastern Shear Zones along the contacts between the youngest and the older intrusive units. Lastly, narrow ultramylonites developed along the contacts between all intrusive units and along country-rock rafts, in particular in the Eastern Domain. Deformation at ~400–280 °C was driven by plagioclase-reaction weakening in the feldspar-rich older intrusive units. Progressive CRMZ deformation therefore transitioned from broadly- and homogeneously-distributed at moderate temperatures—enabled by the abundance and weakness of quartz of the youngest units—to localized at low temperatures, driven by reaction softening of plagioclase in the older units. We conclude that the composition of intrusive units (silicic and peraluminous) has a first-order control on strain localization. The CRMZ is part of a larger N-S striking network of wide ductile shear zones deforming peraluminous Famatinian-aged igneous rocks. Their similar composition likely caused the development of wide shear zones in the eastern part of the Famatinian orogen.