CHARACTERIZATION AND MICROSTRUCTURAL ANALYSIS OF A SHEAR ZONE IN COLALAO DEL VALLE, NW ARGENTINA

Sierra de Quilmes in Northern Sierras Pampeanas, NW Argentina is a N-S trending basement block that exposes polydeformed and polymetamorphosed schists, gneisses, migmatites, and pegmatites that record low-Paleozoic tectonic events critical to deciphering the geodynamic evolution of a portion of the paleo-Pacific continental margin of South America. Sierra de Quilmes contains metasediments that have been interpreted as high-grade metamorphic equivalents of the Neoproterozoic to Early Cambrian Puncoviscana Formation. The Puncoviscana Formation consists of a turbiditic sequence of pelitic beds and banded sandstones that were deposited along the western margin of Gondwana on an originally passive margin. The sequence was chevron folded due to subduction along the Gondwana margin during the mid-Cambrian (Pampean Orogeny). In Colalao del Valle, Sierra de Quilmes, we observe a gradual transition from chevron folded pelitic and compositionally banded psammites with relict sedimentary structures to migmatized garnet-orthopyroxene metasediments. Amphibolite-facies migmatites include xenoliths of chevron folded metasediments and layer-parallel leucosomes that have been intruded by lenses of a dark plagioclase-quartz-biotite-rich rock and discordant pegmatitic bodies. All these rocks have been affected by a predominant NW-SE mylonitic foliation, which is concentrated along narrow bands, 10cm to 2m thick. Based on the geometry of kinematic indicators, displacement along the shear zone occurred in two main directions: E over W and N over S. The mylonitic foliation is defined by biotite and sillimanite grains signifying that they formed synkinematically near or during the metamorphic peak. The bulk composition of these rocks and quartz microstructures indicate that the metamorphic peak occurred at temperatures above 600 °C. The temperature of deformation, geometry, and kinematics of the shear zone are consistent with other Ordovician and Silurian shear zones in Northern Sierras Pampeanas, interpreted as related to a late stage of the Famatinian Orogeny. Following shear, a younger episode of brittle deformation (likely Andean) juxtaposed blocks containing chevron folded metasediments with higher-grade metamorphic rocks suggesting the tilting of blocks along E-W trending horizontal axes.