SILLIMANITE GRADE SYN-KINEMATIC METAMORPHISM IN THE CORDILLERAN INTRA-ARC ASSOCIATED WITH A MESOZOIC PLUTON, EASTERN SILURIAN HILLS, CA

The Silurian Hills, located ~20 miles north of Baker, CA, at the physiographic confluence of the Sevier Fold and Thrust Belt and the coeval magmatic arc, preserve structural and geometric relationships between a cogenetic suite of Mesozoic plutons and metamorphosed Pahrump Group, Ediacaran and early Cambrian sediments. A south-dipping ductile shear zone has previously been interpreted as a ductile back thrust fault (Riggs Thrust) that separates the Mesozoic plutonic complex and highly strained marbles (Riggs Fm) in the hanging wall from domed greenschist to amphibolite grade metasediments in the footwall. New detailed mapping indicates that the shear zone in the Silurian Hills broadly parallels the margin of the heterogeneously strained Owl Canyon Quartz Diorite which has a U-Pb zircon date of ~101 Ma. The shear zone fabric and quartz diorite are cut by undeformed aplite dikes dated to ~98 Ma, providing a minimum timing of shear zone movement. These and other ages we present indicate that magmatism in the eastern Silurian Hills was contemporaneous with the early stages of Teutonia Batholith emplacement nearby. Importantly, our mapping suggests that the hanging wall marbles of the Riggs Fm appear to be stratigraphically continuous with the underlying Neoproterozoic to Cambrian rocks in multiple places, encouraging their correlation with the Cambrian Bonanza King Fm and eliminating the need for a thrust contact. Pervasive grain boundary migration in mylonitic quartzites and abundant syn-kinematic sill + bt + st in pelites within the shear zone footwall suggest that deformation and metamorphism took place at temperatures > 500°C. Due to the doming of stratigraphy in the footwall, units can be traced outside of the shear zone where they host greenschist grade assemblages, preserve original detrital textures and are largely unstrained, necessitating the involvement of an external heat source in deformation and metamorphism. Taken together, these data suggest that the Riggs Thrust represents the deformational aureole of the Owl Canyon Quartz Diorite rather than a back thrust in the Sevier hinterland, demonstrating the caution that must be used when interpreting structures in intra-arc settings and emphasizing the need for a detailed examination of thrusts within the Clark Mountain Thrust Complex ~35 km east which bear similarity to the shear zone of the Silurian Hills.