INVESTIGATING THE CONTACT AUREOLE AND EMPLACEMENT MECHANISMS OF AN UPPER CRUSTAL ULTRAMAFIC PLUTON NEAR EMIGRANT GAP, CALIFORNIA

The emplacement mechanisms of ultramafic plutons into the upper continental crust are puzzling, as the initial composition (mafic vs ultramafic) of the intruding magma is debated and the way in which the upper crust responds to the intrusion of hot mafic/ultramafic magma is unknown. The Emigrant Gap Complex (EGC) is a Middle Jurassic intermediate-ultramafic intrusive complex located in the northern Sierra Nevada that intruded the Shoo Fly complex and overlying Paleozoic sediments and Triassic-Jurassic arc rocks. It has an aureole of increasing temperature and deformation, starting from regional slaty cleavage and culminating in a mylonitic shear zone between country rock and pyroxenite. We have conducted several transects of the aureole, using field mapping, microstructural analysis, Electron Backscatter Diffraction (EBSD), Crystallographic Vorticity Analysis (CVA), and thermobarometry to investigate the emplacement conditions and mechanisms of the EGC. Thermobarometry indicates the EGC intruded at ~1100 ℃, resulting in a very high temperature near the contact. This matches with quartz c-axis distributions from the mylonite showing the presence of prism <c> slip, indicative of deformation at >700 ℃, along with grain boundary migration in quartz and the dynamic recrystallization of plagioclase. Farther away from the contact, deformation drops off to regional slaty cleavage formed at ~300 ℃. Thus, the EGC contact aureole preserves a temperature gradient, from regional low temperature deformation far from the contact to high temperature (> 700 ℃) deformation and mylonitization near the contact with hot mafic/ultramafic magma. This gradient enables us to investigate the development of high temperature microstructures and how the upper continental crust accommodates the intrusion of ultramafic plutons.