CHARACTERIZATION OF QUARTZ DEFORMATION IN THE OROCOPIA SCHIST, NORTHERN PLOMOSA MOUNTAINS USING EBSD AND MICROSTRUCTURAL ANALYSES

In the Plomosa Mountains of west-central Arizona, the Orocopia Schist was subducted beneath and exhumed through the upper plate of a subduction zone back to the surface, undergoing multiple phases of deformation. Deformation microstructures and crystallographic preferred orientations (CPO) of quartz provide information about the temperature at which deformation occurred, and will allow us to test whether the preserved mylonitic fabrics developed during subduction-related peak metamorphism or during Miocene extension and exhumation. Deformation temperature is determined qualitatively with deformation microstructures and electron backscatter diffraction (EBSD), and, where possible, quantitatively determine temperature using c-axis opening angles. We documented shear sense indicators and identified the weakest interconnected phase and quartz deformation recrystallization mechanisms. Microstructural analyses show most samples record subgrain rotation (SGR) and grain boundary migration (GBM) deformation mechanisms, although some samples close to the Plomosa detachment fault record bulge nucleation (BLG). In samples with abundant biotite that defines foliation planes, quartz typically shows more evidence for GBM; where mica is less abundant, quartz typically shows more SGR. EBSD data was collected from quartz-rich areas of thin sections to determine the distribution of quartz CPO’s. Samples with SGR microstructures reveal a strong Y point maxima, representing dominantly prism <a> slip at upper greenschist facies conditions (400–500ºC). CPO within GBM samples mostly indicate rhomb and basal <a> slip at amphibolite facies conditions (500–600ºC). Albite is part of the prograde metamorphic assemblage in the Orocopia Schist, and therefore graphitic inclusions within albite analyzed by Raman spectroscopy of carbonaceous material (RSCM) record upper amphibolite peak metamorphic temperatures of 550–600 ºC during subduction and underplating. Quartz CPO patterns are similar to patterns documented in other metamorphic core complexes of the Lower Colorado River Extensional corridor, and therefore the lower temperatures recorded by quartz deformation mechanisms most likely indicate deformation was related to Miocene extension and exhumation following peak metamorphism.