RECONCILING NORTH AMERICAN METAMORPHIC CORE COMPLEX FORMATION MODELS WITH COUPLED MICROSTRUCTURAL ANALYSIS AND APATITE U-Pb PETROCHRONOLOGY IN THE CHEMEHUEVI MOUNTAINS, CA, USA

Despite many seminal studies regarding North American Cordilleran metamorphic core complexes (MCC), there has been a lack of agreement behind a model of evolution. Of particular importance for reconciling MCC evolution is the temporal and spatial relationship between the brittle faults and ductile mylonitic shear zones. Previous studies suggest that brittle detachment faulting in MCCs exhumes older unrelated shear zones, preexisting buoyant domes, deep mid-crust distributed shear zones, or the coeval downdip continuation of the detachment fault. The Chemehuevi Metamorphic Core Complex (CHM), located in the Colorado River Extensional Corridor, is composed of a footwall of mylonitized Proterozoic crystalline basement and Jurassic to Cretaceous plutons and a hanging wall of brittlely faulted Miocene sedimentary and volcanic rocks. Coupled U-Pb petrochronology, microstructural analyses, and crystallographic vorticity axes (CVA) will help constrain the P-T-t evolution of ductile deformation in exhumed footwall rocks in CHM. Field observations and preliminary EBSD data show dominantly top-to-the-NE shearing in these mylonites, consistent with detachment fault kinematics. Quartz and feldspar microstructures are representative of deformation temperatures from 450-550°C. Apatite and titanite in the mylonitic rocks show evidence of crystal plastic deformation that is characterized by low angle boundaries (~5°) consistent with deformation by dislocation creep, and thus may record the age of ductile deformation. The CVA analyses can directly link the accessory-phase petrochronometers to the bulk fabric forming phases, and in-progress U-Pb analyses are expected to help differentiate differing models for MCC evolution. For example, pre-Miocene deformation ages would reveal that the mylonites are older than regional extension, whereas Miocene-or-younger ages suggest coupled mylonitization and detachment faulting during regional extension. We will compare mylonitization ages with previous low-T data in the footwall to understand timing of magmatism, extension, and faulting to better constrain MCC formation.