GEOCHEMICAL, PETROLOGIC, AND GEOCHRONOLOGIC INVESTIGATION OF THE DEL PUERTO CANYON KERATOPHYRE, UPPER COAST RANGE OPHIOLITE, AND IMPLICATIONS FOR GREAT VALLEY FOREARC BASIN DEVELOPMENT, CALIFORNIA

Del Puerto Canyon, ~35 km southeast of San Jose, California, exposes a 5.5 km-thick remnant of the Jurassic Coast Range Ophiolite (CRO). Within the canyon, the Del Puerto keratophyre is a mappable unit interpreted as the upper volcanic section of the CRO. Consisting of feebly to un-metamorphosed andesite and rhyolite, the keratophyre is folded into an anticline, in contact on its flanks with mudstones of the latest Jurassic-earliest Cretaceous Knoxville Formation of the Great Valley forearc basin. Along Del Puerto Canyon Road, mudstones of the basal Knoxville Formation are interbedded with the keratophyre, indicating a depositional contact. Epidote-hosting shear zones found ~ 1 m below the uppermost keratophyre are crosscut by a series of faults that do not penetrate the overlying interbedded volcanic layers and Knoxville Formation. In contrast, a series of sub-horizontal and high-angle faults offset both units. Whole-rock geochemical analyses of the keratophyre show a similarity to continental arc andesites, including a pronounced negative Eu anomaly, and overlap with geochemical data from the Knoxville Formation. At Del Puerto Canyon, mudstones of the Knoxville Formation follow the broad REE trends of the keratophyre, but show an increase in compositional maturity and evidence for sediment recycling. Petrologic analyses reveal unaltered volcanic lathwork grains and polycrystalline quartz in both units, as well as detrital olivine in the Knoxville Formation. The age of the keratophyre is between ~150 and ~146 Ma, based on fission track and Ar-Ar studies on rhyolitic lavas and andesites and new zircon U-Pb maximum depositional ages from the overlying Knoxville Formation (~146-139 Ma). Integrating these datasets, we hypothesize that the top of the Del Puerto Keratophyre may have formed in a forearc position coeval with the onset of deposition of the Knoxville Formation. Prior deformation of the keratophyre may have generated seafloor topographic relief that accommodated sediments eroded from the CRO and proximal Sierra Nevada magmatic arc, marking the onset of forearc basin development as keratophyre volcanism waned.