IMPLICATIONS OF NEW GEOCHEMICAL AND FIELD DATA FROM THE PEÑON BLANCO ARC, SIERRA NEVADA FOOTHILLS, CA

Continental growth at the Mesozoic Cordilleran margin primarily occurred via continental arc volcanism, off-scraping of sediments at subduction zones, and arc accretion. New geochemical and structural data from the Peñon Blanco arc complex in the central Sierran Nevada foothills suggest that accretion of oceanic arcs may have played an important role in continental growth during the Early Jurassic-Middle Jurassic.

The ~200 Ma Peñon Blanco arc complex comprises the Jasper Point Formation, Peñon Blanco Formation, and the coeval Don Pedro intrusive complex. The Jasper Point volcanics consist of ~900m of massive to pillowed lavas and 50-100m of depositionally overlying chert. The overlying Peñon Blanco Formation comprises approximately 700m of crystal-lithic basaltic tuff, 1-3.5km of augite-rich volcaniclastics, and up to 3.5km of massive to brecciated flows of augite-phyric basalt. Field relationships indicate that the Don Pedro diorite is intrusive into the Jasper Point and lower Peñon Blanco formations, and is likely coeval with upper Peñon Blanco Formation. The Late Jurassic Mariposa Formation unconformably overlies these rocks, possibly along a previously unrecognized low-angle thrust fault.

New geochemical data reveals that the two volcanic formations are geochemically distinct. The massive-pillowed basalts of the Jasper Point Formation are characterized by MORB-like HFSE and HREE abundances and plot in the MORB fields on Hf-Th-Ta and Cr-Y plots. Basaltic flow breccias and volcanoclastics of the Peñon Blanco Formation have high LILE and LREE abundances, and low HFSE and HREE abundances, suggestive of an arc origin. Limited geochemical and petrographic data from the Don Pedro intrusive complex suggest that there are at least two distinct plutonic suites.

The new field and geochemical data, combined with previous geochronological and mapping studies, suggest that the Peñon Blanco arc complex comprises MORB-chemistry lavas formed at a spreading center, upon which a thick arc sequence was built. The arc was subsequently accreted to the margin during the Jurassic before a regional compression event.