NEW GEOCHEMICAL AND GEOCHRONOLOGICAL DATA FROM THE PENON BLANCO ARC AND MARIPOSA FORMATION, SIERRA NEVADA FOOTHILLS, CA

Continental growth at the Mesozoic Cordilleran margin occurred via continental arc volcanism, sediment accretion at subduction zones, and arc accretion. New geochemical data from the Peñon Blanco arc complex suggest that arc accretion played a significant role in continental growth during the Early-Middle Jurassic.

The ~200 Ma Peñon Blanco arc comprises the Jasper Point Formation, Peñon Blanco Formation, and the coeval Don Pedro intrusive complex, which are exposed in the core of the Cotton Creek anticline. The Jasper Point volcanics consist of ~900m of massive to pillowed lavas and up to 50m of depositionally overlying chert and transitional basalts. 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 the upper Peñon Blanco Formation. The Late Jurassic Mariposa Formation comprises nearly 2 km of black shale with minor basal sandstones, and unconformably overlies the Peñon Blanco arc on the east side of the Cotton Creek anticline, and may be in fault contact with the arc on the west side of the anticline.

New geochemical data reveals that the two volcanic formations are geochemically distinct. The massive-pillowed basalts of the Jasper Point Formation are characterized by low LILE and LREE abundances, and moderate HFSE and HREE abundances. Basaltic flow breccias and volcaniclastics of the Peñon Blanco Formation have higher LILE abundances and lower HFSE abundances. Although these formations are geochemically distinct from one another, both are interpreted to represent the remnants of arc volcanism.

New detrital zircon data indicate that (1) The Mariposa Formation received sediment predominantly from the Early-Middle Jurassic arc; (2) detrital zircon distributions in the Mariposa Formation are nearly identical to those from the Great Valley Sequence rocks, indicating source similarities, and (3) the abundance of ~150 Ma zircons suggest that volcano-plutonism was more prevalent than previously suggested at that time.