SIGNIFICANCE OF U-PB ZIRCON AGES FROM THE PESCADERO AND CAMBRIA FELSITES, WEST-CENTRAL CALIFORNIA COAST RANGES

Forty-three oscillatory-zoned zircon crystals from three samples of weakly metamorphosed Pescadero Felsite were analyzed for U-Pb isotopic concentrations, 17 by SHRIMP-RG and 26 by LA-ICPMS. Thirty-three zircons gave late Mesozoic U-Pb ages, ranging from nominal individual values of 81-167 Ma; the other 10 have pre-Mesozoic, chiefly Proterozoic ages. The youngest zircon population (four grains) yields a maximum possible igneous age for the felsite of ~86-90 Ma. Reflecting the broad age scatter, the rest (perhaps all) are interpreted as xenocrysts. The Cambria Felsite is petrographically somewhat similar to the Pescadero Felsite, so zircons were separated and analyzed from two specimens, yielding unimodal U-Pb ages of 27 Ma. Clearly the late Oligocene Cambria unit has a different age and origin than the Late Cretaceous Pescadero Felsite. Relative-probability curves for the Mesozoic and pre-Mesozoic Pescadero zircons compare reasonably well with more abundant U-Pb age data for detrital zircons from Franciscan metagraywackes located ~100 km to the east in the Diablo Range, although peak ages differ slightly. Pescadero zircon ages are also similar to those analyzed from San Francisco Bay area Franciscan metasandstones, and from the San Joaquin turbidites of the Great Valley Group. These quartzofeldspathic sections lie inboard beyond several major dextral strike-slip faults, so a close match is not expected. Based on the new U-Pb age data as well as field and petrographic relations, we interpret the poorly exposed contact of the Pescadero Felsite with overlying, sheared metaconglomerate apparently below the base of the Upper Cretaceous Pigeon Point Formation as a low-angle fault, possibly related to basin margin extension. The Pescadero Felsite erupted and was deposited during Late Cretaceous time, was feebly metamorphosed to pumpellyite-prehnite grade, and veined with laumontite during burial, then finally was uplifted, altered and weathered. The melt apparently formed by anatexis of clastic strata that possessed a nearby calcalkaline arc provenance, consistent with—but not definitive of—the late Mesozoic Salinia-Sierra-Mojave volcanic-plutonic belt. We conjecture that this ~86-90 Ma volcanism reflects a previously unrecognized thermal event in the forearc.