U-PB AGE OF THE PESCADERO FELSITE: LATE CRETACEOUS ARC VOLCANISM IN THE WEST-CENTRAL CALIFORNIA COAST RANGES?

Felsite structurally underlying unmetamorphosed Campanian-Maastrichtian Pigeon Point strata near Pescadero, California contains disaggregated sandstone inclusions, minor neoblastic pumpellyite ± prehnite, and later carbonate + laumontite veining. We analyzed 43 oscillatory-zoned zircon crystals from three felsite samples, 17 by SHRIMP-RG, 26 by LA-ICPMS. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81-167 Ma; ten have pre-Mesozoic, chiefly Proterozoic ages. Four youngest Pescadero zircons yield a maximum igneous age of ~86-90 Ma. Relative-probability curves for these Mesozoic and pre-Mesozoic zircons compare reasonably well with U-Pb age data for detrital zircons from Diablo Range + San Francisco Bay area Franciscan trench deposits, San Joaquin Great Valley forearc turbidites, and Upper Cretaceous Nacimiento block Franciscan + Great Valley clastics of the Transverse Ranges. The source region for the felsite is problematic reflecting poorly constrained pre-San Gregorio-Hosgri fault system displacements by breaks within and bounding the Nacimiento block, but restoration of ~150 km San Gregorio-Hosgri dextral offset would place the Pescadero felsite adjacent to the Nacimiento block, which restores to southern California in pre-San Andreas fault system time. Direct correlation with Diablo Range, San Francisco region, and San Joaquin strata seems unlikely, whereas correlation with Nacimiento block Franciscan seems likely. The felsite erupted in Late Cretaceous time, was metamorphosed at depth, then was uplifted, altered and weathered before Pigeon Point deposition. Pescadero volcanism reflects a previously unrecognized ~86-90 Ma felsic igneous event in the accretionary margin resulting from anatexis of clastic strata sourced from a calcalkaline volcanic-plutonic arc. Evidently this arc source, superposed on ophiolitic + Franciscan rocks, resided south of the Sierra Nevada and Salinia, requiring pre-San Andreas juxtaposition with the Salinian block. East of the San Gregorio-Hosgri fault system, arc volcanics of similar Late Cretaceous age, metamorphism and paleogeographic setting are known only south of the Peninsula Ranges, although their absence to the north might be the result of crustal shortening, uplift and/or erosion.