MINERALOGIC STUDY OF COASTAL BELT FRANCISCAN FELDSPATHIC SANDSTONES, NW CALIFORNIA: DETRITAL ZIRCON U-PB AGES

Oceanward from the Klamath Mountains and adjacent high Coast Ranges of NW California, the Franciscan Complex consists of a stack of east-rooting thrust sheets of the Central and Coastal belts, but also contains small Eastern Belt slivers. Avoiding veined rocks, we studied medium-fine-grained Franciscan graywackes from 41 Coastal Belt localities in the Cape Mendocino-Garberville-Covelo 30' x 60' quadrangles. These K-feldspar-rich sandstones exhibit clear detrital features and are only feebly recrystallized. We performed electron microprobe analyses on newly formed indicator minerals from nine Coastal Belt rocks. Scattered neoblastic prehnite is present in several Coastal terrane sandstones, and traces of possible pumpellyite (?) too fine-grained for microprobe analysis occur in three Yager terrane clastic strata; mm-sized domains in Coastal Belt rocks did not achieve chemical or textural equilibrium. Zeolites were sought but not identified in the matrix of these rocks. Coastal Belt mineral assemblages suggest maximum burial depths of 5-8 km and temperatures of 50-135 °C. Prograde P-T trajectories were ~25-30°C/km. The Coastal Belt accretionary prism evidently was stranded along the North American margin without undergoing appreciable subduction, probably during Tertiary unroofing of older, moderately to deeply subducted Franciscan units lying to the east in dextrally offset, east-vergent crustal wedges. Detrital grains of magmatic zircons separated from three Coastal terrane and three Yager terrane graywackes yielded maximum LA-ICPMS depositional U-Pb ages ranging from ~35-54 Ma, and ~54-57 Ma respectively. Similar zircon-based sedimentation U-Pb ages typify a Coastal Belt Franciscan sandstone in the northern Covelo quadrangle (60 Ma, Blake and Ireland, unpublished) and another at San Bruno Mountain, San Francisco Bay area (52 Ma, Snow et al., 2010). Calcalkaline arc volcanism-plutonism shut down in the Klamath-Sierran system by ~85 Ma, so the source of these young zircons must lie elsewhere along the margin—and except for the distinctly younger 35 Ma zircons from one sample, perhaps from the Idaho Batholith and/or the Challis Volcanic Field of the Pacific Northwest.