SHRIMP-BASED DEPOSITIONAL AGES OF FRANCISCAN COMPLEX METAGRAYWACKES, SAN FRANCISCO BAY AREA, WESTERN CALIFORNIA

U/Pb ages were obtained from detrital zircons separated from six metagraywacke samples collected from Franciscan imbricate nappes around San Francisco Bay. All analyzed samples are dominated by Cretaceous-Jurassic aged grains that were likely shed off the Sierra Nevada volcanic-plutonic arc. The youngest sample, at 52 Ma, is from the structurally lowest sheet at San Bruno Mountain. None of the other rocks yielded zircon ages less than 83 Ma. Zircons from the Upper and Lower El Cerrito units are dominated by ~100-160 Ma grains; in addition, the Upper El Cerrito contains several grains in the 1200-1800 Ma interval. These samples are nearly identical to the 98 Ma metasediment from the structurally low Hunters Point shear zone. The Hunters point zircon ages exhibit a broad distribution ranging from 97-200 Ma; only on grain yielded a pre-Mesozoic age. The Novato Quarry sample has a distribution of pre-Mesozoic grains from 1300-1800 Ma, generally similar to that of the Upper El Cerrito unit. However, this sample contains zircons as young as 83 Ma, suggesting it is significantly younger than the Upper and Lower El Cerrito. The Skaggs Spring Schist is clearly the oldest rock, with youngest analyzed grains at ~144 Ma; this sample is unique, and is the only one displaying a significant Paleozoic component.

All six specimens are quartzose (20-60 vol%) metagraywackes containing trace amounts of detrital biotite, epidote, titanite, and opaque minerals. The structurally high Skaggs Spring and Upper El Cerrito rocks recrystallized at high pressures, and contain neoblastic glaucophane, Na-clinopyroxene, phengitic mica, and minor lawsonite and stilpnomelane; they lack clastic albite and rock fragments. The other structurally lower sheets contain abundant albite, variable amounts of white mica, and a wide range of rock fragments including chert, siltstone, shale chips, volcanic rocks, and serpentinite.

These data support previous fossil data and a structural stacking order from highest to lowest of Skaggs Spring, Upper El Cerrito, Lower El Cerrito, Hunters Point shear zone, Novato Quarry, and San Bruno Mountain.