CRETACEOUS DEPOSITIONAL AGES OF BLUESCHIST FACIES METAGRAYWACKE, TIBURON PENINSULA, CA: IMPLICATIONS FOR EVOLUTION OF THE FRANCISCAN COMPLEX IN THE SAN FRANCISCO BAY AREA

The Franciscan Complex of coastal California records an intricate history of subduction-accretion along the ancestral western North American continent. However, understanding the evolution of the Franciscan is hindered by a lack of reliable fossil and metamorphic ages. We have determined maximum depositional ages (²⁰⁶Pb/²³⁸U zircon, LA-MC-ICPMS) of 107.1 ± 1.8 Ma and 109.1 ± 2.0 Ma (2 σ) for blueschist facies metagraywackes from the Tiburon Peninsula on the west side of San Francisco Bay.

The two samples come from the structurally highest unit that underlies a slab of peridotite and serpentinite matrix mélange and overlies prehnite-pumpellyite facies metagreywackes (MDA ~94 Ma; Bero, 2014; Wakabayashi, 2015). Until now, the only direct age constraint on the Tiburon samples was a single Early Cretaceous (134-145 Ma) macrofossil in a rock found as float (Buchia keyserlingi; Dudley, 1967). Our new ages imply redeposition of the fossil and constrain the emplacement age of the overlying peridotite to ≤ 107 Ma. Wakabayashi (1992) assigned these metagreywackes to the Angel Island nappe, the highest nappe in a succession of downward younging nappes, which he correlated with rocks on the east side of the Bay at El Cerrito. Our data strongly support this correlation based on zircon age distribution, lawsonite-blueschist metamorphic grade, and structural position.

The zircon populations of the Tiburon samples and those at El Cerrito (Snow et al. 2010) show several similarities: (1) the youngest zircon at both localities is ~101 Ma; (2) maximum depositional ages are ~102 and 107 Ma for El Cerrito and Tiburon, respectively; (3) probability density distributions show coincident peaks at 110-120 Ma and 140 Ma at each locality; and (4) both zircon populations have near identical age ranges between 100-170 Ma with a few Precambrian grains (~1100-2500 Ma) and are consistent with a Sierra-Klamath source region. Furthermore, the Tiburon and El Cerrito metagraywackes each contain jadeite and lawsonite, are overlain by serpentinized peridotite and serpentine matrix mélange with exotic high-grade blocks and overlie younger, prehnite-pumpellyite grade metagreywackes.

These similarities strongly support the suggestion that the El Cerrito and Tiburon rocks belong to the same nappe unit and that the nappes are regionally extensive.