DETRITAL ZIRCON GEOCHRONOLOGY OF GREYWACKE BLOCKS WITHIN THE FRANCISCAN MÉLANGE AT SAN SIMEON, CALIFORNIA: DEPOSITIONAL AGE AND PROVENANCE IMPLICATIONS

Franciscan mélange contains tectonically boudinaged and mixed blocks of greywacke, greenstone, chert, graphite schist, and blueschist immersed in a shale matrix. Based on rare fossil occurrences, Franciscan greywackes range from late Mesozoic to early Cenozoic. Deposited in a subduction setting, these greywackes are likely to contain a population of syndepositional volcanogenic zircons. Thus, the youngest zircons provide a maximum age of greywacke deposition, and the age distribution provides information on source terranes. Isoprobe LA-MC-ICPMS technology enables the rapid dating of single zircon crystals allowing a statistically sound determination of maximum depositional age and provenance. In order to fully represent the detrital zircon population of each sample, 150 zircons were selected for analysis based upon proportionate numbers of each morphology and color group. 2100 zircons were analyzed from 13 greywacke blocks collected along a 3 km transect of mélange exposed along the coast of California near San Simeon. 34 zircons from 11 samples have ages between 77 to 90 Ma. The protolith of this part of the mélange was deposited in the Late Cretaceous. The age distribution of concordant grains from each of these samples is similar; with 40-70 % Franciscan age (77-155 Ma) zircons and 20-55 % of pre-Franciscan Mesozoic zircons. The remainder of the sample age distribution typically consisted of 15 to 30% Paleozoic and Precambrian age zircons, except for two samples that contained 30% Paleozoic zircons. A similar distribution of ages was found in the four northwest groups of nine samples in Pelona-type schists of southern California and southwesternmost Arizona (Grove et al., 2003). The protolith of these rocks is also believed to be sediment deposited in the trench outboard of southern California. Proximal forearc basin deposits of Late Cretaceous age are present on the Salinian Block (Barbeau et al., 2005). Detrital zircons from these strata have minimum ages of 78-90 Ma, in agreement with Campanian fossil ages. However, two of the three samples contain abundant grains with ages >1000 Ma.This difference could indicate that source area supplying the proximal part of the forearc basin was areally restricted and included exposures of Precambrian basement.