DETRITAL ZIRCON GEOCHRONOLOGY OF COHERENT JADEITIC PYROXENE-BEARING ROCKS OF THE FRANCISCAN COMPLEX, PACHECO PASS, CALIFORNIA: IMPLICATIONS FOR UNROOFING

The Diablo Range in Central California is a "tectonic window" into the Franciscan accretionary complex, where it is exposed in the core of an anticlinorium surrounded by upturned strata of the Great Valley forearc basin. The area near Pacheco Pass, located along Highway 152, is the world's largest coherently bedded jadeitic pyroxene-bearing terrane. Peak metamorphic temperatures were 150-250ºC at pressures of ~8 kbar. These rocks were subducted to depths of 25-30 km and underplated to the base of the North American plate. Franciscan rocks were deposited during the Late Jurassic to Tertiary. However, no fossils exist in the Pacheco Pass area, and local age constraints are restricted to K-Ar ages of 115 and 122 Ma from one mafic block, and a whole rock age of 83 Ma for a coherent metagraywacke. These data have been interpreted as minimum ages for subduction and metamorphism of these Franciscan rocks. Using U-Pb detrital zircon geochronology by Isoprobe LA-MC-ICPMS, 150 grains have been dated from each of 12 samples distributed over an 8 km2 area. Using the youngest zircons to delimit the maximum age of deposition, 16 zircons from 5 samples have ages between 90-100 Ma. As a syndepositional volcanic component is expected in this environment, a depositional age of ca. 95 Ma (Cenomanian) is proposed. Thus, the older K-Ar ages must be from an exotic block in mélange. Approximately 90% of the zircon data fall between 85 to 250 Ma. These zircons were derived from the Sierran Arc (85-155 Ma) and other previously active Early Mesozoic arcs along the western margin of North America (170-250 Ma). Comparison of detrital zircon populations between Pacheco Pass and the Cretaceous Great Valley Group (DeGraaff-Surpless et al., 2002), show marked similarities. However, a Late Paleozoic-Early Mesozoic component (~5% of the data), present at Pacheco Pass, is sparse in the Great Valley. Apatite fission track data were also obtained for five of the samples from across the field area, and they yield pooled ages ranging between 54 to 68 Ma. This indicates at ca. 60 Ma these rocks cooled though ~110ºC, corresponding to a depth of ~15 km for a thermal gradient of ~8ºC/km. Given a 25 km depth of underplating, the rates of unroofing from 95 Ma to ~60 Ma and from ~60 Ma to ~10 Ma, are estimated to be the same at ~300m/my. These data are consistent with a model of slow synsubduction unroofing.