CORRELATION OF PRE-BATHOLITHIC MIOGEOCLINAL ROCKS IN THE PENINSULAR RANGES OF SOUTHERN AND BAJA CALIFORNIA: IMPLICATIONS FOR THE PALEOGEOGRAPHY OF THE SOUTHWEST CORDILLERAN MARGIN

The pre-Neogene paleogeography of the Peninsular Ranges Batholith (PRB) is an unresolved issue. The PRB sits outboard of the San Andreas fault, the Nacimiento fault and the hypothesized Mojave - Sonora Megashear, therefore proper reconstructions must take into account movement on these structures as well as the Gulf of California rift zone. Correlation of pre-batholithic, miogeoclinal rocks found along the northeast margin of the PRB (Mt. San Jacinto, near Palm Springs, Ca., and San Felipe, Mexico) to potential in situ North American miogeoclinal equivalents is one way to elucidate this issue. Here we present new detrital zircon U-Pb ages from the Desert Divide Group (DDG) at Mount San Jacinto. The DDG is a 2 km thick, east dipping structural package divided into 2 units: (1) the Bull Canyon Formation, a diverse assemblage comprised of marble, schist and gneiss, and (2) the Ken Quartzite, a 750 m thick package of relatively pure, coarse grained quartzite. New laser ablation ICP-MS detrital zircon U-PB ages from these two units yield similar ages spectra that are dominated by ~2.6 Ga, ~1.8 Ga, ~1.4 Ga and ~1.1 Ga peaks, although the relative proportions of grains varies between samples. The zircon age distribution of these samples agrees well with previous detrital zircon age results from the San Felipe quartzite in Baja California. The age spectra of the Desert Divide and the San Felipe samples in turn correlates nicely to Ordovician miogeoclinal rocks of the Cordilleran margin which are characterized by the distinctive Peace River arch signature. Local source terrains are unlikely for the DDG because the abundant 2.6 Ga zircon grains are rare or absent from Precambrian rocks in Mexico or the southwest US. These results are consistent with Laurentian Proterozoic cratonal or transitional basement beneath the NE edge of the PRB. Pre-Ordovician miogeoclinal units may be present in the Peninsular Ranges but are not yet documented.