WHOLE-ROCK GEOCHEMISTRY, PETROGRAPHY AND PRELIMINARY U-PB ZIRCON ANALYSIS OF OLIGOCENE INTRUSIVES IN THE EASTERN SAN GABRIEL MOUNTAINS

We have resurrected a study of mid Tertiary silicic intrusives (e.g. McCulloh et al. 2001) that, in a pre-San Andreas fault reconstruction, occupy a broad region of the eastern San Gabriel Mountains and Pomona Valley coinciding with latitude of the 29 Ma collision between the East Pacific ridge and Farallon trench. Host rocks include Proterozoic gneiss, Jurassic and Cretaceous plutonic rocks, Pelona schist, and mylonite of the Vincent thrust. Petrographic analysis reveals that the Telegraph Peak intrusive is a biotite granodiorite and related quartz-porphyry dikes and Mountain Meadows Dacite of Shelton (1955) are biotite rhyodacites to biotite quartz latite. Preliminary whole rock XRF analysis of 3 samples with an additional 17 samples from an earlier study (Nourse et al. 1998) range in composition from rhyolite to dacite according to Le Bas TAS classification scheme. The Telegraph Peak intrusive and quartz-porphyry dikes belong to the calc-alkaline magma series and to the medium to high- K₂O series. Molar Na₂O+Al₂O₃ +K₂O plots show a metaluminous to weakly peraluminous character and their alumina saturation index varies from 0.98-1.15. We separated zircons from 7 samples for SHRIMP-RG analysis. Preliminary results show that Oligocene ages (46 spots) correspond to high-U zircon overgrowths on xenocrystic cores (43 grains) that exhibit a wide range of Proterozoic and Mesozoic ages. A dike from lower San Antonio Canyon yielded a coherent weighted mean ²⁰⁶Pb/²³⁸U age of 27.9±0.4 Ma (N=10). Several other samples including the Telegraph Peak granodiorite and Mountain Meadows Dacite yielded a wider age range of 32-22 Ma that may record effects of inheritance as well as lead loss associated with mid-Miocene mafic-intermediate intrusions. A composite weighted mean of 36 ages is 27.5±0.7 Ma. These samples are targets for future depth profiling studies to better constrain the young rim ages. The Oligocene intrusive suite represents an important crustal melting event possibly associated with a slab window that formed near the focal site of migrating triple junctions during inception of the Pacific-North American transform plate boundary.