Paper No. 17-5
Presentation Time: 3:25 PM
ZIRCON U-PB GEOCHRONOLOGY AND GEOCHEMICAL ANALYSES ELUCIDATE MAGMATIC PROCESSES IN A TILTED CRUSTAL SECTION: LITTLE SAN BERNARDINO MOUNTAINS, CALIFORNIA
In continental arc settings, zones of melt stagnation provide ample time for magmatic processes to occur. Countless models exist depicting the nature of magmatic processing in such settings, but models vary in methods of magmatic differentiation, depth of magma evolution and the existence of large magma bodies in the mid-crust (Annen et al., 2006, Bachmann & Bergantz, 2008, de Silva 2008). The Little San Bernardino Mountains constitute the lower portion of a tilted crustal section, providing continuous exposure of Proterozoic meta-sedimentary and meta-igneous country rock and magmatic intrusions of Cretaceous and Jurassic age, allowing for the observation of magmatic processes in the Cordilleran Arc. Wide Canyon is an approximately 16 km long, north/south trending canyon that crosscuts the tilted crustal section at approximately 18.65 km projected paleodepth (Needy et al., 2009). U-Pb zircon geochronology yields a striking data set of contemporaneous, yet compositionally and geochemically distinct magmas along the canyon. Country rock samples include alkali-feldspar rich, augen orthogneiss, garnetiferous paragneiss and deformed granodiorite, each of which yielding 1700 Ma cores and 1400 Ma rims. Heavy REE depletions in some Proterozoic zircons indicate the timing of garnet growth. Cretaceous samples include granite, granodiorite and amphibole gabbro compositions yielding core and rim ages of approximately 88 ma and 76 ma respectively. Ti in zircon temperatures range from 630 to 811°C, up to 250 degrees lower than temperatures calculated from plagioclase compositions and via hornblende thermobarometry. U-Pb in zircon geochronology, combined with a new mapping transect, elucidate the relative distribution of Cretaceous vs. Proterozoic units in the Little San Bernardino Mountains. With a well-constrained understanding of host rock/pluton relationships, compositional and geochemical variations among contemporaneous magma bodies and their host rocks are the current focus of XRF bulk rock geochemical analysis. XRF data will enable further testing of existing magma processing models through the identification of assimilation and fractional crystallization prior to or at the level of emplacement and the degree in which these processes alter a magmas composition and chemistry.