THERMAL AND MAGMATIC EVOLUTION OF A CALC-ALKALIC BATHOLITH RECORDED IN ELEMENTAL AND ISOTOPIC COMPOSITIONS OF SILICATE MINERAL ASSEMBLAGES: BIG BEAR LAKE INTRUSIVE SUITE, SAN BERNARDINO MOUNTAINS, CALIFORNIA

The Big Bear Lake intrusive suite (BLIS) is a ~400 km², zoned calc-alkaline suite composed of marginal granodiorites, interior porphyritic granite, and a central leucogranite, emplaced into Paleoproterozoic basement, sedimentary cover, and older Mesozoic plutons. Five granodiorites and granites yield identical SHRIMP ages of ~78±1 Ma. BLIS is structurally and compositionally reminiscent of slightly older Sierran composite batholiths, but emplacement into thick cratonal crust provides added leverage for characterizing magma sources and melt history. Agreement among thermometers indicates most crystallization occurred at ~750-660°C, consistent with experimental phase relations for hydrous granodiorite and granite. About 85% of 58 plag-hbl pairs from seven granodiorites yield T=725-660°C and P increasing from 3 to 4.7 kb southward, consistent with previous work suggesting the batholith is tilted up to the south. Five samples from these same units yielded euhedral sphene, and 126 analyses yield a very similar T range, ~740-665°C, consistent with textures suggesting early crystallization of ilm+mt+hbl and late crystallization of sphene+qz+hbl. Zircons from both granodiorites and granites yield T=750-640°C, consistent with zircon saturation T of ca. 800-740°C, but rare apparently magmatic cores record T as high as 900°C. The presence of high Ti magmatic cores and abundant premagmatic zircon in all analyzed samples suggests melts of compositions similar to those at the level of emplacement were either never above ~800°C, or experienced transient heating episodes too brief to fully digest older zircons. Time-resolved analyses of zircon εHf supports this observation and illuminates batholith assembly. Proterozoic crustal rocks, preserved as zircons cores (εHf= -29 to -54 at 80 Ma), were assimilated by relatively primitive (hydrous basaltic?) and more evolved melts (εHf= -3 to -19) in the deep crust. Late magmatic zircons have more restricted compositions (εHf= -14 to -19), recording homogenization of diverse individual magmatic batches during ascent and emplacement. Zircon and sphene trace element analyses suggest melt homogenization occurred very late in the crystallization history, consistent with late mineral-melt fractionation generating the map pattern.