MAGMATIC EVOLUTION IN THE BIG BEAR LAKE INTRUSIVE SUITE, SAN BERNARDINO MOUNTAINS, CALIFORNIA: CORDILLERAN LITHOSPHERE DYNAMICS RECORDED IN PROVENANCE OF BATHOLITH MAGMAS?

Cretaceous magmatic and structural evolution in California records the transition from frontal Sierra Nevada - Salinia - Penisular Ranges arc magmatism and Sevier retroarc shortening to a migrating Laramide arc and subduction erosion. Early Laramide migratory magmatism is recorded by batholiths emplaced into older Mesozoic arcs. Can we use lithospheric magma source variations recorded in these batholiths to illuminate the dynamics of magma source regions, and gain insight into magmatism and shortening in the deeper lithosphere? We address this question through a study of the Big Bear Lake intrusive suite (BLIS), a ~400 km² upper crustal, compositionally zoned calc-alkaline suite emplaced into Paleoproterozoic basement rocks, sedimentary cover, and older Triassic and Jurassic batholiths. BLIS is composed of marginal granodiorites of Heaps Peak, Angeles Oaks, Hook Creek, and Hanna Flat, interior porphyritic granite of Keller Peak, and central leucogranite of Butler Peak. Oscillatory-zoned magmatic zircon crystallization temperatures are consistent with initial growth <60° above zircon Tsat, and trace element compositions record evolution of an oxidized, calc-alkaline silicate melt similar to that recorded by silicate mineral assemblages. Zircons from five samples yield ages of 78±1 Ma - thus BLIS is structurally and compositionally reminiscent of slightly older composite batholiths of the Sierra Nevada, but was emplaced into thick cratonal crust, providing added geochemical leverage for characterizing magma sources. Texturally and compositionally distinct premagmatic zircon is concordant to ~50% discordant. Paleoproterozoic crystallization ages predominate, yet minor and trace element abundances in these older zircons are distinct – higher Hf/Zr than magmatic zircon, similar Yb/Gd but more subdued Ce anomalies and lower Eu/Eu*, and similar to lower Th/U. About ¼ of premagmatic zircons yield Mesozoic ages and have compositions similar to magmatic zircon, suggesting some Mesozoic hybridization of a largely Paleoproterozoic source. However, this source contrasts with the Paleoproterozoic source of earlier Mesozoic batholiths intruded by BLIS, recording dynamic evolution of the lithospheric source of arc magmas during Mesozoic time.