CRUSTAL MAGMA SOURCES AND ASSEMBLY OF THE CRETACEOUS BIG BEAR LAKE INTRUSIVE SUITE, CENTRAL TRANSVERSE RANGES: LASER ABLATION SPLIT STREAM ANALYSIS

The Late Cretaceous Big Bear Lake Intrusive Suite is a large (~440 km²) zoned calc-alkalic intrusive suite in the Transverse Ranges, occupying a significant part of the upper crust of a tilted and exhumed crustal column of the California arc. The main intrusive mass of this intrusive suite is composed of metaluminous, sphene+hornblende+biotite granodiorite margins surrounding biotite +/- muscovite interior granites that are metaluminous to weakly peraluminous. Petrologically diverse Proterozoic host rocks provide isotopic leverage to characterize differentiation processes in the upper crust as well as crustal magma sources. In this study we utilized split stream LA-ICPMS to characterize zircons extracted from eleven samples across the intrusive suite. All samples record an array of spot ages, ranging from 75 Ma to 2.3 Ga. Ten of the eleven samples yielded magmatic zircon giving relatively precise, weighted mean Pb/U crystallization ages of 77.5 to 79.6 Ma. Interpreted crystallization ages are broadly younger inward and northeastward and suggest rapid assembly of the suite – age patterns that will be refined by CA-TIMS analysis. All eleven samples of granodiorites and granites also yielded premagmatic zircon defining two older age populations, Mesozoic (85-240 Ma) and Paleoproterozoic (1.67-1.78 Ga). Premagmatic zircons with moderate to high Th/U and low Gd/Yb are virtually identical in marginal granodiorites and interior granites, implicating the same crustal sources. The abundance of Paleoproterozoic premagmatic zircons and the paucity of distinctive Mesoproterozoic zircon age populations make magma sources unlikely in either subducted/underplated metasediments or in upper crustal, Neoproterozoic and younger metasedimentary rocks. These observations support a model wherein mantle-derived magma interacted with a chronologically diverse, hybrid lower crustal source region, formed in the Paleoproterozoic and intruded and/or underplated by earlier Mesozoic arc igneous rocks.