COMPOSITIONAL VARIATION OF THE CRETACEOUS BIG BEAR LAKE INTRUSIVE SUITE, CENTRAL TRANSVERSE RANGES: PETROGRAPHIC AND WHOLE ROCK GEOCHEMICAL ANALYSES

The Big Bear Lake Intrusive Suite (BLIS) is a Late Cretaceous granitic intrusive suite in the Transverse Ranges, with interest in this suite stemming from its concentric zonation and large size (~440 km²). Research on the zonation of the Tuolumne Intrusive Suite (TIS) has yielded considerable insight into the construction of large zoned intrusive suites, suggesting many of these suites are formed episodically. This study employed petrographic, whole rock geochemical, and color index analysis on twenty-five BLIS samples across the main intrusive mass and satellite plutons, utilizing the TIS as a comparison site for the geochemical analyses. R (version 4.3.2) was used for visualization of the geochemical data, and the StraboTools app was used for color index analysis. The BLIS samples examined are magnesian calc-alkaline I-type granitoids that range from metaluminous to peraluminous. Marginal samples are metaluminous granodiorites and quartz diorites with color indices ranging from 9-23%, defined by titanite+hornblende+biotite assemblages and relative enrichment in strontium. Interior samples are peraluminous granites with color indices ranging from 7-9%, defined by biotite +/- muscovite assemblages and relative enrichment in rubidium and barium. Accessory minerals such as zircon and allanite were observed in most samples but primarily limited to granite samples. Mineral alteration was frequently observed, primarily manifesting as myrmekite and chloritization of biotite; evidence of mechanical deformation was also frequent, appearing as fractures, deformation twinning, and undulatory extinction. With respect to whole rock geochemistry the TIS and BLIS are nearly indistinguishable; the TIS is slightly less aluminous and more alkaline. Current literature on the TIS and similar sites suggest that large intrusive suites are unlikely to have been comagmatic due to age constraints on rates of cooling and heat sources. The compositional similarities between the BLIS and TIS allow us to focus on the zircon geochronology of the BLIS to better understand how such similar intrusive suites differed in construction.