IN SITU CRYSTALLIZATION DIFFERENTIATION WITHIN A BI-MODAL GRANITE-GABBRO COMPLEX: AN EXAMPLE FROM THE GUADALUPE IGNEOUS COMPLEX, SIERRA NEVADA BATHOLITH, CALIFORNIA

The western Sierra Nevada Batholith (SNB) exposes a bi-modal Jurassic (153 Ma) plutonic suite, the Guadalupe Igneous Complex (GIC), which provides a unique record of granite magma genesis. High-density sampling, field relationships and preliminary isotopic data indicate that some and perhaps all GIC granites were produced in situ, by fractionation of underlying gabbros, with little input from adjacent wall rock or partial melts from the lower or middle crust. GIC gabbros are exposed at the base of the section, as highly mafic sills (MgO = 6-9 wt. %) and fractionated layers, 0.03-3.0 m in thickness; most layers contain abundant hornblende. Granite dikes (70-75% SiO₂; 0.5 – 1.0 m) cut the mafic layers, and point towards the top of the pluton; some dikes have hornblende-rich cumulates at their base, and emanate from gabbro layers lacking hornblende. These felsic dikes are far to small to be far-travelled and so almost certainly formed by in situ fractionation, but very thick anhydrous layers (>3m) attest to the possibility of much larger granitic dikes. High-density sampling verifies a lack of intermediate (56-69% SiO₂) compositions, but mass balance indicates that an intermediate parent (ca. 60% SiO₂) can differentiate to yield an anhydrous layer (53-55% SiO₂) and an expelled granite dike, as in Bachmann and Bergantz (2004). Since all unaltered GIC samples fall on a single Rb-Sr isochron with an age of 152±7 Ma that matches single crystal zircon ages (Ernst et al., 2009) all rock types are allowably co-magmatic. Geochemical relationships also appear to disallow assimilation of upper crust Mariposa Formation or partial melting of older, metabasaltic lower/middle crust. Finally, felsic dikes in the gabbro are compositionally similar to larger granitic masses in the middle and upper portions of the GIC, and mass balance of the pluton as a whole allows all such granites to be generated by fractionation alone. GIC granites likely formed as serial intrusions of granite formed by Bachmann-Bergantz processes operating within the lower gabbroic section, which in turn formed as serial intrusions of mantle-derived gabbroic sills into the upper crust.