285-8 Assimilation, Mixing, and Tholeiitic Versus Calc-Alkaline Trends: Insights From the Emigrant Gap Composite Pluton, California

Abstract:

The Jurassic Emigrant Gap ultramafic to granodioritic composite pluton in the northern Sierra Nevada batholith, California, records tholeiitic crystallization-differentiation of H₂O-poor near-primary arc basalts followed by open-system hybridization and crustal assimilation that generated calc-alkaline diorite to granodiorite. We integrate field relationships, petrography, mineral chemistry, and zircon U-Pb-O-Hf isotopic data to understand the details of assimilation and magma mixing that produced seemingly contrasting rock suites in a single intrusive complex. Zircon oxygen isotope ratios were measured by secondary ion mass spectrometry, followed by U-Pb geochronology and Hf isotope analysis via laser ablation multi-collector mass spectrometry within the same core and rim domains as SIMS spots in zircon from gabbronorite, two-pyroxene diorite, and granodiorite. The tholeiitic gabbronorite crystallized at 165.8±3.3 Ma and contains zircon with depleted mantle-like d¹⁸O_VSMOW = 4.7 to 5.8‰ and eHf_t = +11.1 to +15.2. This mantle-derived mafic intrusion, which crystallized from a basaltic andesitic magma, then hybridized with a separately derived granodioritic magma, producing heterogeneous two-pyroxene diorite. Mixing models suggest approximately equal proportions of granodioritic and gabbronoritic endmembers to produce the hybrid diorite. During hybridization, orthopyroxene from the gabbronorite reacted to form framboidal orthopyroxene clusters surrounded by reaction coronas. The granodiorite contains metasedimentary xenoliths and zircon populations consistent with significant crustal source materials. Jurassic autocrystic zircon from the granodiorite yield ages of 164.3 to 168 Ma with d¹⁸O_VSMOW values of 5.4 to 9.1 and eHf_t values of –36.8 to +16.9. The granodiorite also hosts inherited zircon with concordant older Jurassic (~180–200 Ma), Devonian (~365 Ma), and Proterozoic (~1200–1895 Ma) ages. Locally derived xenoliths indicate some incorporation of metasedimentary rocks at the emplacement level, as prominently exposed along the margins of the granodiorite. However, additional assimilation could have occurred at greater depths—further study is required to clarify the relative importance of shallow contamination versus partial melting of deep crustal sources. Together, these observations show how crystallization-differentiation combined with magma mixing and assimilation created Emigrant Gap’s seemingly conflicting evidence for tholeiitic versus calc-alkaline magmatic lineages.

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