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Description
The Emigrant Gap composite pluton (EGCP) intruded several Paleozoic and Mesozoic units in the northern Sierra Nevada. Detailed mapping and a series of cross-sections constructed across the pluton-wall rock interface along the ~15 km NW wall of the EGCP allows a better understanding of the 3-D geometry as a result of the emplacement of the pluton. Over ~1 km of exposed relief is present along the NW wall of the pluton in which a well developed dynamothermal contact metamorphic aureole with complex geochemical and structural characteristics vary with position and elevation. The EGCP consists of an ~168 Ma quartz monzodioritic unit, undated two-pyroxene gabbronorite and weherlitic units, an ~164 Ma two-pyroxene diorite and an ~163 ± 7 Ma hornblendebiotite granodiorite unit which comprises the greatest volume of the pluton. New U-Pb zircon data from the hornblende-biotite granodiorite suggest this phase may be as young as ~161 Ma. Towards the south, the two-pyroxene diorite is in contact with the Shoo Fly Complex. Structural and cross-sectional data show that Paleozoic structures within the Shoo Fly Complex are deflected from NW strikes outside the aureole to NE strikes parallel to the ~70° NW dipping pluton-wall rock contact. In going from the outer reaches of the aureole to the pluton-wall rock contact, NW striking tabular beds containing disjunctive cleavage are progressively transformed into beds that pinch and swell, exhibiting a pervasive NE striking and NW dipping schistosity. Adjacent to the pluton, a corrugation lineation pitches moderately to the SW within the steeply NW dipping schistosity, and sillimanite and K-spar are relatively common, as are sheath and refolded folds. Estimated pressures are less than ~2.7 kbar. Tracing the western pluton-wall rock contact ~15 km to the NE, up the NW dip of the western wall of the pluton, ductile strains within the wall rock system appear to die out at English Mountain, where wall rocks of the Early to Middle Jurassic Sailor Canyon Formation and Middle Jurassic Tuttle Lake Formation are in contact with the hornblende granodiorite unit. In this area, tabular planar bedding and domainal cleavage, both outside and inside the aureole, strike ~NW and dip ~45°E and are sharply transected by the hornblende granodiorite unit. At English Mountain the pluton-wall rock contact forms a relatively flat surface that may represent the shoulder of the EGCP. Thus, response of the wall rocks to pluton emplacement varies in a spatial sense, a relationship that is consistent with the idea that multiple MTP's must have operated during emplacement of the EGCP.
