The Emigrant Gap composite pluton, northern Sierra Nevada, California is predominantly composed of a hornblende-biotite grandoiorite unit that grades into a two-pyroxene diorite through granodiorite unit along its margms. Field relationships indicate that a previously mapped unit of so called diorite, located in the southeastern portion of the pluton, is in fact a melanocratic granodiorite. Within the southeastern part of the Emigrant Gap composite pluton, the contact separating the two-pyroxene diorite through granodiorite unit from the hornblende-biotite granodiorite unit is gradational. The melanocratic granodiorite unit exhibits similar field relationships with the hornblende-biotite granodiorite unit. In addition, the mineralogy and melanocratic characteristic of the granodiorite unit closely resembles the granodiorite mapped as part of the two-pyroxene diorite through granodiorite unit. On silica variation diagrams, samples from the granodiorite unit plot in a transitional position between fields defined by specimens from the two-pyroxene through granodiorite and hornblende-biotite granodiorite units. Analyzed samples of the granodiorite unit are metaluminous, and may have been derived, along with the Emgirant Gap composite pluton as a whole, from a mantle or mixed sialic-mantle hybrid source.
