Description
The Emigrant Gap composite pluton (EGCP), northern Sierra Nevada, California is subdivided into quartz monzonite, two-pyroxene, and hornblende granodiorite units. The latter two units make up the bulk of the pluton, and based on U-Pb zircon data are about 164.0 ± 1.5 Ma. Field relationships indicate that the quartz monzonite unit is the oldest phase of the EGCP. Using the Streckeisen - IGUS classification scheme, the mean of eighteen point counted samples is a quartz monzonite. Geochemical analysis indicates that the quartz monzonite unit is geochemically unlike the two younger units in terms of K20, Rb, and Sr values. In contrast, silica variation diagrams show little differences in other major and trace elements for the quartz monzonite and the younger two units. Analyzed samples of the quartz monzonite unit are metaluminus, and when plotted on magmatic setting discrimination diagrams they plot in the vocanic arc and vocanic arc + syn-collisional fields. These results suggest that the quartz monzonite unit is a member of the I-type plutonic suite. U-Pb zircon work indicates that one of three analyzed fractions is concordant at 168.0 ± 2.0 Ma. Thus, the quartz monzonite unit is the oldest dated Mesozoic intrusive body in the Sierra Nevada, east of the Melones fault zones at about the latitude of Lake Tahoe.
