A COMPLEX MIXING ORIGIN FOR A SMALL DIORITE PLUTON IN THE SIERRA NEVADA BATHOLITH, CALIFORNIA: EVIDENCE FROM HAND SAMPLES AND PETROGRAPHY

We present a hand sample and petrographic study of mixing textures common in a small (5-6 sq. km) diorite pluton exposed in the Sierra Nevada batholith of California. The samples for this study were collected from locations throughout the diorite of Lake Sabrina in the central Sierra Nevada. Although the pluton is mapped as a single unit, there are distinct heterogeneities present at hand-sample and microscopic scales. Macroscopic textures include distinct clots of hydrous mafic minerals (chiefly biotite and hornblende) enveloped in a felsic, quartz and feldspar-rich host; as well as distinct zones of felsic minerals bounded by zones of hydrous mafic minerals. The boundaries between these zones can be either sharp or gradational. Titanite ocelli (after Hibbard, 1991) are another common macroscopic sign of magma mixing. Microscopically, textures include quartz oikocrysts; complexly zoned plagioclase; feldspar (and/or quartz) mantled with biotite and/or hornblende; aggregates of partially resorbed hydrous mafics +/- titanite; and the presence of large (2-10 mm) xenocrysts that appear to be resorbed and out of equilibrium with the rest of the rock. Additional microscopic textures of interest include titanite ‘proto-ocelli'; subhedral embayed titanite with (or without) included plagioclase grains; and subhedral to euhedral biotite enclosed by anhedral hornblende.

The spatial heterogeneity of the diorite and the presence of textures described above demonstrate that it was mixed. We conclude that two scenarios are possible: 1) mixing and mingling of multiple pulses of magma or 2) complex convection and incomplete mixing of a magma that was originally inhomogeneous. Based upon thin section textures and evidence from other localities in the batholith, we conclude that the diorite of Lake Sabrina is a composite of several distinct mineralogical compositions that represent multiple mixing events. This interpretation is consistent with the hypothesis that continental crust generation in arc settings occurs via incremental emplacement of multiple pulses of magma, rather than via emplacement of a few large plutons.