Geology and geochemistry of a garnet-bearing granitoid in the Southwestern Sierra Nevada, Tulare County, California

The garnet-bearing Tharps Peak granodiorite (TPG), located along the southwestern margin of the Sierra Nevada batholith (SNB), is the most chemically evolved pluton in a suite of intrusions with typical Sierran chemical characteristics. A genetic relationship between the peraluminous TPG and the surrounding metaluminous granitoids (MG) of the batholith is suggested by (1) continuity of chemical trends from the MG to the TPG, (2) a Sr isochron age of 109.5 Ma for both the MG and TPG, which indicates that both were emplaced during the same intrusive event, and (3) an initial 87Sr/86sr ratio for both the MG and TPG that indicates an upper mantle or lower crustal source. In addition, garnet-bearing and inetaluminous granitoids of the SNB both exhibit the same regional chemical variations. The composition and distribution of garnets in the TPG indicate an igneous origin. Fe-Mg partitioning between biotite and garnet indicate decreasing P-T conditions during garnet crystallization of 4.5 kb and 653°C for garnet cores and 3.2 kb and 521°C for garnet rims. Increasing A/CNK in the suite is a result of a decreasing Ca content due to the removal of hornblende and plagioclase. Elevated oxygen isotopic values, xenocrystic zircons, and biotite-rich inclusions with high 87sr/86sr ratios indicate assimilation of crustal material. However, an initial 87Sr/86sr ratio of 0.7049 for the suite indicates that the metasedimentary wall rocks were not the principal source material. The above evidence suggests that the origin for the TPG differs from the proposed models for other garnet-bearing granitoids such as the S-type granites of southeastern Australia and the inner Cordilleran muscovite granites of western North America. In both these regions peraluminous intrusions are not commonly spacially related to more mafic, metaluminous rocks.