PETROLOGY AND GEOCHEMISTRY OF THE JOSHUA FLAT QUARTZ MONZONITE, WHITE-INYO MOUNTAINS, CALIFORNIA

The Jurassic composite EJB pluton, located in the White Inyo Mountains of California, is predominantly composed of the Eureka Valley monzonite (EVM), Joshua Flat quartz monzonite (JFQM), and Beer Creek granite (BCG). The JFQM forms the marginal facies of the EJB along its southern and northeastern borders.

Outcrops of the JFQM are typically rather fresh, and the rock has a gray, often foliated appearance. The typical mineralogy of the JFQM consists of microcline +/- perthite, plagioclase, hornblende, and sphene. Biotite and clinopyroxene may be locally common, and quartz is usually present in small amounts. Apatite, zircon and magnetite typically are common accessory phases. Mafic enclaves and schlieren of varying size and shape are locally abundant within the JFQM and some enclaves appear to be lithologically similar to the Eureka Valley Monzonite. In many hand samples, blotchy areas of fine-grained mafic-rich material suggests enclave disaggregation and assimilation by the JFQM. Contacts between the JFQM and BCG are generally sharp, but locally, the JFQM is cut by aplite dikes assumed to be related to the BCG.

Preliminary geochemical data indicate that the JFQM may be chemically classified as syenodiorite/syenite and has subalkaline-alkaline characteristics. Within the analyzed suite, systematic trends consistent with magmatic differentiation appear for many, but not all, elements. For example, as SiO2 increases, TiO2, Fe2O3, MnO, MgO, CaO, P2O5, Zn and Ni decrease while Na2O, and K2O increase. However, plotted versus increasing SiO2, trace elements such as Ga, Ba, Rb, Sr, Y, Zr, Nb, La, Pb, and Th are characterized by a wide scatter in concentrations. These geochemical data also indicate that JFQM samples from the southern end of the EJB composite pluton tend to be geochemically distinct from samples collected in other areas of the EJB.

Collectively, these observations suggest that the JFQM experienced magmatic differentiation that was locally influenced by assimilation of other preexisting units, possibly including the EVM. However, it is not clear that ALL exposures mapped as JFQM are associated with the main JFQM magma body. Our preliminary data suggests that at least a portion of what has been previously mapped in the EJB as JFQM may have formed from a separate and chemically distinct magma.