PETROLOGY AND GEOCHEMISTRY OF THE EJB DIORITE, WHITE-INYO MOUNTAINS, EASTERN CALIFORNIA

The Jurassic EJB pluton crops out in the Deep Springs Valley/Eureka Valley region of the White-Inyo Mountains of eastern California. A composite pluton, the EJB comprises the Eureka Valley monzonite (EVM), the Joshua Flat quartz monzonite (JFQM), and the Beer Creek granite (BCG). South of the EJB, the JFQM forms a large portion of the Marble Canyon pluton, where it is extensively associated with the more mafic Marble Canyon Diorite (MCD). Diorite also crops out within the EJB pluton – most commonly along the southeastern margin of Deep Springs valley – where it is surrounded by rocks of the BCG. The diorite of the EJB is a dark rock composed of plagioclase feldspar, hornblende, biotite, quartz, K-spar, and sphene. Previous studies (Sylvester and others, GSA Bulletin, 1978; Miller, PhD Dissertation, 1977; Dietl, PhD Dissertation, 2000) have not fully addressed the significance of the diorite within the EJB system, nor of its relationship to other plutonic bodies in the area.

Observations of the contacts between the EJB diorite and the adjacent rocks reveal several different relationships. In at least one case, the contact relationships between the diorite and the surrounding BCG are characterized by apparent magma mingling. Elsewhere, contacts are characterized by elongated xenoliths of the diorite within the BCG suggesting intrusion into sheared ductile diorite. Finally, some contacts are characterized by angular, broken diorite xenoliths surrounded by BCG. Collectively, these observations suggest that the BCG may have intruded a cooling diorite mass of spatially variable rheology.

Preliminary XRF analyses of the EJB dioritic rocks shows them to be enriched in MgO and TiO2, and depleted in Na2O and K2O, relative to other igneous rocks of the EJB. Trace element concentrations of the diorites include the following ranges: V, Cr: 100-300 ppm; Co: 8-50 ppm; Ni: 7-90 ppm; Cu: 25-100 ppm; Zn: 80-120 ppm; Rb: 30-80 ppm; Sr: 700-2000ppm; Zr: 130-400 ppm; and Ba: 800-1800 ppm. Preliminary assessment of these data suggests that the diorite may have either mixed with the BCG to form the JFQM or fractionated to form the JFQM and the BCG.