PETROLOGY AND GEOCHEMISTRY OF THE EUREKA VALLEY MONZONITE, WHITE-INYO MOUNTAINS, CALIFORNIA

The composite EJB pluton is exposed in the Deep Springs Valley-Eureka Valley region of the White-Inyo Mountains of eastern California. The Jurassic EJB is characterized by three principal units: the Eureka Valley monzonite (EVM), the Joshua Flat quartz-monzonite (JFQM), and the Beer Creek granite (BCG). The largest body of EVM outcrops in the central portion of the EJB in contact with both the JFQM and the BCG. Smaller bodies similar in appearance to the EVM outcrop sporadically throughout the area, and the largest of these occur in the northeast portion of the EJB.

In the field, the EV monzonites weather dark brown to black and form rather prominent but bouldery outcrops. Beneath a narrow weathering rind, the rocks are generally fresh. Many EVM outcrops have been intruded by small bodies of BCG. Near contacts with the BCG, intrusive breccias composed of angular EVM fragments surrounded by aplite are common.

The EVM hand samples are very dark to rather light colored rocks, typically with 20-30% mafic minerals. Thin section observations indicate that most EVM samples contain substantial plagioclase and untwinned potassium feldspar, with associated clinopyroxene and olivine. Nephelene and biotite are present in some samples. Magnetite and apatite are quite abundant in many samples. Quartz is rare. No cumulate textures have so far been identified.

Preliminary geochemical data of the EVM rocks shows them to be alkalic and geochemically classified as gabbroic foid diorites. Within the analyzed suite, as SiO2 increases, Fe2O3, MnO, MgO, CaO, and P2O5 decrease, Al2O3, Na2O, and K2O increase, and TiO2 remains constant. Trace element concentrations vary as follows: Ba: 884–1151 ppm; Zn: 104–128 ppm; Ga: 23–26 ppm; Rb: 75–207 ppm; Sr: 1171–1879 ppm; Y: 31–48 ppm; Zr: 231–486 ppm; Nb: 1–25 ppm; La: 38–51 ppm; and Th: 6–28 ppm. The major elements and most trace elements define clear differentiation trends, and suggest the possibility of separate evolutionary trends for other EVM bodies within the EJB. Specifically, EVM samples collected from the east and northeast portions of the EJB appear to be geochemically distinct from those collected from the large body of EVM in the central part of the EJB. These relationships suggest the possibility of multiple intrusions of mafic magma during the development of the EJB system.