GEOCHEMICAL AND PETROGRAPHIC ANALYSES OF THE BASALTS OF THE DARWIN PLATEAU, INYO COUNTY, CA

The Darwin Plateau lies between the Inyo Mountains to the north, and the Coso Range to the south. A series of basaltic flows and cones were emplaced on the plateau from 8 Ma to 4 Ma. The Coso field (2 Ma-Pres.) lies 50 km to the southwest and the Ricardo volcanics (10-8 Ma) 100 km to the southwest. Sixty samples from basalt flows and cones of the Darwin Plateau were analyzed for major, minor and trace elements. On a Le Bas diagram the basalts show a considerable range in composition from basalt and basaltic andesite, to trachybasalt and basaltic trachyandesite; straddling the alkaline-subalkaline boundary line. Major element and trace element geochemistry are remarkable consistent within individual flows, but vary non-systematically between adjacent flows.

When plotted on a basalt tetrahedron the Darwin basalts again show considerable variation in composition, however the majority are olivine tholeiites. This is in marked contrast to the Ricardo volcanics which are quartz-normative tholeiites and the Coso volcanics which are alkali basalts. Petrographic examination of Darwin basalts reveals only small amounts of partially altered olivine, unlike the complete olivine replacement by iddingsite in the Ricardo field and large, unaltered phenocrysts of olivine in the Coso volcanics.

These differences may be related to evolutionary trends for the volcanic fields of the southern Owens Valley. Older Ricardo volcanics are quartz normative, Darwin is neither quartz nor nepheline normative, and Coso is distinctly nepheline normative. This can be attributed to variation in the thermal regime represented by differing degrees of partial melt or depth of melting, and/or dissimilarity in water content and oxygen fugacity of the magma.