A Geochemical and Petrographic Analysis of the Basalts of the Ricardo Formation, Southern El Paso Mountains, CA

ABSTRACT

The Miocene Ricardo Group of the southern El Paso Mountains consists of lucustrine sedimentary rocks interlayered with felsic tuffs and basalt flows. The basalts were sampled and analyzed geochemically and petrographically. The results of those analyses were compared to data from other Mojave/Owens Valley basalt fields. Major elements show dramatic differences. Ricardo volcanics have significantly lower alkali (Na₂O + K₂O) content, plotting as tholeiites on a basalt tetrahedron. This is in marked contrast to the distinctly alkaline character of other fields. CIPW analyses support the geochemical data. Ricardo basalts are quartz normative, while basalts from other fields are typically olivine normative. In a seeming paradox, hand samples of Ricardo basalt appear to contain phenocrysts of olivine. Thin sections reveal, however, that only remnants of olivine remain; most grains having been replaced and pseudomorphed by iron oxides, siderite and iddingsite. Similar remnant olivines from the Big Pine field have been characterized as partially digested xenocrysts. Perhaps the altered olivines of the Ricardo basalts represent a reaction between an earlier alkaline magma that mixed with a pulse of later, more tholeiitic magma.

Trace elements were plotted on a Spider diagram and compared to other fields. The dramatic variations seen for major elements are less apparent. In general, all Mojave fields display a characteristic barium spike and incompatible element enrichment relative to the MORB standard. Ricardo basalts are slightly depleted in some incompatible elements, but the trends are less well defined than those for major elements. Trace element data suggests a common parentage and/or tectonic setting for all Mojave basalts. Minor differences can be attributed to mixing, assimilation or chemical variation of source rocks.

Any model that relates Mojave basalts must explain the major element variations. Wang (2002) proposed that Cenozoic basalt composition varies from east to west across the Mojave as a function of depth of melting. To the east, magmas tapped deeper fertile mantle having higher alkali and incompatible trace element concentrations. To the west, shallower melting resulted in more siliceous, alkali-poor magmas derived from a depleted mantle. My data appears to support this hypothesis.