PETROCHEMICAL TRENDS OF NEOGENE BASALTIC VOLCANISM IN THE SOUTHERN OWENS VALLEY, CA

Three Neogene, basaltic, volcanic fields lie within the southern Owens Valley. Each is characterized by a distinct petrology and geochemistry. The basalts of the Ricardo Group, emplaced between 8.2 and 10.1 Ma, are comprised of 23 flows of dominantly quartz-normative tholeiitic basalt with interbedded rhyolite tuff. Olivine has undergone partial to complete alteration to iddingsite and calcium-siderite. The basalts of the Darwin plateau were emplaced between 4 and 8 Ma. Intermediate and/or felsic volcanics are absent. Basalts have a wide range in composition from tholeiites to alkali basalts, with the majority olivine tholeiites. Petrology shows a similar variation; some flows are comprised of modal olivine while others consist of olivine grains partially altered to iddingsite. The Coso field (<4 Ma) is bimodal, consisting of both basalt and felsic tuff in equal amounts. The basalts have a restricted composition within the alkali basalt field of a basalt tetrahedron. They contain abundant, large phenocrysts of unaltered olivine.

Differences in petrology and geochemistry may be related to tectonic setting and/or oxygen fugacity of the magma. The older tholeiites of the Ricardo Group require a shallower source area than do the younger alkali basalts of the Coso field. The Darwin basalts appear to be transitional between Ricardo and Coso. The older Ricardo basalts may have risen along conduits that tap shallower regions of the lower crust/upper mantle. Younger alkali basalts (Coso) would tap reservoirs deeper within the mantle. Compositional differences may also be related to residence time in the crust. Iddingsite has been shown to be a product of dueteric alteration, resulting from increasing oxygen fugacity during magma crystallization. Basaltic magmas that rise slowly will assimilate oxygen leading to iddingsite formation, whereas magmas rising rapidly will have a lower oxygen fugacity permitting the crystallization of unaltered olivine. Therefore, the basalts of the southern Owens Valley appear to have evolved from magmas generated at shallower depths rising slowly to the surface, to magmas from greater depth that have risen more rapidly. This evolutionary trend may reflect a transition from dip-slip faulting to dextral shear within the Owens Valley.