GEOCHEMICAL DISTINCTION OF RHYOLITE FLOWS IN THE WESTERN SNAKE RIVER PLAIN OWYHEE COUNTY, IDAHO

Rhyolite units within Owyhee County were mapped by Ekren et al. (1984), who grouped large areas of rhyolite into tuff cooling units such as the Tuff of Little Jacks Creek (TLJC) and Tuff of Swisher Mountain (TSM). Later mapping by Kauffman and Bonnichsen (1990) produced a detailed map of the eastern TLJC, differentiating the unit into a series of lavas. As these rhyolites have similar petrography within their respective units, geochemical fingerprinting is a useful method for distinguishing between them. Whole rock and phase chemistry for these samples were obtained for over 100 rhyolite samples collected from Owyhee County, using XRF, ICP-MS, and electron microprobe at the WSU GeoAnalytical Laboratory. Our data suggests that western portions of the TLJC and the entire TSM can be split into at minimum four and two units respectively, all of which can be classified as potassic A-type rhyolites. These units also contain higher concentrations of Fe on average than CSRP rhyolites such as the Cougar Point Tuff. Individual units are near homogeneous and show very little internal variation with respect to both major and trace element chemistry. Silica content of TLJC samples range from 72.1 to 74.7 wt%. Phenocryst mineralogy for TLJC samples include clinopyroxene, Fe-Ti oxides, and plagioclase, which is often sieve textured. TSM samples have slightly lower silica contents of 70.5 to 73.7 wt%, and contain clinopyroxene, Fe-Ti oxides, sanidine, and plagioclase, which also is commonly sieve textured. On normalized trace-element diagrams, all samples show negative P, Ti, and Sr anomalies, representing fractionation of apatite, Fe-Ti oxides, and plagioclase. There is no evidence for zircon fractionation, nor of depletion of light REE by fractionation of chevkinite or similar REE-phases. Most samples exhibit modest negative Ba anomalies, indicating limited removal of alkali feldspar.