PETROLOGY AND GEOCHEMISTRY OF BASALTIC ANDESITE TO RHYOLITE ROCKS FROM BOREHOLE CH-1: IMPLICATIONS FOR QUATERNARY, EASTERN SNAKE RIVER PLAIN-STYLE INTERMEDIATE-COMPOSITION VOLCANISM

The eastern Snake River Plain (ESRP) is one of the best-known late Pliocene-Quaternary bimodal volcanic fields. Here we describe a rare intermediate composition volcanic center near the middle of ESRP, informally named Unnamed Butte (UNB). Largely buried by younger basalts, we have sample UNB in a 610-meter-deep, cored borehole CH-1. Uppermost parts of the center appear to correlate with a nearby, partially buried rhyolite dome, dated in previous studies at 1.4 Ma. Beginning at a depth of 366 meters, the borehole penetrates a ~213-meter-thick comagmatic sequence of lavas. The lavas vary stratigraphically downward, in a compositionally systematic manner, from rhyolite (~76% SiO₂) to trachyandesite, dacite, andesite, and basaltic andesite (~55% SiO₂). Bulk ⁸⁷Sr/⁸⁶Sr_(i) and e_Nd are ~0.7064 and -4, respectively, and exhibit little variation. The most siliceous rhyolite contains euhedral to subhedral phenocrysts of plagioclase (An₁₅), sanidine (Or₄₀), quartz, augite, hypersthene, Fe-Ti oxides, zircon and apatite. More mafic rhyolites contain mixed populations of plagioclase (oligoclase and labradorite) phenocrysts. Dacite, andesite and basaltic andesite contain phenocrysts of labradorite (An_55-63), pyroxenes which are compositionally similar to those in the rhyolite, Fe-Ti oxides (dominantly magnetite_ss), apatite ± zircon. Phenocryst populations are appear bimodal, and do not vary systematically in assemblage or composition with bulk rock composition. In addition, bulk major and trace element covariation plots of CH-1 samples yield linear patterns which overlap on the siliceous end with other high silica rhyolites of the ESRP (e.g., Big Southern Butte), and extrapolate towards the composition of "typical" ESRP olivine tholeiite basalt on the mafic end. Observed compositional variations in CH-1 could be produced by mixing typical ESRP olivine tholeiite with ESRP high-silica rhyolite in proportions increasing from 0 (rhyolite) to 75% (the most mafic basaltic andesite).