GEOCHEMISTRY OF RHYOLITES AT THE 'OXYGEN ISOTOPE STEP', SNAKE RIVER PLAIN, IDAHO

Volcanism associated with the Yellowstone hotspot has produced large volumes of low-δ¹⁸O rhyolite in the central Snake River Plain and Owyhee Plateau (CSRP-OP). Along the southwestern margin of the western Snake River Plain graben, near Oreana, Idaho, there is a sharp transition from low (average of ~2‰) to 'normal' (>6‰) oxygen isotope ratios among otherwise similar rhyolites (Boroughs et al., 2012). This study investigates the rhyolites to the southwest of this transition at the resolution of individual eruptive units. Prominent in this area are the 9.6 Ma Tuff of Little Jacks Creek (TLJC) and the 13.5 Ma Tuff of Swisher Mountain (TSM) (Ekren et al., 1984; Armstrong et al. 1980). Based on field relations, geochemistry, and unit morphology, it is likely that these two map units actually represent multiple eruptive events of both lavas and ignimbrites. Using whole-rock and mineral chemistry, we have sub-divided the TLJC into five chemical types, and the TSM into three, all of which are classified as potassic A-type rhyolites. These sub-groups show little internal chemical variation and have consistent mineralogy. Preliminary oxygen isotope analyses of individual feldspar crystals show that all TLJC and TSM chemical types are low-δ¹⁸O rhyolites. Oxygen isotope ratios for TLJC chemical groups also vary, indicating variable sources or processes operating at the scale of single members. Additionally TLJC and TSM samples from this study are the most northwestern of the CSRP low δ¹⁸O province analyzed to date, further constraining the sharp transition between low and normal-δ¹⁸O rhyolites along the northwestern margin of the CSRP-OP province.