LARGE VOLUME LOW d18O RHYOLITES IN THE BRUNEAU-JARBIDGE VOLCANIC CENTER: EVIDENCE FOR LARGE SCALE MELTING OF HYDROTHERMALLY ALTERED CRUST?
The Bruneau-Jarbidge (BJ) volcanic field is located in southern Idaho at the intersection of the western and eastern arms of the Snake River Plain. The BJ region is an oval structural basin of about 6000 km2, and is likely a system of nested caldera and collapse structures similar to, though larger than, the Yellowstone Volcanic Plateau. BJ rocks are high-temperature rhyolite tuffs, high-temperature rhyolite lavas, and volumetrically minor basalts. Exposed volumes of individual rhyolite units range up to >500 km3. We have analyzed feldspar and, where present, quartz from 30 rhyolite units emplaced throughout the history of the BJ center. All, including the Cougar Point Tuff, are 18O depleted (d18OFSP=-1.3 to 3.7), while petrographically, temporally, and chemically similar lavas erupted along the nearby Owyhee Front have "normal" rhyolite magmatic d18O values of 7 - 9. There is no evidence for significant modification of d18O values by post-eruptive alteration. No correlation exists between d18O and age, magmatic temperature, major element composition or trace element abundances among depleted BJ rhyolites. The B-J and Western Snake River Plain (WSRP) rhyolites possess the major- and trace-element characteristics (depressed Al, Ca, Eu, and Sr contents, high Ga/Al and K/Na) expected of liquids derived from shallow melting of calc-alkaline granitoids with residual plagioclase and orthopyroxene. The total volume of 18O-depleted rhyolite may be as high as 10,000 km3, requiring massive involvement of meteoric-hydrothermally altered crust in rhyolite petrogenesis. We suggest that the B-J and WSRP rhyolites originated by melting of Idaho Batholith granitoids, previously affected by large-scale meteoric-hydrothermal alteration during the Eocene. Therefore, the genesis of the rhyolites, and the origin of the 18O depletion, are the results of two distinct and unrelated thermal events.