LARGE-SCALE LOW d¹⁸O RHYOLITIC VOLCANISM, BRUNEAU-JARBIDGE VOLCANIC FIELD, IDAHO

The Bruneau-Jarbidge (B-J) volcanic field is located in southern Idaho at the intersection of the western and eastern arms of the Snake River Plain. The western arm is thought to be an extensional basin initiated when the Yellowstone hotspot was beneath the B-J area, while the eastern arm is the track of the hotspot as volcanism proceeded ENE from 12 Ma to its present location in western Wyoming. The B-J region is an oval structural basin of about 6000 km², and is likely a system of nested caldera and collapse structures similar to, though much larger than, the Yellowstone Volcanic Plateau. Exposed B-J rocks are high-temperature rhyolite lavas and tuffs, and volumetrically minor basalts. Magmatism occurred between about 12 and 8 Ma in three temporally overlapping episodes: 1) eruption of the volumetrically dominant Cougar Point Tuff; 2) extrusion of large rhyolite lavas; 3) emplacement of a veneer of basalt. We have analyzed oxygen isotope ratios in feldspar and, where present, quartz from 30 rhyolite units emplaced throughout the history of the B-J center. All, including the Cougar Point Tuff, are ¹⁸O depleted (d¹⁸O_FSP=-1.3 to 3.7‰), while contemporaneous petrographically and chemically similar lavas erupted outside the B-J center, along the nearby Owyhee Front, have “normal” rhyolite magmatic d¹⁸O_FSP values of 7 - 9‰. Volumes of individual strongly depleted units are as large as 500 km³. There is no evidence for significant modification of oxygen isotope ratios by post-eruptive alteration. No correlation exists between d¹⁸O and age, magmatic temperature, major element composition or trace element abundances among depleted B-J rhyolites. At Yellowstone, only the volumetrically minor post-caldera collapse lavas show strong ¹⁸O depletion. In contrast, both lavas and caldera-forming ignimbrites are strongly ¹⁸O-depleted at B-J. The minimum total volume of strongly ¹⁸O-depleted rhyolite may be as high as 10,000 km³, requiring massive involvement of meteoric-hydrothermally altered crust in rhyolite petrogenesis throughout the history of the volcanic field. Hydrothermal modification of the crust under the thermal influence of the Yellowstone hotspot apparently preceded voluminous rhyolite generation at Bruneau-Jarbidge.