PRELIMINARY CHARACTERIZATION AND INTERPRETATION OF A THICK RHYOLITE IGNIMBRITE IN BOREHOLE USGS 142 ON THE IDAHO NATIONAL LABORATORY, EASTERN SNAKE RIVER PLAIN

A recently drilled deep borehole in the Eastern Snake River Plain, USGS 142, penetrates ~150 meters of rhyolite beneath 434 meters of interlayered basalt lava flows and sediment interbeds. The borehole is located at 43.646N, 113.038W, in a region that straddles the late Miocene Picabo and Heise volcanic fields, and overlaps with a proposed Picabo volcanic-field-related Big Lost Trough caldera (BLT). Core recovery is nearly 100%. The deposit is a densely welded and rheomorphic, single ignimbrite cooling unit that is bounded by well developed upper and lower vitrophyre zones. The interior of the deposit is devitrified and variably vapor-phase crystallized. Anastomosing zones of weak to moderate argillic alteration occur at some levels. Sparse lithic fragments and prominent eutaxitic textures are preserved in vitrophyre zones. Basal contact of the rhyolite is not penetrated, but rapid downward variations in welding and vitric content indicate that it likely occurs within a few meters of the borehole bottom. Bulk major- and trace-element XrF analyses of the rhyolite indicate that the deposit is chemically homogeneous, containing ~76% SiO2. The rhyolite is moderately porphyritic containing ~7% phenocrysts of plagioclase >> quartz > ferroaugite > sanidine > orthopyroxene >> magnetite > ilmenite and accessory zircon and apatite. Preliminary examination of the bulk major and trace-element composition and petrography of the rhyolite indicates that it is not correlated to any of the known large ignimbrites of the Heise or Picabo volcanic fields. Dating and additional geochemical work on the rhyolite are in progress. The 150 m thick ignimbrite is thicker than most regional outflow ignimbrites. We speculate that the ignimbrite could be an upper cooling unit of the BLT caldera, perhaps from the same sequence that occurs in borehole INEL-1. However it could also be an outflow of the BLT caldera resting on country rocks, or an outflow unit of some as of yet unidentified caldera that erupted soon after the BLT caldera, and that ponded within the moat region of the BLT caldera.