HR: 17:45h
AN: V52G-08 [PDF]
TI: Large Volume $^{18}$O-depleted Rhyolitic Volcanism: the Bruneau-Jarbidge Volcanic Field,
Idaho
AU: * Boroughs, S
EM: entoptics@turbonet.com
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AU: Wolff, J
EM: jawolff@mail.wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AU: Bonnichsen, B
EM: billb@uidaho.edu
AF: Idaho Geological Survey, University of Idaho, Moscow, ID 83014 United States
AU: Godchaux, M M
EM: mgodchau@mtholyoke.edu
AF: Idaho Geological Survey, University of Idaho, Moscow, ID 83014 United States
AU: Larson, P B
EM: plarson@wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AB:
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 km$^{2}$, 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 greater than 500 km$^{3}$. 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 $^{18}$O depleted
($\delta^{18}$O$_{FSP}$ = -1.3 to 3.7$\permil$), while petrographically, temporally, and chemically similar lavas erupted
along the nearby Owyhee Front have "normal" rhyolite magmatic $\delta^{18}$O values of 7 - 9$\permil$. There is no evidence
for significant modification of $\delta^{18}$O values by post-eruptive alteration. No correlation exists between
$\delta^{18}$O and age, magmatic temperature, major element composition or trace element abundances among depleted BJ
rhyolites. The BJ and WSRP rhyolites possess the geochemical 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 (Patino-Douce, Geology v.25 p.743-746, 1997). The classic Yellowstone low $\delta^{18}$O rhyolites are
post-caldera collapse lavas, but at BJ, both lavas and caldera-forming ignimbrites are strongly $^{18}$O-depleted. The total
volume of low $\delta^{18}$O rhyolite may be as high as 10,000 km$^{3}$, requiring massive involvement of
meteoric-hydrothermally altered crust in rhyolite petrogenesis. Regional hydrothermal modification of the crust under the
thermal influence of the Yellowstone hotspot apparently preceded voluminous rhyolite generation at Bruneau-Jarbidge.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1040 Isotopic composition/chemistry
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting