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Large-volume, low-¹⁸O rhyolites of the central Snake River Plain, Idaho, USA

¹ Department of Geology, Washington State University, Pullman, Washington 99164, USA
² Idaho Geological Survey, P.O. Box 443014, Moscow, Idaho 83844, USA
³ Department of Geology, Washington State University, Pullman, Washington 99164, USA

The Miocene Bruneau-Jarbidge and adjacent volcanic fields of the central Snake River Plain, southwest Idaho, are dominated by high-temperature rhyolitic tuffs and lavas having an aggregate volume estimated as 7000 km³. Samples from units representing at least 50% of this volume are strongly depleted in ¹⁸O, with magmatic feldspar ¹⁸O_VSMOW (Vienna standard mean ocean water) values between –1.4 and 3.8. The magnitude of the ¹⁸O depletion and the complete lack of any rhyolites with normal values (7–10) combine to suggest that assimilation or melting of a caldera block altered by near- contemporaneous hydrothermal activity is unlikely. Instead, we envisage generation of the high-temperature rhyolites by shallow melting of Idaho Batholith rocks, under the influence of the Yellowstone hotspot, affected by Eocene meteoric-hydrothermal events. The seeming worldwide scarcity of strongly ¹⁸O-depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths.

Key Words: Snake River Plain • oxygen isotopes • rhyolites • meteoric-hydrothermal alteration

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