Large-volume, low-delta super 18 O rhyolites of the central SnakeRiver plain, Idaho, USA

Abstract

The Miocene Bruneau-Jarbidge and adjacent volcanic fields of thecentral Snake River Plain, southwest Idaho, are dominated byhigh-temperature rhyolitic tuffs and lavas having an aggregate volumeestimated as 7000 km super 3 . Samples from units representing atleast 50% of this volume are strongly depleted in super 18 O, withmagmatic feldspar delta super 18 O sub VSMOW Vienna standard meanocean water values between -1.4ppm and 3.8 ppm. The magnitude of thesuper 18 O depletion and the complete lack of any rhyolites withnormal values 7 ppm-10 ppm combine to suggest that assimilation ormelting of a caldera block altered by near- contemporaneoushydrothermal activity is unlikely. Instead, we envisage generation ofthe high-temperature rhyolites by shallow melting of Idaho Batholithrocks, under the influence of the Yellowstone hotspot, affected byEocene meteoric-hydrothermal events. The seeming worldwide scarcity ofstrongly super 18 O-depleted rhyolites may simply reflect a similarscarcity of suitable crustal protoliths.

78 samples from this reference