CO-CRBG RHYOLITE VOLCANISM REASSESSED

Mid Miocene rhyolite volcanism is widespread across the tri-state area of Oregon, Nevada, and Idaho in the form of rhyolite domes, lava flows, ignimbrites, and fallout tuffs. Our work has focused on mapped and unmapped rhyolite occurrences in eastern Oregon, including those of the Lake Owyhee volcanic field. While this work is still ongoing, we now have data that allow us to reassess some common misconceptions that arose from the lack of modern geochronological and petrological data regarding silicic volcanism associated with flood basalt volcanism of the Columbia River Basalt Group (CRBG).

One misconception is that the oldest rhyolites are those found near the Nevada-Oregon border and that rhyolites are time progressive from south to north. Our oldest Ar-Ar plateau and single crystal ages come from rhyolites in the greater Dooley Mountain area ranging from 16.53±0.14, 16.69±0.01, to 17.00±0.12. In addition, an age of 17 Ma is reported for a rhyolite of the Strawberry Volcanics, but this remains to be confirmed. Such ages are as old or older than what is currently reported for southern rhyolite centers like the McDermitt volcanic field and High Rock caldera complex. Numerous other rhyolite centers have also yielded ages at, or slightly older than, 16 Ma, thus placing the onset of rhyolite volcanism at many centers earlier than suggested by older publications, with the exception of the Littlefield Rhyolite. Therefore, rhyolite volcanism does not indicate a northward migration trend; rhyolite volcanism was active across the province at ≥16 Ma and was centered ~100 km NNE of McDermitt.

A second misconception is that only high temperature, A-type rhyolites are associated with CRBG volcanism. Several rhyolite centers either started out with more calc-alkaline type rhyolites (e.g. with biotite or hornblende), or A-type rhyolites appeared sometime during the activity phase of the same center. Thus, both rhyolite types occur together and require changes in local petrogenetic conditions rather than changes in large-scale tectonic framework conditions.

A key implication of this work is that the widespread, nearly instantaneous thermal footprint of CRBG magmatism, as deduced from rhyolites, raises questions of the significance of the apparent time progressive basaltic volcanism.