NEW 40AR/39AR AND PALEOMAGNETIC DATA FROM THE BISHOP MOUNTAIN FLOW, YELLOWSTONE VOLCANIC FIELD

Whereas the Yellowstone Volcanic Field caldera-forming eruptions have been studied extensively, the small volume, pre- and post-collapse rhyolitic lavas and tuffs and their relationship to the development of the magma chamber that produces the caldera-forming eruptions remains poorly understood. Five small-volume rhyolitic eruptions of the Big Bend Ridge Rhyolites (BBRR) precede the 1.30 Ma Mesa Falls Tuff and occurred on what is now the shared rim of the Henry’s Fork and Island Park Calderas. The largest mapped of these eruptions, the Bishop Mountain Flow, outcrops along the northwestern flank of this shared caldera rim. To test a potential genetic relationship between the Bishop Mountain Flow and the Mesa Falls Tuff, we collected new whole rock samples and drill cores for age, geochemical, and paleomagnetic analyses. Sanidine phenocrysts from the Bishop Mountain Flow were separated for ⁴⁰Ar/³⁹Ar single crystal incremental heating analyses to improve upon and evaluate the accuracy of the published K/Ar age. Similarly, we test the reported reversed polarity direction of the Bishop Mountain Flow with eight laterally spaced samples treated with >15-step alternating field demagnetization. Our data suggest that the Bishop Mountain Flow erupted ~ 150 ka prior to the Mesa Falls Tuff. The significant time gap between the two events, along with interpretations of rapid magma accumulation for super-eruptions, makes it difficult to postulate a relationship between the Mesa Falls Tuff and the Bishop Mountain Flow. Forthcoming zircon geochemistry and U-Pb ages will assist in describing the chemical evolution of the BBRR through time, and how this evolution is related to the broader development of the magmatic system that generates caldera-forming eruptions in the Yellowstone Volcanic Field.