WIDESPREAD GENERATION OF LOW-δ18O MAGMA IN THE CENTRAL SNAKE RIVER PLAIN: COMPARING THE WEST BENNETT HILLS AND BRUNEAU-JARBIDGE RHYOLITES

The West Bennett Hills (WBH), on the north side of the Snake River Plain of southern Idaho, consist of a sequence of high-temperature rhyolitic volcanic units. They share similar ages and chemistry with many of the rhyolites of the central Snake River Plain, including the Cougar Point Tuff (CPT), a series of high-grade ignimbrites associated with the Bruneau-Jarbidge eruptive center 100 km south of WBH (Bonnichsen et al, 2007). The CPT and associated Bruneau-Jarbidge rocks represent the largest known low-δ¹⁸O rhyolite province in the world (Boroughs et al., 2005), and initial oxygen isotope analyses of the WBH units show a similar low-δ¹⁸O signature. This raises the question of whether the West Bennett Hills units were deposited by far-travelled high-grade pyroclastic flows that were erupted from the Bruneau-Jarbidge area, or alternatively, similar rhyolitic magmas were simultaneously generated over a wide area. Detailed comparisons of whole rock trace element, oxygen isotope, and microprobe pyroxene data do not support the presence of CPT or other Bruneau-Jarbidge units at WBH, despite closely similar overall characteristics. For example, it was found that three WBH units (Frenchman Springs, Henley, and Dive Creek) share similar trace element contents with CPT unit XV, but the pyroxene compositions are more similar to those found in CPT units X, XI, and XIII (Cathey & Nash 2004). The data are therefore inconsistent with northward cross-plain travel of Bruneau-Jarbidge flows, and instead support an origin for the central Snake River Plain rhyolites that involves many different rhyolitic magmas of near-identical composition being generated simultaneously over a wide area.

References: Bonnichsen et al. (2007) Bulletin of Volcanology, in press. Boroughs et al. (2005) Geology 33, p. 821. Cathey & Nash (2004) Journal of Petrology 45, p. 27.