Rhyolitic volcanism of the central Snake River Plain: a review

The central Snake River Plain (CSRP) of southern Idaho and northern Nevada, USA, forms part of the Columbia River-Yellowstone large igneous province. Volcanic rocks of the province are compositionally bimodal (basalt-rhyolite), and the rhyolites produce a broadly time-transgressive record of a hotspot which is currently located under Yellowstone. Snake River Plain rhyolites represent hot (>850 degrees C), dry magmas and have field characteristics consistent with high emplacement temperatures. Individual ignimbrite sheets reach 1,000 km(3) and exhibit little to no compositional zonation on a large scale but reveal considerable complexity on a crystal scale, particularly with regard to pyroxene compositions. Multiple pyroxene compositions may exist in a single ignimbrite which, along with multiple glass compositions in widely dispersed fallout tephra, suggests complex storage of rhyolite prior to eruption. Unlike most igneous rocks, the mineral cargo of the CSRP rhyolites exhibits little isotopic variability, with unimodal Sr-87/Sr-86 values returned from plagioclase grains inferred to represent the combination of strong crystal-melt coupling and rapid diffusional re-equilibriation. All the rhyolites within the CSRP have a characteristic low-delta O-18 signature; with >20,000 km(3) of rhyolite exhibiting this depletion, the CSRP represents the largest low-delta O-18 province on Earth. The low-O-18 nature of the rhyolites requires assimilation of hydrothermally altered materials which may be from altered Eocene batholithic rocks or from down-dropped intracaldera tuffs. The wide range of crustal assimilants, with highly variable radiogenic isotope characteristics, available in the CSRP is permissive of a variety of petrogenetic models based on radiogenic isotopic data.