A NEW TYPE OF VOLUMINOUS SILICIC VOLCANISM, ‘SNAKE RIVER (SR)-TYPE VOLCANISM': EVIDENCE FROM SOUTHERN IDAHO AND NORTHERN NEVADA, USA

We define a major new type of large-scale volcanism, ‘Snake River (SR)–type' volcanism, based on volcanic rocks in part of the Yellowstone hot-spot track. Volcanic rocks in southern Idaho and northern Nevada, USA record a distinctive type of large-scale volcanism which differs in several respects from better understood Plinian/ignimbrite eruptions elsewhere. SR-type volcanism involves styles of silicic eruption that have not been witnessed, and so must be inferred from the distinctive association of deposit facies. (1) The ignimbrites, typically large volume (VEI 6-8), contain few pumice lapilli or fiamme, and are much better sorted than are ignimbrites elsewhere. (2) They are characteristically lithic poor, even near inferred source areas, but many of the pyroclastic units contain abundant angular chips of vitric material. (3) Many of the ignimbrites are high to extremely high-grade, and rheomorphic, even where thin. Many are lava-like. Welding appears to be bimodal, in that there are some non-welded ignimbrites, but very few moderately welded ignimbrites. (5) Ashfall deposits are unusual: rather than typical Plinian fall deposits, most are thinly bedded, medium-grained ash fall layers, with unusually large bubble wall shards (large enough to be seen without a hand lens). Many, but not all are thoroughly fused, both beneath and above the ignimbrites, including co-ignimbrite ashfall layers. (6) Ash pellets and accretionary lapilli are common. (7) Rhyolite lavas are unusually large in volume (>200 km3). Smaller-scale domes or coulees such as characterise rhyolite fields elsewhere are absent. (8) Lacustrine facies are abundant (e.g. volcaniclastic sands and silts, pillow lavas, hyaloclastites; peperitic bases of ignimbrites). Work is in progress to investigate the nature and origin of the unusual eruptive styles manifested by SR-type volcanism. Clearly the eruptions were high mass-flux, sustained, and involved unusually fluidal rhyolite magmas (950-1100o C temperatures estimated), and there is an association with basaltic magmatism (the silicic rocks form part of a bimodal association) and the presence of shallow epi-continental lakes. SR-type volcanism is recorded to various degrees at some other volcanic provinces (e.g. Trans-Pecos Texas and Etendeka-Parana).