Rhyolitic Explosive Eruptions of the Central Snake River Plain, Idaho: Investigations of the Lower Cassia Mountains Succession and Surrounding Areas

The Snake River Plain of north-western U.S.A. was the site of voluminous, bimodal, hotspot volcanism in the Miocene. Between c. 12.7-6 Ma silicic volcanism produced an association of deposits so different to typical Plinian and ignimbrite deposits elsewhere it has been termed Snake River (SR)-type. The Cassia Mountains of southern Idaho contain SR-type ignimbrites produced from complex and dynamic magmatic plumbing systems involving multiple magma chambers which gave rise to multiple compositional populations of clinopyroxene that mixed during eruption and were deposited together. The Cassia Mountain ignimbrites become progressively more mafic up-succession in terms of whole rock, glass, feldspar and clinopyroxene compositions, reflecting decreasing time available for fractional crystallisation, as supported by geochronology. Two Cassia Mountain ignimbrites are among three newly discovered ‘super-eruptions’ defined on the basis of phenocryst, glass and whole rock compositions; magnetic polarity; 40Ar/39Ar geochronology; oxygen isotopes; and field data. Erupted volumes range between 640 and 1200 km3, amongst the largest recorded. Intercalated within the Cassia Mountain succession is a newly discovered deposit representing the first recorded explosive, rhyolitic phreatomagmatic eruption from the central Snake River Plain. The fine-grained, non-welded deposit has similar whole rock, glass, oxygen isotope and magmatic temperature characteristics to the surrounding welded ignimbrites, so the unusual deposit facies are interpreted as representing interaction of rising rhyolitic magma with near-surface water. During SR-type volcanism, lavas and ignimbrites of similar chemistry were erupted within a short time. Water contents of melt inclusions were low in both ignimbrites and lavas, consistent with the anhydrous mineralogy and high inferred magmatic temperature. Volatile contents of the magmas (as recorded by the melt inclusions) did not control eruptive style. The intense rheomorphism which characterises SR-type ignimbrites appears to be due to high emplacement temperatures rather than enhanced halogen contents.