CRYSTAL AGGREGATES FROM THE SNAKE RIVER PLAIN: A MICRO-SCALE INVESTIGATION OF A LARGE IGNEOUS PROVINCE

The central Snake River Plain (CSRP) of southern Idaho contains numerous intensely welded, rhyolitic ignimbrites which represent the surface expression of the Yellowstone hotspot. These ‘Snake River-type’ ignimbrites were erupted between c. 13 and c. 7 Ma from sources currently obscured by later eruptions of basalt. The rhyolites are characterised by an anhydrous mineral assemblage variably containing, plagioclase, quartz, sanidine, augite, pigeonite, orthopyroxene, ilmenite, magnetite, fayalitic olivine and accessory zircon and apatite. Magmatic temperatures are inferred to have been high (> 850 ^oC) and magmatic water contents were low (< 3 wt.%). Ignimbrites in the CSRP may contain multiple compositional populations of compositionally unzoned pigeonite and augite. Importantly, in cases where multiple compositions are present, the compositions remain constant throughout a deposit although their relative abundance may vary. Crystal aggregates of varying morphology are virtually ubiquitous in addition to single, euhedral crystals. Where multiple compositions are present as single crystals, the full range of compositions may also be present within crystal aggregates, yet within any one aggregate only a single composition of pigeonite and augite have yet been found. We infer that the restricted compositions of pigeonite and augite in crystal aggregates reflect a pre-eruptive situation whereby magma was stored in multiple discrete batches which were evolving in parallel. In one case, an aggregate contains augite and pigeonite compositions which are not recorded by single crystals. Although ignimbrites of the central Snake River Plain are known to have large volumes (1,000 km³ or more) there is no relationship between the number of compositional modes of pigeonite and augite found in a deposit and the volume of that deposit.