Petrologic constraints on the development of a large-volume, high temperature, silicic magma system: The Twin Falls eruptive centre, central Snake River Plain

Explosive volcanism associated with the Yellowstone hotspot spanning ~ 11.3 to 9 Ma, thought to have erupted from the Twin Falls eruptive centre, is recorded in the Cassia Mountains of southern Idaho and northern Nevada. The stratigraphy contains intensely welded, rhyolitic (SiO 2 69–76 wt.%) ignimbrites with an anhydrous mineralogy: plagioclase, sanidine, quartz, pigeonite, augite, ilmenite, titanomagnetite, accessory zircon and apatite. Several different thermometers indicate high temperature rhyolitic magmas (> 900 °C). All Cassia Mountain ignimbrites show a significant depletion in δ 18O VSMOW with magmatic feldspar values between 1.7 and 3.0‰, reflecting incorporation of a hydrothermally altered protolith. Multiple compositions of both pigeonite (Mg# 30–46) and augite (Mg# 17–53) may occur within an individual ignimbrite while crystal aggregates contain only a single composition of each. The compositional heterogeneity within the ignimbrites reflects a complex magmatic system whereby magma was segregated into multiple smaller chambers prior to eruption. ►Low δ 18O rhyolites from the central Snake River Plain. ►Compositionally heterogeneous, high temperature rhyolitic ignimbrites spanning ~ 2 Ma. ►Crystal aggregates suggest magma was stored in discrete batches prior to eruption and mixed during eruption.