Voluminous low {delta}18O magmas in the late Miocene Heise volcanic field, Idaho: Implications for the fate of Yellowstone hotspot calderas

摘要

We report oxygen isotope compositions of phenocrysts and U-Pb ages of zircons in four large caldera-forming ignimbrites and post-caldera lavas of the Heise volcanic field, a nested caldera complex in the Snake River Plain, that preceded volcanism in Yellowstone. Early eruption of three normal 18O voluminous ignimbrites with 18O_quartz = 6.4 and 18O_zircon = 4.8 started at Heise at 6.6 Ma, and was followed by a 2–3 18O depletion in the subsequent 4.45 Ma Kilgore caldera cycle that includes the 1800 km3 Kilgore ignimbrite, and post-Kilgore intracaldera lavas with 18O_quartz = 4.3 and 18O_zircon = 1.5. The Kilgore ignimbrite represents the largest known low-18O magma in the Snake River Plain and worldwide. The post-Kilgore low 18O volcanism likely represents the waning stages of silicic magmatism at Heise, prior to the reinitiation of normal 18O silicic volcanism 100 km to the northeast at Yellowstone. The occurrence of low 18O magmas at Heise and Yellowstone hallmarks a mature stage of individual volcanic cycles in each caldera complex. Sudden shifts in 18O of silicic magmas erupted from the same nested caldera complexes argue against any inheritance of the low 18O signature from mantle or crustal sources. Instead, 18O age trends indicate progressive remelting of low 18O hydrothermally altered intracaldera rocks of previous eruptions. This trend may be generally applicable to older caldera complexes in the Snake River Plain that are poorly exposed.