|Title:||Large–volume Rhyolite Genesis in Caldera Complexes of the Snake River Plain: Insights from the KilgoreTuff of the HeiseVolcanic Field, Idaho, with Comparison toYellowstone and Bruneau–Jarbidge Rhyolites|
|Authors:||K. E. Watts, I. N. Bindeman, and A. K. Schmitt|
|Publication:||Jour. Petrol., v. , p. 1‐34.|
Generation of large–volume rhyolites in the shallow crust is an important, yet enigmatic, process in the Snake River Plain and worldwide. Here, we present data for voluminous rhyolites from the 6.6–4.5Ma Heise volcanic field in eastern Idaho. Heise is arguably the best site to evaluate shallow rhyolite genesis in the Snake River Plain; it is the youngest complete record of caldera cluster volcanism along theYellowstone hotspot track and it culminated with the eruption of the most voluminous low–d18O rhyolite known on Earth: the 1800 km3 KilgoreTuff (d18O=3.4‰). Such low–d18O values fingerprint meteoric waters, and thus the shallow crust. New oxygen isotope data for phenocrysts, obtained by laser fluorination, correspond to a low–d18O magma value of 3.4±0.1‰ (2 standard error) for Kilgore Tuff samples erupted4100 km apart; however, ion microprobe data for single zircon crystals show significant diversity, with d18O values that range from –1.3‰ to 6.1‰. U–Pb zircon ages, mineral chemistry, whole–rock major and trace element geochemistry, Sr and Nd isotope data, and magmatic (liquidus) temperatures are similar and/or overlapping for all studied samples of the KilgoreTuff. Normal–d18OHeise tuff units that preceded the KilgoreTuff define a temporal compositional trend in trace element concentrations, trace element ratios, and Sr and Nd isotope ratios that is consistent with fractional crystallization from a common reservoir, whereas low–d18O Kilgore cycle units have compositions that define a sharp reversal in the temporal trend back towards the composition of the first normal–d18OHeise tuff (6.62Ma Blacktail Creek Tuff). The data support derivation of the voluminous low–d18O Kilgore Tuff from remelting of hydrothermally altered (18O depleted) intracaldera and subvolcanic portions of the Blacktail CreekTuff. Single pockets of melt with variable low–d18O values were assembled and homogenized on a caldera–wide scale prior to the climactic Kilgore Tuff eruption, and the best record of this process is provided by the d18O diversity in KilgoreTuff zircons. Temporal trends of oxygen isotopic depletion and recovery in rhyolite eruptions of the Heise volcanic field are clearly linked to caldera collapse events, and remarkably consistent with trends in the Yellowstone Plateau volcanic field. At Heise and Yellowstone, magmatic d18O values can be predicted on the basis of cumulative eruptive volumes, with a decrease in d18O by 1‰ for every 1000 km3 of erupted rhyolite.The KilgoreTuff of the Heise volcanic field has the same timing, magnitude of d18Odepletion, and cumulative eruptive volume as the youngest phase of voluminous rhyolitic eruptions in the Yellowstone Plateau volcanic field, indicating that the KilgoreTuff may serve as a useful analog for these and perhaps other large–volume low–d18O rhyolites on Earth.