Rapid extraction of discrete magma batches from a large differentiating magma chamber: the Central Plateau Member rhyolites, Yellowstone Caldera, Wyoming

Abstract

The Central Plateau Member rhyolites have been erupted between 173 and 70 ka and are the youngest Yellowstone intracaldera rhyolites. They mostly comprise very voluminous lava flows totaling ~600 km3 in volume. Their eruptive vents define two NNW-trending lineaments which are aligned with regional faults. We present new whole rock, glass, and mineral analyses and propose a petrogenetic and volcano-tectonic model for these rhyolites. At a caldera-wide scale, there is a temporal enrichment in elements such as Nb, Y and HREE, and a depletion in Sr, Ba, and Ce/Yb. Simultaneously, clinopyroxene becomes less magnesian while Ti contents in quartz decrease. By contrast, quartz in all rhyolites is rounded and bears long glass re-entrants, suggesting heating. Based on these data and observations, we propose that the Central Plateau Member rhyolites have been generated as follows. A hydrothermally altered low-δ18O rhyolitic protolith beneath the Mallard Lake Resurgent Dome in the southwestern part of the caldera started to melt at ~250 ka. Repeated heating pulses caused the melting front to expand radially, and a large crystal mush formed beneath much of the caldera. The mush was able to differentiate but not erupt due to its high crystallinity and viscosity. Further inputs of heat and silicic magma in this mush increased the degree of melting, forming crystal-poor magma batches which erupted a few hundred to a few thousand years later through regional faults to form the Central Plateau Member rhyolites.