Paper No. 2
Presentation Time: 8:15 AM
DISSECTED CALDERA PROVIDES A WINDOW INTO THE LINKED VOLCANIC-PLUTONIC ORIGINS OF THE 34 MA CAETANO TUFF SUPERERUPTION, NEVADA
Post-collapse extension and faulting have exposed a >5 km cross-section of the late Eocene (34 Ma) Caetano caldera, source of the >1,100 km3 Caetano Tuff supereruption. A volcanic and plutonic rock suite, consisting of intracaldera tuff >3 km thick, pre- and post-caldera lavas, silicic ring-fracture intrusions, a tuff dike that fed the early eruption, and two generations of resurgent granitic plutons, is exposed in tilted fault blocks. Petrologic, isotopic, and geochronologic datasets, synthesized with geologic mapping, constrain genetic relationships between volcanic and plutonic caldera components. The Caetano Tuff is a crystal-rich (~30-50 vol%) rhyolitic ignimbrite zoned upward from ~77 to 72 wt% SiO2. Whole-rock major and trace elements, mineral chemistry, O isotopes, and U-Pb and Ar-Ar geochronology indicate a genetic connection between the less-evolved upper Caetano Tuff and the two granitic plutons that intrude the caldera, suggesting that the plutons represent unerupted, mushy equivalents of the caldera-forming eruption. Zircon U-Pb geochronology obtained by SHRIMP-RG show that magma assembly time scales for all extrusive and intrusive rocks spanned several million years (~37-34 Ma), with a major crystallization pulse within ~100 ka of the caldera’s 40Ar/39Ar sanidine eruption age (34.01±0.05 Ma). Recharge of the Caetano magma chamber shortly prior to eruption is evident by sharp rim-ward increases in Ba concentrations in sanidine phenocrysts, up to ~30,000 ppm. Crystal “memory” of the recharge event(s) is most pronounced in the less-evolved upper Caetano Tuff, which represented the deeper parts of the Caetano magma chamber. Reactivation time scales were sufficiently short to mobilize >1,100 km3 of crystal-rich rhyolite while preserving compositional gradients in the erupted deposits. Caetano demonstrates that "super-sized" magma chambers do not require mutually exclusive volcano-pluton origins, and calls into question the prevailing paradigms of crystal-poor rhyolites extracted from crystal mushes or crystal-rich “monotonous intermediates” sourced from homogeneous dacitic magma reservoirs.