Evolution of high-temperature, large-volume silicic magmas of the Miocene Yellowstone hotspot: The record of the Bruneau-Jarbidge eruptive center

The 12.7--10.5 Ma Cougar Point Tuff in southern Idaho, USA consists of ten large volume (>102--103 km3 each), high temperature (800--1000°C), rhyolitic ash flow tuffs erupted from the Bruneau-Jarbidge volcanic center of the Yellowstone hotspot. In addition, multiple rhyolite lava flows were erupted from ∼11.2--8.1 Ma, with volumes of <10 km3 to 200 km3 each. Units of the Cougar Point Tuff provide evidence for compositional and thermal zonation in pre-eruptive rhyolite magma, and suggest the presence of a long-lived reservoir that was tapped by numerous large explosive eruptions. Pyroxene compositions exhibit discrete compositional modes with respect to Fe and Mg that define a linear spectrum punctuated by conspicuous gaps. Airfall glass compositions also cluster into modes, and the presence of multiple modes indicates tapping of different magma volumes during early phases of eruption. Equilibrium assemblages of pigeonite and augite are used to reconstruct compositional and thermal gradients in pre-eruptive magma. The recurrence of identical compositional modes and of mineral pairs equilibrated at high temperatures in successive eruptive units is consistent with the persistence of their respective liquids in the magma reservoir. Recurrence intervals of identical modes range from 0.3 to 0.9 Ma and suggest possible magma residence times of similar duration.; Pyroxene compositions in the lavas are unimodal, and reproduce and extend the range of Fe-Mg modes observed in the tuffs to more Mg-rich varieties. Pyroxene thermometry yields pre-eruptive magma temperatures of 905 to 980°C, and individual modes consistently record higher temperatures than their preceding counterparts in the Cougar Point Tuff. Pyroxene compositions are homogeneous and independent of their textural environment. Zonations in plagioclase suggest that magmatic temperatures and/or H2O contents varied over a limited extent through time.; Collectively, the data suggest that compositional diversity was produced early in the history of the eruptive center and persisted through much of its lifetime. Mineral compositions and thermometry of the multiple lavas suggest that residual magmas may have been stored at high temperatures subsequent to the explosive phase of volcanism. Such persistent high temperatures and large eruptive magma volumes likewise require an abundant and persistent supply of basalt magmas to the lower crust, consistent with the tectonic setting of a continental hotspot.