COMPOSITIONAL RELATIONSHIPS AMONG THE MULTIPLE RHYOLITE LAVAS AND THE COUGAR POINT TUFF OF THE MIOCENE BRUNEAU-JARBIDGE ERUPTIVE CENTER, YELLOWSTONE HOTSPOT

The Bruneau-Jarbidge eruptive center of the Miocene Yellowstone hotspot generated large volumes of high-temperature silicic magma for an extended interval from 12.7 to ~8 Ma, represented by the ten eruptive units of the Cougar Point Tuff (CPT) and intercalated lavas, and subsequent eight to twelve rhyolite lava flows. First order estimates of the average volume of tuffs are 100–1000 km³ each and 10–100 km³ each for the lavas. These tuffs and lavas are characterized by a sparse anhydrous crystal cargo including equilibrium assemblages of pigeonite and augite. Pyroxene thermometry yields pre-eruptive magma temperatures of 800–1000 °C for both tuffs and lavas, and the compositional ranges of individual phases (feldspars, pyroxenes, Fe-Ti oxides) in the lavas overlap those found in the CPT. Whereas the CPT displays a characteristic polymodal behavior in individual eruptive units that is clearly evident in glass and pyroxene compositions, the lavas each are unimodal. Detailed compositional investigations of pyroxene in thin section by electron microprobe demonstrate no detectable core-to-rim zonation, and pyroxene compositions are independent of their textural relationships to glass and other minerals. Textural variety in individual samples is represented by apparent equilibrium (e.g. solitary euhedral crystals in glass) as well as reaction and disequilibrium textures (e.g. plagioclase cores with extremely fine-grained intergrowths of pigeonite, titanomagnetite and glass). Although the post-CPT lavas do not share common compositional modes with one another, many of them do reproduce the discrete modes that characterize the CPT and intercalated lavas. These modes have χ _En and χ_Fs identical to their counterparts in earlier eruptions, but typically have slightly higher χ_Wo and thus record higher pre-eruptive temperatures. The detailed similarities in composition and temperature among the tuffs and lavas suggest intimately related, complex histories of generation, differentiation and storage for silicic magmas of the Bruneau-Jarbidge eruptive center.