PRELIMINARY TEXTURAL CHARACTERIZATION OF PHENOCRYSTS IN 57 KA TOPAZ RHYOLITE LAVA DOMES IN THE BLACKFOOT VOLCANIC FIELD, SE IDAHO

China Cap is one of three coeval topaz rhyolite lava domes erupted at 57 ka near the center of the Blackfoot volcanic field (BVF), SE Idaho. Bimodal volcanic rocks of BVF closely resemble coeval Quaternary rocks erupted to the north along the Eastern Snake River Plain segment of the Yellowstone-Snake River Plain volcanic track. However rhyolites in BVF are distinguished by having more evolved isotopic ratios, as well as having a more complex assemblage of phenocrysts that includes hydrous phases (biotite and hornblende), thorite, and vapor-phase topaz. This study seeks to improve our understanding of the unique conditions of magma evolution that led to these differences. The robust phenocryst assemblages have the potential for well constraining magma evolution via quantitative thermodynamic modeling, but there is currently not enough information for assessing which were in equilibrium with their glass matrix prior to eruption (autocrysts), and which may have been inherited from older cycles of magmatism (antecrysts). As a preliminary basis for assessing the origin of the phenocryst cargo we have focused on quartz phenocryts. Well preserved quartz phenocrysts separates have be produced by acid dissolution methods (e.g., Bindeman, 2003). Early results suggest that there are two size populations of quartz grains 1.0-1.2 and ~0.5 mm. Both exhibit euhedral prismatic and subhedral rounded morphology. Different size populations of quartz suggest that at least one was inherited, possibly from earlier formed parts of the magma reservoir system, prior to eruption.