CONSTRAINING THE PETROGENESIS OF THE QUATERNARY TOPAZ RHYOLITE LAVA DOMES OF THE BLACKFOOT VOLCANIC FIELD, SOUTHEASTERN IDAHO

The China Hat lava dome field, a set 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 more crustal 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 via integrated field, petrographic, and numerical modeling methods. 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 inherited from older cycles of magmatism (antecrysts). We focus on thin sections from the three lava domes of the China Hat lava dome field. Results indicate an early population of quartz and plagioclase antecrysts formed and underwent resorption with no later crystal growth. Other phenocrysts are termed as a later population of antecrysts that are euhedral with no resorption or skeletal textures. Late population phenocrysts are termed as autocrysts that have skeletal textures indicative of oversaturation with the surrounding melt prior to volatile venting.