| 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 (ESRP) 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 of the rhyolites 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). I focus on thin sections from the three lava domes of the China Hat lava dome field (CHDF). Results indicate three populations of phenocrysts in the rhyolites of the CHDF. The first population of phenocrysts are autocrysts that are in equilibrium with the surrounding melt and euhedral or skeletal textures because of oversaturation of the melt during magma ascent. The second population are antecrysts that come from other high melt zones or border zone mush within the magma reservoir and underwent resorption events during magma transport and magma mixing that resulted in felsic magmatic enclaves in the erupted melt. The third population are xenocrysts that are from mafic compositions from magma recharge event(s) in which xi olivine, plagioclase, and pyroxene underwent resorbtion and resulted in mafic magmatic enclaves in the final erupted melt. In addition, I conduct quantitative hypothesis testing of selected mafic parental magma compositions of the ESRP and CHDF rhyolite magma systems using thermodynamic modeling to a more felsic composition similar to those of the Craters of the Moon-Cedar Butte (COM-CB) trend and the topaz rhyolites of the CHDF. Hypotheses include equilibrium and fractional crystallization of parental magmas and wallrock assimilation of felsic Archean upper crust along with fractional crystallization of parental magmas. Evaluation of phase equilibria, major element oxides, and chemical evolution pathways is included. Best fit for the China Hat magma system is further constrained by heavy isotope ratios. While the modeling does not fully follow the fractional crystallization trends of COM-CB or CHDF with respect to major oxides and mineral phase precipitation events and amounts, similar mineral phases are produced by modeling and therefore aides in the hypothesis that between 15-30% assimilation of felsic Archean upper crust along with fractional crystallization is required to produce felsic magmas from a mafic parental basalt based on the most primitive olivine tholeiite of the BVF. |