CHARACTERIZING CRYSTAL ASSEMBLAGES FOR THE PETROGENESIS OF POST-COLLAPSE RHYOLITES IN THE LONG VALLEY CALDERA, CALIFORNIA

Post-collapse rhyolites erupted from the Long Valley caldera have long been considered the product of a single, long-lived, voluminous magma body characterized by thermal and compositional stratification (Bailey, 1976). However, recent geobarometric data (Gualda and Ghiorso, 2013) combined with Nd and Hf isotopes and geochronology from zircon populations (Simon et al., 2014) suggest a system involving several independent magma chambers. Despite recent work, questions remain regarding the characteristics of individual magma batches and whether or not each successive eruption incorporates recycled material from previous events. Therefore a detailed study of crystal assemblages from the Resurgent Dome Rhyolite (700 ka), Moat Rhyolites (576 ka), the Hot Creek Flow (335 ka), and the Deer Mountain Rhyolite (100 ka) has been undertaken to constrain on the evolution of the post-collapse magma system beneath the Long Valley caldera. This study examines crystal assemblage morphology, geochemistry, and geochronology.

Initial petrographic observations suggest complex magmatic origins for the post-collapse rhyolites. The absence of resorption textures coupled with oscillatory and normal zoning of feldspar populations within the Resurgent Dome Rhyolite suggest a simple evolutionary history with small scale compositional changes within the magma chamber. Distinct textural differences within crystal populations from the Moat Rhyolite, Hot Creek Flow, and Deer Mountain Rhyolite suggest different magmatic origins. Moat Rhyolites show intense resorption and sieve textures within plagioclase populations but also exhibit distinct rim overgrowths. Glomeroporphyritic sanidine, biotite, plagioclase, and pyroxene from the Hot Creek Flow similarly show high degrees of resorption and embayment. Additionally plagioclase grains display sieve textured cores. Intense resorption is also apparent in Deer Creek Rhyolite where plagioclase populations exhibit skeletal textures. An increase in disequilibrium and dissolution textures with decreasing age may indicate changes in the pressure, temperature, compositional parameters of the post-collapse plumbing system. Distinct populations of crystals and textures within each unit suggest different pre-eruptive evolutionary paths.