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

Post-collapse rhyolites in the Long Valley caldera have been considered the product of a single long-lived, voluminous magma chamber. However, recent work suggests a system involving several independent magma bodies. Questions remain regarding the characteristics of individual magma batches and whether or not each successive eruption incorporates residual material.

New ⁴⁰Ar/³⁹Ar dates have been obtained for Moat Rhyolites (525 ka, 343 ka, and 94 ka), the Hot Creek Flow (298 ka), and the Deer Mountain Rhyolite (65 ka). Initial post-collapse eruptions produced the Resurgent Dome Rhyolite (700 ka). This unit lacks resorption textures within feldspar populations suggesting a simple pre-eruptive history. Plagioclase textures and glomeroporphyritic clusters of plagioclase and pyroxene crystals suggest equilibrium fractional crystallization. This coincides with geochemistry of Resurgent Dome plagioclase crystals, which display modest normal zonation with slightly more calcic cores (An₃₄) and sodic rims (An₃₂). Orthopyroxene crystals also indicate crystallization from a rhyolitic melt (Fs₅₁).

Conversely, petrographic observations suggest complex magmatic origins for <525 ka post-collapse rhyolites. Distinct disequilibrium textures and phenocryst abundances within crystal populations from the Moat Rhyolite, Hot Creek Flow, and Deer Mountain Rhyolite suggest different evolutionary histories for each eruptive unit. Porphyritic Moat Rhyolites (18% phenocrysts) show extensive resorption and sieve textures within plagioclase populations, and also exhibit distinct rim overgrowths. Hot Creek Flow (2% phenocrysts) sanidine populations are often euhedral while resorbed plagioclase and biotite crystals form as glomeroporphyritic clusters. However, a population of plagioclase crystals display sieve textured cores and antirapakivi textures (core: An₂₇; rim: An₂). Extensive resorption is apparent in the porphyritic Deer Creek Rhyolite (22% phenocrysts) as plagioclase crystals exhibit skeletal textures. Following the eruption of the Resurgent Dome rhyolite, distinct phenocryst abundances and variable disequilibrium textures within crystal assemblages may indicate separate magma chambers are responsible for each post-collapse unit with little reincorporation of residual material.