Characterizing Crystal Populations for the Petrogenesis of the Post-Collapse Rhyolites in The Long Valley Caldera, California

摘要

Post-collapse rhyolites of the Long Valley in eastern California are the product of remelting of crystal mush bodies via mafic rejuvenation following the eruption of the Bishop Tuff. This is supported by mineral textures and major element geochemistry from the Resurgent Dome rhyolite, the Moat rhyolite, the Hot Creek Flow, and the Deer Mountain rhyolite. New 40Ar/39Ar eruptive ages are reported for the Moat rhyolites (525 ka, 333 ka, 118 – 94 ka), Hot Creek Flow (312 – 295 ka), and the Deer Mountain rhyolite (65 ka). The initial post-collapse eruptions, resulting in the Resurgent Dome rhyolite, appear to be derived from the residual Bishop Tuff magma chamber and are characterized by euhedral, juvenile plagioclase and orthopyroxene phenocrysts in crystal poor rhyolites.At 525 ka, Moat rhyolites mark a major increase in mafic rejuvenation inducing rhyolitic magma production. Reverse zonation (An18 – An39) present in plagioclase populations indicates mafic magma injecting into the crystal mush zone. Similarly, anti-rapakivi textures and plagioclase cores with labradorite compositions (An56) found within 333 ka Moat rhyolites indicate mixing between mafic and rhyolitic melts. Spatially and temporally related Hot Creek Flow rhyolites appear to be the results of crystal poor rhyolitic melt extracted from the same magma body. A decrease in mafic inputs is exhibited the youngest Moat rhyolite where andesine core and oligoclase rim overgrowths are separated by dissolution surfaces in plagioclase populations. This relationship is displayed again in the culminating Deer Mountain eruption, but decrease in mafic input indicates a waning of the Long Valley magmatic system.