GLASS AND CRYSTAL CHEMISTRY FROM MIOCENE JARBIDGE RHYOLITE LAVAS (NEVADA, USA): CONSTRAINTS ON CRYSTAL-RICH RHYOLITE PETROGENESIS AND EFFUSIVE VOLCANISM

Understanding how large volumes of rhyolite magma form, are stored, and erupt are important questions in volcanology and petrology. Addressing these issues depends on decoding how erupted rhyolite resembles and/or is related to possible equivalent plutonic rocks, as well as fully constraining the physical, geochemical, and temporal characteristics of the rhyolites. To address these issues, we have been studying ~16.1 to ~13.6 Ma Jarbidge Rhyolite domes and lavas that crop out across northern Nevada (USA). The most voluminous exposures are in the Jarbidge Mountains, where at least 500 km³ of metaluminous to slightly peraluminous, ferroan calc-alkalic, and phenocryst-rich (up to 40 modal%) A-type rhyolite erupted from 16.1 to 15.0 Ma. Younger eruptions are time-transgressive and concurrent with eastward-younging Basin and Range extension. Previous studies, based on limited geochemical data, classified the Jarbidge Rhyolite as a high-Si topaz rhyolite. Cenozoic western USA topaz rhyolites are enriched in F (typically >0.2 wt%) and incompatible lithophile elements. Genetic models for topaz rhyolite formation invoke melting of Precambrian crust by mafic magmas and an intimate spatial link to lithospheric extension, which likely provide conduits for magma ascent and eruption. New electron microprobe analyses (n=85) of F (0.01 to 0.34 wt%; avg. = 0.14 wt%) and Cl (0.04 to 0.12 wt%; avg. Cl = 0.07 wt%) from four Jarbidge Rhyolite lava vitrophyres overlap with the lowest Cl and F concentrations of western USA Topaz rhyolites and verify that Jarbidge Rhyolite units should be considered topaz rhyolites. Vitrophyre glass chemistry ranges from ~73 to 78 wt% SiO₂, which is generally more silicic than bulk rock SiO₂ values from the same samples. Chemical data from sanidine is consistent with cumulate heating and thermal rejuvenation affecting a mushy magma reservoir in the upper crust. The presence of abundant sanidine+quartz+oxide±pyroxene glomerocrysts in all samples are also consistent with this interpretation. LA-ICP-MS Ti-in-quartz analyses yield temperatures from ~1046 to 819°C. These temperatures, coupled with the F and Cl concentrations, would effectively lower viscosity and can explain how crystal-rich Jarbidge rhyolite magmas could actually erupt effusively.