INSIGHTS ON ERUPTIONS OF CRYSTAL-RICH RHYOLITE MAGMA: CRYSTAL CHEMISTRY CONSTRAINTS ON MID-MIOCENE JARBIDGE RHYOLITE (NEVADA, USA) VOLCANISM

The Jarbidge Rhyolite crops out across northeastern Nevada (USA) and consists mainly of quartz-rich porphyritic (up to 40%) lavas. The thickest and most extensive exposures are found in the vicinity of the Jarbidge Mountains. There, at least 500 km³ of metaluminous to slightly peraluminous ferroan calc-alkalic rhyolite erupted from 16.1 to 15.0 Ma, at times coeval with local basalt crustal input (Brueseke et al., 2014). Jarbidge Rhyolite volcanism gets younger to the east; eruptions in the Utah-Nevada border region are ~3-4 Ma younger. This time-transgressive behavior mirrors major caldera-forming silicic activity just to the north on the Snake River plain, but predates that magmatism at any given longitude by ~2-3 myr. In order to better understand how Jarbidge Rhyolite magmas evolved and erupted, we present new mineral (individual phenocrysts and glomerocrysts) and glass chemistry obtained via electron microprobe from four Jarbidge Rhyolite lava vitrophyres. Analyzed feldspars are euhedral to subhedral, with sieve textures common in only one of the rocks. Alkali feldspars consist solely of the mineral sanidine (Or_46-65) and include one analyzed phenocryst with a distinct, reversely zoned, Ba-rich rim. Plagioclase feldspars are predominately An_18-42 and show little compositional zonation. Pyroxenes are only present in one sample and show exsolution intergrowth between clinopyroxene and orthopyroxene. Quartz grains in all samples are mostly subhedral and typically lack zoning. Glass chemistry ranges from ~73 to 78 wt. % SiO₂, which overlap, but are generally more silicic than bulk rock SiO₂ values from the same samples. Ti-in-quartz analyses via LA-ICP-MS (aTiO₂ = 0.55) yield temperatures from ~1046 to 819°C. Avg. F (1300 ppm) and Cl (700 ppm) from 85 glass microprobe analyses overlap with some western USA Topaz rhyolites. Glass chemistry also denotes Ba-rich (600-1000 ppm Ba, and < 500 ppm Cl) and Ba-poor (<600 ppm Ba, and 1000-1600 ppm Cl) populations. These F and Cl concentrations, coupled with the high temperatures recorded by quartz phenocrysts, likely explain the ability of crystal-rich Jarbidge rhyolite magmas to actually erupt and flow as lava. The alkali feldspar with a Ba-rich rim is consistent with cumulate heating via injection of a higher-T, less evolved magma into a crustal mushy magma reservoir.