GEOLOGICAL AND PETROLOGICAL OBSERVATIONS OF THE LONG CANYON DOME RHYOLITE, SIERRA NEVADA

Long Canyon Dome (LCD, 2880 m elevation) is located at the northern edge of Long Canyon in California’s south central Sierra Nevada, approximately 30 km west of the northern limit of the Coso Volcanic Field. LCD is situated within a few kilometers of 3 other rhyolitic domes that erupted more than 2 ma known as Monache Mtn, Templeton Mtn, and Little Templeton Mtn (Bacon and Duffield, 1981). Previously dated sanidine crystals (Bacon and Duffield, 1981) and 238U-230Th and U-Pb dating of LCD zircons supports the 185 ka eruption age from Bacon and Duffield (1981). Geologic mapping indicates that LCD is composed of at least two separate lobes of porphyritic rhyolite lava, which are distinguishable based on vesicularity and biotite abundance. The larger lobe, measuring ~35 x 10⁶ m³ in volume, contains sanidine (10 vol%), plagioclase and quartz (5 vol% each), and traces of biotite, magnetite, ilmenite, allanite and rutile. Exposed on the westernmost edge of this larger lobe of rhyolite is a 12-m-thick outcrop of jig-sawed rhyolite fragmental debris characterized by black and glassy obsidian flakes interbedded with 15-25-cm-thick lenses of clast-supported blocky and poorly vesicular pumice clasts ranging in diameter from 2-25 cm, indicating that LCD was initially emplaced under thick glacial ice. The smaller eastern lobe accounts for ~15 x 10⁶ m³ of rhyolite and contains equivalent amounts of sanidine, plagioclase, quartz, allanite and oxides as the larger western lobe, but approximately double the abundance of biotite and is more vesicular. Geothermometry calculations of magnetite-ilmenite and Ti-in-Zircon (Watson et. al, 2006) yield pre-eruptive temperatures of 676-723°C and 664-789°C, respectively, however more analyses are needed in order to statistically evaluate if differences exist in pre-temperatures for the different rhyolite lava lobes. LCD is partially enclosed by a poorly consolidated tuff ring composed of blocky pumice fragments, ash and accidental lithics. Extending away from LCD to the north lies a 12 x 10⁴ m² area devoid of virtually any plant material due to the poorly sorted nature of the pyroclastic flow deposit that was emplaced during the LCD eruption there.