HR: 0800h
AN: V51C-0720    [Abstracts]
TI: High-silica rhyolite magmatism in the Big Pine volcanic field, eastern California
AU: * Lidzbarski, M I
EM: marsha.lidzbarski.97@csun.edu
AF: California State University, Northridge, Dept of Geological Sciences, Los Angeles, CA 91330, United States
AU: Vazquez, J A
EM: jvazquez@csun.edu
AF: California State University, Northridge, Dept of Geological Sciences, Los Angeles, CA 91330, United States
AB: The Quaternary Big Pine volcanic field (BPVF) located in the Owens Valley of eastern California is dominated by basaltic cinder cones and associated lava flows, but contains a single rhyolite lava erupted at circa 1 Ma. Despite its uniqueness, the petrogenesis of this rhyolite is poorly known. At nearby Coso volcanic field, an abundance of rhyolite relative to basalt suggests crustal melting by mafic magmas stalled in mid to upper crustal reservoirs, whereas the paucity of rhyolite relative to basalt at BPVF suggests only brief crustal residence of ascending mafic magmas (Mordick and Glazner, 2006). In order to determine the origin of rhyolite magmatism at BPVF (e.g., crustal melting versus extreme fractionation), we have examined the geochemical and petrographic characteristics of the Fish Springs high-silica rhyolite. The Fish Springs rhyolite comprises a single thick coulee with a volume of at least 0.05 km³ (DRE) of highly evolved (~76 wt.% SiO₂) magma. The outer portions of the coulee are composed of autobrecciated and felsitic rhyolite, and internal portions, as exposed by quarrying, are pumiceous perlite with local obsidian. Fish Springs rhyolite is crystal poor (~1%), with small (<0.5 mm) phenocrysts of generally euhedral to subhedral plagioclase, sanidine, quartz, orthopyroxene, clinopyroxene, biotite, hornblende, Fe-Ti oxides, apatite, pyrrhotite, and zircon, as well as apparent xenoliths and xenocrysts of metamorphic and igneous wallrocks. Orthopyroxene phenocrysts show compositional zoning, with rims that contain higher Mg and lower Fe concentrations than cores. Trace element concentrations in Fish Springs rhyolite are characterized by very low concentrations of typically compatible elements such as Ba (~15 ppm), Sr (~8 ppm), La (~10 ppm) and Zr (~80 pm), as well as a pronounced europium anomaly, comparable to other high-silica rhyolites elsewhere in the Owens Valley, and suggesting high degrees of feldspar and accessory mineral fractionation. Samples taken from different portions of the coulee are essentially identical in composition, suggesting effusion of relatively homogenous rhyolite. Zircon saturation thermometry yields temperatures of approximately 730° C, comparable to highly evolved rhyolites at nearby Coso and Long Valley. Taken together, the relatively low temperature, multi-phase saturation, and low crystallinity of the Fish Springs rhyolite suggest an origin via fractionation of highly evolved melt from granitic mush, similar to high-silica rhyolites from voluminous caldera systems, rather than simply by localized crustal melting. Nevertheless, scattered xenoliths and xenocrysts record at least minor contamination of the rhyolite. Near-rim reverse zoning in orthopyroxenes suggests that heating and/or magma mixing affected the late history of the Fish Springs rhyolite, perhaps serving to catalyze magma ascent and eruption. Ref: Mordick and Glazner, 2006, CMP 152: 111
DE: 1036 Magma chamber processes (3618)
DE: 3640 Igneous petrology
DE: 3641 Extrusive structures and rocks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting