HR: 1330h
AN: V12A-0563    [PDF]
TI: Transient Rhyolites: a Fresh Perspective on the Generation of Silicic Magmas Associated with Long Valley Caldera
AU: * Simon, J I
EM: jisimon@ucla.edu
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States
AU: Reid, M R
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States
AU: Reid, M R
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86011 United States
AB: The paradigm for the evolution of rhyolitic magma in the Long Valley caldera system includes episodic generation and protracted residence ($<$350 k.y) of precaldera (Glass Mountain) rhyolites that eventually served as forerunners to the nascent caldera magma chamber that erupted at 760 ka and produced the Bishop Tuff (BT). We present in situ 238U-206Pb ages and U concentrations for zircons from the "late" Bishop Tuff ("late"=Ig2NW) and from representative Glass Mountain rhyolites (domes OD, YG, and YA). Zircons in the three precaldera rhyolites crystallized from 2024$\pm$33-1675$\pm$43 ka (dome OD), 1023$\pm$35-857$\pm$27 ka (dome YG), and 1109$\pm$34-885$\pm$35 ka (dome YA) but, except for dome OD, most crystals grew within $<$100 k.y. of eruption. These crystallization intervals are independent of assumptions about the affinities between crystals and their host melts and, notably, they support Rb-Sr isotope evidence for differentiation and crystallization well before eruption. Zircons from the "late" Bishop Tuff (LBT) yield a mean age of 824$\pm$8 ka that is indistinguishable from that for the "early" Bishop Tuff (EBT). Zircons ages like those expected if there was a significant contribution from the precaldera rhyolites are absent and there is no significant evidence for zircon crystallization $>$200 k.y. prior to eruption, suggesting that the BT is a new batch of rhyolite. Uranium contents of zircon from the late and early BT pumice are distinct ($\sim$500-3600 ppm and $\sim$1000-5000 ppm, respectively) even though their age distributions are the same, suggesting that compositional zoning of the Bishop Tuff developed before most zircon crystallized. Given (1) this apparent lack of carry-over of older zircons between older and younger precaldera rhyolites, as well as between precaldera rhyolites and the Bishop Tuff and (2) the compositional differences between precaldera rhyolites and the Bishop Tuff (Metz and Mahood, 1991; Davies et al., 1994; Davies and Halliday, 1998), we envision a magma system where the pre- and caldera-related rhyolites represent isolated and/or transient magmas in a voluminous system of crystal mush rather than periodic tapping of a largely liquid, albeit stratified, magma reservoir.
DE: 1010 Chemical evolution
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 8400 VOLCANOLOGY
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting