HR: 10:30h
AN: V22A-01    [Abstracts]
TI: Insights into the near-solidus evolution of an erupted batholith: the trace element record in the Fish Canyon magma body, Colorado
AU: * Bachmann, O
EM: olivier.bachmann@terre.unige.ch
AF: University of Geneva, 13, rue des Maraichers, Geneva, 1211 Switzerland
AU: Dungan, M A
EM: michael.dungan@terre.unige.ch
AF: University of Geneva, 13, rue des Maraichers, Geneva, 1211 Switzerland
AU: Bussy, F
EM: Francois.Bussy@unil.ch
AF: University of Lausanne, Dorigny, Lausanne, 1015 Switzerland
AB: Highly evolved rhyolite glass plus near-solidus mineral assemblages in voluminous, dacitic, crystal-rich ignimbrites provide an opportunity to evaluate the late magmatic evolution of granodiorite batholiths. This study reports laser-ablation ICP-MS analyses of trace element concentrations in feldspars, hornblende, biotite, titanite, zircon, magnetite, and interstitial glass of the crystal-rich Fish Canyon magma. The high-silica rhyolite glass is characterized by relatively high concentrations of feldspar-compatible elements (e.g., 100 ppm Sr and 500 ppm Ba) and low concentrations of Y (<7 ppm) and HREE (~1 ppm Yb), hence high LREE/HREE (Ce/Yb>40) compared to many well-studied high-silica rhyolite glasses and whole-rock compositions. Most minerals record some trace element heterogeneities, with, in particular, one large hornblende phenocryst showing 4-6 fold core-to-rim increases in Sr and Ba coupled with a decrease in Sc. The depletions of Y and HREE in the Fish Canyon glass relative to the whole-rock composition (concentrations in glass ~30% of those in whole-rocks) reflect late crystallization of phases wherein these elements were compatible. As garnet is not stable at the low-P conditions at which the Fish Canyon magma crystallized, we show that a combination of modally abundant hornblende (~4%) + titanite (~0.5-1%) and the highly polymerized nature of the rhyolitic liquid led to Y and HREE depletions in melt. Relatively high Sr and Ba contents in glass and rimward Sr and Ba increases in euhedral, concentrically-zoned hornblende suggest partial feldspar dissolution and a late release of these elements to the melt as hornblende was crystallizing, in agreement with textural evidence for feldspar (and quartz) resorption. Both observations are consistent with thermal rejuvenation of the magma body prior to eruption, during which the proportion of melt increased via feldspar and quartz dissolution, even as hydrous and accessory phases were crystallizing. These thermal oscillations may be common in batholithic chambers, and could provide a mechanism to stir the magma and induce the whole-rock homogeneity that characterises these bodies.
DE: 1020 Composition of the crust
DE: 1065 Trace elements (3670)
DE: 3620 Crystal chemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly