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Insights into shallow magmatic processes in large silicic magma bodies: the trace element record in the Fish Canyon magma body, Colorado

Bachmann, O. ; Dungan, M. ; Bussy, F.

In: Contributions to Mineralogy and Petrology, 2005, vol. 149, no. 3, p. 338-349

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    Titre
    • Insights into shallow magmatic processes in large silicic magma bodies: the trace element record in the Fish Canyon magma body, Colorado
    Auteur
    • Bachmann, O.. Section des Sciences de la Terre, Université de Genève, 13, rue des maraîchers, 1211, Genève 4, Switzerland
    • Dungan, M.. Section des Sciences de la Terre, Université de Genève, 13, rue des maraîchers, 1211, Genève 4, Switzerland
    • Bussy, F.. Faculté des géosciences et de l'environnement, Université de Lausanne, Switzerland
    Type de document
    • Postprint
    Langue
    • Anglais
    Publié dans
    • Contributions to Mineralogy and Petrology, 2005, vol. 149, no. 3, p. 338-349. Springer-Verlag; http://www.springer.de
    Autre version électronique
    Classification
    • Sciences de la terre
    Identifiant OAI-PMH
    • oai:doc.rero.ch:311032
    Summary
    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 Tuff. The high-silica rhyolite glass is characterized by relatively high concentrations of feldspar-compatible elements (e.g., 100ppm Sr and 500ppm Ba) and low concentrations of Y (<7ppm) and HREE (~1ppm 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 four- to six-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. Sr/Y in Fish Canyon glass (13-18) is lower than the typical "adakitic” value (>40), confirming that high Sr/Y is a reliable indicator of high-pressure magma generation and/or differentiation wherein garnet is implicated