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

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State: Public
Version: Final published version
Serval ID
serval:BIB_3ADA40D4EC93
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Insights into shallow magmatic processes in large silicic magma bodies: the trace element record in the Fish Canyon magma body, Colorado
Journal
Contributions to Mineralogy and Petrology
Author(s)
Bachmann O., Dungan M.A., Bussy F.
ISSN-L
0010-7999
Publication state
Published
Issued date
2005
Peer-reviewed
Oui
Volume
149
Pages
338-349
Language
english
Abstract
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., 100 ppm Sr and 500
ppm Ba) and low concentrations of Y (< 7 ppm) and HREE (&SIM; 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 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 &SIM; 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 (&SIM; 4%) + titanite (&SIM; 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.
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01/10/2012 19:07
Last modification date
20/08/2019 13:30
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