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| Titel |
Disequilbrium plagioclase in Icelandic magma chambers |
| VerfasserIn |
Sigurjón B. Thórarinsson, Christian Tegner, Tod E. Waight |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091673
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| Publikation (Nr.) |
 EGU/EGU2014-5977.pdf |
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| Zusammenfassung |
| The Austurhorn intrusive complex of southeast Iceland represents the exhumed roots of a ~6
Ma central volcano. A modally layered gabbro occupies the central part of the complex. The
outer part is dominated by granophyre which frequently carries pillow- or sheet-like enclaves
of basalt, interpreted to reflect mafic magma intruded into a molten rhyolitic magma
chamber. Some of the basaltic pillows are highly plagioclase-phyric (20–30%) and
carry macrocrysts with cores that are more primitive (An85–75) than overgrowth
rims and groundmass grains (An67–43), indicating disequilibrium with the host
melt.
The layered gabbro comprises more than 800 m of cpx + plag ± ol ± Fe-Ti oxide
cumulates and can be divided into at least 8 macrorhythmic units formed by repeated
injections of mafic magma. Two distinct populations of high- and low-An plagioclase crystals
have also been identified in the gabbro. The high-An grains are typically 2–4 mm, prismatic
to tabular in shape and have bytownite cores (An87–69) abruptly zoned to labradorite
rims (An65–55). The low-An grains (An69–55) are smaller (1–2 mm), typically
lath-shaped and weakly zoned (ΔAn -¤ 7 mol.%). Microdrilling followed by TIMS Sr
isotope analyses of high- and low-An plagioclase grains indicate that the rims of the
high-An grains (87Sr/86Sr = 0.70337-0.70340) are in isotopic equilibrium with the
low-An grains (0.70338). The cores of the high-An grains, however, are generally
more radiogenic (0.70342-0.70344) than coexisting rims, indicating a xenocrystic
origin.
We suggest that the high-An plagioclase macrocrysts of the basalt enclaves and the
layered gabbro are two of kind and grew in a deeper, more primitive magma chamber. The
macrocrysts were entrained by genetically unrelated magmas and were carried into the
Austurhorn plumbing system during magma recharge events. The low-An grains, on the other
hand, are interpreted to record in situ growth. |
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