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| Titel |
Submicron scale 3D investigation of kelyphyites after garnet:
thermodynamics, crystallographic orientation, microstructure evolution and
fluid-rock interactions |
| VerfasserIn |
Júlia Dégi, Kalman Török, Bianca Nemeth, Dieter Rhede, Agnes Takacs, Gerlinde Habler, Rainer Abart |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122117
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| Publikation (Nr.) |
 EGU/EGU2016-1062.pdf |
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| Zusammenfassung |
| Complex kelyphitic rims after garnet were studied in a lower crustal mafic granulite xenolith
from the Bakony–Balaton Highland Volcanic Field, W-Hungary. The xenolith is dominated
by a garnet granulite mineral assemblage equilibrated at 900 ˚ C, 1.4 GPa within the
overthickened orogenic root of the Alps. Garnet breakdown was initiated during the extension
of the Pannonian Basin and remained continuous until the xenoliths reached the surface. This
resulted in the formation of various microstructural domains within the kelyphitic rims which
were distinguished to three main types: fine-grained symplectites, recrystallized
symplectites and cyrstallized melts. Fine-grained symplectites are the products of
isochemical breakdown of garnet to pure anorthite, Al-orthopyroxene and hercynitic
spinel. Nanoscale topography built up by curved chains of humps on the garnet
surface showing regular spatial distribution is observed at the reaction front in 3D
reconstructions. These patterns follow the contours of anorthites nucleating at the
reaction front. This suggests that diffusion-controlled material transfer in solid state
together with surface energy minimization determines symplectite microstructure. The
latter leads to continuous isochemical coarsening getting further from the reaction
front.
Slight increase of Na and Ti-content in fine-grained symplectites is associated with
sudden changes in 3D microstructure reflecting the effect of aquaeous fluids infiltrating to the
reaction rim. A similar aquaeous fluid may have induced the formation of nearly isochemical
melting and in situ recrystallization of the symplectites to form recrystallized symplectites.
Some of these remained pristine, while some experienced ductile deformation and re-melting
due to a reaction with an external melt in the lower crust. Well-crystallized melt pockets
consisting of complexly zoned pyroxene, spinel and plagioclase grains were formed around
1000 ˚ C in this stage.
Following this, interaction with the host basalt took place and resulted in chemical
modification of recrystallized symplectites and melt pockets as well and the formation of melt
channels at the kelyphite boundaries partly penetrating into the reaction rim. Although
orthopyroxene should form during garnet breakdown, according to our EBSD studies, all
pyroxenes in the kelyphite have clinoenstatite microstructure. This is explained by the
transition of orthopyroxene to protopyroxene above 1100 ˚ C due to the heating effect of the
host magma, which was followed by protopyroxene to clinopyroxene transition during
quenching. |
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