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| Titel |
Growth mechanisms for decompression plagioclase rims around metastable kyanite from high-pressure felsic granulites (Bohemian Massif): A thermodynamic approach |
| VerfasserIn |
Lucie Tajcmanova, Rainer Abart, Gregor Neusser, Dieter Rhede |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250050798
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| Zusammenfassung |
| Samples of high-pressure felsic granulites from the Bohemian Massif (Variscan belt of
Central Europe) characterized by a peak metamorphic (high pressure) mineral assemblage of
Grt-Ky-Pl-Kfs-Qtz±Bt contain various locally equilibrated domains which where developed
during decompression around metastable kyanite crystals. The variability depends on the
local source of important components in the vicinity of the metastable kyanite grain,
which in some case, may result in crystallization of spinel or spinel-plagioclase
symplectite if former garnet or biotite was present in the kyanite domain. All these
domains are silica undersaturated and it was shown that plagioclase rim serves as an
important barrier which conserves the silica undersaturated conditions and thus
allows for further local crystallization of spinel. The present work is focused on the
development of the plagioclase rim as the mineral reaction responsible for the formation
of decompression plagioclase and the mechanisms that lead to the typical corona
structures are still enigmatic. In particular, cases, where plagioclase crystals from the
rock matrix are decomposed in the vicinity of metastable kyanite porphyroblasts
and the so liberated components serve for crystallization of the decompression
plagioclase rim at the kyanite interface, are puzzling, because the driving force for
such material transfer in a system that is saturated with respect to plagioclase is not
obvious.
The studied samples show well developed plagioclase reaction rims around kyanite grains
in two microstructural settings where kyanite is randomly distributed in the polyphase matrix
in one setting and enclosed within large perthitic K-feldspars in the other setting. Kyanite is
regarded as a relic of an earlier high-pressure metamorphic assemblage, which became
metastable during low pressure overprint. Plagioclase rims from both microstructural
settings show continuous changes in composition from An32-25 at the contact to
kyanite to An20-19 at the contact to the matrix or to the K-feldspar respectively.
Mass balance considerations show that either only a small amount of kyanite or no
kyanite was consumed to provide the Al2O3 component for the plagioclase growth.
In majority, Al2O3 and SiO2 were supplied together with CaO and Na2O from
the surrounding matrix material. A thermodynamic analysis reveals that kyanite
became metastable at the interface with the host perthite at pressures as high as
the peak metamorphic pressure and therefore the plagioclase rim started to grow
immediately at high-pressure conditions. On the contrary, kyanite remained stable in the
polyphase matrix down to pressures of about 16 kbar and started to grow only at
a relatively late stage during the decompression. The chemical variation across
the plagioclase rim from more anorthitic at the kyanite interface to more albitic
at the interface to the host or matrix is similar for both microstructural settings.
From the phase equilibria point of view, the most anorthitic part of the rim (i.e. the
inner part) should have crystallized under lower-most pressure conditions. The
epitactic nature of the plagioclase rim however indicates outward growth which is
documented by the inheritance of structural elements form the host feldspar. A kinetic
fractionation model is suggested here to explain the chemical zoning across the rim. |
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