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| Titel |
Ultra Sodic gedrite and micro-scale metasomatic processes in granulitised kyanite eclogites from the Rhodope UHPM Province, Greece |
| VerfasserIn |
Evangelos Moulas, Dimitrios K. Kostopoulos, James A. D. Connolly, Jean-Pierre Burg |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043706
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| Zusammenfassung |
| The Rhodope Massif occupies most of northeastern Greece and southern Bulgaria and
comprises high-grade metamorphic rocks such as metapelites with microdiamond inclusions.
The (U)HP mineral paragenesis underwent extensive metamorphic overprint at
granulite-facies followed by amphibolite-facies retrograde metamorphism.
A kyanite-eclogite that occurs as lenses in orthogneisses from Thermes village was used
to unravel the pressure-temperature-time path of the (U)HP rocks from the Rhodope.
The peak-pressure mineral assemblage is omphacite, garnet, kyanite, phengite,
rutile, apatite and zircon. Quartz is absent from the matrix and it can be found either
as inclusions in garnet or as post-peak veins. This late quartz contains primary
and secondary fluid inclusions implying the presence of a fluid phase during post
high-pressure metamorphism. Kyanite is never observed in direct contact with quartz being
armoured by an intervening stripe of plagioclase which itself shows zoning, becoming
increasingly albitic towards quartz. Plagioclase formation at the Ky-Qtz interface
requires certain Na and Ca influx that was provided by matrix omphacite. Two types
of symplectites were formed by reaction between omphacite and kyanite. Firstly,
corundum+plagioclase symplectites were formed at the expense of the jadeitic component of
omphacite during decompression. Subsequently, the residual diopsidic component of
omphacite reacted with the already formed corundum to give rise to spinel+plagioclase
symplectites. The previous mechanisms demonstrate metasomatism in the micro-scale
by diffusion controlled processes. During decompression matrix omphacite was
decomposed to amphibole+plagioclase symplectites which reacted with garnet to form
coronas consisting of two amphiboles (ortho- and clino-), plagioclase, ilmenite and
magnetite. Biotite and plagioclase are also found as symplectites replacing phengite
during decompression. Thermodynamic modelling of the symplectitic domains
that replace kyanite shows that the stability of these domains is sensitive to the
effective local chemical composition; in addition, analysis of phase relationships
demonstrated the existence of the observed assemblages at pressures lower than
1.3GPa.
The orthoamphibole identified in the garnet corona is sodic gedrite and classifies amongst
the most sodic compositions published in the literature. Sodic gedrite is a phase commonly
found in high-temperature metamorphosed rocks. The mechanism responsible for the
incorporation of sodium in gedrite is the extensive edenite substitution:
[|_|(A)+Si(T)=Na(A)+Al(T)].
Zircon U/Pb geochronology using SHRIMP II revealed an Eocene (42Ma) age for zircon
recrystallisation. We suggest that the latter occurred during granulitisation. This age is in
agreement with a coeval age obtained for apophyses of the huge Skaloti granitic
batholith occurring in the area and is consistent with the evolutionary history of the
Rhodope Massif undergoing general exhumation and crustal collapse in Eocene times. |
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