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| Titel |
Intracrystal microtextures in alkali feldspars from fluid deficient felsic granulites: A chemical and TEM study |
| VerfasserIn |
Lucie Tajcmanova, Rainer Abart, Richard Wirth, 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 |
250050854
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| Zusammenfassung |
| Samples of essentially “dry” high-pressure felsic granulites from the Bohemian Massif
(Variscan belt of Central Europe) contain large perthites with several generations of
exsolution features. A first generation of albite-rich plagioclase precipitates takes the form of
spindles or lenses up to 30 micrometers long with an aspect ratio of 12 to 6. The precipitates
show strong shape preferred orientation. A second generation of albite-rich precipitates takes
the form of thin (around 30 nanometers wide) up to several 10’s of micrometers long lamellae
or spindles. In the vicinity of large kyanite, garnet or quartz inclusions, the orthoclase-rich
host shows diffuse exsolution features taking the shape of short and narrow spindles
(tails) aligned in two directions at an angle of about 45Ë . The contacts between
the orthoclase-rich host and the plagioclase precipitates of the first generation and
the contact between the orthoclase-rich host and large kyanite, quartz and garnet
inclusions is lined with a thin rim of albite This albite was formed at a late stage of the
petrogenetic history probably related to fluid infiltration and associated albitization. In
the vicinity of the large inclusions the plagioclase precipitates of the first perthtite
generation become significantly depleted and the perthite microstructure coarsens
composed dominantly of tweed orthoclase. The integrated composition corresponds to
Or59Ab36An5 in the inclusion free domains and slightly decreases in plagioclase content
(Or63Ab33An4) towards the kyanite or quartz inclusion. This chemical change is very
gentle.
The primary exsolutions probably formed by spinodal decomposition at around 850-900Ë
C during the high pressure stage (16-18 kbar). The primary exsolution was followed by
primary coarsening and albite twinning of the plagioclase precipitates. The second generation
of albite-rich precipitates was formed at around 650-700 Ë C. TEM investigations revealed
that the interfaces between the second generation plagioclase lamellae and the orthoclase rich
host are coherent or semi coherent. The formation of albite linings at phase boundaries and
of patch perthite produced incoherent interfaces. The patch perthites, albitization
and secondary coarsening in the vicinity of large inclusions developed below 400Ë
C contemporaneously with fluid infiltration in the course of deuteric alteration. |
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