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| Titel |
Phase equilibrium modeling of Pan-African incipient charnockite from southern Madagascar |
| VerfasserIn |
Toshiaki Tsunogae, Takahiro Endo, Mw. Santosh, N. O. Thierry Rakotonandrasana, Erath Shaji, Roger A. Rambeloson |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250084771
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| Zusammenfassung |
| Dark brownish patches and/or veins of coarse-grained orthopyroxene-bearing felsic granulite
(charnockite) within foliated amphibolite-facies gneiss/migmatite, are considered
as examples of ‘metamorphic’ charnockite, and represent the transformation of
amphibolite-facies rocks to dry granulites on a local scale. Such ‘incipient’ charnockites have
been reported so far from many localities in southern India and Sri Lanka which corresponds
to the central part of the East African – Antarctic Orogenic Belt related to the assembly of
the Gondwana Supercontinent. Detailed petrological investigations of incipient
charnockites therefore provide important insights into granulite-forming processes
in the lower crust during Neoproterozoic to Cambrian. Here, we report the first
occurrence of incipient charnockite from Ihosy area in southern Madagascar, and
discuss the petrogenesis of granulite formation in an arrested stage on the basis of
petrography, geothermobarometry, fluid inclusion study, and mineral equilibrium
modeling.
In the study area, patches of brownish charnockite (Pl+ Qtz + Kfs + Bt + Grt + Opx +
Ilm + Mag) of about 20 to 50 cm in length occur within host orthopyroxene-free
garnet-biotite gneiss (Pl + Qtz + Kfs + Bt + Grt + Ilm + Mag). The application of mineral
equilibrium modeling on charnockite assemblage in NCKFMASHTO system to
constrain the conditions of charnockitization defines a P - T range of 8-10.5 kbar and
820-880Ë C, which is broadly consistent with the results from the conventional
geothermobarometry (820-880Ë C at 9 kbar) on Grt-Bt gneiss. The result of T versus mole
H2O (M(H2O)) modeling demonstrated that orthopyroxene-free assemblage in Grt-Bt
gneiss is stable only at M(H2O) >0.1 mol.%, while orthopyroxene in charnockite
occurs as a stable mineral at very low M(H2O) condition of |
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