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| Titel |
Petrology and mineral equilibrium modeling of incipient charnockite from the Trivandrum Granulite Block, southern India: implications for granulite formation in a Gondwana fragment |
| VerfasserIn |
T. Endo, T. Tsunogae, M. Santosh |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250071383
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| Zusammenfassung |
| The Southern Granulite Terrane (SGT) in India is known for its classic exposures of
regionally metamorphosed granulite-facies rocks formed during the collisional orogeny
related to the amalgamation of Gondwana supercontinent. The SGT is composed of a collage
of Proterozoic crustal blocks dissected by large Late Neoproterozoic shear/suture zones. The
Trivandrum Granulite Block (TGB) is comprises dominantly metasedimentary sequence with
khondalites, leptynites and charnockites with subordinate quartzite, mafic granulite,
calc-silicate rocks, and meta-ultramafic rocks. The TGB is known as one of the
classic examples for the spectacular development of ‘incipient charnockites’ within
orthopyroxene-free felsic gneisses as exposed in several quarry sections in the states of
Kerala and Tamil Nadu. The charnockite-forming process in the TGB is considered to have
been triggered by the infiltration of CO2-rich anhydrous fluids along structural pathways
within upper amphibolite facies gneisses, resulting in the lowering of water activity and
stabilization of orthopyroxene through the breakdown of biotite. However, no quantitative
study on the stability of charnockitic mineral assemblage using mineral equilibrium modeling
approach has been done so far. In this study, we report a new occurrence of incipient
charnockite from Mavadi in the TGB and discuss the petrogenesis of granulite formation in
an arrested stage on the basis of petrography, geothermobarometry, and mineral equilibrium
modeling.
In Mavadi, patches and lenses of charnockite (Kfs + Qtz + Pl + Bt + Grt +
Opx + Ilm + Mag) of about 30 to 120 cm in length occur within Opx-free Grt-Bt
gneiss (Kfs + Qtz + Pl + Bt + Grt + Ilm) host rocks. The application of mineral
equilibrium modeling on charnockite assemblage in NCKFMASHTO system to
constrain the conditions of charnockitization defines a P - T range of 800Ë C at 4.5
kbar to 850Ë C at 8.5 kbar, which is broadly consistent with the results from the
conventional geothermobarometry (810-880Ë C at 7.7-8.0 kbar) on these rocks. The
P - T conditions are lower than the inferred peak metamorphic conditions from the
ultrahigh-temperature granulites of the study area (T >900Ë C), which might
suggest heterogeneity in peak P - T conditions within this crustal block in relation
to local buffering of metamorphic temperature by Opx-Bt-Kfs-Qtz assemblage.
The result of T versus mole H2O (M(H2O)) modeling demonstrated that Opx-free
assemblage in Grt-Bt gneiss is stable at M(H2O) = 0.3 to 1.5 mol.%, and orthopyroxene
occurs as a stable mineral at M(H2O) |
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