A. Mohan et al., FLUID INCLUSIONS IN CHARNOCKITES FROM KODAIKANAL MASSIF (SOUTH-INDIA)- P-T RECORD AND IMPLICATIONS FOR CRUSTAL UPLIFT HISTORY, Mineralogy and petrology, 57(3-4), 1996, pp. 167-184
The Kodaikanal massif is part of a Precambrian terrane characterized b
y granulite facies rocks. It is dominated by the widespread occurrence
of charnockites. The observed textural relationships in these rocks a
re consistent with the following main reactions: i. Biotite + Quartz =
Orthopyroxene + Alkali feldspar +/- Garnet + Vapour ii. Garnet + Quar
tz = Orthopyroxene + Plagioclase iii. Pyrope = Mg Tschermaks + Enstati
te (in Opx) Garnet consuming reactions and the preservation of biotite
-quartz-orthopyroxene-plagioclase symplectites are indicative of a dec
ompression event. Progress of such reactions with decreasing pressure
together with fluid inclusion data has implications for the constructi
on of P-T vectors. Quartz from the charnockites contains the following
fluid inclusions: (1) monophase high density CO2-rich (0.968/-1.014 g
/cm(3)) as the dominant fluid phase; (2) aqueous biphase CO2-H2O (0.88
8-0.915 g/cm(3)) and (3) late minor aqueous H2O inclusions with no vis
ible CO2. CO2-isochores for the high density fluid inclusions yield a
pressure limit of ca. 6.5 kbars, at granulite facies temperatures of c
a. 800 degrees C, which is in accordance with the estimation from mine
ralogical thermobarometry. The P-T path delineated from combined miner
alogical and density data on carbonic inclusions is characteristically
T-convex suggesting an isothermal decompression path and rapid uplift
followed by cooling of a tectonically thickened crust.