Granulite Metamorphism, Fluid Buffering, and Dehydration Melting in the Madras Charnockites and Metapelites

Interlayered and cofolded charnockites and metapelites of the type charnockite area near Madras were metamorphosed under granulite fades conditions. Fe-Mg partitioning between orthopyroxene, garnet, and biotite indicates that chemical equilibrium was approached under similar P-T conditions in the two rock suites. Several geothennometers and geobarometers give P-T values which converge at 750–800°C and 6.5–7.5 kb. Computations utilizing data from high pressure phase equilibrium experiments of Bohlen et al. (1983a) and Wones & Dodge (1977) point to several significant relations regarding the behaviour of H₂O during the granulite metamorphism. aH₂O values, computed from Bohlen et al.'s (1983a) reversal data and the a-X model for phlogopite after Bohlen et al. (1980), show distinctly lower magnitudes in metapelites (0.10–0.16) than in charnockites (0.23–0.34). No systematic spatial gradients exist within the charnockites or metapelites, and aH₂O has similar values in metapelite exposures widely separated in the field. These imply an internal, rather than an external (e.g., by CO2 influx), control of the fluids. Applying the algebraic method developed by Rumble (1976), Gibbs analysis in the system K₂O-MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂-TiO₂-H₂O shows that the chemical potentials of H₂O and to O₂, as monitored against biotite composition $(∂μH2O∂XMgbt)$ and $(∂μO2∂XMgbt)$, exhibit gradients with respect to X_Mg in the two rock suites under isothermal-isobaric conditions. μH₂O was found to decrease with X_Mg^bt in both, while μO₂ increases with decreasing X_Mg^bt in metapelites but increases sympathetically with X_Mg^bt in charnockites. These findings point out again that μH₂O and μO₂ were internally buffered. The absence of graphite in the metapelites, at an estimated fO₂ = 10^−14.7 b, also argues against an external influx of CO₂ and, inter alia, supports internal buffering. A complementary enquiry into variations of aTIO₂ reveals an inverse relation between aTIO₂ and aH₂O, suggesting a similar control for aTIO₂. The inferences from biotite dehydration equilibria, when combined with the P-T data and with several field and chemical features of these rocks noted earlier (Sen, 1974), make dehydration melting a distinct possibility for the Madras rocks. It is argued that the low aH₂O and high aTIO₂ (˜ 0.9) observed in the metapelites have been caused by a greater extent of melting in the precursors of metapelites, which were more hydrous than those of charnockites, coupled with preferential partitioning of Ti into the residual rocks—thus strengthening the case for dehydration melting.