Phase equilibrium modeling of incipient charnockite formation in NCKFMASHTO and MnNCKFMASHTO systems: A case study from Rajapalaiyam, Madurai Block, southern India

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Incipient charnockites represent granulite formation on a mesoscopic scale and have received considerable attention in understanding fluid processes in the deep crust. Here we report new petrological data from an incipient charnockite locality at Rajapalaiyam in the Madurai Block, southern India, and discuss the petrogenesis based on mineral phase equilibrium modeling and pseudosection analysis. Rajapalaiyam is a key locality in southern India from where diagnostic mineral assemblages for ultrahigh-temperature (UHT) metamorphism have been reported. Proximal to the UHT rocks are patches and lenses of charnockite (Kfs + Qtz + Pl + Bt + Opx + Grt + Ilm) occurring within Opx-free Grt-Bt gneiss (Kfs + Pl + Qtz + Bt + Grt + Ilm + Mt) which we report in this study. The application of mineral equilibrium modeling on the charnockitic assemblage in NCKFMASHTO system yields a p-T range of ∼820 °C and ∼9 kbar. Modeling of the charnockite assemblage in the MnNCKFMASHTO system indicates a slight shift of the equilibrium condition toward lower p and T (∼760 °C and ∼7.5 kbar), which is consistent with the results obtained from geothermobarometry (710–760 °C, 6.7–7.5 kbar), but significantly lower than the peak temperatures (>1000 °C) recorded from the UHT rocks in this locality, suggesting that charnockitization is a post-peak event. The modeling of T versus molar H2O content in the rock (M(H2O)) demonstrates that the Opx-bearing assemblage in charnockite and Opx-free assemblage in Grt-Bt gneiss are both stable at M(H2O) = 0.3 mol%–0.6 mol%, and there is no significant difference in water activity between the two domains. Our finding is in contrast to the previous petrogenetic model of incipient charnockite formation which envisages lowering of water activity and stabilization of orthopyroxene through breakdown of biotite by dehydration caused by the infiltration of CO2-rich fluid. T-XFe3+ (=Fe2O3/(FeO + Fe2O3) in mole) pseudosections suggest that the oxidation condition of the rocks played a major role on the stability of orthopyroxene; Opx is stable at XFe3+ <0.03 in charnockite, while Opx-free assemblage in Grt-Bt gneiss is stabilized at XFe3+ >0.12. Such low oxygen fugacity conditions of XFe3+ <0.03 in the charnockite compared to Grt-Bt gneiss might be related to the infiltration of a reduced fluid (e.g., H2O + CH4) during the retrograde stage.