Examples of apparent exsolution lamellae and lenticular blebs of chalcopyrite in pyrrhotite are described in orthopyroxene-bearing granulite facies rocks from two, oxidized (log10fO2 = −14 to −11), widely separated, well characterized high grade terranes: the Bamble Sector, SE Norway (795°C, 7.5 kbar) and the Shevaroy Hills Massif, Tamil Nadu, S India (750°C, 7.5 kbar). These exsolution features only occur in isolated pyrrhotite grains and not in integral pyrrhotite-pyrite-chalcopyrite-magnetite grain clusters which essentially represent an oxidation equilibrium. Reintegration of these chalcopyrite exsolution features back into the pyrrhotite host indicate Cu contents ranging from 1 to 5 wt.% in good agreement with experimental observations which indicate that pyrrhotite can take up to 7 wt.% Cu at temperatures above 800°C at pressures of ∼1 bar. This suggests that under high grade conditions these chalcopyrite exsolution features were in solid solution with pyrrhotite. Whether Cu stabilizes pyrrhotite at higher oxygen fugacities or these chalcopyrite-pyrrhotite grains represent a metastable phase is uncertain. One possibility is that the isolated pyrrhotite grains with chalcopyrite lamellae could represent grains that were preferentially not exposed to infiltrating fluids, which oxidized the pyrrhotites in other areas of the sample. A second possibility is that either these grains had enough Cu to stabilize them during pervasive infiltration of oxidizing fluids or that they represent a metastable phase with respect to the overall oxygen fugacity of the sample. The two conclusions that can be drawn from these observations are, firstly, that it is possible for pyrrhotite and chalcopyrite to form a limited solid solution at granulite facies temperatures and pressures under relatively high oxidizing conditions, i.e. 1.5 log units above fayalite-magnetite-quartz, at 800°C and 8 kbar. Secondly, this limited solid solution should have some bearing on the stability of pyrrhotite with respect to co-existing magnetite and pyrite as a function of the oxidation state of the rock, be it inherited or fluid induced.

In the southern part of the Archaean craton of South India, an approximately 3.4–2.9 b.y. old migmatite–gneiss terrane (Peninsular gneiss complex) has been subjected to granulite facies metamorphism about 2.6 b.y. ago. During this event, the extensive charnockite-khondalite zone of southern India developed. A younger metamorphism (Proterozoic?) led to retrogression of the charnockites and khondalites, mainly under the conditions of the amphibolite facies.

The physical conditions of metamorphism have been evaluated by applying methods of geothermobarometry to the widespread charnockitic assemblages with garnet, orthopyroxene, clinopyroxene, plagioclase, and quartz. The interpretation of the P–T estimates includes a critical discussion of potential error sources, e.g. errors of the analytical data and the calibrations of the models, and takes into account the complex metamorphic history of the rocks and the kinetics of the mineral equilibria.

P-T estimates were obtained for seven subareas from the rim compositions of the coexisting minerals: Shevaroy Hills 680±55°C—7·4±1 kb; Kollaimalai area 680±40°C—8·6± 1 kb; Nilgiri Hills 680±90°C—6·6±0.8kb (upland massif) and 705±60°C—9·3±0.8 kb (northern margin); Bhavani Sagar area 650±50°C—7·2± 1 kb; Sargur-Mysore area 690±60°C—7·6 kb; Bangalore-Kunigal-Satnur area 760±50°C—6 kb. Except for the last subarea, the P-T model data reflect the conditions of a late annealing stage probably related to the retrogressive metamorphism. Conditions near the peak of granulite facies metamorphism (730–800°C—6·5–9·5 kb) are recorded by the core compositions of the minerals. Although a rather uniform cooling history of the main part of the charnockite-khondalite terrane is suggested from the temperature data, differential uplift of smaller blocks is indicated by the regional variation of the pressure data.