A reappraisal of the pressure-temperature path of granulites from the Kerala Khondalite Belt, Southern India

The Kerala Khondalite Belt (KKB) of South India is a pan-African ica. 600-500 Mai regional granulite terrain that is composed of charnockites, khondalites, and migmatitic leptinitic gneiss. Peak assemblages in the KKB are characterized by orthopyroxene + garnet + K-feldspar + magnetite + biotite + plagioclase + quartz in felsic to intermediate rocks and by orthopyroxene + cordierite + plagioclase + quartz or garnet + cordierite + sillimanite + feldspars + quartz in semipelites and pelites. Postpeak reaction textures in these assemblages often involve the production of cordierite and are consistent with a predominantly decompressional pressure- temperature history at temperatures in excess of 700 degreesC. Pressure-temperature estimates based on garnet-orthopyroxene thermobarometry are in the range of 6.5-7.5 kbar and 860 degrees -920 degreesC for localities close to and within the Achankovil Shear Zone, a prominent high-grade, high-strain zone on the northeastern margin of the KKB. For this northeastern subarea, peak temperatures of 900 degrees +/- 20 degreesC at 6.5-7.0 kbar are increased to 925 degrees +/- 25 degreesC when retrieval calculations are applied to account for postpeak Fe-Mg exchange. Such near-ultrahigh-temperature conditions, however, are not recorded from the central KKB, where pressure-temperature estimates based on a range of thermobarometers and data sets are 4.8-5.7 kbar and 830 degrees -860 degreesC. In contrast to earlier studies that suggested that peak pressure-temperature conditions were rather uniform across the whole RICE, these new results, coupled with those in other recent studies, demonstrate that peak metamorphic temperatures varied across the KKB. The highest temperatures attained may reflect the thermal input of dry magmas now exposed as massive charnockites within and to the northeast of the Achankovil Shear Zone, if these are synmetamorphic and similar in age to the main metamorphic event.