Metamorphic P-T Conditions of the Kerala (South India) Khondalite Belt, a Granulite Facies Supracrustal Terrain

The Khondalite Belt of southernmost India is an immense section of granulite facies metasediments of probable early Proterozoic age. Dominant rock types are migmatized khondalites (garnet-sillimanite ± cordierite metapelites), acid biotite-garnet graphitic gneisses and charnockites (quartzofeldspathic gneisses with orthopyroxene). Throughout much of southern Kerala, the biotite gneisses are in a state of arrested conversion to charnockite. Partially decrepitated CO₂ inclusions in quartz at the Ponmudi mixed biotite gneiss-charnockite quarry have significant CH₄ and/or N₂ and are texturally late. The probable sequence of events in metasediments was (1) migmatization, (2) charnockite formation, (3) symplectitic cordierite formation, and (4) late CO₂ entrapment. Phase equilibrium diagrams of anatexis in aluminous systems and Fe-Mg exchange between garnet and biotite indicate metamorphic temperatures of 650-850°C for the khondalites. Temperatures based on garnet-orthopyroxene Fe-Mg exchange in charnockites are 755 ± 60°C, which probably refers to a vapor-induced charnockitization event. Paleopressures of khondalites and charnockites are mostly 5-6 kbar over the entire Belt, with a few pressures over 6 kbar adjacent to the charnockitic orthogneiss massif which borders the Belt on the northeast. The texturally-deduced sequence of events can logically be related to a continuous P-T cycle, with maximum pressures of 6.5 kbar generated in the deepest parts of the metasedimentary section overlying an orthogneiss basement. Paracharnockites and cordierite symplectites may have formed at the beginning of the pressure-release cycle of uplift. CO₂ influx, possibly from below the crust, facilitated charnockite formation. Entrapment of late CO₂ in quartz occurred during uplift at 3-4 kbar pressure. Some mechanism of burial of a vast tract of sediments at a nearly uniform depth under nearly a continental thickness of overburden is necessary to explain the monotonous paleopressure pattern. An "A-subduction" model used for the northern Norwegian Cale-donides, in which a continental shelf sequence was entrained at the flat interface of colliding continents in near-horizontal subduction, may be a plausible mechanism. Radioactive self-heating of a doubly-thick continent or pervasive streaming of hot CO₂ from the mantle through tectonic shear zones could have raised regional temperatures to the granulite facies.