Pressures, Temperatures and Metamorphic Fluids Across an Unbroken Amphibolite Facies to Granulite Facies Transition in Southern Karnataka, India

Abstract

A sampling traverse has been made across the late Archaean regional amphibolite-facies to granulite-facies transition in southern Karnataka, India. The traverse extends from the Peninsular Gneiss-Closepet Granite terrane in the north, through the incipient charnockite localities near Kabbaldurga and southwards into the charnockite massifs of the Biligirirangan and Andhiyur Hills.

Geobarometry based on the temperature-insensitive indicator assemblage garnet-pyroxene-plagioclase-quartz shows that there is a smooth increase of pressure from near 5 kb at Channapatna (12°40′N) to 7.5 kb in the highest-grade massif areas near 12°00′N. Geobarometry based on the densities of CO₂ fluid inclusions in quartz, determined from hundreds of homogenization temperature measurements at seven localities in the transition zone, agrees quantitatively with the mineralogic geobarometry. The average palaeo-temperature in the transition zone is 750°C, determined from garnet-pyroxene Fe/Mg distributions. The quality of the geothermometry is not sufficient to establish with certainty a temperature increase southward over the transition zone.

A reconnaissance profiling of the K/Rb whole rock ratios of harnockites and gray gneisses shows normal upper-crustal ratios near 300 for gneisses and charnockites in the northern half of the traverse and onset of a few very high ratios, indicating extreme depletion of Rb southward from about 12°20′N where charnockite becomes the dominant country rock. Even in the southernmost high-grade massif areas the depletion is patchy, with many rocks in the relatively low range 300–600. No correlation of Rb depletion with Fe/Mg ratio, which is a good indicator of partial melt/magmatic residue relations, was found.

The results establish that the metamorphic gradient south of Kabbal is an unbroken prograde transition to granulite facies. A depth-zone arrangement of amphibolite facies and granulite facies is indicated. Most of the CO₂ inclusions observed were entrapped at peak metamorphic pressures. Therefore CO₂ streaming is implicated as the major agency of metamorphic dehydration (charnockitization). The source of CO₂ was deep-seated, below the deepest level sampled in this traverse. Partial melting, manifest in migmatization in the transition interval, probably helped lower H₂O activity and may have been instrumental in Rb depletion, but this process is, by itself, inadequate to account for charnockite formation and Rb depletion patterns. Carbonic fluid streaming was a more fundamental agency.