The Nagercoil Granulite Block, southern India: petrology, fluid inclusions and exhumation history

Abstract

The Nagercoil Granulite Block (NGB) constitutes the southernmost domain in the high-grade metamorphic terrain of southern India and dominantly comprises orthopyroxene–K-feldspar–plagioclase–quartz–biotite–ilmenite bearing massive charnockites. Where they are mixed with aluminous supracrustals, the charnockites contain garnet, often-displaying typical decompression textures with coronal orthopyroxene and plagioclase surrounding relict garnet grains. Orthopyroxene is preserved in near-pristine state in the charnockite suggesting low water activity. Garnet–orthopyroxene thermometry of NGB charnockites yields temperatures of 691–934 °C. Garnet–biotite thermometry yield values that broadly overlap (538–864 °C) with the above estimate, although some lower values indicative of retrograde Fe–Mg exchange are also obtained. The presence of mesoperthite in the rock and high fluorine content in biotites also suggest high metamorphic temperatures. Charnockite barometry using garnet–orthopyroxene–plagioclase–quartz assemblage yields 4.0–6.3 kbar. An overall drop in pressure by over 2 kbar correlate with the isothermal decompression history inferred from textural criteria. Detailed fluid inclusion petrographic, microthermometric and laser Raman spectroscopic data presented here allow an evaluation of the fluid evolution history of NGB charnockites. We distinguish four groups of inclusions, with the earliest generation characterised by CO 2 with variable but minor proportion of H 2O. Subsequent generations are dominated by CO 2. Locally, CH 4 and N 2 occur as trace components. CO 2-rich inclusions dominate in all minerals: garnet, K-feldspar, plagioclase and apatite. Homogenisation temperatures assign densities of 0.92–0.93 g/cm 3 for the CO 2 involved in charnockite formation. A close correspondence of the isochores for CO 2+H 2O and CO 2 inclusions with P–T data derived from mineral phase equilibria suggests fluid entrapment at peak metamorphic conditions. The metamorphic evolution of NGB and its exhumation path characterised by isothermal decompression are comparable with those of charnockites from Pan-African terrains elsewhere in East Gondwana.