Garnet in the Neoproterozoic Oddanchatram Anorthosite in the Southern Granulite Terrain, India: A Petrological Puzzle

Abstract

Garnet is a commonly occurring accessory mineral in many anorthosite plutons, and it is variably interpreted as a metamorphic, magmatic, or xenocrystic mineral. The Neoproterozoic Oddanchatram anorthosite in the South Indian granulite terrain is one such example. Igneous textures are remarkably well preserved, with large grains (up to 5 cm) of garnet in the margins of the pluton. Typical prograde metamorphic textures, such as garnet formation by isobaric reaction between mafic silicate and plagioclase or by dehydration breakdown of amphibole, are lacking, and Fe-Ti oxides mostly occur as exsolutions in the associated plagioclase or scarce disseminated oxide grains. The garnets are almandine rich in composition, with higher CaO (>5 wt%) than the low-CaO (<2 wt%) almandine of garnets in the adjoining metamorphic assemblages. They cannot be described as xenocrysts because of their large size, compared to their smaller size in the country rocks, apart from the chemical distinctions. A reaction of hydrous fluid influx with plagioclase and mafic phases in the anorthosite, as described for similar large-garnet formation in the Adirondacks by McLelland and Selleck, appears to be a more reasonable explanation. The fluid source may be external, released from hydrous minerals of the hornblende-biotite-bearing gneiss country rock during the prograde metamorphism that the terrain experienced, or internal, from the late magmatic enriched hydrous liquids, as evident from primary amphibole. Formation of garnet and exsolution of Fe-Ti oxides appear to have developed when the anorthosite was still at a depth corresponding to upper-amphibolite-facies metamorphic conditions and experienced isothermal decompression and exhumation subsequently, as evidenced by the plagioclase-orthopyroxene symplectite development from garnet.