Mafic granulites from the Eastern Ghats, India: further evidence for extremely high temperature crustal metamorphism

Abstract

Mafic granulites occurring as conformable lenses within khondalite, leptynite, and calc-silicate rocks from the Anantagiri-Araku areas of the Eastern Ghats belt contain orthopyroxene, clinopyroxene, garnet, quartz, ilmenite, and plagioclase as major constituents. Orthopyroxene contains (100) lamellae of clinopyroxene, and clinopyroxene contains (100) orthopyroxene lamellae and (001) spindles of pigeonite. Reaction textures indicate formation of garnet through coupling of two reactions, viz. orthopyroxene + anorthite = garnet + clinopyroxene + quartz and anorthite + ilmenite - hematite_ss + quartz = garnet + ilmenite + O₂. Thermobarometry indicates exsolution of pigeonite at ~950°C, of (100) clinopyroxene in orthopyroxene and vice versa at 750-820°C and formation of garnet at around 700-725°C, 7 kb, all in response to cooling of the rock. This shows a nearly isobaric cooling path subsequent to peak metamorphic conditions. Subsequently, the rock suffered nearly isothermal decompression to 5 kb. Hydration and K-metasomatism of the assemblage occurred at lower temperatures (~500°C) in the final phases. The structural characteristics and thermal history of the mafic granulites suggest that the mafic rocks were emplaced during the culmination of prograde metamorphism accompanying a compressive orogeny. Mafic magmas could have contributed significantly to enhance the thermal budget for granulite facies metamorphism. This study, therefore, attests to the very high metamorphic temperatures (as high as 1000°C regionally) attained by the Eastern Ghats granulites.