Metamorphism of the Koraput Alkaline Complex, Eastern Ghats Province, India—Evidence for reworking of a granulite terrane

An assemblage of predominantly metasedimentary rocks in the Eastern Ghats Province, India, underwent granulite facies metamorphism and deformation in early Neoproterozoic times, and was subsequently intruded by the Koraput alkaline complex. The intrusion was earlier believed to be syn- to late tectonic. The gabbroic core of the complex hosts nepheline-bearing syenitic dykes and veins. Following emplacement, magmatic amphibole within the syenites, and early orthopyroxene in feldspathic gneisses within the country rocks were retrogressed to biotite during pervasive solid-state deformation. Subsequent prograde metamorphism resulted in the formation of anhydrous assemblages at the expense of relict magmatic amphibole within the syenites, and metamorphic biotite in both the complex and the country rocks. Reactions reconstructed from textural observations indicate breakdown of biotite and amphibole to garnet + clinopyroxene ± orthopyroxene-bearing assemblages. Schreinemakers’ analysis on the relevant mineral associations suggests that heating was followed by loading of the region. This indicates thermal rejuvenation of the complex and the host granulites during an intracrustal orogeny that post-dates emplacement and cooling of the pluton. Available ages suggest that this event occurred in the mid-Neoproterozoic, and is probably unrelated to the amalgamation of the granulite belt with the Archaean Bastar/Dharwar craton.