Alkaline magmatism versus collision tectonics in the Eastern Ghats Belt, India: Constraints from structural studies in the Koraput Complex

Linear domains of deformed alkaline rocks and carbonatites have recently been identified as representing sites of ancient suture zones. In peninsular India, the western margin of the Proterozoic Eastern Ghats Belt (EGB) is characterized by a series of alkaline plutons that are aligned close to the contact with the Archaean Craton. Most of the complexes were deformed and metamorphosed during a subsequent orogenic event. Unlike other plutons in the belt, the alkaline complex at Koraput reportedly escaped deformation and granulite facies metamorphism forming an anomalous entity within the zone. Multiply-deformed country rocks hosting this complex underwent syn-D-1CR granulite facies metamorphism followed by D-2CR thrusting, with pervasive shearing along a NE-SW trending foliation. A second granulite facies event followed localized D-3CR shearing. Within the Koraput Complex, strain partitioning was responsible for preserving igneous textures in the gabbroic core, but aligned magmatic amphibole needles and plagioclase laths occasionally define a S-1AC fabric. Along the margins, S-1AC is rotated parallel to a NE-trending, east-dipping S-2AC fabric in the gabbro, fringing syenodiorite and nepheline syenite bands. Locally, D-3AC shearing follows D-2AC deformation; S-2AC and S-3AC parallel S-2CR and S-3CR in the country rocks. High-grade metamorphism represented by recrystallization of amphibole and plagioclase, and breakdown of amphibole and biotite to garnet, pyroxene and K-feldspar in the complex follows D-3AC. Unlike earlier reports, therefore, the Koraput body is also deformed and metamorphosed. The aligned alkaline complexes in the EGB probably represent deformed alkaline rocks and carbonatites formed by rifting related to an earlier episode of continental break-up that were deformed during subsequent juxtaposition of the EGB with the Archaean Craton. This supports the contention that the western margin of the EGB and its contact with the Archaean Craton is a suture zone related to the Indo-Antarctica collision event.