Post-magmatic Metamorphic Evolution of the Koraput Anorthosite Pluton, Eastern Ghats Granulite Belt, India: Tectonic Implications

ABSTRACT This study investigates the metamorphic history of the Koraput Anorthosite Pluton in India’s Eastern Ghats Granulite Belt using petrographic, mineralogical, and thermodynamic analyses. Two post-emplacement metamorphic events, M1 and M2, were identified. M1 is characterized by biotite layering over orthopyroxene and clinopyroxene in meta-anorthosite and amphibole formation in metadiorite, including amphibole corona over clinopyroxene. The M2 peak metamorphic event is characterized by the formation of anhydrous garnet porphyroblasts in meta-anorthosite and small garnets in the metadiorite. Petrographic investigations reveal that meta-anorthosite contains two forms of garnet, Grt-I (coronal), and Grt-II (porphyroblast). Grt-I appear as necklace-type minerals that occur at the biotiteplagioclase interface, whereas Grt-II appear as large porphyroblasts associated with biotite and plagioclase (Pl-III) in a cuspate-lobate form. The metadiorite garnets are small and have well-developed faces. Thermobarometric studies estimate P-T conditions for the M1 metamorphic events in meta-anorthosite at T: 800 ±50°C and in metadiorite at T: 750±40°C, P: 0.4–0.7 GPa, respectively. Similarly, thermobarometric studies estimate P-T conditions for the M2 metamorphic event in meta-anorthosite at T: 850 ±50°C, P: 0.8–1.2GPa. The P-T pseudosection for metaanorthosite, based on its effective bulk composition, estimates peak metamorphic conditions (M2) at T: ~920°C, P: ~1 GPa. The post peak metamorphic cooling and decompression stage (under dry conditions and low (aH2O)) is marked by the formation of garnet corona (Grt-I) at the biotite-plagioclase interface, with conditions (M2R) of P: ~0.7 GPa and T: ~850°C. Similarly, P-T pseudosection analysis of metadiorite predicts M2 conditions at T: ~800°C, P: ~0.8 GPa. Finally, integrating these metamorphic events with available monazite geochronological data suggests that the M2 metamorphic event likely represents a loading and heating event associated with the thrusting of Eastern Ghats - Jeypore Province composite over Bastar Craton during Pan-African Gondwanaland assembly.