Ultrahigh-temperature granulites from the Shillong-Meghalaya Gneissic Complex, NE India: Implications for the Indo-Antarctic Correlation

The Shillong-Meghalaya Gneissic Complex (SMGC) in the Riangdo region, northeastern India, is mainly composed of metamorphic rocks from upper amphibolite to ultrahigh temperature (UHT) granulite facies with several igneous intrusions. The pelitic granulite comprises biotite, garnet, K-feldspar, sillimanite, spinel, quartz, and biotite. This study is the first to report ultrahigh-temperature (UHT) metamorphism in the Riangdo (Sonapahar) block of the SMGC. Metamorphic pressure-temperature conditions estimated from the Spinel + Quartz bearing pelitic granulite using conventional thermobarometer (THERMOCALC) and pseudosection modelling in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 system are more than 900 C and pressure of about 8 kbars, representing UHT metamorphism. The sequence of reactions, constructed from the post-peak textural relationship, along with petrogenetic grid and pseudosection modelling, records a clockwise P-T evolution. This indicates an isothermal decompression path associated with rapid uplift before cooling of the tectonically thickened crust. Available geochronological ages suggest the presence of widespread Pan-African tectonothermal events in the SMGC. The K-Ar isotopic ages obtained from biotite suggest a cooling age of 407.20 +/- 3.49 Ma. Given the general acceptance of UHT with clockwise isothermal decompression in PanAfrican age metamorphism in the East-African-Antarctic Orogen (EAAO) and Eastern Ghat Mobile Belt (EGMB), the Sonapahar UHT metamorphic history is considered to be part of this record.