Petrological Significance and Geochemical Characteristics of Metabasic Rocks in Central Shillong Plateau, Meghalaya, India

ABSTRACT The metabasic rocks from the central Shillong Plateau occur as bands interspersed within quartzo-feldspathic gneiss and as substantial rock masses along with Pan-African granitoids. Petrographic and mineral chemical analyses reveal that these rocks originated as igneous intrusions and were subsequently metamorphosed under amphibolite facies conditions. Wholerock geochemistry indicates a tholeiitic affinity with a subalkaline basaltic composition, characterised by high Al2O3, Fe2O3, CaO, and MgO, and low SiO2, TiO2, and K2O contents. Trace element patterns, including LREE enrichment, negative Nb and Ti anomalies, and low Nb/Th and Nb/La ratios, suggest a subduction-related origin involving slab dehydration and mantle wedge metasomatism. Thermobarometric estimations indicate crystallisation temperatures of 601–682 °C and emplacement pressures between 5.2 and 6.8 kbar, consistent with amphibolite facies metamorphism. REE patterns and partial melting models suggest a mantle source transitional between spinel and garnet peridotite. Tectonic discrimination diagrams indicate an island arc tholeiitic affinity, supporting derivation from a lithospheric mantle source. The data collectively suggest that these metabasic rocks originated in a convergent tectonic setting during the Proterozoic, probably related to crustal thickening associated with, or prior to, the Rodinia assembly.