Petrogenesis of Fractionated High Ba–Sr Granitoids from the Southern Kyrdem Pluton, Northeast India: Evidence for complex Mantle–Crust Interaction

Abstract

The high Ba–Sr granites as indicators of crust–mantle interaction processes is becoming more widely acknowledged. These uncommon melts typically originated from hydrous mafic sources in the lower crust or from partial melting of basaltic underplates. The present investigation focuses on Late Cambrian southern Kyrdem Granitoid (KG) in the Shillong Plateau, India, using field, petrographic and geochemical studies to depict its tectonics, magmatic processes, and genesis of one such uncommon rock suite. The KG has porphyritic and non-porphyritic varieties, with patches of nepheline syenites, suggesting magma mixing, hybridization, and multiple intrusive episodes. Hydrothermal inputs are indicated through petrographic investigations showing different intergrowths textures. Whole-rock geochemical studies suggest that granitoids are alkalic to alkalic-calcic and metaluminous in nature. With high concentrations of Ba (709-3000 ppm) and Sr (103-586 ppm), moderate Rb (~295 ppm), low Y, U (1.9–4.7 ppm), and negative Eu anomalies, the KG are comparable to other high Ba-Sr granites. Such enrichment of Ba, Sr is likely controlled by feldspar, micas and accessory mineral phases. This elevated concentration suggesting a highly evolved, fractionated magma, potentially with subduction-modified mantle contributions, akin to sanukitoid-type granites. The low Ba/Th (avg.~58.5), U/Th (~0.15), along with the low Mg# values (~18.7) and La/Sm ratios, suggest that these melts may originated from an enriched lithospheric mantle that had been metasomatized by subducted sediment-derived melts with a minimal crustal input. The tectonic diagram infers emplacement during a late-collisional to post-collisional stage. Thus the findings improved our understanding of multiple genesis of high Ba-Sr granitoid systems.