Trans-crustal evolution of enriched high-K melts through the crystal mush mechanism: A case study from the Mikir Hills, NE India

Abstract The origin and association of high-silica granites (HSGs; SiO2 ≥ 71 wt%) and residual intermediate to felsic cumulates remain enigmatic due to their rare coexistence. These limitations complicate their characterization, parental source, and tectonic setting identification. This work presents a case study from the Mikir Hills, NE India, utilizing zircon U-Pb geochronology, mineral chemistry, and geochemical thermodynamic modeling constraints to understand the origin and relationship of ca. 508 Ma high-K monzogranites, granites, and HSGs. The monzogranites show accumulated plagioclase crystals and high Ba, Sr, and CaO, whereas the HSGs show depleted Ba, Sr, and CaO, representing fractionated melt. The redox calculations (ΔNNO −0.3 to −0.1) and thermometry results (~880–902 °C) using amphibole chemistry show oxidized and high-temperature conditions for monzogranites. The monzogranites’ high K2O and large-ion lithophile elements suggest the role of a sediment-derived enriched mantle source in their origin. The rhyolite-MELTS and Magma Chamber Simulator modeling results suggest the monzogranites’ parental melt resulted from a silicic metasomatized phlogopite-stabilized lithospheric mantle wedge that underwent subsequent lower crust assimilation. The polybaric (~6–4 kbar) crystallization of the generated assimilated melt formed the monzogranites with plagioclase and K-feldspar as the dominant crystallizing phase (>50%). Plagioclase-melt equilibrium tests yield slightly higher liquid-only saturation temperatures than plagioclase-liquid pairs, indicating progressive crystallization and melt extraction. The rhyolite-MELTS geobarometer suggests the highly felsic melts separated from the crystal mush between ~3.8 kbar and 2.8 kbar and emplaced at subvolcanic levels as failed rhyolite eruptions. We suggest the HSGs represent extracted evolved melt, while monzogranites represent residual silicic cumulates formed in a syn- to postcollisional setting during Gondwana assembly.