Petrography and Geochemical Characteristics of Aplites from Kyrdem Granitoids, Meghalaya Plateau, Northeast India

Abstract

This study investigates aplites associated with the Kyrdem granitoids of the Meghalaya Plateau, Northeast India, using field observations, petrography, and whole-rock geochemical analyses. The aplites are fine-grained, leucocratic rocks that occur as small stocks, narrow veins, or minor dykes and represent late-emplaced products of a silica-rich residual melt associated with the host granitoids. Their mineral assemblage consists predominantly of quartz, plagioclase, and alkali feldspar, with biotite, zircon, apatite, and opaque minerals as accessory phases. Whole-rock geochemistry indicates high silica contents (average 68.78 wt.%), a granitic composition, calc-alkaline affinity, and a peraluminous character. Harker variation trends show decreasing Fe₂O₃, MgO, CaO, TiO₂, and P₂O₅ with increasing SiO₂, indicating progressive fractionation involving plagioclase, apatite, titanite, biotite, and hornblende. Rare earth element and trace element patterns indicate late-stage granitic differentiates derived from subduction-modified magmas that evolved through extensive fractional crystallisation with minor crustal assimilation. The aplites have an average total REE concentration of approximately 637.37 ppm and display strong LREE fractionation. Their moderate Rb/Sr ratios (0.40–4.02) and variable Nb/Ta ratios (13.77–39.38) indicate crystallisation from moderately evolved felsic melts with heterogeneous Nb–Ta behaviour. Tectonic discrimination suggests a predominantly late-orogenic affinity with a minor syn-collisional component. These integrated observations provide a petrographic and geochemical framework for understanding late-stage felsic magmatism in the Shillong–Meghalaya Plateau.