Abstract

In this paper, detailed petrology, mineral chemistry and geochemistry of lamprophyres from the Rongjeng–Nongchram area, Meghalaya, northeast India, is being reported. These dykes are intruded into the Proterozoic Assam Meghalaya Gneissic Complex and Neoproterozoic porphyritic granites with limited field exposures in two separate clusters controlled by lineaments. These lamprophyres are alkaline, potassic to ultra-potassic, and metaluminous and have been classified into camptonite and sannite from their petrography and geochemical affinity. Resorption texture of biotite in camptonite and compositional zoning in clinopyroxene of sannite indicate their crystallization under in-equilibrium conditions and subsequent interaction with changing composition of the magma(s). Detail geochemistry indicates fractionation as the imperative mechanism during the evolution of the magma(s) and shows heterogeneity at the mantle source and evidence for minimal crustal contamination. Various tectonic discrimination diagrams indicate the emplacement of the lamprophyres in a post-collisional and within plate setting, signifying a possible plume mechanism involved in their emplacement. The affinity of the lamprophyres towards an OIB setting was recorded from the chondrite-normalized REE and primitive mantle-normalized multielement patterns. The geochemistry of the lamprophyres points towards the presence of possible enriched metasomatic pods in the mantle, which might have melted due to the heat from the mantle plume and triggered the emplacement of the lamprophyres in the Rongjeng–Nongchram area.

Research highlights

• New lamprophyre bodies from Garo Hills, Meghalaya Plateau, NE India, are being reported with detailed petrology, geochemistry, and mineral chemistry.

• The lamprophyres are of alkaline affinity.

• The Kerguelen plume played a vital role in the generation of these lamprophyres.

• There are geochemical signals of the presence of enriched metasomatic pods in the mantle, which were melted due to plume interaction.

• The study also indicates a distinctive mantle source heterogeneity across the Meghalaya Plateau.