South Khasi batholith (SKB) and its nearby smaller granitoid plutons of Umdulun, Phlangdiloin (West Khasi) and Mylliem (East Khasi) of Meghalaya are Neoprotcrozoic intrusives in the Archaean (?) gneisses and Proterozoic Shillong Group metasedimentary rocks, and are overlain by the Cretaceous-Tertiary sediments. Granitoids of SKB and Umdulun are essentially high-Ca, metaluminous, biotite (±
... [Show full abstract] hornblende and cpx.) granodiorite-adamellite, whereas those of Phlangdiloin and Mylliem are low-Ca, weakly peraluminous, biotite (± muscovite) adamellite, all being calc-alkaline. They show many similarities with the I-colIisional-type granitoids. Their field, petromincralogical and geochemical data indicate that they are genetically related, with the evolution of the younger Mylliem granitoid (610 Ma) from an older SKB (760 Ma) mainly by fractional crystallisation and longer crustal residence.time. Their petrogenesis involves generation of metaluminous granitic melt by low-degree partial melting of a lower crustal basic rock like amphibolite or hornblende gneiss at low water-pressure with hornblende (± garnet) being residual phase, together with "plagioclase effect", followed by fractional crystallisation, with fractionation of metaluminous phases like hornblende and pyroxene, resulting in a weakly peraluminous melt. High Th/ U value (6-24) of the fertile granitoids of SKB and Umdulun suggests that much of their U was probably lost due to its labile nature, which, on release from these provenance granitoids and meeting a strong reducing environment in sediments downstream, has led to the formation of sandstone-type U-deposit at Domiasiat; south of SKB.