Felsic magmatism in the South Khasi Hills of the Meghalaya Plateau, NE India, referred herein as the South Khasi granitoids (SKG: 519.5±9.7 Ma), invariably contains rounded to elongate, fine- to medium-grained, mafic to porphyritic microgranular enclaves (ME: 515±13 Ma) showing sharp to crenulate contacts with the host SKG. Compositions of plagioclase, amphibole and biotite in the ME are slightly distinct or similar to those of the host SKG, which appear re-equilibrated through diffusion mechanisms during partial liquid (semi-solid) conditions prior to the complete solidification of the mafic–felsic interacting system maximum at shallow continental crustal depths of approximately 9.5 km (c. 250 MPa) under oxidizing conditions. Although the ME are chemically modified, both the ME and SKG exhibit a wide chemical variation as high-K2O metaluminous (I-type) granitoids. Linear to near-linear variations of chemical elements against SiO2 may suggest the origin of the ME as the result of the mixing of crystal-charged mafic and felsic magmas in various proportions. However, the data scatter and ill-defined chemical variations can be attributed to chaotic chemical mixing, diffusion and, to some extent, mechanical sorting of the crystals. The identical trace element patterns of the ME and the respective SKG have strengthened the idea of chemical re-equilibration at varying levels between them through diffusion during synchronous mixing–fractionation and mingling. Mean zircon 207Pb/206Pb ages from the ME (515±13 Ma) and SKG (519.5±9.7 Ma) underline the co-existence of Cambrian mafic and felsic magmas formed during the later stages of the assembly of East Gondwanaland as an integral part of the Pan-Indian–African–Brasiliano orogenic cycle. The ME in SKG thus represent mingled, undercooled, heterogeneous hybrid magma globules formed by linear to chaotic mixing that was synchronous with fractionation of coeval crystal-charged mafic (enclave) and felsic (SKG) magmas, which experienced differential degrees of chemical exchange through diffusion with the surrounding felsic host in an open magma system.

Neoproterozoic (690±19 Ma) felsic magmatism in the south Khasi region of Precambrian northeast Indian shield, referred to as south Khasi granitoids (SKG), contains country-rock xenoliths and microgranular enclaves (ME). The mineral assemblages (pl-hbl-bt-kf-qtz-mag) of the ME and SKG are the same but differ in proportions and grain size. Modal composition of ME corresponds to quartz monzodiorite whereas SKG are quartz monzodiorite, quartz monzonite and monzogranite. The presence of acicular apatite, fine grains of mafic-felsic minerals, resorbed maficfelsic xenocrysts and ocellar quartz in ME strongly suggest magma-mixed and undercooled origin for ME. Molar Al 2 O 3 / CaO+Na 2 O+K 2 O (A/CNK) ratio of ME (0.68-0.94) and SKG (0.81-1.00) suggests their metaluminous (I-type) character. Linear to sub-linear variations of major elements (MgO, Fe 2 O 3 t , P 2 O 5 , TiO 2 , MnO and CaO against SiO 2 ) of ME and SKG and two-component mixing model constrain the origin of ME by mixing of mafic and felsic magmas in various proportions, which later mingled and undercooled as hybrid globules into cooler felsic (SKG) magma. However, rapid diffusion of mobile elements from felsic to mafic melt during mixing and mingling events has elevated the alkali contents of some ME.

The Meghalaya Plateau including the Mikir Hills represents the northeastern extension of the Precambrian Indian Shield and mainly comprises the Proterozoic basement granite gneisses, granites (sensu lato), granulites, metasediments, Cambrian granites, and Mesozoic‐Tertiary lithounits. A new whole‐ rock geochemical dataset of Proterozoic and Cambrian granites is presented and investigated to decipher the petrogenesis of these granites with its implications on understanding the crustal growth history of Meghalaya Plateau. Cambrian granites commonly intrude the Proterozoic basement granite gneisses and Shillong Group of rocks. Both the Proterozoic and Cambrian granites exhibit similar mineral assemblages (Bt ± Amp‐Pl‐Kf‐Qz‐Zrn‐Mag‐Ttn‐Ap ± Ilm), but they are texturally distinct. Microgranular enclaves are ubiquitous in Cambrian plutons but are devoid or rare in the Proterozoic granites. Cambrian granites are medium‐ to coarse‐grained, inequigranular, hypidiomorphic, frequently porphyritic, and undeformed to mildly deformed, whereas Proterozoic granites are medium‐ to coarse‐grained, less frequent porphyritic, and mildly to strongly deformed. Geochemically, the Proterozoic (molar Al2O3/CaO + Na2O + K2O (A/CNK) = 0.86–1.15; FeOt/FeOt + MgO = 0.60–0.93) and Cambrian (A/CNK = 0.77–1.20; FeOt/FeOt + MgO = 0.56–0.90) granites are nearly identical showing strongly metaluminous to moderate peraluminous, magnesian to ferroan, and alkali‐calcic to calc‐alkaline and transitional character between I‐type and A‐type oxidized granites formed in a post‐collision tectonic environment. Comparison of studied granites with experimental melt compositions derived from various protoliths suggests that they are sourced from metabasics to tonalites. Harker bivariate plots demonstrate fractional differentiation as a dominant process in the evolution of these granites. However, occurrence of hybrid microgranular enclaves and geochemical features manifest that the mixing and fractionation of coeval mafic and felsic magmas have also played a key role in the evolution of some Cambrian plutons. A slightly higher zircon saturation temperatures for Cambrian (700–950°C) than the Proterozoic (675–900°C) granites characterize a relatively higher‐T melting regime for the generation of Cambrian than the Proterozoic granites. Petrogenetic modelling constrains that the parental magma to the Proterozoic granites can be generated by about 25.5% melting of heterogeneous lower crustal sources with low maficity, which subsequently had undergone fractional crystallization involving bt‐amp‐pl‐Kf‐qz assemblage. However, parental to Cambrian granites can be produced by about 32% melting of the amphibolitic lower crust that subsequently evolved through synchronous fractional crystallization and mixing with a mantle‐derived mafic melt. The chronological records and the present petrogenetic findings on Proterozoic granites propound a viable geodynamic model that the assembly and growth (thickening) of the Columbia Supercontinent during ca. 1,800–1,600 Ma caused a hi...