Petrography and Major Elements Geochemistry of Microgranular Enclaves and Neoproterozoic Granitoids of South Khasi, Meghalaya: Evidence of Magma Mixing and Alkali Diffusion

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 Al2O3/CaO+Na2O+K2O (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, Fe2O3t, P2O5, TiO2, MnO and CaO against SiO2) 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.