SHORT COMMUNICATION 629 MICROGRANULAR ENCLAVES IN NEOPROTEROZOIC GRANITOIDS OF SOUTH KHASI HILLS, MEGHALAYA PLATEAU, NORTHEAST INDIA: FIELD EVIDENCE OF INTERACTING COEVAL MAFIC AND FELSIC MAGMAS SANTOSH KUMAR', THEPFUVILIE PIERU' , VIKOLENO RINO', and B.C. LYNGDOH~ '~epartment of Geology, Kumaun University, Nainital - 263 002, Uttranchal '~irectorate of Mineral Resources, RISA Colony, Shillong, Meghalaya Email: sky adavan @ y ahoo.com The microgranular enclaves (ME) in south Khasi granitoids (SKG) are mesocratic to melanocratic, fine- to medium-grained and porphyritic, and occur abundantly between Mawthaphdahand Jashiar villages. The ME are 3 to 30 cm across, and their shapes vary from subrounded, ovoid, to ellipsoidal and show sharp and crenulate contacts with the SKG without deformational and/or reaction signatures. The mega- scopic features of ME and their relationships with SKG reveal that rnafic and felsic (SKG) magmas having some initial crystailinities co-existed, mixed and generated a hybrid (ME) magma. Minerals such as K-feldspar megacrysts, mafic and felsic xenocrysts were transferred and disequilibrated, and became corroded in the new hybrid magma. The hybrid magma further evolved as ME globules mingled and undercooled into relatively cooler SKG magma. Introduction Medlicott (1869) had described granitoid masses within Shillong Group. Mazumder (1986) discussed the Precambrian geology of the Khasi hilIs, and observed numerous enclaves sometimes occurring parallel to the foliation in the granitoids defined by the elongation of K- feldspar megacrysts. Ghosh et al. (1991) carried out geochronological and geochemical studies of granitoids including the Neoproterozoic south Khasi granitoids (690+19 Ma, with an initial 87Sr/86Sr=0.71074f0.00029), and have mentioned the presence of enclaves of arnphibolite, granite gneiss and diorite. The French term encIaves include xenoliths, surmicaceous enclaves (mica-rich restite from metasedimentary source region), cognate or autolith (cogenetic affiliation with host rocks), and microgranular (fine grained, mafic magmatic) enclaves (Didier, 1984). In this communication, we report the field relations and types of microgranular enclaves (ME) hosted in south Khasi granitoids and discuss their implication for rnafic and felsic magma mixing and mingling processes. GeoIogical Framework The Neoproterozoic felsic magmatism in the southern Khasi hills (West Khasi Hill district), referred to as south Khasi granitoids (SKG), intrudes a gneissic complex and the rocks of the Shillong Group of Meghalaya plateau (Fig. lc). The SKG cover an area of about 600 sq km, and are overlain by Cretaceous-Tertiary sediments. The SKG and other granitoids of Mylliem, Kyrdem, and Nongpoh regions of Meghalaya plateau (Fig. I b) are considered to be late- to post-tectonic, fracture controlled diapirs (Mazumder, 1986) that most likely evolved as a result of a protracted Pan-African thermal event during Neoproterozoic-Early Palaeozoic that was related to mantle upwelling (Ghosh et al. 1991). The granitoids in the southern part of the SKG pluton have a foliation, defined by the elongation of K-feldspar megacrysts (phenocrysts), and are oriented parallel to the contact of the granitoids with the gneissic country-rocks. The foliation has formed during flow of the SKG magma during emplacement. The granitoids in the central part are dominated by very coarse-grained and porphyritic granitoids, in which tabular, rounded to subrounded and elongated K-feldspar megacrysts are randomly distributed, but locally show a crude alignment. Some megacrysts of K-feldspar reach a size of 7 cm. However, fine to medium grained equigranular granitoids do occur amidst the porphyritic granitoids. Quartz-aplite and micropegmatite veins cut across the granitoids at several places. Field Relations of Enclaves and their Types The enclaves hosted in SKG can be broadly classified into two types: country rock xenoliths and microgranular enclaves (ME). The former are confined to the margin of the SKG pluton whereas ME ubiquitously occur in equigranular and porphyritic varieties of SKG but are more frequent in porphyritic SKG. Mafic aggregates and quartz pods are locally present in SKG. The ME hosted in SKG are mesocratic to melanocratic, fine- to medium-grained and phenocryst-bearing or phenocryst-free. Based on JOUR.'GEOL.SOC.INDIA, VOL.65, MAY 2005