Simple Statistical and Mineralogical Studies as Petrogenetic Indicator for Neoproterozoic Mylliem Porphyritic Granites of East Khasi Hills, Meghalaya, Northeastern India

The porphyritic granite body (∼600 Ma) around Mylliem (25°32'N:91°52'E), east Khasi hills district, Meghalaya occurs as a distinct intrusive body into the host rock of low-grade meta-sediments belonging to Proterozoic Shillong Group. The porphyritic granite body manifests prominent lath-shaped feldspar phenocrysts giving rise to dominant porphyritic texture. Further, this porphyritic granite body is characterized by primary foliation (defined by parallely arranged feldspar laths), tongues and appophyses and xenoliths of older metamorphics. Petrographically, the Mylliem porphyritic granite is grey leucocratic, coarse grained, phanerocrystalline with modal variants ranging from granite to granodiorite tending to tonalite. Simple statistical studies based on correlation coefficient values involving modal variables of the Mylliem porphyritic granite bodies appear significant in majority of the cases indicating magmatic crystallization. Use of relevant thermometric method indicates temperature of equilibration of the body in the range of 369°C to 507°C. The crystallization of the Mylliem porphyritic granite pluton initiated at an average lithostatic pressure in the tune of ∼11 Kb followed by dominant P_H2O controlled milieu. Spatially projected mineralogical parameters suggest a general trend of the magmatic cooling of the body from margin inward with sudden influx of volatile matters that occasionally offsets mineralogical trends.