Petrography
Our field and petrographic studies show that the granitoids are less deformed and are invariably porphyritic. The granitoids contain large phenocrysts of potash feldspars and at places the potash feldspars are zoned (Fig. 2.D). Isolated, irregularly shaped enclaves of mafic rocks and quartzo-feldpathic gneisses and long tabular enclaves of aplites are common (Fig. 2). The granitoids show varying degrees of alteration, including chloritisation of biotite and replacement of plagioclase by sericite.
The granitoids are composed of quartz, microcline, orthoclase and plagioclase in varying amounts along with biotite, apatite, sphene, zircon and opaques as accessories. K-feldspars of the granitoids are mostly microclines, although orthoclase is present but constitutes only 5-10% of the total mode. Quartz grains show deformation bands that exhibit undulatory extinction. Orthoclase is commonly associated with exsolved plagioclase in the form of perthite. Perthites of different varieties such as string, rod and flame type are seen. Micrographic intergrowths between quartz and feldspars sometimes occur.
Figure 3. IUGS (modal) classification scheme
IUGS (modal) classification scheme (after LeMaitre et al., 1989) for the granitoids of Mikir Hill massif, Shillong plateau.