Field and Petrographic Studies on Granitoids of the Bibinagar-Bhongir Area, Yadadri District, Telangana State, India

The granitoids of the Bibinagar- Bhongir area from the Yadadri district are confined to a part of the Eastern Dharwar craton. The major rocks types are in the present study area are volumetrically in decreasing order of granite, granodiorite, quartzmonzonite and diorite. They form massive weathered boulders, pointed hillocks and batholithic domes. All these rock types are described as felsic granitoids displayed two main Phaneritic texture features: (1) Porphyritic (inequigranular) and (2) non-porphyritic (equigranular), and these textural features have a regional variation from south to north from the study area. Non-porphyritic granitoids (grey granite gneiss, granodiorite and diorite) have fine to medium grained equigranular-hypidiomorphic texture, while the porphyritic varieties (pink granite and quartzmonzonite), and show very coarse to coarse-grained inequigranular porphyritic texture. Field relationships of the granitoids indicate that the porphyries are younger than the non-porphyries due to cross-cutting relations between them. The granitoids are characterized by quartz, K-feldspar (microcline) and plagioclase as essential minerals and hornblende and biotite as minor minerals. Augite, Fe-Ti oxides, apatite, zircon and epidote form important accessories. They show massive form with inter locking mineral grains in the field and exhibit typical perthitic, myrmekitic and hypidiomorphic textures under the microscope. Petrographic features of granitoids indicate that they are of “subsolvus granites” by the presences of two feldspars in them. The field relationships and petrographic features of the granitoids suggest that these felsic granites are formed by differentiation and fractional crystallization of felsic magma that is generated by partial melting of lower Amphibolitic or diorite crust.