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M. Narsimha Reddy ¹, E. V. S. S. K. Babu ², C. Leelanandam ¹

Affiliations
1 Department of Geology, Osmania University, Hyderabad - 500 007, IN
2 Present address National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

The Inukurti Anorthosite Complex (IAC) forms a small elliptical dome (15 x 10 km) within the Archaean metamorphosed volcano-sedimentary sequences of the Nellore Schist Belt (NSB) The IAC comprises dominantly of anorthositic rocks, which are cut by mafic sills and dykes, volumetncally insignificant lenses and bands of dionte occur along its periphery All the rocks are deformed and metamorphosed under amphibohte-facies conditions but exhibit relict magmatic textures despite metamorphic recrystallisation and deformation The minerals from anorthosites diontes and amphibohtes exhibit compositional characteristics distinct to each group

Garnet-hornblende plagioclase-quartz equilibria from the IAC indicate a narrow range of metamorphic equilibration temperature between 644 and 570 °C, but a broad range of pressure between 76 and 27 kbar Geothermobarometry on zoned garnet porphyroblasts indicates that the rocks of the IAC were subjected to decompression of the order of about 5 kbar from 76 to 27 kbar related to rapid exhumation

Keywords

Anorthosite, Dionte Amphibohte, Geothermobarometry, Amphibohte facies, Inukurti Complex, Nellore schist belt, Andhra Pradesh.

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C. H. Ramakrishna ¹, M. Narsimha Reddy ²

Affiliations
1 Department of Geology, University College of Science, Osmania University, Hyderabad-500007, IN
2 Department of Geology, Mahatma Gandhi University, Nalgonda-508254, Telangana State, IN

Abstract

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.

Keywords

Granitoid Rocks, Perthite, Myrmekite, Subsolvus and Felsic Magma.

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