Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

K. Srinivasa Rao ¹, T. Sreenivasa Rao ², S. Rajagopalan Nair ³

Affiliations
1 6-3-628/8/1, Ravindra Nagar, Khairatabad, Hyderabad 500004, IN
2 Barkatpura, Hyderabad, IN
3 Ananda Nagar, Khairatabad, Hyderabad 500004, IN

Abstract

The lithostratigraphic succession worked out for the Upper Precambrian of Venkatapuram-Cherla area includes Cherla Formation, Somandevara Quartzite, Tippapuram Shale Utlapalli Conglomerate, Albaka Sandstone, Nambi Breccia and Doli and Usur Sandstones. Three disconformities, one each at the base of the Somandevara Quartzite, Utlapalli Conglomerate and the Nambi Breccia, have been inferred. Of these, the one at the base of the Nambi is believed to be the most significant one. Based on gross lithological similarity and relative stratigraphic position, the different Upper Precambrian units of this area are correlated with those of the already established Pakhal and Sullavai Groups of the main Upper Precambrian belt, west of the Godavari River. The correlation implies that the nature of the contact between the Pakhal and Sullavai Groups of the, main Upper Precambrian belt on the western side of the Godavari, a much debated question, is a profound unconformity.

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A. T. Rao ¹, K. Srinivasa Rao ¹, S. Srinivas ¹

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam - 530 003, IN

Abstract

Basic granulites frequently occur in garnetiferous quartzo-feldspathic gneisses and rarely in garnetiferous sillimanite gneisses, calc-silicate rocks and quartzites in the entire Eastern Ghats Granulite Belt (EGGB). Commonly these basic layers are parallel to the foliation and are occasionally boudinaged. These share a common tectonic and metamorphic history with those of the associated litho-units. There are also unusual basic granulite dykes which crosscut the foliation in quartzo-feldspathic gneisses, folded calc-silicate rocks and basic granulites at Maruturu near Anakapalle in the EGGB. The two generations of basic granulites are even grained with a polygonal annealed mosaic and characterised by the mineral assemblage: coexisting pyroxenes and plagioclase. The mineral composition data of coexisting pyroxenes indicate large scale equilibrium at granulite facies conditions. Both types of basic granulites have almost similar bulk composition and have tholeiitic affinity. The basic granulite dykes have low contents of REE with a smooth distribution pattern and a general decrease from LREE to HREE. The revival of basic magmatism during post D1 and D2 at Maruturu reveals that extensional set-up recurs in the EGGB. The basic granulites of Maruturu are correlated with Proterozoic metabasic dykes in East Antarctica and Western Australia. The 1300 Ma basic granulites reported in Maruturu and Rayagada in the vicinity of Maruturu indicate that these intrusive basic bodies are coeval with the dyking events in Eastern Albany Mobile Belt in Western Australia and Vestfold Hill Block in Prydz Bay region in East Anatarctica.

Keywords

Petrology, Geochemistry, Basic Granulite, Mineral Compositions, Eastern Ghats Granulite Belt, Andhra Pradesh.

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At. Rao ¹, Y. Srinivasa Rao ¹, K. Srinivasa Rao ¹

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam - 530 003, IN

Abstract

No Abstract.

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K. Srinivasa Rao ¹, D. B. Ghosh ², Chamanlal ²

Affiliations
1 6-3-609/195, Ananda Nagar, Khairatabad, Hyderabad, IN
2 Professors Colony, Bhopal (M.P.), IN

Abstract

The rocks of the Vindhyan supergroup show evidences of deposition in a shallow marine environment, the depositional basin being similar to 'epeiric sea'. The dark grey limestone unit of the Nagod Limestone Formation is a low magnesian limestone and is invariably an intrasparite or oosparite with variable amounts of authigenic cement spar, neomorphic spar and micrite occupying intra- and inter-allochemical pore spaces. Fibrous nonferroan calcite, dog tooth fringe of nonferroan calcite, drusy mosaic of ferroan calcite and syntaxial rim cement, constitute the different types of cements noticed. Authigenic minerals like dolomite, quartz, plagioclase and pyrite are present in small quantities. Based on the precipitational and recrystallisation fabrics of the calcite and the presence of different authigenic minerals, the diagenetic history of the limestone is worked out.

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A. T. Rao ¹, K. Srinivasa Rao ¹, V. Vijayakumar ¹

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam - 530 003, IN

Abstract

Basalts near Kateru, north of Rajahmundry in the East Godavari district and near Pangidi in the West Godavari district of Andhra Pradesh along the East Coast of India are of particular interest to investigators on account of their separation from the well known West Coast Deccan Basalts, localised around Bombay. The basalts around Rajahmundry are tholeiitic with equigranular, subophitic to ophitic textures. The principal constituents are plagioclase (An_55-56) and brownish nonpleochroic augite. The REE patterns of Rajahmundry basalts have a marked increased enrichment in LREE due to crustal contamination. The K-Ar age of Kateru basalts is 75.1±4.9Ma. The reported biochronostratigraphy of microfauna in the sediments closely associated in time and space with basalt flows in the Krishna-Godavari basin, east of Rajahmundry indicate that the age of basic volcanism is around 67 Ma. The lower limit of 70 Ma for the Kateru basalts supports that these basalts have evolved along with West Coast Deccan Basalts (69-65 Ma), where, the intensity of basic magmatism is more. The geological setting of Rajahmundry basalts with coastal sedimentary rocks indicate that the basic magmatism and sedimentation had taken place during the Cretaceous-Tertiary boundary.

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