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

A. T. Rao ¹, V. Divakara Rao ², M. Yoshida ³, M. Arima ⁴

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam- 530 003, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN
3 Department of Geosciences, Osaka City University, Osaka - 558, JP
4 Department of Geological Science, Yokhoma National University, Yokahoma, JP

Abstract

Oxygen isotope values in metapelites including garnet-sillimanite gneisses (khondalites), quartzo-feldspathic gneisses (leptynites), cordierite-sapphirine gneisses and the associated marbles, calc-silicates, quartzites, basic granulites and charnockites from the Eastern Ghats Granulite Belt are presented and their significance discussed. Metapefites show a wide variation in δ¹⁸O values (4.50 to 13.90‰) that is controlled largely by the mineralogy of the rocks. Coarse grained porphyritic charnockites have higher δ¹⁸O values (8.60-11.20‰) than that of medium grained ones (5.90-6.60‰) Basic granulites have the minimum δ¹⁸O values (∼5.0‰). Oxygen isotope values of different rock units in the granulite belt are constrained mostly by the composition of the source rocks of the sediments and of the protoliths in the case of charnockites. Post-depositional metamorphic and metasomatic fluids had little contribution to the δ¹⁸O variations in these rocks.

Keywords

O-Isotopes, Eastern Ghats Granulite Belt, Khondalites, Charnockites, Andhra Pradesh.

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A. T. Rao ¹, C. V. Raman ¹

Affiliations
1 Department of Geology, Andhra University, Waltair 530003, IN

Abstract

Charnockites in parts of the Eastern Ghats show indications of sulphide mineralisation. Pyrrhotite, chalcopyrite and galena have formed in paragenetic sequence replacing blue quartz, black feldspars, hypersthene and pink garnet of the charnockites near Kondapalli in Andhra Pradesh.

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A. T. Rao ¹

Affiliations
1 Department of Geology. Andhra University, Waltair 530003, IN

Abstract

The bad lands or high lands referred earlier from Visakhapatnam are now termed as 'Red Sediments'. The mineralogical approach with reference to statistical studies of zircon from inland and coastal red sediments of Visakhapatnam supports their khondalite parentage. The pebbly and lateritic gravel-silt zones at the bottom of coastal red sediments indicate the different stages of deposition by fluvial processes. The climatic changes from cold to arid along the East Coast of India indicate that the process of formation of red sediments has set in around 11,000 years B.P. The recent archaeological evidence of microliths in the bottom lateritic gravel-silt zones and top red sediments towards Coast reveals the period of transportation and deposition during 7,000 years B.P. and 3,000 years B.P.

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A. T. Rao ¹, C. V. Raman ¹

Affiliations
1 Department of Geology, Andhra University, Waltair, IN

Abstract

Vemparala in northernmost Precambrian Nellore schist belt is made up of banded quartz-magnetite rocks, quartzites, pyroxene granulites, gabbros and granites. These were subjected to folding and refolding resulting in F₁, F₂, F₃ folds. Spinel-bronzite pyroxenites were structurally emplaced along the crests of F₂ folds and are the first known occurrences in Nellore Precambrian belt.

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A. T. Rao ¹

Affiliations
1 Department of Geology, Andhra University, Waltair 3, (A.P.), IN

Abstract

No Abstract.

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A. Sriramadas ¹, A. T. Rao ¹

Affiliations
1 Department of Geology, Andhra University, Waltair 530003, IN

Abstract

The tonalitic charnockites from Visakhapatnam, Andhra Pradesh, belong to 2,600 m y. age. They were structurally emplaced along the axes of NE and E plunging overturned isoclinal synclines exhibited by khondalites. The palingenetic charnockites of granodioritic composition have indicated an age of 2,000 m.y., which corresponds to the age of NW-SE cross-folds in the Eastern Ghats of Andhra Pradesh.

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C. V. Raman ¹, A. T. Rao ¹

Affiliations
1 Department of Geology, Andhra University, Waltair 530003, IN

Abstract

The red sediments of Recent age confined to the slopes of hill ranges and along the coast of Visakhapatnam and Bhimunipatnam are classified into inland and coastal red sediments. The textural analysis reveals that the inland red sediments are poorly sorted with negative skewness, and the coastal red sediments are better sorted and positively skewed. Beach samples are well sorted Showing both positive and negative skewness. In the light of skewness patterns it is concluded that the coastal red sediments were laid down in a depositional environment, whereas inland red sediments belong to an erosional or non-depositional environment and further that these clastic red sediments are immature and have formed under a fluvial environment.

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A. T. Rao ¹, K. V. S. Acharyulu ¹, K. Sanyasi Rao ¹

Affiliations
1 Department of Geology, Andhra University, Waltair, IN

Abstract

No Abstract.

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A. T. Rao ¹

Affiliations
1 Department of Geology, Andhra University, Waltair 530003, IN

Abstract

The chromite pyroxenite nodules of Kondapalli region were brought from deeper levels of the earth by spinel pyrofenite magma during cross folding in the Eastern Ghats. The spinel pyroxenites were transformed into the mineral assemblage: serpentine-gamet-magnesite-calcite-quartz, due to the activity of volatiles derived from the regions of magma generation. The Ni coptent (1,050-1,100 ppm) in the serpentines and magnesites support pyroxenite parentage.

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A. T. Rao ¹, P. Sankara Pitchaiah ¹

Affiliations
1 Department of Geology, Andhra University, Waltair, IN

Abstract

White sands in Prakasam District, Andhra Pradesh, occur as a narrow zone extending 45 km in length and 3-4 km in width between Karlapalem and Chinnaganjam. The origin and provenance of white sands are worked out based on geology and textural analysis of the white sand belt. The textural parameters and the gradational changes in the white sands support interpretation of the depositional environment as belonging to a beach system. The sub-angular to sub-rounded shape of the grains and appreciable amount of feldspars in the white sands suggest that they have undergone transportation locally and originated from a nearby source. It is established that the Gondwana sandstones exposed around Pavalur and Budavada were the source rocks for the white sands and they were deposited as a near shore bar.

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V. Venkateswara Rao ¹, I. V. Radhakrishna Murthy ², A. T. Rao ³

Affiliations
1 Dept. of Geo-Engineering, Visakhapatnam 530003, IN
2 Dept. of Geophysics, Visakhapatnam 530003, IN
3 Dept. of Geology. Andhra University, Visakhapatnam 530003, IN

Abstract

Magnetic anomalies in the Gosrhani river basin in Andhra Pradesh indicate a suite of charnockitic rocks at depth. The emplacement of these rocks is explained by folding leading to an apical graben development followed by subsidence, uplift and erosion.

Keywords

Geophysics (General), Gostbani River Basin, Charnockite, Eastern Ghats, Andhra Pradesh.

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I. V. Radhakrishna Murthy ¹, B. Polinaidu ¹, A. T. Rao ²

Affiliations
1 Department of Geophysics, Andhra University, Visakhapatnam 530003, IN
2 Department of Geology, Andhra Univeristy, Visakhapatnam 530003, IN

Abstract

Magnetic anomalies in an area of about 1500 sq km around Vizianagaram town in Andhra Pradesh bring out a shallow magnetic basement of charnockitic rocks, which is highly faulted. The magnetic basement exhibits horst and graben-like structures and broadly maintains conformable relations with the antiformal and synformal structures of the overlying khondalite suite of rocks. The development of these faults is attributed to the earlier folding of the Eastern Ghats and their refolding about a N-S axis to change their initial NE-SW trend to E-W and NW-SE in the nonhern part of Andhra Pradesh. The development of the horst-graben type structure is explained by movements of the faulted blocks along the fault planes at different stages or tectonic activity.

Keywords

Geophysics. Magnetic anomalies, Vizianagaram, Andhra Pradesh, Basement Structure.

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A. T. Rao ¹, V. Divakara Rao ², M. Yoshida ³, M. Arima ⁴

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam-530 003, IN
2 National Geophysical Research Institute, Hyderabad- 500 007, IN
3 Department of Geosciences, Osaka City University, Osaka 558, JP
4 Department of Geological Sciences, Yokohama National University, Yokohama, JP

Abstract

The whole rock analyses confirm the basic to intermediate nature of the protolith (SiO₂, > 53% - < 67%) with high alumina (highest 22.18%) and high K₂O (∼ 4%). Metamorphic segregation as well as source rock compositions appear to have contributed -to the formation of high alumina granulites. The temperatures of metamorphism estimated using garnet - hypersthene/cordierite pairs range from 680-980°C, The gamet-cordierite reference temperatures with narrow range 700-750°C, indicate a pressure range of 5-7 kbar for the garnet-orthopyroxene-plagioclase-quartz assemblage in the aluminous granulites. Textural relationships of cordierite breakdown and reappearance of cordierite supplement the two generations of cordierite from aluminous granulites in the Eastern Ghats Granulites Belt.

Keywords

Geochemistry, Aluminous Granulites, P-T Estimates, Eastern Ghats Granulite Belt (EGGB), Andhra Pradesh.

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V. P. Kovach ¹, E. B. Salnikova ¹, A. B. Kotov ¹, S. Z. Yakovlevn ¹, A. T. Rao ²

Affiliations
1 Institute of Precambrian Geology and Geochronology, Russian Academy of Sceince, Makaarova emb., 2, St. Petersburg 199 034, RU
2 Department of Geology, Andhra University, Visakhapatnam 500 003, IN

Abstract

²⁰⁶Pb/²³⁸U ages of zircon correspond to 502 ± 3 Ma and ²⁰⁷Pb/²⁰⁶Pb ages are equal to 508 ± 14 Ma. U-Ph isotopic study of the Kasipatnam zircon has led to the recognition of pan-African thermal event in the Eastern Ghats Granulite Belt, India.

Keywords

Geochronology, Easetern Ghats Granulite Belt, Pan-African.

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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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A. T. Rao ¹, V. I. Fonarev ², A. N. Konilov ², M. Romanenko ²

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam-530 003, IN
2 Institute of Experimental Mineralogy, 1142432, Chernogolovka, Moscow District, RU

Abstract

Electron microprobe dating method is same as chemical aging method of U-Th minerals used in geochronology. The Th-U-Pb concentrations obtained from a single electron probe analysis allow calculation of geologically meaningful age. Based on this method monazite from spinel granulite of Visakhapatnam in the Eastern Ghats Belt (EGB) gave ∼1,000 Ma, corroborating the late Proterozoic metamorphism and charnockite emplacement, established in parts of the EGB and Antarctica. Microgeochronology of commonly occurring accessory mineral such as monazite in granulite variants is recommended for quick reconnaissance correlation in Gondwanian fragments in East Gondwanaland.

Keywords

Micro-Geochronology, Electron Microprobe Dating, Monazite, Eastern Ghats Belt, A. P.

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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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