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

B. L. Sreenivas ¹, R. Srinivasan ¹

Affiliations
1 Department of Geology, Central College, Bangalore University, IN

Abstract

Compositional layering of pebbles has provided a positive structural criterion in distinguishing the tectogenic (autoclastic) conglomerates from the sedimentogenic ones in the Dharwars of Mysore. Making use of current bedding and graded bedding as markers of two different tectonic environments of arenite sedimentation, the stable areas of deposition in Dharwars of Shimoga, Chital drug and Bababudan schist belts are separated from the main geosynclinal regions. Stability in the depositional environment is found to be characteristic of the parageosynclinal stage; and the conglomerates deposited in stable regions are oligomictic as against the polymictic nature of the geosynclinal conglomerates.

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R. Srinivasan ¹, B. L. Sreenivas ²

Affiliations
1 Department of Geology, Mysore University, Mysore, IN
2 Bangalore, IN

Abstract

Reconsideration of the stratigraphy of the Dharwars of Mysore State in the light of new data suggests that they are Paleoproterozoic and overlie Charnockites of Archean age. Classification of Dharwars based on tectonomagmatic and atmospheric evolution leads to a four fold divison. Pregeosynclinal shelf stage iS represented by two cycles of platformal sedimentation and magmatism; the earlier, under oxygen deficient conditions, on mafic platforms built by subaerial lavas, Is represented by Bababudan Series, and the latter under oxygenic conditions accompained by alkaline ultramafic intrusions gave rise to Dodguni Series. The geosynclinal Stage represented by pillow-Iava-greywacke-Iron ore deposition (third cycle of magmatism and sedimentation) constitutes the Grey trapRambennur Series. Early tectonic plagioclase granites-Champion gneisses syntectonic migmatites-Peninsular gneisses and late to post-tectonic alkali metasomatic granites Closepet granites (fourth, fifth and sixth magmatic cycles) developed during the folding and metamorphism of Dharwar geosynclinal pile. The inversion of geosyncline witnessed apogeosynclinal molasse red bed Sedimentation (fourth cycle of sedimentation) represented by G. R. Series. Paleoproterozoic epidiorites and post-Paleoproterozoic dolerites cratonised the area.

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B. L. Sreenivas ¹, R. Srinivasan ²

Affiliations
1 Geomysore Services, Bangalore
2 Geomysore Services, Bangalore, IN

Abstract

A critical evaluation of the available major and trace element data on rocks of the granite-greenstone belts or South India leads to the following inferences: Barrovian and Abukuma facies series of metamorphism are recognizable in the terrain. Rocks of peridotitic and basaltic komatiite affinities occur in the pre-Dharwar and early Dharwar ultramafic-mafic sequence. The bulk of the greenstones are island arc tholeiites with a strong calc-alkaline affinity. The presence of detrital pyrites in the oldest conglomerates, and the occurrence of red beds in the topmost Dharwar sequence, indicate that an oxygenic atmosphere evolved through the Dharwar times. The pre-Dharwar granitoids were probably tonalitic, and through time, alkali fractionation gave rise to the Champion gneisses, Peninsular gneisses and Closepet granites. The available data are inadequate to elucidate the fractionation pattern of the granitic crust of Peninsular India.

Pyrolite and eclogite mantle are invoked for tectonomagmatic events during the pre-Dharwar and Dharwar times, respectively. Convection, and eclogite sinkers, are proposed as alternative geodynamic models.

From the geochemical evolution, it is contended that the granite-greenstone belts of Peninsular India represent a more evolved crustal condition as compared to the Archaean granite greenstone belts of South Africa and Western Australia. Obviously, the granite-greenstone belts of all the shield areas, did not evolve simultaneously.

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R. Srinivasan ¹, B. L. Sreenivas ¹, V. N. Vasudev ²

Affiliations
1 Geomysore Services, 12, Palace Road, Bangalore 560 052, IN
2 Chitradurga Copper Co., Ali Asker Road, Bangalore 560052, IN

Abstract

No Abstract.

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R. Srinivasan ¹, B. L. Sreenivas ¹

Affiliations
1 Geomysore Services, 12, Palace Road, Bangalore 560052, IN

Abstract

A preliminary interpretation of black and white LANDSAT IMAGERY of a major part of Karnataka state on 1:1 million scale has been attempted to interpret structure, landforms, mineral and groundwater occurrence, and engineering and environmental conditions. Ground truth of the interpretations needs to be investigated.

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R. Srinivasan

Abstract

No Abstract.

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V. N. Vasudev ¹, R. Srinivasan ²

Affiliations
1 Chitradurga Copper Co. Ltd., 16/1, Ali Asker Road, Bangalore-560052, IN
2 Geomysore Services, 12, Palace Road, Bangalore-560052, IN

Abstract

The vanadium bearing titaniferous magnetite deposits of Karnataka occur within gabbro-anorthosite members, spatially associated with the ultramafic rocks emplaced in the platformal miogeosynclinal regions of the Dharwar schist belts. They are generally conformable, syngenetic magmatic deposits, although instances of magmatic injection are known. Vanadium occurs in magnetite lattice substituting for ferric iron. The vanadium bearing titaniferous magnetites often carry copper and iron sulphide mineralization. The association of titaniferous magnetite with the ultramafic-mafic suite and their tectonic setting suggest, that the ultramafic-mafic sequence of rocks of Dharwar greenstone belts are probably not alpine, but compare better with concentric complexes.

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B. Krishna Rao ¹, R. Srinivasan ², B. L. Ramachandra ³, B. L. Sreenivas ⁴

Affiliations
1 Mineralogical Institute, University of Mysore, Manasagangotri, Mysore 570 006, IN
2 INSA Project, Dept. of Mines and Geology, Race Course Road, Bangalore 560 001, IN
3 Mysore Minerals Ltd., 39, Mahatma Gandhi Road, Bangalore 560 001, IN
4 Geomysore Services, 12, Palace Road, Bangalore 560 052, DE

Abstract

Four types of manganese ores are distinguished: 1. Reworked metasedimcntary ore in Precambrian banded manganese formation; 2. Sedimentary oolitic/pisolitic ore associated with claystone; 3. Cavity filling/replacement type ore in weathered Precambrian phyllitic schists and laterites; 4. Bouldery-pebbly float ore in soils and laterite. The reworked metasedimentary ore is low in silica (∼6%), high in Fe content (∼26%) and its Mn/Fe ratio varies between 0.75 and 1.2. The oolitic/pisolitic ore on the other hand is high in silica (∼14%), low in Fe ( ∼17%) and Mn/Fe ratio of 1.7 and 2.1. Supergene processes, during Tertiary-Quaternary (7) have contributed significant quantities of sedimentary oolitic/pisolitic ore and minor amount of cavity filling/replacement type ore. The surficial exposures of reworked metasedimentary and oolitic/pisolitic ores have been subjected to mechanical disintegration and detrital accumulation with repeated breaks leading to extensive development or float ore in both laterite and soil profile.

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R. Srinivasan ¹, S. M. Naqvi ¹, B. Uday Raj ¹, D. V. Subba Rao ¹, V. Balaram ¹, T. Gnaneshwar Rao ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Turbidite greywackes of the northwestern part of the Chitradurga Schist Belt constitute a part of >2.6 Ga Cbitradurga Group of Dharwar Supergroup. They consist of a detrital assemblage of mono- and poly-crystalline quartz, microcline and plagioclase feldspar. and volcanic rock fragments. Quartz content and Na₂O/K₂O ratios show their quartz intermediate character. In this respect and also in their high FeO(t)+ MgO content they are similar to other Archaean greywackes. The TiO₂ content. K₂O/Na₂O, Al₂O₃/CaO+Na₂O and Al₂O₃/SiO₂ ratios plotted against Fe₂O₃(t)+MgO, although assign an island arc environment for these greywackes, do not unequivocally discriminate between continental and oceanic island arc setting. The Chitradurga greywackes are highly enriched in Zr, Cr and Ni indicating a mixed felsic and mafic source. The contribution from contemporary volcanism in the basin could be significant. The QFL proportions indicate a dissected arc and/or recycled orogenic nature of the provenance. The chemical index of alteration varying from 58 to 63 and Al₂O₃/Na₂O ratio less than 6, indicate relatively unweathered nature of source rock and chemical immaturity of the sediment respectively. The REE patterns show wen-defined negative europium anomalies which reflect granitoid upper crust in the provenance during the Archaean.

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R. Srinivasan ¹, S. M. NAQVI ¹, B. Vasantha Kumar ²

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN
2 Department of Mines and Geology, Race Course Road, Bangalore 560001, IN

Abstract

The stromatolitic cherty dolomite of KaIche is a lithostratigraphic equivalent of the carbonate rocks of the Joldhal Formation of Archaean age. Morphotypes suggest development in subtidal to lower intertidal zone of the Dharwar seas. Some of the forms observed near Kalche are comparable to the domical forms observed near Kumsi in the Shimoga schist belt and Vanivilaspura in the Chitradurga schist belt. The morphotypes observed in these regions contrast with the actively and passively branched columnar stromatolites observed near Dodguni and Bhimasamudra in the Chitradurga schist belt where the stromatolites evolved in an intertidal environment. Diversity of morphotypes of stromatolites appears to have occurred in the Archaean itself, although such diversity appears to have attained stratigraphic significance by the middle Proterozoic.

Keywords

Archaean, Palaeobiology, Stromatolites, Dharwar Supergroup, North Kanara, Karnataka.

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Anil Kumar ¹, R. Srinivasan ¹, K. Gopalan ¹, D. J. Patil ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN

Abstract

Field observations, trace element and isotopic results are reported for rocks classified as carbonatites from Vinjamur area in the Nellore schist belt. These carbonate rocks occur either inter-bedded with metasedimentary quartzites and pelites or in association with calc-alkaline rather than alkaline volcanic rocks. No fenitised contacts are seen. Their Nb and REE contents are too low for carbonatites. Samples free from baryte veins have less than 150 ppm of Sr with an isotopic ratio greater than 0.7100 δ¹³C_PDB range from -1.58 to - 0.56 ‰ and δ¹⁸O_SMOW from +14.00 to +16‰ , All these features, individually and collectively, show that the carbonate rocks of Vinjamur are most probably sedimentary and not igneous in origin.

Keywords

Carbonatite, Nellore Schist Belt, Andhra Pradesh.

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S. T. Sambandam , C. Devaprasad , R. Srinivasan , K. S. Krishnamurthy

Abstract

No Abstract.

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M. Hanuma Prasad ¹, B. Krishna Rao ¹, V. N. Vasudev ², R. Srinivasan ³, V. Balaram ³

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006, IN
2 Mineral Resources Consultant, 120/45(A), 3^rdBlock, TR Nagar, Bangalore - 560 028, IN
3 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

In the late Archaean Sandur supracrustal belt of the Dharwar craton, Southem India, bimodal (mafic-felsic) volcanic rocks are encountered in its eastern region (Copper Mountain region). The mafic volcanic rocks are represented by Al-depleted picritic basalt and tholeliitic basalt. Pieritic basalt has a HREE - depleted pattern. Tholeliitic basalt exhibits two types of REE patterns; (a) slightly depleted - to flat - LREE and unfractionated HREE patterns, and (b) LREE - enriched and HREEdepleted patterns. The felsic volcanic rocks are Na-Rhyolites, which show calc-alkaline affinity and LREE - enriched and HREE-depleted chondrite-normalised patterns. Geochemical signatures show that the picritic basalt owes its origin to melting of a mantle diapir at depths around 100 km. The LREE - depleted tholeiites are not genetically related to the picritic basalt. They were derived through high degree partial melting of a depleted shallow mantle source. The internal variations observed in the LREE - depleted tholeiitic suite are due to derivation of magma through different degrees of melting of the same mantle source. REE-based petrogenetic modelling shows that the LREE - depleted and LREE - enriched tholeiites are not related to a common parent magma. The LREE - enriched tholeiites were generated from a mantle source, which has been metasomatized by subduction related melt/fluid phase. REE chemistry shows that the mafic and felsic volcanic rocks, though spatially and temporally associated, are not genetically related. The HREE depleted rhyolites were derived by partial melting of a garnet - amphibole bearing source, possibly a subducting oceanic crust. Feild, geochemical and petrogenetic aspects suggest that the magmatic rocks of the study area evolved in an active plate margin environment.

Keywords

Geochemistry, Meauics, Late Archaean Sandur Belt, Kamataka.

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P. Senthil kumar ¹, G. Parthasarathy ¹, S. Das Sharma ¹, R. Srinivasan ¹, P. Krishnamurthy ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad-500 007, IN
2 Atomic Minerals Directorate for Exploration and Research, AMD Complex, Begumpet, Hyderabad-500 01 6, IN

Abstract

Carbonate veins up to few metres thick have been recognised in the E-W striking Salem-Attur deep fault zone which represents the eastern extention of the Moyar fault in the southern granulite terrane of India. X-ray diffraction, FTIR spectroscopy and chemical analyses show calcite, ankerite, siderite and benstonite as dominant minerals besides minor apatite and green biotite. Whole rock compositions show enrichment of light REE, Ba, Sr and Nb. C and 0 isotopic data determined for calcite (δ¹³C= -4.9 to -5.1%0 V-PDB and δ¹⁸0 = 9 to 9.2%0 V-SMOW) points to a mantle origin. The chemical characteristics therefore suggest that the carbonate veins have carbonatitic affinity. The FTIR spectra provide evidence for presence of H₂0 and CH₄ in addition to CO₂ in the carbonate minerals of these veins.

Keywords

Ferrocarbonatites, Benstonite, FTIR Spectroscopy, X-Ray Diffraction, Geochemistry, Isotopes, Tamil Nadu.

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R. Srinivasan ¹, B. L. Sreenivas ², B. Krishna Rao ²

Affiliations
1 Geomysore Services, 12 Palace Road, Bangalore-560 052, IN
2 Geomysore Services, 12 Palace Road, Bangalore-560 052., IN

Abstract

Peninsular Gneiss' is a product of syntectonic remobilization of some sialic basement (Fundamental gneiss?) with reference to Dharwar folding. The preservation or obliteration of the basement relation with respect to Dharwars is a function of varying degrees of remobilization characteristic of low pressure and high pressure metamorphic belt regions of Karnataka. By remobilization the basement highs within the schist belts were transformed to mantled gneiss domes, and tonalites were made over to granodiorites and adamellites. With a common source of heat, accompanying the remobilization, retrogression of high grade metamorphic rocks of older' Sargur Supergroup' and prograde metamorphism of volcanics and sediments of 'Dharwar Supergroup' were accomplished.

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