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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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, 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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