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

S. A. Drury ¹

Affiliations
1 Department of Earth Sciences, The Open University, Milton Keynes, NK7 6AA, Great Britain, GB

Abstract

Major and trace element data for metabasaits and metaperidotites from the littleknown Archaean volcanic-sedimentary belt at Kudremukh are presented. The dominant rocks are low K tholeiites showing evidence of partial melting from a Zr, Y and Nb enriched, P and Ta depleted mantle source, at depths where garnet was stable in peridotitic compositions. The action of olivine, pyroxene and possible spinel fractionation from a high MgO parent magma can account for many of the chemical variations within the suite. Clasts and matrix from an associated agglomerate prove to be the most evolved rocks of the suite and evidence strong plagioclase feldspar fractionation. No precise tectonic setting for the volcanics is attempted; this must await more information on age, structure and volcanic geochemistry for the rest of the Karnataka craton.

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Y. J. Bhaskar Rao ¹, S. A. Drury ²

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN
2 Department of Earth Sciences, The Open University, Milton Keynes, GB

Abstract

Data on the incompatible trace elements Ti, Rb, Sr, Ba, Zr, Nb, Y, rare-earth elements, Sc, Hf, Ta and Th are presented for basalts, andesites and rhyodacites from the Archaean Bababudan greenstone belt. All samples are genetically related and show general calc-alkaline affinities. The parent magma is proposed to have been formed by about 15 per cent partial melting of mantle at shallow depths. The source mantle is shown to have been enriched in light REE and Zr relative to primitive mantle with 2-3 times chondritic abundances. The parent magma evolved by progressive fractionation of, first olivine and pyroxene to produce incompatible element-enriched alld Cr, Ni and Co depleted tholeiites, which were then accompanied by plagioclase feldspar and possibly magnetite to generate the andesites and rhyodacites. Standard tectonic characterization parameters do not give consistent results for this suite, and until more is known about overall structure, stratigraphy and relative ages within the South Indian Archaean craton, a precise setting cannot be ascribed.

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S. A. Drury ¹

Affiliations
1 Department of Earth Sciences, The Open University, Milton Keynes, GB

Abstract

The usefulness of LANDSAT images, both photographic and digitally processed, for reconnaissance structural studies is demonstrated. To a certain extent lithological discrimination is possible by these techniques, but stratigraphic interpretation is impossible other than in conjunction with detailed field studies of structure and way-up indications.

The tectonic boundary between the two cratonic sub-blocks of eastern and western Karnataka, identified by deep seismic sounding and recognised in a few places by field studies is delineated, as are other major faults. Major Archaean fold forms in the greenstone and gneisses are traced throughout the belt and interpreted as westward verging lobes of basement separated by synclinal cusps of greenstone belt cover, which have been disrupted and further deformed by smaller NNE-SSW folds and the major fault lineaments. Varying crustal level within this sequence of lobes and cusps is suggested to explain the variable state of deformation and metamorphism of greenstones in the craton, and N-S shear zones in the gneisses are thought to be expressions of shear strains beneath eroded cusps.

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S. A. Drury ¹, R. W. Holt ¹, P. C. Van Clasteren ¹, R. D. Beckinsale ²

Affiliations
1 Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, GB
2 Institute of Geological Sciences, Exhibition Road, London, SW7 2DE, GB

Abstract

A Sm-Nd whole-rock isochron age of 3020 ± 230 Ma for basal rnetavolcanics of the Kudremukh-West Coast greenstone belt and a Rb-Sr whole rock age of 3280 ± 230 Ma for the underlying granitoid gneisses are presented. These ages indicate that the filling of an ensialic basin with volcanic and sedimentary rocks began at around 3000Ma and confirm earlier studies which concluded that the dominant gneisses of Karnataka represent a major period of primary crust formation in the South Indian Craton at about 3300 Ma. Both the early granitoid gneisses and the younger metavolcanics were derived from unfractionated mantle sources which had not previously been depleted or enriched in LIL elements.

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S. A. Drury ¹

Affiliations
1 Department of Earth Sciences, The Open University. Milton Keynes, MK7 6AA, GB

Abstract

The spatial and temporal relationships between mid-Proterozoic, intracratonic sedimentation in the Cuddapah Basin and earlier Proterozoic basic dyke swarms are analyzed in the context of thermal and mechanical models for crustal doming, erosion and subsidence. The dyke orientations imply that the Archaean crust of South India was thermally warped into an elliptical dome with a roughly E-W axis, under varying regional tectonic conditions. Thermal relaxation of this dome, after erosion and crustal thinning, laid the basis for sedimentation in the Cuddapah Basin, whose deepest part lay to the SW of its present outcrop. Late-Proterozoic sedimentation of the Kurnool rocks must have been initiated by a later, separate thermal event.

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