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

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On Some Geochemical Aspects of the Sevattur Carbonatite Complex, North Arcot District, Tamil Nadu


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1 Atomic Minerals Division, Department of Atomic Energy, Hyderabad 16, India
     

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The major and trace element compositions of the Sevattur carbonatites are broadly comparable with the 'typical carbonatite abundance' values compiled by Gold (1966). The mean values for the Sevattur carbonatites are as follows (in weight per cent): SiO2-2.60, Al2O3-2.49, TiO2-0.46, Fe2O3 + FeO-9.64, MnO-0.52, MgO-10.14, CaO-32.84, Na2O-0.85, K2O-0.49, P2O5-3.48, CO2-36.15, H2O±0.32, with trace elements in parts per million as: Ba-1663, Sr-6427, Zr-763, Rb-2, La-271, Ce-641, Y -41, Nb-78, Ta-34, SC-16, Zn-69, Cu-78, Ni-18, Pb-27.

There is an enrichment of total iron, MgO, MnO, P2O5 and Na2O with successively younger phases within the main carbonatite body and such a feature is comparable with the liquid lines of descent, observed in synthetic carbonatite systems. Fenitisation of the country rocks (granitic gneisses) along the southern border of the carbonatite body is indicated from the enrichment of K2O, Na2O, Al2O3 and impoverishment in SiO2, total iron and MgO in the fenites when compared with the country rocks.

Sr (4109-9375 ppm, mean 6427) exceeds Ba (1000-2420 ppm, mean 1663) in all the types of carbonatites by a factor of 2 to 5. The lighter rare earths such as La and Ce are enriched in the calcitic types (La + Ce, 892-1351 ppm) when compared to the beforsites (647-920 ppm). In general, the beforsites are richer in Ba, Nb, Ta and Sc.

Among the carbonatitic minerals, early calcites are much enriched in Sr (5500-6200 ppm) in comparison with Ba (1230-1700) by a factor of 3 to 5. Sr with successively younger fractions of the carbonatite, seems to show a preference to the dolomites (3500-4000 ppm in the early phases and 9000-11000 ppm in the later phases) rather than to the apatites (6200 ppm). No systematic variation has been found in the Sr content of the different carbonatitic minerals within a particular carbonatite type.

The study emphasizes the importance of chemical data as one of the essential parameters in identifying carbonatite complexes from doubtful occurrences and also in elucidating their evolution. The study confirms the earlier observation that the carbonatites of Sevattur belong to the apatite-magnetite type.


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  • On Some Geochemical Aspects of the Sevattur Carbonatite Complex, North Arcot District, Tamil Nadu

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Authors

P. Krishnamurthy
Atomic Minerals Division, Department of Atomic Energy, Hyderabad 16, India

Abstract


The major and trace element compositions of the Sevattur carbonatites are broadly comparable with the 'typical carbonatite abundance' values compiled by Gold (1966). The mean values for the Sevattur carbonatites are as follows (in weight per cent): SiO2-2.60, Al2O3-2.49, TiO2-0.46, Fe2O3 + FeO-9.64, MnO-0.52, MgO-10.14, CaO-32.84, Na2O-0.85, K2O-0.49, P2O5-3.48, CO2-36.15, H2O±0.32, with trace elements in parts per million as: Ba-1663, Sr-6427, Zr-763, Rb-2, La-271, Ce-641, Y -41, Nb-78, Ta-34, SC-16, Zn-69, Cu-78, Ni-18, Pb-27.

There is an enrichment of total iron, MgO, MnO, P2O5 and Na2O with successively younger phases within the main carbonatite body and such a feature is comparable with the liquid lines of descent, observed in synthetic carbonatite systems. Fenitisation of the country rocks (granitic gneisses) along the southern border of the carbonatite body is indicated from the enrichment of K2O, Na2O, Al2O3 and impoverishment in SiO2, total iron and MgO in the fenites when compared with the country rocks.

Sr (4109-9375 ppm, mean 6427) exceeds Ba (1000-2420 ppm, mean 1663) in all the types of carbonatites by a factor of 2 to 5. The lighter rare earths such as La and Ce are enriched in the calcitic types (La + Ce, 892-1351 ppm) when compared to the beforsites (647-920 ppm). In general, the beforsites are richer in Ba, Nb, Ta and Sc.

Among the carbonatitic minerals, early calcites are much enriched in Sr (5500-6200 ppm) in comparison with Ba (1230-1700) by a factor of 3 to 5. Sr with successively younger fractions of the carbonatite, seems to show a preference to the dolomites (3500-4000 ppm in the early phases and 9000-11000 ppm in the later phases) rather than to the apatites (6200 ppm). No systematic variation has been found in the Sr content of the different carbonatitic minerals within a particular carbonatite type.

The study emphasizes the importance of chemical data as one of the essential parameters in identifying carbonatite complexes from doubtful occurrences and also in elucidating their evolution. The study confirms the earlier observation that the carbonatites of Sevattur belong to the apatite-magnetite type.