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

G. Sarkari ¹, D. K. Pauli ², J. R. De Laeter ³, N. J. Mcnaughton ², V. P. Misra ¹

Affiliations
1 Geological Survey of India, 29, J. L. Nehru Road, Calcutta 700016, IN
2 Department of Geology, The University of Western Australia, Nedlands WA 6009, AU
3 School of Physics and Geosciences, Curtin University of Technology, Bentley WA 6102, AU

Abstract

Preliminary Pb-Pb and Rb-Sr geochronology of granitic and gneissic rocks from the Sukma area of the Bastar craton, Central India, provides important constraints on crustal evolution, Much of the craton is made up of felsic orthogneisses and younger granitic intrusives, compositionally ranging from tonalite to granite. Pb-Pb isotopic data suggest the presence of ca. 3.0 Ga old gneisses. Younger granitic intrusives have been dated at ca. 2.6 Ga which represents a widespread resetting and/or emplacement event. Comparison of the Pb-Pb and Rb-Sr whole rock ages suggests that the latter were more perturbed after the gneiss-forming or emplacement events. All rock suites show significant geological scatter of isotopic data probably because of sampling on a regional scale, and reflect multi-stage isotopic evolution in a complex terrain.

The present isotopic data indicate the presence of Archaean rocks in the Bastar craton and suggest temporal similarity with the oldest crustal rocks in the Singhbhum and Dharwar cratons.

Keywords

Bastar Craton, Geochemistry, Geochronology, Isotope Geology, Madhya Pradesh.

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A. F. Trendall ¹, J. R. de Laeter ¹, D. R. Nelson ², Y. J. Bhaskar Rao ³

Affiliations
1 School of Physical Sciences, Curtin University of Technology, GPO Box 1987, Perth 6000, Western Australia, AU
2 Geological Survey of Western Australia, Mineral House, I Plain Street, East Perth 6004, Western Australia, AU
3 National Geophysical Research Institute, Hyderabad 500 007, IN

Abstract

SHRIMP U-Pb analyses of zircons from a rhyolite sample collected from the Daginkatte Formation of the Chitradurga Group (Dharwar Supergroup), close to the type locality, give an age of 2601 ± 6 Ma (1σ 23 of 24 analysed grains). This is within error of the 2614 ± 8 Ma (2σ 11 grains) recently reported by A.P. Nutman and his colleagues in 1996 from a sample of acid volcanic schist of the same formation, about 25 km along strike. Combining Nutman et aI's (1996) published data with the new analyses reported here gives a pooled age of 2606 ± 6 Ma (n = 34, 1σ, χ² = 1.31), within error of both of the separately determined ages. Having regard to the reported uniform lithology of the Daginkatte Fonnation, the pooled age is probably that of a short magmatic event during which it was formed, and provides a firm constraint relevant to this stratigraphic level of the Chitradurga Group; however. it is now important to determine whether the rhyolitic rocks of the Daginkatte Formation are intrusive or extrusive.

Keywords

Geochronology, Zircon U-Ph Age, Rhyolite, Dharwar Supergroup, Karnataka.

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