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

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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A. F. Trendall ¹, J. R De Laeter ¹, D. R. Nelson ², D. Mukhopadhyay 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, 1 Plain Street, East Perth 6004, Western Australia, AU
3 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road. Calcutta 700 019, IN

Abstract

SHRIMP U-Pb analyses of zircon grains from two thin dark shales believed to be syndepositional ashfall tuffs close to the base of the banded iron-formation (BIF) within the Mulaingiri Formation of the Bababudan Group (Dharwar Supergroup) of the western Kamataka Craton provide a precise age for the onset of BIF deposition in the basin. Sample 120019C, from the exact base of the BIF, gave an age of 2720 ± 7 Ma, while sample 120018C, 14.5m above the base, gave an age of 2718 ± 6 Ma. The second sample contained younger grains which may have been affected by a late thermal event.

Keywords

Geochronology, Bababudan Schist Belt, SHRIMP, BIF, Karnataka.

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