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

N. V. Chalapathi Rao ¹, J. A. Miller ², S. A. Gibson ³, D. M. Pyle ³, V. Madhavan ⁴

Affiliations
1 Mineralogy Laboratory, Ore Dressing Division, Indian Bureau of Mines, Hingna Road, Nagpur- 440016, IN
2 Bullard Laboratories, Madingley Road, Cambridge- CB3 9BB, GB
3 Department of Earth Sciences, Downing Street, Cambridge- CB2 3EQ, GB
4 Department of Geology, Kakatiya University, Warangal- 506 009, IN

Abstract

⁴⁰Ar/ ³⁹Ar age spectra of groundmass phlogopite separates from the Kotakonda kimberlite (Eastern Dharwar craton) and the Chelima lamproite (Cuddapah basin) are dominated by major plateaus at 1401.4±4.6Ma and 1417.8±8.2 Ma respectively. These ages are consistent (within their error limits) with the conventional K-Ar dates on phlogopite separates from the same samples earlier reported by us. This study supports our contention that the emplacement of Kotakonda kimberlite and Chelima lamproite was contemporaneous and these pipes are older than the Anantapur kimberlites (-1090 Ma) of the Eastern Dharwar craton. The agreement of ages obtained by employing more than one technique is a testimony of confidence in our results and, therefore, firmly establishes an episode of Proterozoic mafic potassic magmatic activity in the Eastern Dharwar craton and adjacent Cuddapah basin at ca. 1400 Ma. The concordance of plateaus and correlation of isochron plots for both these bodies suggests that at least these parts of the Eastern Dharwar craton and Cuddapah basin have not been subjected to any subsequent tectono-thermal activity. Our results contrast with the recent suggestion that a ∼ 1000 Ma thermal event in and around the Cuddapah basin led to argon loss and was responsible for the variable K-Ar ages of dyke swarms in the Eastern Dharwar craton.

Keywords

⁴⁰Ar/ ³⁹Ar Ages, Kimberlite, Lamproite, Dyke Swarms, Dhanvar Craton, Andhra Pradesh.

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N. V. Chalapathi Rao ¹, S. A. Gibson ², D. M. Pyle ², A. P. Dickin ²

Affiliations
1 Mineralogy Laboratory, Ore Dressing Division, Indian Bureau of Mines, Nagpur - 440 016, IN
2 Department of Geology, McMaster University, Hamilton L8S4M1, Ontario, CA

Abstract

Kimberlites intruding the Precambrian basement towards the western margin of the Cuddapah basin near Anantapur (1090 Ma) and Mahbubnagar (1360 Ma) in Andhra Pradesh have initial ⁸⁷Sr/⁸⁶Sr between 0.70205 to 0.70734 and εNd between +0.5 to +4.68. Mesoproterozoic lamproites (1380 Ma) from the Cuddapah basin (Chelima and Zangamarajupalle) and its NE margin (Ramannapeta) have initiail ⁸⁷Sr/⁸⁶Sr between 0.70520 and 0.7390 and εNd from -6.43 to -8.29. Combined Sr- and Nd-isotopic ratios suggest that lamproites were derived from "enriched" sources which have time-averaged higher Rb/Sr and lower Sm/Nd ratios than the Bulk Earth whereas kimberlites were derived from "depleted" source with lower Rb/Sr and higher Sm/Nd ratios. Calculated T_DM model ages suggest that the lamproite source enrichment (∼2 Ga) preceded that of kimberlites (∼1.37 Ga). Our work demonstrates the existence of isotopically contrasting upper mantle sources for southern Indian kimberlites and lamproites and provides evidence for a lateral, isotopically heterogeneous mantle beneath the Cuddapah basin and eastern Dharwar craton. The significance of our results in the context of diamond exploration is also highlighted.

Keywords

Petrology, Geochemistry, Kimberlite, Lamproite, Isotopes, Mantle, Andhra Pradesh.

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