Petrography, Sr-isotope geochemistry and geochronology of the Nxau Nxau kimberlites, north-west Botswana

Abstract

The Nxau Nxau kimberlites in northwest Botswana belong to the Xaudum kimberlite province that also includes the Sikereti, Kaudom and Gura kimberlite clusters in north-east Namibia. The Nxau Nxau kimberlites lie on the southernmost extension of the Congo Craton, which incorporates part of the Damara Orogenic Belt on its margin. The Xaudum kimberlite province is geographically isolated from other known clusters but occurs within the limits of the NW-SE oriented, Karoo-aged Okavango Dyke Swarm and near NE-SW faults interpreted as the early stages of the East African Rift System. Petrographic, geochronological and isotopic studies were undertaken to characterise the nature of these kimberlites and the timing of their emplacement. The Nxau Nxau kimberlites exhibit groundmass textures, mineral phases and Sr-isotope compositions (87Sr/86Sri of 0.7036 ± 0.0002; 2σ) that are characteristic of archetypal (Group I) kimberlites. U-Pb perovskite, 40Ar/39Ar phlogopite and Rb-Sr phlogopite ages indicate that the kimberlites were emplaced in the Cretaceous, with perovskite from four samples yielding a preferred weighted average U-Pb age of 84 ± 4 Ma (2σ). This age is typical of many kimberlites in southern Africa, indicating that the Xaudum occurrences form part of this widespread Late Cretaceous kimberlite magmatic province. This time marks a significant period of tectonic stress reorganisation that could have provided the trigger for kimberlite magmatism. In this regard, the Nxau Nxau kimberlites may form part of a NE-SW oriented trend such as the Lucapa corridor, with implications for further undiscovered kimberlites along this corridor.

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Acknowledgements

We thank Graham Hutchinson, Alan Greig and Erin Matchan and Stan Szczepanski for assistance with EMP, LA-ICP-MS and 40Ar/39Ar analysis, respectively and to The University of Melbourne Noble Gas and Isotope Geochemistry Labs. We are also grateful to Tsodilo Resources for access to samples and logistical support. The primary author is a grateful recipient of an Australian Government Research Training Program Scholarship. The authors would like to thank Bruce Kjarsgaard, Sebastian Tappe and an anonymous reviewer for their thoughtful and constructive comments on this paper and Angus Fitzpayne for his assistance in proof-reading.

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Correspondence to Henrietta Farr.

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Farr, H., Phillips, D., Maas, R. et al. Petrography, Sr-isotope geochemistry and geochronology of the Nxau Nxau kimberlites, north-west Botswana. Miner Petrol 112, 625–638 (2018). https://doi.org/10.1007/s00710-018-0593-8

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Keywords

  • Kimberlite
  • Congo Craton
  • U–Pb geochronology
  • 40Ar/39Ar geochronology
  • Perovskite