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A late Archaean radiating dyke swarm as possible clue to the origin of the Bushveld Complex

Nature Geoscience volume 4, pages 865–869 (2011)Cite this article

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Abstract

The Bushveld Complex in South Africa represents the world’s largest intrusion of magnesium- and iron-rich magmas. The Bushveld magmas were emplaced beneath the Transvaal basin1 ∼2.06 billion years ago2,3, but their origin remains elusive. The magmas may have formed in response to an upwelling mantle plume4, ancient subduction5 or melting triggered by a meteorite impact6. Here we use U–Pb dating of baddeleyite crystals to date a series of mafic magmatic dykes located east of the Transvaal basin. We find that these dykes formed between 2.70 and 2.66 billion years ago, roughly 600 million years before the Bushveld magmas were emplaced. Collectively, the geometry of the dykes forms a radiating swarm converging towards a focal point in the eastern part of the Bushveld Complex. Such radiating swarms typically record the impact of a mantle plume head that injected large volumes of magma into the crust and at the base of the lithosphere. We propose that subsequent cooling and metamorphism of these mantle-plume-derived rocks caused them to increase in density and sink, triggering subsidence of the Transvaal basin. The dense rocks may later have sunk away into the mantle, with the delamination causing the inflow of hot mantle that initiated production of the voluminous Bushveld magmas about 600 million years after the mantle plume impact.

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Figure 1: Geological overview of the Kaapvaal craton.
Figure 2: U–Pb geochronology of mafic dykes.
Figure 3: The origin of the Bushveld Complex.

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Acknowledgements

We acknowledge funding from the Crafoord Foundation and the Swedish Research Council via grants to U.S. We thank W. L. Griffin for constructive comments that improved the manuscript. Thanks to D. Conley at Lund University for commenting on the manuscript and linguistic help. J.R.O. thanks the laboratory staff at the Jack Satterly Geochronology Laboratory for assistance during his visit in 2009.

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Authors and Affiliations

  1. Department of Earth and Ecosystem Sciences, Division of Geology, Lund University, Sölvegatan 12, SE 223 62 Lund, Sweden

    Johan R. Olsson & Ulf Söderlund

  2. Department of Geology, Jack Satterly Geochronology Laboratory, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada

    Michael A. Hamilton

  3. Department of Earth Sciences, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa

    Martin B. Klausen

  4. School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK

    George R. Helffrich

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  1. Johan R. Olsson
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  5. George R. Helffrich
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Contributions

M.B.K. and U.S. initiated the study and sampled the targeted dolerite dykes. J.R.O. processed the dyke samples and extracted the baddeleyite separates. M.A.H. and J.R.O. performed the U–Pb ID-TIMS analyses at the Jack Satterly Geochronology Laboratory, University of Toronto. G.R.H. provided important re-interpretations of published seismic data. J.R.O. and U.S. wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Johan R. Olsson.

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Olsson, J., Söderlund, U., Hamilton, M. et al. A late Archaean radiating dyke swarm as possible clue to the origin of the Bushveld Complex. Nature Geosci 4, 865–869 (2011). https://doi.org/10.1038/ngeo1308

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