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 basin(1) similar to 2.06 billion years ago(2,3), but their origin remains elusive. The magmas may have formed in response to an upwelling mantle plume(4), ancient subduction5 or melting triggered by a meteorite impact(6). 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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