The Bushveld Complex in South Africa records 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 magma 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 minute baddeleyite crystals to date a series of magmatic dykes located east of the Transvaal basin. We find that all of the dykes formed between 2.70 and 2.66 billion years ago, some 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. We suggest that the radiating swarmrecords 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 have later fallen away into the mantle, with the delamination causing the inflow of hot mantle that initiated production of the voluminous Bushveld magmas 600 million years after the mantle plume impact.