U–Pb baddeleyite ages linking major Archean dyke swarms to volcanic-rift forming events in the Kaapvaal craton (South Africa), and a precise age for the Bushveld Complex

Abstract

The Archean basement in the northeastern part of the Kaapvaal craton is intruded by a large number of mafic dykes, defining three major dyke swarms, which collectively appear to fan out from the Bushveld Complex. Herein we present U–Pb baddeleyite ages for two of these dyke swarms, the northwest trending Badplaas Dyke Swarm and the east-west trending Rykoppies Dyke Swarm, and infer their correlation with tectonic events in the Kaapvaal craton. We also present a U–Pb baddeleyite age for a noritic phase of the Marginal Zone of the Rustenburg Layered Suite (Bushveld Complex). The age of the Badplaas swarm is determined from two dolerites dated at 2965.9 ± 0.7 Ma and 2967.0 ± 1.1 Ma. These ages coincide with units of the Nsuze Group lavas (2967–2985 Ma), which constitute the world's oldest preserved rift basin, and suggest that dykes of this swarm are feeders to basaltic units of this group. Similarly, the E–W trending Rykoppies swarm has earlier been interpreted as a potential feeder system to the Bushveld Complex. However, the emplacement ages of six dolerites fall in the range 2.66–2.68 Ga, thus ∼600 Myr earlier than the intrusion of the Bushveld Complex (herein dated at 2057.7 ± 1.6 Ma). Rather, these ages coincide with the Allanridge Formation at the uppermost part of the Ventersdorp Supergroup as well as volcanic rocks of the “protobasinal” sequences preserved at the base of the overlying Transvaal Basin. The Rykoppies Dyke Swarm probably marks the initial stages of rifting of the Transvaal Basin and reflects a major shift from a NW–SE to an E–W trending tectonic setting. The origin of the Rykoppies Dyke Swarm can be linked either to prolonged mantle plume activity or to the onset of back-arc extension associated with south-directed subduction of oceanic lithosphere in a compressional setting along the northern margin of the Kaapvaal craton.