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
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. © 2010 Elsevier B.V.