POST-BUSHVELD PALEOPROTEROZOIC INTRAPLATE MAGMATISM IN SOUTHERN AFRICA

Understanding Proterozoic mantle dynamics requires delineation of mantle-derived intraplate igneous provinces in ancient cratons worldwide. This is best accomplished using precise U-Pb geochronology coupled with paleomagnetic correlation of widely distributed dike swarms, sill complexes, remnants of flood lavas, and related rocks. In southern Africa, published and ongoing work provides evidence for two widespread Paleoproterozoic mafic magmatic events that postdate the better known, 2.06 Ga Bushveld event. Remnants of lavas and intrusions emplaced during these events encompass large areas in the Archean Kaapvaal (KC) and Zimbabwe (ZC) cratons and intervening Limpopo belt (LB). In the KC, the first event is represented by the Moshaneng diabases in SE Botswana, the Hartley basalts along the western KC margin, and the buried Trompsburg lopolith in the southern KC, all of which have U-Pb baddeleyite or zircon ages of ~1928-1915 Ma. A second event in the KC is represented partly by widespread diabase sills that intrude the Waterberg Group in South Africa and have U-Pb baddeleyite ages of ~1879-1872 Ma. Flood basalts and sills in the Soutpansberg rift along the LB to the NE have yielded a pole antipodal to that for the Waterberg sills, indicating emplacement as part of the same event but during a reversed polarity chron. In the ZC, the widespread Sebanga dikes have yielded two distinct pole positions. One pole is statistically indistinguishable from the pole for the Moshaneng diabases, whereas the other pole is indistinguishable from the pole for the Waterberg sills. These data suggest that the 1928-1915 Ma and 1879-1872 Ma events both extended well into the ZC to the north. Precise geochronology is lacking for the extensive Mashonaland diabase sheets in the ZC, but Rb-Sr isotopic data are consistent with emplacement at ~1.9 to ~1.8 Ga. The Mashonaland pole is distinct from the poles for the Sebanga dikes, the Moshaneng diabases in Botswana, or the Waterberg/Soutpansberg sills and basalts, suggesting that the Mashonaland diabases represent a third intraplate magmatic event that may be restricted to the ZC. Other dike swarms of inferred Paleoproterozoic age occur in the KC and ZC, and future work on these swarms should help resolve the time-space distribution of Paleoproterozoic intraplate magmatism in both cratons.