New geochronological constraints on the Klipriviersberg Group: defining a new Neoarchean large igneous province on the Kaapvaal Craton, South Africa

Abstract: In recent years, there has been a significant improvement in our ability to determine the ages of extensive short-lived magmatic events (large igneous provinces) dominated by rocks of mafic (silica-poor) compositions. This has been aided by targeting the magmatic feeders to these large igneous provinces (e.g., dykes and sills), which often host trace amounts of datable zirconium minerals such as baddeleyite and zircon. These age determinations are of great importance for unravelling the geological history and paleogeography of our planet. The Kaapvaal Craton in southern Africa hosts a rich and pristine geological history with many geological units and events in need of better age constraints. One is the Neoarchean Ventersdorp Supergroup, with the Klipriviersberg Group, and underlying successions of the Meso- to Neoarchean Witwatersrand Supergroup, of which the latter is world-wide known for its association with gold findings. By extracting baddeleyite from intrusive mafic sills from the Witwatersrand sediments, we can obtain critical age constraints for these successions using U–Pb geochronology on baddeleyite. Here we show that feeders to the Klipriviersberg Group are ca. 2790 Ma using ID-TIMS and complimentary LA-ICP-MS on baddeleyite from two mafic sills. This age makes the Klipriviersberg Group magmatic event almost 80 Myr older than previously thought, and is in agreement with several indirect studies which have suggested a similar age in Ventersdorp Supergroup temporally equivalent basins. Our results imply that there is a significant time gap in the Ventersdorp Supergroup between the deposition of the Klipriviersberg Group and the Platberg Group, and is associated with craton-wide mafic and felsic magmatism. This conclusion implies that the deposition of the Witwatersrand Supergroup ceased before 2790 Ma, which is when the eruption of the Klipriviersberg Group basalts commenced. Our results define a new large igneous province on the Kaapvaal Craton, and redefine the timing of both the beginning of Ventersdorp and termination of Witwatersrand supergroups. This new timing indicates that world class gold-bearing conglomerates within the Witwatersrand succession were deposited before 2790 Ma.