NEW INSIGHTS INTO 1.87 GA INTRAPLATE MAGMATISM AND THE APPARENT POLAR WANDER PATH OF THE KAAPVAAL CRATON AS DRAWN FROM THE SOUTPANSBERG GROUP

The stratigraphic relationship between red bed and volcanic successions of the Soutpansberg and Waterberg Groups of southern Africa has long been disputed. Most recent contributions to the problem show that the Waterberg Group is in part older than the Soutpansberg Group (e.g. Hanson et al., 2004, South African Journal of Geology, 107, 233-254). Those authors illustrated that lavas of the Soutpansberg Group as well as dolerite sills intruding the Soutpansberg Group are magnetized antipodal to the ~1.87 Ga post-Waterberg sills. For this reason they provisionally assigned that age to the whole of the Soutpansberg Group.

Results from our study indicate that the Sibasa Fm. lavas, stratigraphically near the base of the Soutpansberg Group, record at least two magnetically similar, but temporally discrete thermal events. The magnetically more stable component (named HIG) is indistinguishable from the ~1.87 Ga post-Waterberg sills. The second component recorded by the Sibasa Fm. is oriented southeasterly and upwards (SEu), and apart from being roughly antipodal to the HIG-component it overprints the HIG-component as well. The SEu-component is also associated with sills that intruded the Soutpansberg Group after the deposition of Wylies Poort Fm. (stratigraphically above the Sibasa Fm and containing detrital zircon populations as young as 1.85 Ga). None of these sills displayed HIG components. The post-Wylies Poort sills are interpreted to represent a discrete thermal event younger than the ~1.87 Ga post-Waterberg sills and are believed to be responsible for overprinting the Sibasa Fm. lavas.

A small, but significant, adjustment is made to the conclusion of Hanson et al. (2004), i.e. an age of ~1.87 Ga for the basal Soutpansberg seems paleomagnetically justified, but not for the whole Soutpansberg (the upper parts of which have been suggested to be as young as 1.76 Ga based on Rb-Sr age data). The paleomagnetic results from the Soutpansberg Group together with that from the Waterberg Group add four paleopoles to the existing database and extend the apparent polar wander path of the Kaapvaal Craton from ~2.05 billion years to ~1.87 billion years (and possibly as far as ~1.76 Ga).