PROTEROZOIC INTRAPLATE IGNEOUS PROVINCES IN SOUTHERN AFRICA: THE NEED FOR INTEGRATED PALEOMAGNETIC AND GEOCHRONOLOGICAL STUDIES

The Kalahari craton in southern Africa contains several spatially overlapping Proterozoic intraplate igneous provinces that may hold clues to ancient supercontinent configurations. Delimiting the time-space relations of these provinces is complicated by the fact that they are mostly defined by scattered erosional remnants or hidden by younger cover. One of the best known examples includes the massive Bushveld Complex and coeval plutonic and volcanic units in South Africa and Botswana, which have crystallization ages of 2061-2054 Ma. Poles for these rocks show significant variations, implying poorly understood differences in the age of magnetization. Other portions of the Kalahari APWP are constrained by poles for a recently recognized tholeiitic intraplate province emplaced at 1879-1872 Ma in South Africa, and for the Mashonaland dolerites in Zimbabwe, which have an imprecise Rb-Sr date of ~1.8 Ga. The younger Umkondo LIP extends over much of the Kalahari craton and includes remnants of plateau basalts and widely dispersed dolerite intrusions, as well as major layered complexes. Single-crystal U-Pb zircon and baddeleyite ages and paleomagnetic data from >100 sites indicate that the main mafic magmatism occurred at 1112-1108 Ma, primarily during a single polarity chron. Close similarity in timing between the Umkondo LIP and intraplate magmatism in Laurentia (especially parts of the Midcontinent rift) suggests that both LIPs may have been linked within the Rodinia supercontinent.

Widespread ~900-720 Ma anorogenic felsic and mafic rocks and more limited alkaline/carbonatite complexes occur near the Kalahari and Congo craton margins, recording protracted intraplate magmatism that may be related to Rodinia breakup. Many of these rocks bear a strong Pan-African overprint and are unsuitable for paleomagnetic work. Better preserved, ~800-750 Ma igneous units in the Congo craton in Tanzania have yielded two widely separate poles that have contributed to the debate on rapid, large-magnitude TPW. In Kalahari, subvolcanic felsic intrusive complexes and the extensive Gannakouriep dolerite dike swarm near the SW craton margin, which have zircon and baddeleyite ages of ~890-780 Ma (based on our new data), appear to be the best targets for constraining broadly coeval parts of the Kalahari APWP.