f Active vs. Passive Rifting in the South Atlantic from a Petrologic Perspective: an Upwelling Controversy

The Atlantic margin of southern Africa is a classic example of a volcanic rifted margin, but the strong gradient in the intensity of magmatism from north to south challenges a conventional plume model. A petrologic study of breakup-related mafic dikes from the northern and southern margin was undertaken to provide data on magma compositions and P-T conditions of mantle melting with which to test the plume (excess heat) hypothesis. The results demonstrate not only more magma but greater diversity of magma types and stronger crustal influence in the dikes from high-flux northern margin in Nambia. Some of these dikes have picritic bulk compositions (>12 wt.% MgO), and olivines Fo90-93, indicating about 200°C excess mantle temperature. Dolerite dikes from the Cape peninsula derive from a single tholeiite series and yield petrologic temperature estimates in favor of a passive upwelling model.