Controls on post-Gondwana alkaline volcanism in Southern Africa

A comprehensive database of high precision (± 5 Ma) radiometric ages has been assembled for kimberlites and other alkaline volcanic pipes erupted in southern Africa during the period immediately preceding and following the disruption of Gondwana. These ages show that alkaline volcanic activity in southern Africa has been episodic since Gondwana break-up, and was broadly coeval with episodes of alkaline volcanism elsewhere in Africa. In southern Africa, offshore sedimentation patterns were also episodic, with periods of essentially continuous sedimentation broken by series of major unconformities, linked to plate spreading regimes. The volcanic episodes followed major changes in plate spreading histories and the associated unconformities in the offshore sedimentary record with a lag of 5–13 Ma. On a regional scale, there is an overall younging of volcanic activity from the interior to the margins of southern Africa. Some of the volcanic pipe clusters define a major igneous lineament, with a general age progression younging to the southwest. Parts of this alkaline lineament coincide with major rift zones, but the volcanism predated the surface expression of rifting by as much as 90 Ma. These relationships point to tectonic triggers for alkaline volcanic activity, rather than plumes initiated near the core–mantle boundary. There is also a systematic geographic variation in alkaline magma type across southern Africa, with Mg-rich, less evolved kimberlites emplaced into the Archean craton of the central portions of the sub-continent, and more evolved compositions emplaced into the marginal younger fold belts. These chemical trends are interpreted in terms of decreasing depth of intersection between the CO2–H2O peridotite solidus and the geotherm with increasing geothermal gradient from the Archean craton to the younger marginal fold belts.