| Summary: | The separation of Africa and South America about 130 million years ago was accompanied by huge volumes of magma derived from the mantle. Erosion has removed many of the surficial lavas but dolerite dyke swarms beneath them are preserved. These dykes are under study to determine the composition of magmas, the age of magmatism, and the direction of extensional stress in the crust. Here, we compare two dolerite dyke swarms from the southern African margin that are separated by about 2000 km. The northern area is the Etendeka Province of NW Namibia with the Henties Bay-Outjo dyke swarm (HOD). The southern area, at the tip of Africa, hosts the False Bay dyke swarm. The dolerites from these two areas differ in compositional diversity and magmatic temperatures, both being higher in the north. Three magma types are distinguished in the northern area and only one in the south. Temperatures calculated from olivine-melt equilibrium show a north-south decrease by about 150 °C. Converted to mantle potential temperature and compared with global averages, the data indicate “normal” upper mantle temperatures in southern region like in the mid-ocean ridge systems (1380 °C), and mantle temperatures in the northern area (1520 °C) similar to hotspot islands like Hawaii and Iceland. We conclude that breakup-related magmatism in the south was related to extension and passive decompression of an upper mantle with normal temperature and composition. The Tristan mantle plume greatly enhanced magma production in the Etendeka Province but was not the cause of continental breakup.
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