A new way to map old sutures using deformed alkaline rocks and carbonatites

Using a recent compilation of African alkaline igneous rocks and carbonatites, we show that nearly 90% (28 of 32 occurrences) of nepheline syenite gneisses and deformed carbonatites are concentrated within known or inferred Proterozoic suture zones. Given the wellestablished intracontinental rift setting for these rocks and the likely continental collisional setting for their subsequent deformation, we suggest that deformed alkaline rocks and carbonatites (DARCs) represent the products of two well-defined parts of the Wilson Cycle. DARCs mark the places where vanished oceans have opened and then closed. We further postulate that DARCs taken into the mantle lithosphere to ca. 100 km depths at collision could provide source material for later alkaline magmatic activity. This could account for the observation of recurrent alkaline magmatic activity over hundreds of m.y. in provinces such as that of southern Malawi.