Magmatism and rifting in Western and Central Africa, from Late Jurassic to Recent times

Abstract

Mesozoic-Cenozoic magmatic activity in West and Central Africa is reviewed, with particular emphasis on the relationship between Mesozoic magmatism, major phases of continental rifting and the opening of the Equatorial Atlantic. It is suggested that during the initial stages of rifting, the activity of a mantle plume, the St. Helena hotspot, may have been important in weakening the lithosphere across the region. Evidence for magmatism concurrent with the onset of rifting in some basins supports such an active rifting model. Magma compositions range from alkali to tholeiitic basalts and their differentiates. Transitional to tholeiitic basalts are comparatively rare and are generated by greater degrees of partial melting, at probably shallower mantle depths, than associated alkali basalts. In some instances their occurrence may be correlated with higher amounts of lithospheric extension. However, in other instances they appear early in the rift sequence when overall amounts of extension were small. These tholeiitic basalts often have geochemical characteristics dominated by an ancient sub-continental lithosphere isotopic signature, which may have been introduced by crustal contamination. Cenozoic magmatism of alkaline affinity is widespread in West and Central Africa. In many instances, sites of activity appear to be structurally controlled by pre-existing basement fractures/lineaments of Mesozoic-Precambrian age. Most of the volcanic fields lie outside the boundaries of the Cretaceous rifts and many are associated with broad basement uplifts. However, there has also been a rejuvenation of tectono-magmatic activity within several of the Cretaceous rift basins during the Neogene. The parental magmas are considered to be generated mainly by partial melting of a zone at the base of the sub-continental lithosphere, which was variably metasomatised by the activity of mantle plumes beneath the African plate during the Mesozoic.