A general account of Karoo vulcanicity in southern Africa

Volcanic rocks of late Triassic to early Cretaceous age which today cover more than 140 000 km² of the southern African sub-continent are discussed. The rocks occur as scattered outliers and represent small eroded remnants of an originally more extensive volcanic province. The rocks are best preserved in three main areas of southern Africa, in Namibia, in central South Africa, Transkei and Lesotho and in the southeastern areas of the sub-continent including South Africa, Swaziland, Mozambique and Zimbabwe. A history of previous work done on Karoo volcanism is presented and accounts of the geological relationships of the main, and some subsidiary, areas as well as events preceding the major phase of extrusive vulcanism are discussed. Relationships between dolerite dykes and basaltic lavas are noted. Volcano-stratigraphic and geochemical mapping have been used to subdivide the volcanic successions found in the different areas, and recently adopted nomenclature is presented. Considerable more variability has been recognised in the volcanic succession than was previously recognised, the flood basalt pile being more complex than was originally thought. Rocks from the central Karoo areas are primarily of basaltic composition whereas those from the western and eastern marginal areas include basic, and felsic types. Age relationships of the volcanics based on recent studies are outlined and geochemical variations within the entire province discussed. The classification system adopted for the Karoo volcanics is described briefly and chemical relationships of extrusive and intrusive rocks are included. In concluding some new ideas concerning the overall development of the Karoo volcanic province and related fragmentation of Gondwanaland are presented.