The Atlantic island of Maio in the Cape Verde archipelago has a Basement Complex of Mesozoic and Tertiary age which is invaded by an intense swarm of sheet intrusions ranging in age from c. 16 to 8 Ma. These cut an alkaline ultramafic, mafic and felsic plutonic complex (Central Igneous Complex) and its host, a segment of Jurassic ocean floor with pillow lavas and abyssal sediments. The Basement Complex is dissected and unconformably overlain by a sequence of lavas of basanitic and nephelinitic composition.
The sheet intrusions comprise sills and dykes of alkaline lamprophyre, many of which are aphyric or sparsely porphyritic. Phenocrysts are predominantly clinopyroxene, with smaller amounts of olivine and Ti-magnetite. Microphenocrysts are kaersutite, apatite and biotite. The groundmass is dominated by potassium feldspar and analcime. Calcic plagioclase is extremely rare. Chemically the intrusions are basanitic, with major and minor element compositions directly comparable with the Neogene and Pleistocene basanitic lavas which cap the island. Calculations based on observed phenocryst phases suggest that the liquids involved in the crystallization of the sheet intrusions may have been derived by crystal fractionation from a range of basanitic parents. These were similar to those from which the later lavas were generated, but modified to some extent by variable water-rock interactions enhanced by trapped volatiles.