Granitic rocks of magmatic origin form about 60 percent of the crystalline basement in the northeastern Sudan. Although divisible on field evidence into older (batholithic) and younger granites and overlying, horizontally disposed rhyolitic volcanic rocks, most units have an isotopic age of 700 m.y.; quantitatively minor intrusive masses have ages of 500 and 100 m.y. Field, geochemical, and, to a lesser extent, petrographic and mineralogic evidence indicates that the Eocambrian granitic rocks represent the cratonization of an island arc. The distribution of ultramafic (ophiolite) masses in northeastern-Africa and western Arabia suggests that several arc systems have been swept together. Paleomagnetic parameters that preclude extensive horizontal movement and closely spaced arc systems, like those of the present-day southwestern Pacific, are envisaged. The later minor magmatic activity was within-plate; a hot-spot origin is thus postulated.