The Sept Iles anorthosite complex is located in the Grenville (structural) Province of the Canadian Shield. Geochronological studies of both the basic and acidic portions of the complex show that it was intruded approximately 540 Ma ago and thus post-dates the Grenville orogeny by at least 500 Ma. Field relationships confirm the contemporaneity of the basic (anorthosite and gabbro) and acidic (syenite and granite) rocks. Differentiation of the basic magma was controlled by preferential nucleation in the boundary layer. Crystal separation due to gravity and possibly liquid immiscibility caused differentiation once intermediate or acidic magma compositions were formed. There is no evidence that crustal contamination has played a role in the generation of the acidic rocks. The complex has been intruded along the St. Lawrence graben fault, known to have been active at this time from the profusion of igneous rocks elsewhere along this structure, which are contemporaneous with the Sept Iles complex. Activity on this fault has disrupted the complex so that rocks found on the mainland were originally formed at the base of the complex, whereas rocks exposed on the islands were formed at the top of the complex. This study of the Sept Iles complex shows that anorthosite is not restricted to Precambrian terranes, and may be associated with acidic alkaline rocks and large-scale ensialic rifting.