The Michikamau Intrusion is a large, unmetamorphosed anorthositic mass lying within the broad belt of anorthositic bodies extending from southeastern Ontario to Labrador. Potassium–argon biotite dates place the time of crystallization of the intrusion at approximately 1 400 million years ago.The main rock units of the intrusion are leucotroctolite, anorthosite, and leucogabbro; essential minerals are plagioclase, olivine, clinopyroxene, and orthopyroxene. The bulk of the plagioclase falls in the range An52–An62 and olivine in the range Fo60–Fo70. The rocks are remarkably fresh and free from secondary alteration products.A number of features characteristic of large, layered basic plutons are present in the Michikamau Intrusion. The chilled margin has the composition of olivine basalt. However, the bulk composition of the mass is highly feldspathic, thus leading to the proposition that the basaltic liquid was heavily charged with plagioclase crystals. Cumulate structures and textures indicate that bottom accumulation of crystals may have played an important part in the solidification and differentiation of the intrusion.The course of differentiation of the magma was toward extreme iron enrichment with the development of ferrous-rich dioritic, granodioritic, and syenitic rocks as end products. It is unlikely that water played an important role in the crystallization of the intrusion.