The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100 500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An68 An60), trace elements (Sr: 1023 1071 ppm; Ba: 132 172 ppm) and Sr isotopic ratios (87Sr/86Sri: 0.70356 0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An60 An38). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm3 during early fractionation of troctolites and then decreased continuously to 2.16 g/cm3 with fractionation of Fe Ti oxide-bearing gabbros. Plagioclase (An69 An60) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (