New observations on the French River alkaline syenite complex, Bigwood township, Ontario

The French River alkaline syenite complex comprises two lenses of nepheline-bearing gneiss and associated peralkaline syenite within migmatitic biotite granite gneiss of the Britt Domain of the Grenville Province. The southern lens forms a shallow synform of biotite dominant gneiss about 4 km long and 800 m wide. The northern, amphibole-dominant lens forms part of the western limb of a synform. The equivalent horizon on the eastern limb is occupied by peralkaline amphibole syenite and garnet-pyroxene granulite, and the core of the synform is formed of megacrystic granite. Neither lens exhibits relicts of igneous texture. Both are associated with peralkaline syenite and appear to be structurally disconformable with surrounding gneiss. These data suggest that much of the nepheline gneiss may have formed by sodium metasomatism along low-angle fault planes with alkalies possibly derived from peralkaline syenite.