Stratigraphy, structure and metamorphism of Archean rocks at Rainy Lake, Ontario

Abstract

The rocks of the Rainy Lake area have been deformed during three distinctive episodes. Minor structures provide the geometry which characterizes each episode. The youngest structures include regional faults, a crenulation cleavage, kink bands and minor F[subscript 3] folds. These D[subscript 3] structures are superimposed on structures of the D[subscript 2] episode. These include dominant F[subscript 2] folds having axes lying in a penetrative cleavage which parallels the axial surfaces of the folds. Some F[subscript 2] folds have a downward structural facing which is evidence that the stratigraphic succession at Rainy Lake is overturned at a regional scale. It is proposed that this inversion took place during a D[subscript 1] deformation by the formation of large F[subscript1] fold nappes. Minor D[subscript1] structures are difficult to document. The rocks of the region were metamorphosed simultaneously with much of the deformation. The distribution of index minerals defines the boundaries of the biotite, staurolite-cordierite and sillimanite-muscovite zones. The non-parallel distribution of metamorphic minerals may be explained by the non-parallelism of isotherms and isobars during medium grade metamorphism. These new data support the view that the Coutchiching biotite schists at Rainy Lake are stratigraphically younger than metavolcanic rocks of the Keewatin Group although they presently underlie the Keewatin structurally. This observation resolves a part of the historically important ”Seine-Coutchiching problem”.