Foliation patterns in large gneiss bodies of the Archaean Wabigoon Subprovince, southern Canadian shield

Abstract

Two areas of gneissic tonalite-granodiorite are being studied in the western Wabigoon Subprovince, the Rainy Lake gneiss body and the southwestern Irene-Eltrut Lakes gneiss body. The Rainy Lake gneiss body is an aggregate of large oval structures outlined by foliation trajectories. The body is dominated by the Ash Bay dome whose leuco-gneiss core is characterized by highly foliated rocks and surrounded by a zone of mafic gneiss rich of greenstone relics. This oval dome seems to have distorted the adjacent oval structures while growing to its present size. Of the various hypotheses of doming considered herein, only two can be eliminated from further consideration (1) large-scale buckling of the gneiss-greenstone interface and (2) bending of thrust sheets while moving over rigid ramps. Diapirism sensu lato remains an attractive mechanism for the growth of the oval structures under consideration. Whether the tonality-granodiorite became a gneiss while rising actively or deforming passively, as the wall rock of magmatic diapirs, remains to be determined.