Magnetic fabrics of granitic plutons and gneisses, Northwestern Ontario

Abstract

Specimens of plutonic granitic gneisses have been collected from different locations of the Superior Province. The study of their Koenigsberger ratio (related to natural remanent magnetization) and the theoretical Koenigsberger ratio (related to maximum artificial remanent magnetization) shows that the theoretical Koenigsberger ratio of the specimens is less dispersed than the Koenigsberger ratio. There is a power law relation between the remanent magnetization and induced magnetization for the Koenigsberger ratio and the theoretical Koenigsberger ratio and this power law relation is due to variation of amount of ferromagnetic minerals in the specimens. A structural study of the McKenzie granite (NE of Thunder Bay), the Rice Bay dome and the Sawhill dome (NE of Fort Frances) have also been performed. The McKenzie granite magnetic fabrics cannot be used as kinematic indicators because of the non-coaxiality of the direction of the magnetic ellipsoid axes and also because a primary fabric has been overprinted by a secondary one. The Rice Bay dome fabrics are noncoaxial but are clearly related to the regional strain and mineral lineation: they may be related to the diapiric emplacement of the dome. Sawhill dome fabrics are not related to the diapiric emplacement of the dome but to a later event related to the southern border of the dome adjacent to the Quetico fault.