Paleomagnetism, structure and magnetic fabrics in a traverse of the Quetico subprovince between Atikokan and Kashabowie, NW Ontario

Abstract

The metasedimentary rocks along the traverse reveal low to high grade metamorphism from chlorite schist near Atikokan, through biotite schist to migmatites in the west within the centre of the Quetico Belt. Continuing towards the ESE, near Huronian Lake, the metamorphic grade decreases symmetrically but with somewhat less steep gradients to Kashabowie. The metamorphism was syn- to late- tectonic. Only one pervasive tectonic fabric was observed in the interior of the belt, with NE-SW striking S1 foliations (mean direction: 256/85° NW, n=121) and extension lineations L1 plunging shallowly to the NE (mean direction: 70-20°, n=52). In migmatite and pegmatite zones foliation was often less steep. It was probably deflected due to the intrusion of pegmatite or granitoid bodies. Minerals contributing to ferromagnetic properties are mostly monoclinic ferrimagnetic pyrrhotite within the belt with some magnetite in medium grade amphibolite - rich outcrops in metavolcanics of Shebandowan Belt. Anisotropy of magnetic susceptibility fabric (AMS} is mainly controlled by paramagnetic biotite or chlorite and subordinate ferromagnetic pyrrhotite in metasediments of the Quetico Belt. In some outcrops of mafic metavolcanics near Kashabowie, the high magnetite content controls bulk susceptibility more than its anisotropy. Variations in AMS fabric are largely due to variations in relative composition of magnetite, pyrrhotite and paramagnetic sheet silicates, not due to changes in strain along the traverse.