Structure and magnetic fabric of the Quetico metasedimentary rocks in the Calm Lake - Perch Lake area, near Atikokan, Northwestern Ontario

Abstract

The Quetico metasedimentary rocks are the metamorphosed equivalents of a turbidite sequence, comprised of a repetative interstratification of sandstones and mudstones. The rocks are metamorphosed to lower greenschist facies, but metamorphic grade increases progressively from north to south. Anchimetamorphosed phyllites and slates in the north part of the present study area grade progressively into biotite schists in the south part. Interpretation of structural features in the study area, mainly bedding(S0) -cleavage(S1) relationships and structural facing directions, has led to the delineation of a number of major folds. These are tight to isoclinal, asymmetric sheath folds, with axial planes arranged en echelon and slightly oblique to the dominant eastwest, vertically-dipping, structural trend. The orientation, geometry and disposition of the F1 folds suggest the Quetico rocks of the present study area have experienced a regional dextral transpressional tectonic evolution, with components of north-south regional shortening and east-west regional dextral shear, with a possible component of vertical (south side up) displacement. The Quetico metasedimentary rocks have a polyminerallic magnetic mineralogy comprised of significant proportions of a ferrimagnetic (magnetite and pyrrhotite) and a paramagnetic (chlorite, biotite, muscovite) component. The rocks possess a predominantly tectonic magnetic fabric, which consists of a deformational, and a metamorphic, magnetic fabric. In some rocks (especially coarse-grained sandstones with wide-spaced cleavage planes) a depositional magnetic fabric is partially preserved. The complexities involved in having component deformational, metamorphic and depositional magnetic fabrics Indicate that the principal magnetic susceptibility directions of the rock's magnetic susceptibility anisotropy cannot be considered reliable Indicators of principal finite strain directions in the Quetico metasedimentary rocks. Great care must be taken in interpreting the significance of principal magnetic susceptibility directions.