Plastic behavior of magnetite and high strains obtained from magnetic fabrics in the Parry Sound shear zone, Ontario Grenville Province

Abstract

Magnetic anisotropy and quantitative petrology of a transition from relatively undeformed metaanorthosite protolith to highly deformed ultramylonite in the Parry Sound shear zone provide information on magnetite as a kinematic indicator. Rock magnetic experiments show that anisotropy of magnetic susceptibility (AMS) is controlled by magnetite in the mylonites and ultramylonites, and by paramagnetic minerals (hornblende, biotite, ilmenite) in the protolith. Geothermometry in the ultramylonite indicates a temperature of 630 ± 50 °C during deformation. A deformation mechanism map calculated from published experimental work for 630 °C indicates that magnetite in the Parry Sound shear zone deformed plastically rather than via rigid-body rotation. The AMS orientations accurately track a foliation trajectory in a portion of the shear zone, which allows the utilization of orientation-based strain models to determine shear strains. Shear strains (γ) of 1–9 were obtained, and an empirical correlation of strain with the ellipsoid shape resulted in the logarithmic relationships: Mmax = 0.15 ln( X), and Mmin = 0.13 ln( Z), where Mi = ( ki/ kmean) describes the principal AMS axes, or ( k max/k min) = ( X Z )0.14 . These AMS-strain correlations were subsequently applied to mylonites and ultramylonites elsewhere in the shear zone, predicting shear strains as high as γ = 13. These observations also reveal that AMS fabrics do not saturate at high strains, as would be expected for rigid-body rotation, due to the plastic behavior of magnetite during deformation.