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

Abstract

Abstract-Magneticanisotropy 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 uhramylonites, 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 (y) of l-9 were obtained, and an empirical correlation of strain with the ellipsoid shape resulted in the logarithmic relationships: Mmax = 0.15 In(X), and Mmin = 0.13 In(Z), where Mi = (kilkmean) describes the principal AMS axes, or (kmaxlkmin) = (X/Z)O.14. These AMS-strain correlations were subsequently applied to mylonites and ultramylonites elsewhere in the shear zone, predicting shear strains as high as y = 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.

Fractal analysis of magnetite grains — implications for interpreting deformation mechanism

“…As stated in the introduction, previous studies indicate that magnetite can undergo intracrystalline deformation by dislocation creep. The deformation mechanism map of magnetite presented by Housen et al (1995) and used by Ferré et al (2003) advocates ductile deformation by dislocaton creep as the dominant mechanism at high T (630 o C and above) . The present author has plotted the magnetite grain size data from Godhra Granite on the same magnetite deformation mechanism map (Housen et al 1995) (Fig.…”

“…Experimental studies have revealed that magnetite can undergo intracrystalline deformation by dislocation creep (Müller and Siemes, 1972;Atkinson, 1977;Hennig-Michaeli and Siemes, 1982). Housen et al (1995) prepared a deformation mechanism map of magnetite for 630 o C based on their study of magnetite grains (20-40 µm size) from the Parry Sound Shear Zone (PSSZ, Ontario), and concluded that magnetite undergoes ductile deformation by dislocation creep. Ferré et al (2003) used this deformation mechanism map to infer dislocation creep in magnetite grains (20-200 mm size) from migmatites (deformation T ≤ 800 o C) of Morton, Minnesota (Superior Province) and stated that the map is valid between 600-800 o C. Sitzman et al (2000) studied magnetite grains in a granulite facies marble under a transmission electron microscope (TEM) and also measured the δ 18 O distribution within them.…”

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

“…Many studies identify correlations between the shape of the magnetic susceptibility ellipsoid and the shape of the strain ellipsoid (Borradaile, 1988;Borradaile, 1991;Rochette et al, 1992;Cañón Tapia et al, 1997;Cañón Tapia, 1994;Cañón Tapia & Castro, 2004;Almqvist et al, 2012), however they are not necessarily equal (Borradaile, 1988;Rochette et al, 1992). Magnetic anisotropy is affected by magnetic grain shape and distribution as well as the deformation mechanism (e.g., Housen et al, 1995;Evans et al, 2003). Therefore in order to interpret the magnetic fabric of a rock in terms of its strain history it is vital to determine the identity and properties of the magnetic minerals contributing to the magnetic susceptibility (Ferré et al, 2014).…”

Evolution of magnetic fabrics during incipient deformation of mudrocks (Pyrenees, northern Spain)

“…In deformed rocks, the potential of susceptibility anisotropy as a strain gauge has received a lot of attention (e.g., Owens, 1974;Singh et al, 1975;Hrouda, 1976;Wood et al, 1976;Kligfield et al, 1977;Goldstein, 1980;Rathore et al, 1983;Henry and Daly, 1983;Borradaile, 1987;Cogné and Perroud, 1988;Hirt et al, 1988;Ruf et al, 1988;Pearce and Fueten, 1989;Kodama and Sun, 1990;Housen et al, 1995). The main reasons for this application are: (1) the relative ease by which the AMS ellipsoid can be established; (2) the sensitivity of the method to weakly developed rock fabrics; and (3) that essentially all rocks are magnetically anisotropic and therefore magnetic anisotropy offers a nearly universally applicable method to analyze deformation state of rocks, as opposed to the much smaller population of samples that may contain other strain indicators.…”