Compositional control of magnetic anisotropy in the Thomson formation, east-central Minnesota

Anisotropy of magnetic low-field susceptibility (AMS) and anisotropy of anhysteretic susceptibility (AAS) of the middle Precambrian Thomson formation, east-central Minnesota, generally record structural features, but orientations and axial ratios of the susceptibility ellipsoids depend on local magnetic mineralogy. The northern structural zone of the Thomson formation is characterized by an upright, east-west slaty cleavage. The susceptibility ellipsoids plot in the flattening field of a Flinn diagram, with minimum axes normal to the cleavage. AAS ellipsoids are coaxial with AMS ellipsoids, but are generally more prolate. Anisotropy increases with mean susceptibility at one outcrop, interpreted as mixing of weakly anisotropic, paramagnetic chlorite with more strongly anisotropic magnetite. A second outcrop shows the opposite pattern: anisotropy decreases with increasing mean susceptibility, suggesting that here the magnetite may be relatively isotropic, or possibly post-deformational. The southern structural zone of the Thomson formation is characterized by a subhorizontal foliation (S₁) and an upright crenulation cleavage (S₂). Here, the magnetic susceptibility minima are coincident with poles to S₁, and maxima are parallel to the S₁S₂ intersection. Correlation between directional susceptibilities and mean susceptibility (technique of Henry and Daly, 1983) suggests that paramagnetic minima are poles to the early foliation, while (possibly interdeformational) magnetite minima record the second deformation. This interpretation, however, is not supported by the AAS fabric, by thin section observations, or by high-field measurements of paramagnetic susceptibility. Alternatively, both directional and mean susceptibilities may be controlled by syndeformational chlorite.