Petrophysical constraints on magnetic anomalies associated with metamorphic reactions in northern Saskatchewan, Canada

Abstract

The western Chipman domain of the east Athabasca mylonite triangle in northern Saskatchewan, Canada, displays a large positive aeromagnetic anomaly that is the result of retrograde magnetite production during exhumation. Petrologic, magnetic coercivity, and hysteresis analyses indicate that multidomain magnetite is the primary magnetic phase in rocks of the region. Measurements of magnetic susceptibility from the western Chipman domain document five orders of magnitude variation, while rocks from the eastern Chipman domain are paramagnetic. The distribution of Koenigsberger ratios is approximately a mixed bimodal lognormal distribution with peak ratios at 0.039 and 0.73, suggesting that magnetic susceptibility is more significant than remanent magnetization. However, remanent magnetization is an important contributor to total magnetization. Petrographic observations indicate that magnetite is primarily produced from the breakdown of hornblende. The consumption of hornblende is also texturally associated with the production of actinolite and the hydration-related breakdown of granulite facies mineral phases such as garnet and clinopyroxene. Based on the proximity of the positive aeromagnetic anomaly to the Cora Lake shear zone, late-stage deformation along the shear zone during exhumation of the east Athabasca mylonite triangle may have structurally controlled the infiltration of fluids resulting in the heterogeneous production of magnetite. These results document the utility of integrating aeromagnetic, petrologic, and rock magnetic data to transcend observational scales and better understand regional tectonometamorphic history.