METAMORPHIC AND STRUCTURAL CONTEXT OF MAGNETISM WITHIN THE LOWER CONTINENTAL CRUST, OBSERVATIONS AND IMPLICATIONS FROM THE CANADIAN SHIELD

Processes that govern the origin and stability of Fe-Ti oxides directly influence the magnetic character of the lithosphere. Bulk composition is typically assumed to be a first-order control of Fe-Ti oxide abundance and is generally invoked to explain aeromagnetic anomalies. However, the metamorphic and structural history of a region can influence the occurrence of ferrimagnetic minerals such as magnetite, which may be reflected in the architecture of aeromagnetic anomalies. The Chipman domain of the Athabasca granulite terrane in northern Saskatchewan, Canada is an archetypal example of the metamorphic and structural control on aeromagnetic anomalies. This lithologically homogenous domain is characterized by an aeromagnetic dichotomy defined by significantly elevated total field intensity to the west. Volumetrically minor mafic lozenges and schlieren hosted by the 3.2 Ga tonalite occur throughout the domain and display significant variations in magnetic susceptibility with observed values in excess of 0.5 SI to below 0.0005 SI. Although low magnetic susceptibilities occur throughout the entire domain, elevated values are restricted to the west. Furthermore, the Chipman mafic dike swarm shows this dichotomy despite an increased abundance of dikes to the east. Finite element modeling of aeromagnetic anomalies across the Chipman domain suggest the occurrence of four distinct petrophysical domains that mimic the trace of the Legs Lake shear zone. Preliminary results suggest that the production of magnetite within mafic lithologies may be partially controlled by hydration reactions associated with deformation along the shear zone. However, continued hydration appears to favor the removal of magnetite. As such, the western Chipman domain likely displays a heterogeneous network of magnetite-poor, anhydrous, granulite facies assemblages cut by partially hydrated, magnetite-rich zones. The eastern Chipman domain is characterized by extensively hydrated lithologies that lack significant concentrations of magnetite. These results suggest that the aeromagnetic architecture observed across the Athabasca granulite terrane, and potentially other regions, may be used to constrain the role of fluid migration associated with deformation within the lower-to-middle continental crust.