MAGNETIC ANOMALIES RELATED TO METAMORPHIC REACTIONS: EVIDENCE FROM LOWER CRUSTAL ROCKS IN THE ATHABASCA GRANULITE TERRANE, NORTHERN SASKATCHEWAN

Aeromagnetic surveys are invaluable resources for interpreting crustal-scale geological features throughout the Canadian Shield. Although many anomalies can be attributed to boundaries between lithotectonic domains, numerous regions are characterized by significant variations in magnetization not associated with lithological variation. Field-based structural, petrological, and geophysical studies targeted at understanding the origin and occurrence of these anomalies can provide information to define specific geological processes. The Chipman domain of the Athabasca Granulite Terrane, a section of now-exposed lower crust between the Rae and Hearn domains in northern Saskatchewan, is an excellent example. Aeromagnetic anomalies in excess of 1800 nT are observed within a single tonalitic batholith, with magnetic highs concentrated in the western portion of the domain as NE striking lineaments truncated by the Cora Lake shear zone. The eastern portion of the domain is characterized by homogenous lower field strength. Previous interpretations have suggested that amphibolite and pyroxenite layers within the 3.2 Ga tonalite host, as well as the occurrence of the Chipman dike swarm are responsible for the aeromagnetic anomalies. Field investigations do not support these simple interpretations. Magnetic susceptibility measurements on outcrops and samples within the western portion of the batholith display orders of magnitude variations within single lithologies. Magnetite is commonly found in thin section as interstitial cuspate grains associated with coronas of fine-grained aggregates of clinopyroxene and hornblende that suggests metamorphic reactions involving silicate minerals may produce magnetite. Thus aeromagnetic highs may be functions of pressure, temperature, fluid composition, and bulk composition. Simple two-dimensional finite element modeling based on field relations and rock properties can mimic the observed regional anomaly throughout the Chipman domain and may be used to constrain the architecture of specific metamorphic reactions. This ability to link aeromagnetic data to geological processes is a powerful resource for both geologists and geophysicists with implications for anomalies beyond the Athabasca Granulite Terrain.