SPATIAL CORRELATION OF MAGNETIC PROPERTIES, DEFORMATION FABRICS, AND PARAGENESIS: INSIGHTS FROM THE ATHABASCA GRANULITE TERRANE, NORTHERN SASKATCHEWAN

Many high-grade gneiss terrains contain regional magnetic anomalies that are interpreted to delineate disparate lithotectonic domains. However, in many regions magnetic anomalies cannot be directly correlated to these boundaries. The distribution of magnetic properties, as a function of mineral assemblage, may provide insight to the architecture of metamorphism at a regional scale within exhumed regions of the lower continental crust. The Chipman domain of the Athabasca granulite terrane is an excellent location to investigate the origin of magnetic patterning within a single lithotectonic unit because it is characterized by some of the largest and smallest aeromagnetic anomalies that are regionally recognized. Our results indicate that the anomalies are not directly correlative to distinctive changes in rock type, deformation fabric, or remnant magnetism but likely reflect changes in mineral assemblage as a function of metamorphism. The mapping of magnetic susceptibility on slabbed samples at the centimeter scale has revealed large variations in susceptibility associated with changes in mineralogy. Many of the high susceptibility regions (k ~ 0.1 SI) occur as layers that track local variations in composition commonly distinguished by modal changes in garnet. Mineral associations observed in thin section indicate that magnetite may be a common product of garnet breakdown during retrograde metamorphism. This relationship implies that the occurrence of some magnetic anomalies within high grade gneiss terrains may reflect reequilibration within specific pressure-temperature-composition fields. The presence of magnetite within low pressure features such as strain shadows on garnet porphyroclasts indicates that in some circumstances magnetite is a fabric forming constituent. Therefore, the recognition of ferromagnetic minerals within the context of rock fabrics may yield kinematic information that can relate metamorphism to deformation with the potential for relative dating by association with other minerals including monazite and zircon. Linking magnetism to metamorphism and deformation may yield geometric constraints on the evolution of ancient orogens that can provide a context for understanding magnetic anomalies in actively deforming regions of the continental crust.