FROM THIN SECTION TO AEROMAGNETIC ANOMALY, LINKING PETROLOGIC DATA AND GEOPHYSICAL SURVEYS TO HELP CONSTRAIN THE RETROGRADE HISTORY OF THE WESTERN CHURCHILL PROVINCE, CANADA

Extrapolation of Pressure-Temperature-deformation-time (P-T-d-t) histories across extensive regions of high-grade gneiss terrains can be speculative. In areas of limited access or poor exposure, sparse tectonometamorphic constrains may not be representative of regional metamorphic conditions or deformation regimes. However, geophysical data such as aeromagnetic surveys can provide a wealth of information with near complete coverage over large areas of interest. The spatial distribution of aeromagnetic anomalies can be a function of not only bulk composition, but also P-T-d-t histories, and may provide a powerful tool to aid in the extrapolation of local information to the regional scale. We apply this concept to the Chipman domain of the Athabasca granulite terrane in northern Saskatchewan, Canada. This domain is characterized by a 3.2 Ga tonalitic gneiss that hosts a diverse, yet volumetrically minor, array of mafic rock types. Despite its homogeneous nature, significant aeromagnetic anomalies occur throughout the tonalite unit. Petrologic studies indicate that an increase in the mode of magnetite (from <1% to locally >24%) is associated with retrograde metamorphism particularly in the western Chipman domain. Retrograde metamorphism is also associated with the influx of CO₂ and H₂O, and the partial oxidation of hbld + qtz to yield mt + act + pl. Finite element modeling of aeromagnetic data coupled with rock-magnetic analyses can reasonably account for the observed aeromagnetic anomalies and suggest that the spatial extent of these anomalies directly relate to the regional extent of retrograde metamorphism. Furthermore, thermodynamic modeling of representative petrologic systems along reasonably constrained P-T-t paths suggest that the production of magnetite is temporally bracketed from ~1882 Ma to ~1870 Ma within a pressure range of ~599 MPa to ~740 MPa and a temperature range of ~524ºC to ~617ºC. We propose a model that involves late-stage left-lateral deformation along the Cora Lake shear zone with a releasing-bend geometry. This geometry enabled infiltration of CO₂ + H₂O fluids under elevated oxygen fugacities to drive retrograde reactions that resulted in the production of magnetite and ultimately the observed aeromagnetic anomalies in the western Chipman domain.