METAMORPHIC CONTROL ON THE FORMATION OF AEROMAGNETIC ANOMALIES; USING GEOPHYSICAL DATA TO EXTRAPOLATE LOWER CRUSTAL TECTONIC PROCESSES WITHIN THE ATHABASCA GRANULITE TERRANE, CANADA

The analysis of Pressure-Temperature-deformation-time (P-T-d-t) histories within high-grade gneiss terrains is fundamental to understanding tectonic and geodynamic processes of the mid-to-lower continental crust. However, in extensive regions with limited access or poor exposure, correlation of these histories between isolated outcrops can be tenuous. Aeromagnetic surveys are typicaly used to correlate rock units, but they may also be used to correlate aspects of tectonic history. We suggest that these anomalies may reflect aspects of the metamorphic, and structural history of a region. Therefore, magnetic information may provide a means to bridge discrete outcrop analyses by understanding the magnetic implications of P-T-d-t histories, enabling regional scale interpretations of tectonic and geodynamic processes.

The Chipman domain of the Athabasca granulite terrane is characterized by a 3.2 Ga tonalitic gneiss that hosts a diverse, yet volumetrically minor, array of mafic material. Despite its lithologically homogeneous nature, significant aeromagnetic anomalies occur throughout the tonalite unit, with the western half defined by a large plateau of elevated total field intensity. Petrological investigations indicate that variations in the volume percent of magnetite are likely responsible for most of these anomalies. Several key reactions that involve the prograde and retrograde production of magnetite have been identified. However, the removal of magnetite during retrograde hydration is locally observed overprinting higher grade assemblages. We suggest that the western and eastern portions of the Chipman domain experienced distinct metamorphic histories that are expressed by the regional aeromagnetic data. Further characterization and modeling of magnetite-producing and consuming reactions will enable the correlation of local petrophysical observations to larger scale interpretations. This approach of integrating P-T-d-t histories and magnetic properties may provide key insight into the four dimensional architecture of specific metamorphic and structural processes by documenting gradients in peak metamorphism, crustal fluid infiltration, and deformation at a regional scale.