Bailey Drover HBSc thesis abstract

The Gunflint Formation near Thunder Bay, Ontario has undergone very low grades of metamorphism. In this study, two outcrops of the Gunflint Formation near Thunder Bay were analyzed for their deformation microstructures, which are used to constrain temperature conditions of the Gunflint Formation during deformation.  Common deformation microstructures within this Formation are pressure-solution stylolites, deformed calcite veins, deformed quartz veins and kinked illite-mica laths.  All processes that formed these microstructures operate at temperatures between 150°C to 300°C, providing a temperature constraint of the system during deformation. In addition to deformation microstructures, semi-quantitative x-ray diffraction (XRD) phase analysis was completed on one sample from a hanging wall ramp of the Gunflint Formation.  XRD analysis shows likely minor interlayered illite-smectite, as well as the ordered illite-muscovite 2M1 polytype.  These phases are interpreted to represent metamorphic temperatures between 200°C and 300°C.  Combining this with the deformation microstructures, all structures and phases present are satisfied by a temperature range between 200°C and 300°C, corresponding to the anchizone of metamorphism (sub-greenschist facies metamorphism, specifically the prehnite-pumpellyite facies).

Given this temperature range, and assuming an average geothermal gradient of 30°C/km, the depth of the Gunflint Formation during deformation and metamorphism is shown to be anywhere between 6 km and 10 km.  This depth is consistent with tectonic loading, due to bed-parallel thrust faults.  Based on the findings of this thesis, the Gunflint Formation near Thunder Bay is interpreted to represent an eroded foreland thrust and fold belt.