Metamorphic and structural evolution of the Flin Flon - Athapapuskow Lake area, west-central Manitoba

The Flin Flon - Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (similar to 1.86-1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite-pumpellyite to amphibolite facies. Regional metamorphism post-dates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite-pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, predating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure-temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon - Athapapuskow Lake area reached about 430-480 degrees C and 3.0-4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 degrees C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250-300 degrees C/1.5-2.3 kbar to 430-480 degrees C/3-4.5 kbar (100-150 degrees C/kbar), defining a geothermal gradient of approximately 25-31 degrees C/km.