ANCESTRY OF THE LEGS LAKE SHEAR ZONE, SASKATCHEWAN: EVIDENCE FOR MULTIPLE STAGES OF TECTONISM IN THE SNOWBIRD TECTONIC ZONE

The Snowbird tectonic zone forms the boundary between the Rae and Hearne domains of the Canadian Shield. There have been many hypotheses for the timing and significance of this enigmatic lineament. The Snowbird tectonic zone is well exposed in the Athabasca granulite terrain (AGT), in Saskatchewan, where it coincides with the Legs Lake shear zone, an oblique thrust-sense shear zone that served to uplift the AGT, over the mid-crustal Hearne domain to the east. The Legs Lake shear zone was active at 1.85 Ga, and was possibly localized in an area that was thermally weakened by the 1.9 Ga Chipman mafic dike swarm. One major tectonic question is whether the Chipman dike swarm and Legs Lake shear zone are the only expression of the Snowbird tectonic zone or whether the zone had an earlier, possibly Archean, history. This question has been investigated in the southern part of the Legs Lake shear zone, where it juxtaposes the 2.6 Ga Fehr granite in the west with the Hearne domain in the east. Field mapping suggests that the Fehr granite may already have contained NE-striking, possibly tectonic, contacts before it was tectonized by the Legs Lake shear zone, i.e. a previous Snowbird-related tectonic fabric. The Fehr granite is a megacrystic granite with both non-mylonitic and intense mylonitic domains. Microstructural analysis, petrologic analysis, and insitu geochronology are currently being used to investigate whether boundaries and strain gradients within the mylonite reflect earlier tectonic contacts or strain localization within a homogeneous granite. If early tectonic contacts are present, these may reflect an early event (possibly Archean juxtaposition) along the Snowbird tectonic zone.