THE BURNTSIDE LAKE AND SHAGAWA / KNIFE LAKE SHEAR ZONES: DEFORMATION KINEMATICS, GEOCHEMISTRY AND GEOCHRONOLGOY; WAWA SUBPROVINCE, ONTARIO, CANADA

The Canadian Shield and Superior Province contain the greatest amount of exposed Archean crust on the continent. This region has been interpreted to have recorded tectonics from the early growth of North America. The Superior Province was constructed by regional folding, faulting, intrusion by large volumes of tonalite-granodiorite, and deposition of sediments in intervening basins. Meta-igneous and volcaniclastic units are well exposed from Pleistocene glaciation, providing an excellent area for investigating kinematics of deformation during Archean tectonics.

The southern Superior Province provides a number of well-exposed, northeast-striking shear zones through meta-igneous and meta-sedimentary rocks are present. The two major structures of interest were the Burntside Lake Shear Zone (BLSZ) in the north, and the Shagawa/Knife Lake Shear Zone (SKLSZ) in the south. This study focused on characterizing these two shear zones based on deformation kinematics, chemistry and geochronology. The BLSZ passes through granite to granodioritic gneisses and migmatites, while the SKLSZ has formed within thick sequences of intermediate to mafic volcaniclastics with banded iron. Samples from the field area had whole-rock initial epsilon Nd values (@ t=2.75Ga) from 3.13 to 5.21, corresponding to model ages from 2982±30 to 2781±17Ma. Analyzed zircons had simple grain systematics, yielding 207Pb/206Pb crystallization ages of 2686±9, 2691±14, and 2740±34Ma (2 sigma errors).

The BLSZ accommodated shear over a 3.5 km zone through an anastomosing pattern. Rotated porphyroblasts, stretching lineations and outcrop-scale folds demonstrated dextral-normal motion within the shear zone between the lower-grade units of the Wawa Subprovince and the overriding Quetico subprovince. The SKLSZ accommodated shear in two discrete shear zones 10's meters wide through primarily sinistral-thrust motion. The similarity in deformation mechanisms and unit ages can be attributed to the two shear zones forming either (1) at the same time or (2) under similar temperature/pressure/strain regimes, thereby accommodating strain in similar ways. The competence contrast due to variation in host lithologies could certainly account for the variation in map-pattern of the shear zones and rare earth element chemistry.