RELATIONSHIPS BETWEEN REGIONAL METAMORPHISM AND TRANSPRESSION IN SHEAR ZONES OF THE ARCHEAN NORTH CARIBOU GREENSTONE BELT, NW SUPERIOR PROVINCE

The 2.8-3.0 Ga North Caribou greenstone belt, host to the Musselwhite BIF-hosted Au deposit, possesses abundant crustal-scale shear zones on the northern and eastern margins of the belt, which appeared to have formed under amphibolite facies conditions. Protracted deformation and regional metamorphism are coeval with widespread magmatism and accretion events during cratonization of the Superior Province, and are responsible for folding the ore-hosting BIF and channeling fluids. The importance of these shear zones in the history of the NCGB and their spatial relationship with metamorphism and fluid flow is still equivocal, but higher grade metamorphic mineral assemblages (Grt + And + St ± Sil) seem to correlate with loci of higher strain. Structural analyses support transpressive collision that produced steeply-dipping planar fabrics and shallow plunging lineations that trend broadly parallel to the doubly arcuate shape of the belt. Electron back scatter diffraction analyses were conducted on strategic samples across one shear zone in order to characterize transpressive deformation, as well as the strain gradient across the shear zone. The Dinnick Lake shear zone cuts through mafic metavolcanics that have a flat REE pattern typical of the NCGB. At its core, an L-tectonite developed on the margins of a granitic batholith is composed of recrystallized feldspars and quartz bands with an abundance of subgrains and illustrates the highest strain conditions. Microstructural analysis show subgrain rotation recrystallization and deformation by prism a- and c-slip in Qz, as well as Hbl oriented growth suggesting temperatures above 500°C. Qz in mafic rocks along the margins of the shear zone exhibit a basal a-slip component, indicating a slight decrease in strain or temperature. Additionally, geochemical analyses of the L-tectonite show very little variation from the unstrained granitoid suggesting deformation in a dry environment. Although the NCGB exhibits some evidence of vertical tectonism (dome and keel structures), the dominant strain record within the shear zones is that of horizontal (transpressive) displacement. This is in agreement with other greenstone belts in the Western Superior (e.g. Hemlo greenstone belt) where vertical and horizontal tectonism were coeval during late stages of cratonization.