Variations in metamorphic grade in metapelites in transects across the Quetico Subprovince north of Thunder Bay, Ontario

Abstract

The Quetico subprovince is a northeast-southwest striking linear belt of migmatites, gneisses, and metasedimentary rocks. These Archean rocks form part of the southern Superior Province- This study involves an examination of variations in metamorphic grade along cross-strike transects in an area north of Thunder Bay, Ontario. The rocks of the Quetico subprovince include metasedimentary rocks with well preserved primary structures, knotted schists, gneisses, migmatites, and anatectic granitic rocks. Metamorphic porphyroblasts include muscovite, biotite, garnet, staurolite, cordierite, andalusite, and sillimanite. Chemical analyses of garnets, geothermobarometry, and mineral assemblage data were used to determine variations in metamorphic grade in transects across the subprovince. Mineral assemblages characteristic of low to high grade metamorphism are exposed along an across-strike transect. Metamorphic grade rises gradually from low grade (521°C) to high grade (714°C) northwards along Highway 527. North of the peak conditions, the grade drops off sharply. Garnet-biotite geothermometry confirms this pattern. Maximum pressure reached in the study area is approximately 5 kbar. The model proposed to account for the distribution of metamorphic assemblages and minerals involves transpression of the Quetico accretionary prism between the Wabigoon volcanic cratonic margin to the north and the docking Wawa volcanic complex to the south. Buckling and folding of the sedimentary rocks was accompanied by thrusting. Erosion has exposed high grade migmatitic and anatectic rocks within the Quetico fold belt which developed as a result of thermal relaxation of depressed isotherms. The boundaries between metavolcanic and metasedimentary terranes are structurally complex. Boundaries may be best described as geometrically complex zones up to several kilometres in extent in which various rock types representative of the adjacent terranes have been folded, faulted, and intruded.