An empirical model for metamorphic evolution of the Archaean Slave Province and adjacent Thelon Tectonic Zone, north-western Canadian Shield

Abstract

Summary On the basis of P-T estimates derived from metamorphic assemblages and the geological setting before and after peak metamorphism, Archaean thermal evolution is attributed to overthickening of previously thinned sialic crust and overlying supracrustal basins. The extensional phase increases geothermal gradients and forms relatively ductile zones within which subsequent compression transports supracrustal rocks to metamorphic pressures, contributes to the rise of isotherms to high levels in the crust, and causes the moderate uplift and erosion required to bring low- P -high- T rocks to the surface of crust 35 km thick. Proterozoic upper amphibolite- and granulite-facies metamorphism and plutonism in the western Thelon Tectonic Zone is attributed to a similar thermal evolution 500 Ma later. Continued convergence across a long linear belt of overthickened, partially melted crust caused hot upper and middle crustal-level rocks to be thrust on to the Slave Province and formation of a medium pressure overprint on the Archaean. The empirical models derived for the region generate low- P -high- T metamorphism without excessive overthickening or major heat transfer by granitic magmatism. For both terrains, metamorphic data, considered in their geological context, are consistent with an ensialic intra-plate tectonic setting rather than an ensimatic, subduction-related interplate setting.