Thermo-tectonic evolution of an Archean accretionary complex: U–Pb geochronological constraints on granulites from the Quetico Subprovince, Ontario, Canada

Abstract

U–Pb geochronological data have been obtained for a granulite zone in the Quetico Subprovince (an Archean accretionary complex) of the Superior Province, north of Manitouwadge, Ontario, Canada. This granulite zone represents the culmination of a low-pressure, high-temperature regional metamorphism in the Quetico Subprovince and an apparent continuation of a northward increase in metamorphic grade in the Wawa Subprovince and across the Quetico–Wawa boundary. A U–Pb zircon age of 2666±1 Ma from a concordant magnetite-bearing granitic pegmatite in the Quetico granulite zone is consistent with the inferred timing of regional metamorphism in amphibolite-facies zones (2671–2665 Ma), and indicates that the low-pressure, high-temperature metamorphism in the Quetico Subprovince commenced during the main compressional deformation. Titanite in a calc–silicate rock from low leucosome-fraction, layered migmatites, that might represent incipient melting, gave a 207Pb/ 206Pb age of 2658±1 Ma. The timing of the granulite-facies metamorphism is constrained by U–Pb zircon ages of 2650±2 and 2651±3 Ma from mafic granulite and garnet-bearing tonalitic leucosome, respectively, that are similar to reported U–Pb ages for mafic granulites in the upper levels of the Kapuskasing Structural Zone. The present U–Pb data suggest a diachronous development of peak metamorphic conditions of the regional metamorphism in the Quetico and Wawa Subprovinces. This diachronous development and the zonal distribution of the regional metamorphism are probably related to oblique collision at a convergent plate margin and differential rates of erosional uplift away from the Quetico–Wawa boundary.