THERMO-TECTONIC EVOLUTION OF AN ARCHEAN ACCRETIONARY COMPLEX - U-PB GEOCHRONOLOGICAL CONSTRAINTS ON GRANULITES FROM THE QUETICO SUBPROVINCE, ONTARIO, CANADA

U-Pb geochronological data have been obtained for a granulite zone in the Quetico Subprovince (an Archean accretionary complex) of the Super ior Province, north of Manitouwadge, Ontario, Canada. This granulite z one represents the culmination of a low-pressure, high-temperature reg ional metamorphism in the Quetico Subprovince and an apparent continua tion of a northward increase in metamorphic grade in the Wawa Subprovi nce and across the Quetico-Wawa boundary. A U-Pb zircon age of 2666 +/ - 1 Ma from a concordant magnetite-bearing granitic pegmatite in the Q uetico granulite zone is consistent with the inferred timing of region al metamorphism in amphibolite-facies zones (2671-2665 Ma), and indica tes that the low-pressure, high-temperature metamorphism in the Quetic o Subprovince commenced during the main compressional deformation. Tit anite in a calc-silicate rock from low leucosome-fraction, layered mig matites, that might represent incipient melting, gave a Pb-207/Pb-206 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 fro m mafic granulite and garnet-bearing tonalitic leucosome, respectively , that are similar to reported U-Pb ages for mafic granulites in the u pper levels of the Kapuskasing Structural Zone. The present U-Pb data suggest a diachronous development of peak metamorphic conditions of th e regional metamorphism in the Quetico and Wawa Subprovinces. This dia chronous development and the zonal distribution of the regional metamo rphism are probably related to oblique collision at a convergent plate margin and differential rates of erosional uplift away from the Queti co-Wawa boundary. (C) 1998 Elsevier Science B.V.