PRECISE U-PB AGES FOR GRENVILLIAN AND PRE-GRENVILLIAN THRUSTING OF PROTEROZOIC AND ARCHEAN METAMORPHIC ASSEMBLAGES IN THE GRENVILLE FRONT TECTONIC ZONE, CANADA

While the position of the Grenville Front is placed at a structural feature associated with the onset of high-grade rocks when approached from the north, the time of first metamorphism in these rocks is, in general, pre-Grenvillian and highly variable. At Killarney, amphibolite to granulite grade rocks were metamorphosed, lineated, and foliated to their present ''front-parallel'' configuration mainly by 1453 +/- 6 Ma. North of North Bay, amphibolite grade migmatites have been subjected to a pre-2650 Ma metamorphism. Granulite grade gneisses south of the Grenville Front near Val d'Or record a 2640 Ma metamorphic event. South of Chibougamau, mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 Ma. In central Labrador a 1650 Ma foliated granite is present south of the Grenville Front, and at least some of the banding is this old. At the Atlantic coast, granulite facies metamorphism in front-parallel gneisses predates a 1790-1662 Ma deformation. In contrast, each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies NW directed thrusting or crustal loading. Near Killarney on Georgian Bay, titanite records an episode of 977 +/- 12 Ma partial resetting, the degree of which increases with distance into the Grenville Province. Here Grenvillian deformation, where present, is manifest as feldspar-porphyroclastic fabric with strong SE plunging lineations. Near North Bay, Archean pegmatites in the Grenville Front zone were similarly deformed at 988 +/- 10 Ma, and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987 +/- 2 Ma. A 991 +/- 4 Ma cooling age for titanite in this melt is analytically identical to the zircon growth age, hence rapid cooling occurred. South of Chibougamau, new zircons grew in high-pressure coronite in response to tectonic loading at 995 +/- 6 Ma, and in Central Labrador, melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995 +/- 2 Ma. Titanite in the granitic host rock has an analytically identical age of 992 +/- 4 Ma, again indicating rapid cooling. This second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651 +/- 5 Ma to 988 +/- 5 Ma two-age mixing line. Grenville resetting at about 1037 +/- 19 Ma in the Grenville Front zone on the Labrador coast is indicated, but major thrusting is pre-1662 Ma. U-Pb isotopic results indicate that the Grenville Front zone is characterized along its length by partially reset U-Pb systems, high-pressure assemblages, local melting, and rapid cooling. Crustal loading and shearing, with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling, are required.