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 fe ature associated with the onset of high-grade rocks when approached fr om the north, the time of first metamorphism in these rocks is, in gen eral, pre-Grenvillian and highly variable. At Killarney, amphibolite t o 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. Sout h of Chibougamau, mafic enclaves in tonalite south of the front were c onverted to migmatitic amphibolite at about 2680 to 2690 Ma. In centra l Labrador a 1650 Ma foliated granite is present south of the Grenvill e Front, and at least some of the banding is this old. At the Atlantic coast, granulite facies metamorphism in front-parallel gneisses preda tes a 1790-1662 Ma deformation. In contrast, each of these areas share s a near common age of second metamorphism that is variable in degree and accompanies NW directed thrusting or crustal loading. Near Killarn ey on Georgian Bay, titanite records an episode of 977 +/- 12 Ma parti al resetting, the degree of which increases with distance into the Gre nville Province. Here Grenvillian deformation, where present, is manif est as feldspar-porphyroclastic fabric with strong SE plunging lineati ons. Near North Bay, Archean pegmatites in the Grenville Front zone we re similarly deformed at 988 +/- 10 Ma, and strongly rodded ductile am phibolite 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 gro wth age, hence rapid cooling occurred. South of Chibougamau, new zirco ns grew in high-pressure coronite in response to tectonic loading at 9 95 +/- 6 Ma, and in Central Labrador, melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 9 95 +/- 2 Ma. Titanite in the granitic host rock has an analytically id entical age of 992 +/- 4 Ma, again indicating rapid cooling. This seco nd 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 m ixing line. Grenville resetting at about 1037 +/- 19 Ma in the Grenvil le Front zone on the Labrador coast is indicated, but major thrusting is pre-1662 Ma. U-Pb isotopic results indicate that the Grenville Fron t zone is characterized along its length by partially reset U-Pb syste ms, high-pressure assemblages, local melting, and rapid cooling. Crust al loading and shearing, with lead loss related to mechanically induce d recrystallization and local heat that was rapidly terminated by rebo und and cooling, are required.