MONAZITE AS A METAMORPHIC CHRONOMETER, SOUTH OF THE GRENVILLE FRONT, WESTERN QUEBEC

Monazite was utilized as a chronometer to examine the effects of high- grade metamorphism across the Parautochthonous Belt and Allochthon Bou ndary Thrust of the Grenville Province in western Quebec. This study, in addition to previous geochronological studies, indicates an Archean component in the gneisses, which is consistent with the presence of m ore than one set of peak metamorphic conditions. Single-grain monazite analyses from metasedimentary gneisses from four locations within the Parautochthonous Belt yielded Grenvillian U-Pb dates of 1000 +/- 5 to 1006 +/- 2 Ma. The location farthest to the northwest, 45 km southeas t of the Grenville Front, included monazite with a distinct Archean si gnature. Southeast of this point an Archean signature was not detected in the monazite. At 70 km southeast of the Grenville Front, monazite yielded two discrete ages of 1005 +/- 2 and 1020 +/- 3 Ma. Xenotime fr om one location indicated that the closure temperature of this mineral may be equivalent to that of monazite (725 +/- 25-degrees-C). Monazit e from the Allochthon Boundary Thrust, 135 km southeast of the Grenvil le Front, yielded Pb-207/Pb-206 dates of 1049-1092 Ma, indicating earl ier cooling than rocks closer to the Grenville Front. The monazite age was combined with that of rutile from the same location to determine a cooling rate of 2-degrees-C/Ma following cooling through the closure temperature of monazite. The abrupt transition from Archean to Grenvi llian ages some 45 km southeast of the Grenville Front is consistent w ith tectonic transport in the form of northwest-directed thrusting.