Abstract

U–Pb data provide new constraints on the age of sedimentation, metamorphism, magmatism, and deformation in the Grenville Province of western Quebec. A metapelite, an alaskitic gneiss, and an amphibolite were sampled within an area of 1 km2 in the Mont-Laurier terrane. The metapelite yielded detrital-metamorphic zircons that gave 207Pb/206Pb ages of ca. 1205–2200 Ma. The youngest detrital components, between 1210 and 1300 Ma and possibly as old as graphic Ma, provide a maximum age range for the deposition of this rock. Data for the alaskitic gneiss suggest that it is either derived from an igneous (volcanic) protolith with a minimum age of ca. 1250 Ma and a maximum age of graphic Ma, or is a dyke emplaced at ca. 1140–1170 Ma. The amphibolite yielded zircon interpreted as metamorphic, with a minimum age of 1118 Ma, and a maximum age not likely older than ca. 1160 Ma. Zircons from charnockites and monzonites of the Morin plutonic complex gave zircon igneous ages between ca. 1157 and 1165 Ma. High-grade metapelites of the Réservoir Cabonga terrane yielded metamorphic zircon ages of 1140-1160 Ma. Metamorphic monazites from both the Réservoir Cabonga and the Mont-Laurier terranes yielded ages of 1138−1182 Ma, interpreted as the crystallization age or the time that significant Pb loss ceased. These ages indicate that the two terranes underwent the same long-lasting metamorphic event. The overlap between ages of metamorphic zircons and monazites on the one hand and the age of anorthosite–charnockite magmatism on the other hand suggests a long-lasting high-grade metamorphism with heat contribution from crystallizing plutons. A posttectonic aplite dyke from the interior of the Mont-Laurier terrane gives a zircon minimum age of 1054 Ma, considered a minimum age for penetrative deformation in this part of the Grenville Province. Rutile ages of 945–955 Ma record cooling through about 400 °C in both the Réservoir Cabonga and the Mont-Laurier terranes.

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