LATE LABRADORIAN METAMORPHISM AND ANORTHOSITE-GRANITOID INTRUSION, CAPE-CARIBOU RIVER ALLOCHTHON, GRENVILLE PROVINCE, LABRADOR - EVIDENCE FROM U-PB GEOCHRONOLOGY

In the Cape Caribou River allochthon (CCRA), metaigneous and gneissic units occur as a shallowly plunging synform in the hanging wall of the Grand Lake thrust system (GLTS), a Grenvillian structure that forms t he boundary between the Mealy Mountains and Groswater Bay terranes. Th e layered rocks of the CCRA are cut by a stockwork of monzonite dykes related to the Dome Mountain suite and by metadiabase-amphibolite dyke s that probably form part of the ca. 1380 Ma Mealy swarm. The mafic dy kes appear to postdate much of the development of subhorizontal metamo rphic layering within the lower parts of the CCRA. The uppermost (leas t metamorphosed) units of the CCRA, the North West River anorthosite-m etagabbro and the Dome Mountain monzonite suite, have been dated at 16 25 +/- 6 and 1626 +/- 2 Ma, respectively. An amphibolite unit that con cordantly underlies the anorthosite-metagabbro and is intruded discord antly by monzonite dykes has given metamorphic ages of 1660 +/- 3 and 1631 +/- 2 Ma. Granitoid gneisses that form the lowest level of the CC RA have given a migmatization age of 1622 +/- 6 Ma. The effects of Gre nvillian metamorphism become apparent in the lower levels of the alloc hthon where gneisses, amphibolite, and mafic dykes have given new gene ration zircon ages of 1008 +/- 2, 1012 +/- 3, and 1011 +/- 3 Ma, respe ctively. A posttectonic pegmatite has also given zircon and monazite a ges of 1016(-3)(+7) and 1013 +/- 3 Ma, respectively. Although these re sults indicate new growth of Grenvillian zircon, this process was gene rally not accompanied by penetrative deformation or melting. Thus, the formation of gneissic fabrics and the overall layered nature of the l ower CCRA are a result primarily of Labradorian (1660-1620 Ma) tectoni sm and intrusion, and probably reflect early movement on an ancestral GLTS. Grenvillian heating and metamorphism (up to granulite facies) wa s strongly concentrated towards the base of the CCRA and probably occu rred during northwestward thrusting of the allochthon over the Groswat er Bay terrane.