Late Labradorian metamorphism and anorthosite–granitoid intrusion, Cape Caribou River allochthon, Grenville Province, Labrador: evidence from U–Pb geochronology

Abstract

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 the boundary between the Mealy Mountains and Groswater Bay terranes. The layered rocks of the CCRA are cut by a stockwork of monzonite dykes related to the Dome Mountain suite and by metadiabase–amphibolite dykes that probably form part of the ca. 1380 Ma Mealy swarm. The mafic dykes appear to postdate much of the development of subhorizontal metamorphic layering within the lower parts of the CCRA. The uppermost (least metamorphosed) units of the CCRA, the North West River anorthosite–metagabbro and the Dome Mountain monzonite suite, have been dated at 1625 ± 6 and 1626 ± 2 Ma, respectively. An amphibolite unit that concordantly underlies the anorthosite–metagabbro and is intruded discordantly by monzonite dykes has given metamorphic ages of 1660 ± 3 and 1631 ± 2 Ma. Granitoid gneisses that form the lowest level of the CCRA have given a migmatization age of 1622 ± 6 Ma. The effects of Grenvillian metamorphism become apparent in the lower levels of the allochthon where gneisses, amphibolite, and mafic dykes have given new generation zircon ages of 1008 ± 2, 1012 ± 3, and 1011 ± 3 Ma, respectively. A posttectonic pegmatite has also given zircon and monazite ages of [Formula: see text] and 1013 ± 3 Ma, respectively. Although these results indicate new growth of Grenvillian zircon, this process was generally not accompanied by penetrative deformation or melting. Thus, the formation of gneissic fabrics and the overall layered nature of the lower CCRA are a result primarily of Labradorian (1660–1620 Ma) tectonism and intrusion, and probably reflect early movement on an ancestral GLTS. Grenvillian heating and metamorphism (up to granulite facies) was strongly concentrated towards the base of the CCRA and probably occurred during northwestward thrusting of the allochthon over the Groswater Bay terrane.