A Reassessment of the Grenvillian Orogeny in Western Labrador

Abstract

Recent mapping and radiometric dating in the Grenville Province of western Labrador indicate two major metamorphic events, one Early Proterozoic (circa 1650 Ma) and one Grenvillian (circa 1000 Ma). A threefold tectonic subdivision of the Grenville Orogen is proposed, consisting of an autochthon, a parautochthon and a number of allochthons. The parautochthon in the west consists of a previously unmetamorphosed sedimentary succession, whereas in the east it consists of paragneiss, metamorphosed in the Early Proterozoic, and a granitoid batholith. From the Grenville Front southwards, metasediments of both the autochthon and parautochthon increase in Grenvillian metamorphic grade from greenschist to upper amphibolite facies. These rocks are imbricated by northward-directed thrusts. In the interior Grenville Province, the parautochthonous sequences are tectonically overlain by Lower Proterozoic gneisses of upper amphibolite to granulite facies. These were thrust into place during the Grenvillian orogeny, when they were structurally reworked but only mildly metamorphosed. Upper amphibolite and granulite facies rocks of an Early Proterozoic metamorphism are thus adjacent to those at equivalent and lower grades of a Grenvillian metamorphism.