U [sbnd]Pb geochronology of the northern Torngat Orogen, Labrador, Canada: a record of Palaeoproterozoic magmatism and deformation

Abstract

The Palaeoproterozoic Torngat Orogen, located in northeastern Quebec and northern Labrador, Canada, records the sinistral transpressional collision of the western margin of the Archaean Nain Province and the eastern margin of the Rae Province. U Pb age determinations are presented for twelve samples from the northern part of the orogen, and used to evaluate models for its tectonic evolution. Voluminous granitoid plutonic rocks which in part intrude gneisses interpreted as the northern extension of the Nain Province and are possibly related to subduction below it, have zircon ages of 1910 ± 2 Ma, 1895 ± 3 Ma, 1891 ± 2 Ma, 1890 ± 2 Ma, 1869 +3 −2 Ma, and 1864 ± 2 Ma, suggesting at least 45 m.y. of convergence between the Rae and Nain Provinces. Granulite-facies conditions may have existed in the northern part of the area at ∼ 1.84 Ga. These plutonic rocks, as well as tonalitic gneisses of the Nain Province, were reworked at amphibolite-facies conditions within a north-south corridor referred to as the Komaktorvik zone. Zircon age determinations of two granitic veins which were intruded synchronously with ongoing deformation in the Komaktorvik zone are 1789 ± 2 Ma and 1719 ± 2 Ma, súggesting that deformation within the zone may have lasted for up to 70 m.y. Metamorphic titanite from samples within the Komaktorvik zone have ages of 1776 ± 4 Ma, 1774 ± 4 Ma, 1767 ± 4 Ma, 1710 ± 4 Ma and 1632 ± 4 Ma, and support the interpretation of long-lived deformation in the Komaktorvik zone. Palaeoproterozoic subduction beneath the Nain Province has not been widely recognized in southern parts of the orogen, suggesting that a distinct tectonic regime existed in the study area.