Crustal evolution in eastern labrador: Constraints from precise U-Pb ages

Abstract

New U-Pb ages for rocks of eastern Labrador complement previous U-Pb dates for the Grenville and Makkovik provinces and substantiate the following conclusions. Eastern Labrador consists of two distinct crustal blocks that are different in age as well as magmatic, metamorphic and structural overprint. The northern block includes Archaean to Lower Proterozoic basement rocks (> 1900 Ma) and two distinct episodes of rhyolitic volcanism at 1860 and 1807 Ma. High-grade metamorphism, deformation and anatexis affected this block between 1860 and 1760 Ma ago (Makkovikian Orogeny). The southern block comprises the Trans-Labrador Batholith and the various terranes of the Grenville Province. In contrast to the northern region, the southern block shows no proof for the existence of continental crust older than 1710 Ma. This suggests that large amounts of juvenile material were added between 1710 and 1630 Ma ago, as dated by a large series of precise rock ages. At least towards its end, this crust-forming event was associated with high-grade metamorphism, deformation and anatexis (Labradorian Orogeny). The 1710-1630 Ma period of activity was followed by three distinct phases of anorogenic plutonism causing the emplacement of pegmatites (1500 Ma), a widespread suite of cotonitic metagabbros (Michael Gabbro, 1426 Ma) and granitic-syenitic bodies (1296 Ma). During the Grenvillian Orogeny, the region achieved its present structural configuration including the emplacement of granitoid plutons. The structurally highest regions, the Mealy Mountains and the Hawke River terranes were thrust over the Lake Melville terrane and part of the Groswater Bay terrane and they were only marginally affected by Grenvillian metamorphism. The more deeply buried Lake Melville and Groswater Bay terranes, however, were significantly modified during tectonothermal events at 1030 Ma and 980-960 Ma respectively.