ND ISOTOPE MAPPING OF CRUSTAL BOUNDARIES WITHIN THE EASTERN GRENVILLE AND MAKKOVIK PROVINCES, SOUTHERN LABRADOR

Abstract

The geological history of southeast Labrador has been resolved into four major orogenic events based on U-Pb dating: the Makkovik orogeny (1880-1790 Ma), the Labradorian orogeny (1710-1600 Ma), the Pinwarian (1520-1460 Ma) and the Grenville orogeny (1085-985 Ma). Although U-Pb ages represent the time of igneous and metamorphic events, they do not necessarily reflect the time of extraction of crustal material from the mantle. Nd isotopic data provide a tool for doing this, hence permitting regions having differing (and perhaps older) crustal formation ages to be recognized, even if this is not apparent in U-Pb geochronological data. In this study, suites of Depleted Mantle model (TDM) ages were determined for three large regions of southern Labrador. These comprise an east-west suite across the eastern Makkovik Province (Cape Harrison domain); a north-south suite along the Labrador coast of the Grenville Province (Groswater Bay, Hawke River, Lake Melville, Mealy Mountains and the Pinware terranes); and a reconnaissance suite from the Grenville Province in the interior of southern Labrador (Mealy Mountains, Pinware and Wilson Lake terranes, as well as the Trans Labrador Batholith). New Nd isotope data for the eastern Makkovik Province suggest that accreted juvenile Makkovik crust was generated in the Cape Harrison domain during a single crust-forming event around 2.0 Ga. Based on these data, a tectonic model is proposed for the Paleoproterozoic Makkovikian orogeny that is similar to the Ketilidian orogeny. In the eastern Grenville Province, Nd data indicate a southern extension of juvenile Makkovik crust, but with strong Labradorian reworking. A WNW-ESE boundary is proposed between Makkovik crust and juvenile Labradorian crust within the northern part of the Pinware terrane near Red Bay. This boundary separates the Pinware terrane into a Pinware North block (TDM ages often above 1.85 Ga), consisting of reworked Makkovik crust, and a Pinware South block (TDM ages below 1.85 Ga), representing juvenile Labradorian arc crust. A new tectonic model is then proposed for the accretion of this juvenile terrane during the Labradorian orogeny. Finally, Nd isotope data were determined in the interior of southern Labrador in order to trace the crustal boundary seen at the coast westwards. To the north of the Labrador-Quebec border, reworked Makkovik crust (TDM ages > 1.85 Ga) is present within a large regional fold in the north-western Pinware terrane, indicating that Makkovik basement does continue westwards in the northern part of this terrane. The new Nd data indicate that juvenile Makkovik arc crust, of the type seen north of the Grenville Province, extends southwards across much of the Grenville Province of southern Labrador, but with increasing degrees of Labradorian reworking southwards. It is believed that the original edge of the Makkovik continental margin was located just north of Red Bay, with an approximately WNW-ESE trajectory. This crustal boundary is believed to continue westwards approximately along the provincial boundary, reflecting the presence of Makkovik basement in the northern Pinware terrane.