Fifty new Nd isotope analyses are presented for a 230 km north-south suite along the eastern Labrador coast of the Grenville Province. Depleted mantle model T(DM) ages for foliated pre-Grenvillian orthogneisses display an overall decrease in age from ca. 2 Ga in the north to ca. 1.8 Ga in the south, but with considerable variability. The noise is attributed to variable mixing of Nd from Makkovik and Labradorian-aged sources, with an increasing Labradorian component to the south. The variability of T(DM) ages decreases markedly in the Pinware terrane south of Red Bay, suggesting that this represents the approximate limit of reworked Makkovikian crust. To the southwest, the total area of juvenile Labradorian crust in Eastern Quebec is substantial (>100,000 km2), suggesting the accretion of a juvenile Labradorian arc. However, the suture between heavily reworked Makkovik crust to the north and juvenile Labradorian crust to the south is unclear. Meanwhile, it is likely that back arc rifting in the hinterland of the orogen led to a second collisional suture ≈ 250 km to the north, in the vicinity of the Trans-Labrador Batholith. The occurrence of ensialic arc magmatism on the Makkovik continental margin prior to these collisional events can explain the intensity of the Labradorian magmatic event. Conversely, a model involving two opposite-dipping subduction zones can explain the relatively sudden cessation of Labradorian magmatism and rapid transition to a post-Labradorian passive margin.
Moumblow et al. (Mon,) studied this question.