Nd-Isotope Mapping of the Grenville Province in Southern Labrador

Abstract

<p>New Nd isotopic data is presented, with previously published Nd model ages, to create a crustal formation age map that demonstrates the extent of crustal terranes of different ages within the Grenville Province in southeastern Labrador. An unpublished Nd data set from C. Gower was used for comparison with the data from this study. The previous UPb data has provided ages that define the geological history of southeast Labrador through four major orogenic events: the Makkovik orogeny (1860-1790 Ma), the Labradorian orogeny (1710-1600 Ma), the Pinwarian (1520-1460 Ma) and the Grenville orogeny (1085-985 Ma) (Gower, 2008). However, the high-grade metamorphism as a result of the Grenville orogeny has obscured the true age of the crustal material that formed the Grenville Province. The crustal formation ages detelmined through Nd analysis can distinguish boundaries between crustal terranes, even in areas with a complex geological history. <br />Previous Nd isotopic data included both the Grenville Province to the south, and the Paleoproterozoic Makkovik Province to the north, and demonstrated that older Nd values were located farther south than previously established but were unsuccessful in determining a crustal terrane boundary. <br />The data from this study provided depleted Mantle model (T DM) age ranges that overlapped from the Hawke River, Lake Melville and Mealy Mountains terranes which suggested that they are representative of one crustal block, which was renamed the Caliwright terrane. This terrane presented an age range of 1.87-1.94 Ga, which may reflect mixing between Pre-Labradorian and Labradorian magmatism. The data from Gower which included the Groswater Bay telTane, displayed a similar age range of 1.90 to 1.97 Ga, also indicating a single crustal source. In contrast, the Pinware terrane falls into two categories in which the northerly portion has Nd signatures similar to those of the Cartwright terrane, and the southern portion has ages indicating a juvenile Labradorian source. A mean Nd model age of 1.77 Ga was recorded for this data set as well as the data from Gower. Therefore the Pinware terrane was divided into north and south domains based on a proposed suture boundary between Gilbeli River and Red Bay. The data presented suggests that the original edge of the Makkovik continental margin passed just north of Red Bay and trends westwards into the interior of southern Labrador. I expect that relatively homogeneous Nd isotope signatures are located to the south of this boundary, whereas heterogeneous signatures are expected to the north.</p>