U-Pb ages of syndeformational dykes associated with the Mesoproterozoic Nain Plutonic Suite, Labrador

Field and U-Pb zircon geochronological studies in a suite of crosscutting, deformed, and metamorphosed basic dykes show that they were emplaced in a s yndeformational environment between 1328 +/- 2 and 1316.5 +/- 1.6 Ma. Earli er dykes in the suite are granulitic, commonly folded along north-south axe s or strongly rotated into north-south-trending shear zones. Later dykes ar e metamorphosed to amphibolite grade. These sometimes retain igneous porphy ritic textures and occasionally crosscut the granulite dykes. The amphiboli te dykes were emplaced in a less ductile crustal environment and a structur al environment less dominated by shear deformation. Both granulitic and to a lesser extent amphibolite dykes show evidence of synshear emplacement. Th e episode of emplacement is broadly coeval with the injection of the mid-Pr oterozoic Nain Plutonic Suite (NPS) and suggests that the dykes were inject ed as a series of leaks from NPS feeder chambers. If so, the granulite dyke s may represent leaks intruded near the culmination of NPS activity, wherea s the amphibolite dykes were probably injected during the waning phase of t ectono-thermal conditions.