Thermotectonic evolution of the Grenville Province of western Labrador

The Grenville Province in western Labrador comprises three lithotectonic terranes, structurally stacked from northwest to southeast, whose geometric relationships evolved over a time interval from the Early Proterozoic to the Grenvillian Orogeny (circa 1.0 Ga). Gagnon terrane lies immediately southeast of the Grenville Front and is dominated by Early Proterozoic sediments that were deposited unconformably on the Archean basement of the Superior Province. It is overlain to the southeast by the Labradorian magmatic arc of Molson Lake terrane which is itself overlain by the highest structural deck comprising Labradorian migmatites and intrusive rocks of Lac Joseph terrane. The Labradorian magmatic arc of Molson Lake terrane was founded on the margin of mid‐Proterozoic Laurentia and was fueled by northwest directed subduction of oceanic crust that existed between Molson Lake and Lac Joseph terranes. Labradorian magmatic rocks of the exotic Lac Joseph terrane were produced in a second, independent subduction zone of undetermined dip direction. No post‐Labradorian magmatic arc is known to have existed in western Labrador, implying that all three terranes were proximal (part of the same plate) by the end of the Labradorian Orogeny. Rifting of the post‐Labradorian configuration, perpendicular to the Labradorian Front led to the emplacement of the Shabogamo‐Michael Gabbro dyke swarm in a linear belt across the future northern Grenville Orogen. The present terrane geometry of western Labrador is a product of northwest directed thrusting that followed an initial period of crustal thickening during the Grenville Orogen at circa 1000–995 Ma, an event accompanied by penetrative deformation and metamorphism only in Gagnon and Molson Lake terranes.