The location of the Allochthon Boundary Thrust and the Archean–Proterozoic suture in the Mattawa area of the Grenville Province: Nd isotope evidence

Over 100 Nd isotope analyses are used to map the crustal formation ages of Grenville orthogneisses in a 10 000 km2 area near Mattawa on the Ontario–Quebec border. Lithotectonic terranes with mapped outcrops of eclogite or coronitic metagabbro have Nd model ages less than 1.8 Ga, and are interpreted as fragments of the allochthonous polycyclic belt. However, large areas of orthogneiss to the north of Mattawa have model ages of 1.9–2.9 Ga, interpreted as Archean crust that was extensively plutonised in Paleo- and Mesoproterozoic magmatic events. Based on this evidence, a revised location is proposed for the Allochthon Boundary Thrust (ABT) which passes to the south of the Bonfield batholith and Mattawa but then trends to the north and is correlated with the Lac Watson shear zone. Some allochthonous klippen are identified northwest of the main trace of the ABT, representing the dissected remains of what was originally a large allochthonous thrust sheet. Within the parautochthon, a major step in depleted mantle model ages is observed in the vicinity of the Mattawa fault, separating Archean and Paleoproterozoic crustal blocks. This boundary is interpreted as an ancient crustal suture, reactivated by faulting associated with the Ottawa–Bonnechere graben. It is concluded that Nd isotope analysis is a useful technique for detailed mapping of major tectonic boundaries in highly metamorphosed orogenic belts such as the Grenville Province.