The majority of gray gneisses from an 80 000 km2 area in the central (Quebec) Grenville province define a Sm-Nd isochron that yields an age of 1.53 ±0.07 Ga. Major element, trace element, and initial Nd isotopic data all point to a subduction-related (orogenic) source for this terrane. It is attributed to a major crustal extraction event that formed a mid-Proterozoic island arc. The age of the terrane agrees with published data from the eastern and western Grenville province in Newfoundland and Ontario, respectively. We suggest that 1.5 Ga are fragments were accreted along much of the length of the Laurentian foreland within 100 m.y. of their formation, in an event marked by widespread granitoid plutonism.