The southern limit of Archean crust and significance of rocks with Paleoproterozoic model ages: Nd model age mapping in the Grenville Province of western Quebec

Detailed Nd model age mapping in the Grenville Province of western Quebec has identified biotite gneisses with crustal formation age intervals of 2.5-2.9 Ga and 1.8-2.0 Ga. Although the presence of Archean crust in the Grenville Parautochthonous Belt has been known for some time, the results of Nd model age mapping, combined with geochemical and aeromagnetic data, indicate that the Archean parautochthon extends more than 20 km to the south of the Renzy Shear Belt. This belt, which was previously defined as the southern limit of Archean crust, corresponds to an isolated allochthon with Nd model ages ranging from 2.0 to 1.4 Ga and does not demarcate the Allochthon Boundary Thrust.Rocks with model ages of 1.8-2.0 Ga exposed to the south of the Archean crust geochemically possess an arc affinity and are interpreted as a Paleoproterozoic arc. A model age boundary between Archean crust and this arc has been defined by this study. The age boundary probably represents a suture along which the southern margin of Laurentia and the arc converged.