Nd isotope mapping of the Lac Dumoine thrust sheet: implications for large‐scale crustal structure in the SW Grenville Province

Terra Nova, 24, 363–372, 2012 Abstract Over 100 new Nd model ages are presented for orthogneisses from the Lac Dumoine region, western Quebec, interpreted as a thrust sheet of the Grenvillian Allochthonous Polycyclic Belt. The thrust sheet has an isotope signature consistent with other terranes of the allochthonous belt, with TDM ages <1.8 Ga in the main allochthon, but ages of 1.8–2.0 Ga in an underlying tectonic unit, itself thrust over parautochthonous Archean crust. However, the new data also reveal a half‐window of Palaeoproterozoic crust that divides the allochthon into two lobes, showing them to be relatively thin‐skinned nappes originating from the Baskatong crustal‐scale ramp. When combined with new Nd isotope mapping for Algonquin Park, Ontario, these results suggest that the main ramp of the Allochthon Boundary Thrust is located further to the SE than previously thought, with a strong SW‐NE linear trend that coincides with dipping reflectors imaged on Lithoprobe seismic line 32 south of Algonquin Park.