THE EXTENT OF JUVENILE GRENVILLIAN-AGE CRUST IN THE GRENVILLE PROVINCE

The extent of juvenile Grenvillian-aged crust in the Grenville province is a long-standing problem that has never been conclusively resolved. Rivers et al. (1989) proposed limits on the extent of juvenile crust when they defined the Allochthonous Monocyclic Belt (AMB) as part of their overall tectonic model for the Grenville Province. This model was refined by Carr et al. (2000), who sub-divided the AMB into the Composite Arc Belt and the Frontenac-Adirondack belt. However, based on a limited data set of published Nd isotope evidence, both belts were regarded as mostly juvenile. In contrast, Hanmer et al. (2000) interpreted the Composite Arc Belt as a back-arc basin that formed by rifting of a pre-existing continental margin, comprising both the Central Gneiss Belt and the Adirondack Highlands. To resolve this problem, Nd isotope data were obtained from a new regional sampling of orthogneissic rocks across the SW Grenville Province, as well as the Grenvillian inlier of Vermont. The results reveal a distinct difference between the Nd model ages of the Composite Arc Belt and the Frontenac-Adirondack belt. Orthogneissic samples from the Adirondack Highlands yield an average TDM model age of 1.48 Ga, similar to the Muskoka and Shawinigan domains of the Allochthonous Polycyclic Belt, as well as the Green Mountains of Vermont. In contrast, orthogneissic rocks from the Composite Arc Belt (Bancroft and Elzevir domains) yield an average TDM model age of 1.27 Ga. We interpret the difference of 200 Myr between the average model age of orthogneisses from the Frontenac-Adirondack belt and the Composite Arc Belt as a real difference in crustal formation age in the two belts. Hence, we argue that only the latter belt can be considered as juvenile Grenvillian-aged crust, thus supporting the tectonic model of Hanmer et al. (2000).