Isotopic mapping of the Allochthon Boundary Thrust in the Grenville Province of Ontario, Canada

Nd isotope analyses of orthogneissic rocks were conducted in the vicinity of the Allochthon Boundary Thrust (ABT) in the Central Gneiss Belt of Ontario, Canada. Gneisses from the parautochthonous belt have relatively homogeneous Nd isotope signatures with an average TDM model age of 1.91 Ga, consistent with an origin as an accreted Paleoproterozoic arc terrane (Barilia). Orthogneissic rocks from the overlying allochthonous belt have a wider range of model ages from 1.46 to 1.79 Ga, consistent with a more complex geological history that may involve reworking of sedimentary material in a long-lived ensialic arc (Algonquia). The isotopic boundary between these terranes, represented by the ABT, is consistent with the distribution of distinct types of metabasic rock, previously used as a criterion for recognising rocks of the allochthonous and parautochthonous belts of the Grenville Province. However, the location of the ABT proposed on the basis of isotopic analysis has been modified from that of other workers, because Nd isotopes provide a much more detailed tool for mapping the original zone of convergence (thrusting) between allochthonous and parautochthonous terranes with distinct crustal formation ages. The isotopic boundary is also supported by field-based mapping of high-strain zones in critical areas. Other mapped shear zones may represent later extensional structures with lesser geological significance.