Abstract
We report new coupled zircon U-Pb age and Hf isotope data, along with whole-rock Hf-Nd isotope data for 20 samples from the Priest River and Clearwater complexes-Neoarchean and Paleoproterozoic basement exposures in northern Idaho, northwest Laurentia. The new zircon data reveal two well-defined periods of magmatism at similar to 2.66 and similar to 1.86 Ga in each complex, with no evidence of any intervening or older magmatic events. Positive zircon initial epsilon(Hf) values of the Neoarchean rocks (+1 to +3) indicate juvenile magmas derived from a depleted mantle source. The Paleoproterozoic rocks, in both complexes, share a much wider range of initial zircon epsilon(Hf) values (-7 to +6) which suggests magmas derived from a depleted mantle source intruded and assimilated preexisting Neoarchean continental crust. The shared age and isotope compositions in the Priest River and Clear-water complexes suggest that the two complexes likely belong to a single crustal block-the Clear-water block.
Based on these new zircon U-Pb ages and Hf isotopic data of the Neoarchean and Paleoproterozoic rocks, the Clearwater block is shown to be distinct from the nearby Wyoming Province, the Medicine Hat block, and components of the Great Falls tectonic zone. We also report new zircon Hf isotope data for the similar to 1.58 Ga Laclede orthogneiss in the Clearwater block, which along with published U-Pb and Hf isotope constraints, support a paleogeographic connection between the Gawler craton, East Antarctica, and northwestern Laurentia during similar to 1.9 Ga to similar to 1.6 Ga. Comparison with other examples of co-existing Neoarchean and Paleoproterozoic igneous rocks present in other continents suggests that the global record of 2.7-2.5 Ga magmatism often represents new crustal additions from a depleted mantle reservoir. The record of 2.0-1.7 Ga rocks also represent additions of juvenile magma, but with significantly larger contributions from pre-existing continental crust. In many cases the 2.0-1.7 Ga crust preserved on Earth today was added during the assembly of the supercontinent Nuna.