CRYSTALLINE BASEMENT ROCKS OF NORTHERN IDAHO: A DISTINCT ARCHEAN TERRANE IN NW LAURENTIA?
In this study, nine new samples from the PR complex agree with the well-defined bimodal ages of ~2.66 Ga and ~1.86 Ga noted in the CW. Zircon Hf analyses of Archean samples from both the CW and PR complexes yield a tight range of positive εHf(t) values (~ 0 – +4), while Paleoproterozoic samples show a spread of εHf(t) values from ~ -7 to +5. The Hf isotopes of the 1.87 Ga – 1.84 Ga rocks from the PR complex are more negative than those of CW, indicating involvement of more evolved continental crust, although Hf signatures of two complexes overlap. These results suggest the ~ 2.6 Ga Archean basement was derived from a relatively homogenous depleted mantle source, and the ~1.8 Ga Paleoproterozoic orthogneisses resulted from the mixing of various amounts of depleted mantle input and evolved continental crust. Our new age and isotopic results suggest the CW and PR complexes represent the same basement terrane. In comparison with other basement terranes in northwestern Laurentia, the crystalline basement in the PR and CW complexes appear to be distinct from the surrounding Wyoming province and the Medicine Hat block, although the latter is poorly characterized. If the PR and CW complexes indeed represent a distinct Archean-Paleoproterozoic crustal block—the Clearwater block—it may provide important paleogeographic constrains.