CRYSTALLINE BASEMENT ROCKS OF NORTHERN IDAHO: A DISTINCT ARCHEAN TERRANE IN NW LAURENTIA?

Basement rocks are important for constraining events occurring in the earlier part of Earth’s history. Newly recognized ancient basement has been identified in NW Laurentia, which was exposed during Eocene unroofing of “core complexes” in present day Idaho and northeastern Washington. Here we discuss the age and origin of the Priest River (PR) and Clearwater (CW) core complexes, which expose crystalline basement across a 115 km-long belt in north-central Idaho and northern Washington. These exposures represent the largest belt of Neoarchean and Paleoproterozoic continental crust in the northwest U.S., outside the Wyoming Province. Recent U-Pb geochronology studies have documented two discrete pulses of magmatism, ~2.66 Ga and ~1.86 Ga, in the CW complex (Vervoort et al., 2016). However, the age and origin of the PR complex to the northwest is less understood.

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 ε_Hf(t) values (~ 0 – +4), while Paleoproterozoic samples show a spread of ε_Hf(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.