THE BASEMENT TERRANES OF NORTH CENTRAL IDAHO: FILLING IN THE GAP BETWEEN THE PRIEST RIVER AND CLEARWATER COMPLEXES

The Priest River and Clearwater complexes are basement rocks and high-grade cover rocks uplifted inside Eocene metamorphic core complexes. Recent U-Pb zircon dating of rocks in the Clearwater complex document a previously unknown 115-km band of crystalline basement rocks in north central Idaho. This represents the largest belt of Neoarchean and Paleoproterozoic crust exposed in the Northwest U.S. The presence of Archean crust in the Priest River complex has been known for nearly two decades, but until recently no other basement has yet to have been documented in this complex. The region south of Coeur d’Alene contains areas that have been mapped as either Belt Supergroup or pre-Belt metamorphic rocks intruded by plutons of presumed Cretaceous age. In order to explore the age relationships of this crust to both the Priest River complex to the north and the Clearwater complex to the southeast we have analyzed six samples from this region.

The U-Pb ages of zircon from these 6 samples were determined by LA-ICP-MS: an amphibolite, two granite orthogneisses, two megacrystic granite orthogneisses, and a migmatite granitic orthogneiss. The amphibolite sample yields a weighted average ²⁰⁷Pb/²⁰⁶Pb age of 2.64 ± 0.03 Ga; the granitic orthogneisses yield weighted average ²⁰⁷Pb/²⁰⁶Pb ages of 1857 ± 6 and 1870 ± 7 Ma; the megacrystic orthogneisses have ages of 1869 ± 13 and 1858 ± 9 Ma; and the migmatite gneiss yields a date of 1875 ± 7 Ma. Collectively, the orthogneisses all yield ages within a narrow time frame from ~1875 to 1857 Ma, and the amphibolite provides evidence for another occurrence of Neoarchean crust in the Priest River complex.

These ages are identical, within error, to the ages of the basement rocks of the Clearwater complex. Similarly, the Archean age (amphibolite) is identical, within error, to the age of the Pend Oreille gneiss in the Priest River complex. Based on our results, we suggest that both the Clearwater and Priest Lake complexes—as well as the newly dated terranes of this study—represent different pieces of the same basement province and may be a southern extension of the poorly known Medicine Hat block in Canada. This data not only documents the basement ages in this region but also provides valuable piercing points for tectonic reconstructions.