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