THE CLEARWATER COMPLEX: UNCOVERING THE NEOARCHEAN AND PALEOPROTEROZOIC BASEMENT OF NORTH CENTRAL IDAHO

The geology of north-central Idaho, broadly known as the Clearwater Country, has long been enigmatic. While there have been suggestions of older crystalline basement rocks in the region (e.g., Reid et al., 1973) this basement was covered by thick strata in the Meso- and Neoproterozoic, subjected to metamorphic events in the Mesoproterozoic and Cretaceous, intruded by the Idaho batholith in the Cretaceous, and finally extended and intruded in Eocene. In total, this complex geologic history—and the rugged nature of and difficult access to much of this terrain—has made this region the terra incognita of Northwest U.S. geology. Recent U-Pb zircon dating of rocks in this region, however, has now documented the existence of crystalline basement rocks, which extend along a 115-km stretch of north central Idaho. This represents the largest belt of Neoarchean and Paleoproterozoic crust exposed in the Northwest U.S.

Here we present U-Pb zircon data for 18 basement gneisses from this region that have been analyzed over the past several years. These samples fall into two discrete age groups: at 2.66 and 1.86 Ga. Four Neoarchean orthogneisses have ²⁰⁷Pb/²⁰⁶Pb dates that range from 2670 to 2651 and, when taken together, define a weighted mean age of 2659 ± 15 Ma (2s). These orthogneisses are broadly granitic in composition and show no involvement of older crust in their genesis in either inherited zircons or variable Hf isotope compositions. The 14 Paleoproterozoic gneisses are more common throughout the region and range in age from 1876 to 1837 Ma with a weighted mean age from all samples of 1861 ± 6 Ma (2s). These orthogneisses are all broadly granitic but range in composition from leucocratic granite to biotite tonalite. In contrast to the Neoarchean samples, the Proterozoic rocks show considerably more evidence for the involvement of older crust in their genesis, both in terms of xenocrystic cores and less radiogenic and more variable Hf isotope compositions.

Collectively, these ages and Hf isotope compositions indicate that the Clearwater complex basement was produced by two periods of magmatism of relatively short duration: one representing production of juvenile crust in the Neoarchean at 2.66 Ga; the other in the Paleoproterozoic at 1.86 Ga involving a mixture of juvenile and pre-existing Neoarchean crustal sources.