GEOCHRONOLOGIC AND HF ISOTOPE CONSTRAINTS ON THE FORMATION AND EVOLUTION OF THE CLEARWATER AND SOUTHERN PRIEST RIVER COMPLEXES, NORTHERN IDAHO-EASTERN WASHINGTON

The nature of the Precambrian crystalline basement underlying the North American Cordillera in northern Idaho provides an important record for understanding not only western Laurentia, but also for the growth, assembly, and modification of the North American continent. Recent geochronology and isotope geochemistry have helped to greatly improve our understanding of the origin and evolution of the crystalline basement rocks in this region.

We have examined two basement gneiss exposures in Idaho and eastern Washington, Clearwater and Priest River complexes. These have crystallization ages that define two discrete periods in the Neoarchean and Paleoproterozoic. Neoarchean rocks range from 2.67 to 2.65 Ga (mean, 2.66 Ga), are broadly granitic in composition, and have slightly positive initial Hf isotope compositions (ε_Hf +2 to +4). Coupled with the lack of zircon inheritance, this indicates derivation from a depleted mantle source with little, if any, input of older crustal material. Paleoproterozoic orthogneisses are more abundant in both complexes and range from 1876 to 1837 Ma (mean,1.86 Ga), are also broadly granitic, and show clear evidence for older crust in their genesis, including xenocrystic zircons and less radiogenic and more variable Hf isotope compositions (initial ε_Hf -8 to +8). These data indicate these gneisses were produced by two discrete periods of magmatism: initial formation of juvenile crust at 2.66 Ga and subsequent crustal addition at 1.86 Ga involving a mix of juvenile mantle, and pre-existing Neoarchean crustal, sources.

In addition, more detailed study of ortho- and paragneisses within the Clearwater complex provide evidence of several metamorphic episodes. Garnet Lu-Hf ages throughout the Clearwater complex range from 1.46 Ga to 1.06 Ga. The best-constrained garnet ages define two major tectonothermal events at ~ 1.33 and 1.08 Ga. The 1.33 garnet age is corroborated by monazite U-Pb ages that show a dominant age range of 1.38 to 1.30 Ga (Baldwin, this meeting). The younger 1.08 garnet age, in contrast, has a weak to absent counterpart in the monazite data, and the Eocene metamorphic ages, so prominent in the monazite data, are completely lacking in the garnet data. In total, these data illustrate the rich tapestry of geologic events that have occurred in this region over the past 2.6 Ga.