A BIMODAL BELT OF ~1.86 TO 2.66GA TONALITIC GNEISSES IN NORTHWEST LAURENTIA: U-PB AND LU-HF CONSTRAINTS ON THE EVOLUTION OF NORTH AMERICA

We have found a belt of orthogneissic crust which extends 115km from the North Fork of the Clearwater river in Idaho to the Priest River metamorphic complex in northern Washington. Continued reconnaissance work has made it clear that this bimodal belt of 1.86 and 2.66 Ga orthogneissic rocks is an important component of northwest Laurentia, and provides crucial contraints on the crustal evolution in this region. Here we present the age and Hf isotopic composition of these rocks. Along with adding to the record of North American crustal evolution, constraining the age and isotopic signatures of these basement rocks, which lie near the rifted western edge of Laurentia, will help identify its conjugate margin and therefore will be useful in tectonic reconstructions. However, this area still suffers from a lack of robust geochronologic and radiogenic tracer isotope data. In order to improve this database, we report concurrently determined U-Pb age and Hf isotope compositions of zircon from nearly 20 orthogneiss samples from this region using the Laser Ablation Split Stream (LASS) method. Data from these samples show a strongly bimodal age distribution (~2.66 Ga and ~1.86 Ga) and distinct Hf isotope signatures.

The ~2.66 Ga samples from this belt of rocks, which we refer to as the “Clearwater belt” have a very restricted range of initial ε_Hf from +2 to +3, documenting derivation from a depleted mantle source, with little input from pre-existing crustal material. In contrast, the ~1.86 Ga orthogneisses from the Clearwater belt collectively have a wide range of initial ε_Hf from about -8 to +10. The initial ε_Hf from the ~1.86 Ga orthogneisses are consistent with mixing between a depleted mantle end member and the preexisting 2.66 Ga crust that their protoliths intruded. These results demonstrate production of juvenile material at ~2.66 Ga, followed by further, and likely larger, input of juvenile material into this preexisting crust at ~1.86 Ga. Importantly, there is a striking similarity between the U-Pb age and Hf isotope signature of the these orthogneisses and new data from granitoid clasts collected from glacial catchments in central East Antarctica, consistent with the proposed SWEAT reconstruction of Rodinia.