Paper No. 10
Presentation Time: 10:50 AM


FISHER, Christopher1, VERVOORT, Jeff1, JANSEN, Andrew C.2, LEWIS, Reed S.3, GASCHNIG, Richard M.4 and GOODGE, John W.5, (1)School of the Environment, Washington State University, Pullman, WA 99164, (2)Newmont Mining Corporation, Twin Creeks Operations, Golconda, NV 89414, (3)Idaho Geological Survey, University of Idaho, 875 Perimeter Drive MS3014, Moscow, ID 83844-3014, (4)Department of Geology, University of Maryland, College Park, MD 20742, (5)Department of Geological Sciences, University of Minnesota, Duluth, MN 55812,

Constraining the nature of ancient crystalline basement underlying western North America provides an important record of the formation and crustal evolution of the continent. 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. One region important for addressing this record of continental growth, yet which suffers from a lack of robust geochronologic and radiogenic tracer isotope data from exposed basement, is the Clearwater metamorphic complex of northern Idaho. In order to improve this geochronologic and radiogenic tracer isotope database, we report concurrently determined U-Pb age and Hf isotope compositions of zircon from 12 orthogneiss samples from this region using the Laser Ablation Split Stream (LASS) method. Data from these samples show a strongly bimodal age distribution (~2.65 Ga and ~1.86 Ga) and distinct Hf isotope signatures that provide useful constraints for understanding crustal growth in western Laurentia and isotopic fingerprints for Proterozoic tectonic reconstructions.

The ~2.65 Ga samples have a very restricted range of initial εHf from +2 to +3, suggesting derivation from a depleted mantle source, with little input from pre-existing crustal material. In contrast, the ~1.86 Ga orthogneisses have a wide range of initial εHf from about -8 to +10. The initial εHf from the ~1.86 Ga orthogneisses are consistent with mixing between a nearly pure depleted mantle end-member and the preexisting 2.65 Ga crust that their protoliths intruded. These results demonstrate production of juvenile material at ~2.65 Ga, followed by further input of juvenile material into this preexisting crust at ~1.86 Ga. Moreover, there is a striking similarity between the U-Pb age+Hf isotope signature of the Clearwater orthogneisses and new LASS U-Pb+Hf isotope data from granitoid clasts collected from glacial catchments in central East Antarctica, consistent with the proposed SWEAT reconstruction of Rodinia.