GEOLOGY AND U-PB GEOCHRONOLOGY OF ARCHEAN BASEMENT AND PROTEROZOIC COVER IN THE PRIEST RIVER COMPLEX, NORTHWESTERN UNITED-STATES, AND THEIRIMPLICATIONS FOR CORDILLERAN STRUCTURE AND PRECAMBRIAN CONTINENT RECONSTRUCTIONS

Precambrian basement rocks exposed within tectonic windows in the Nort h American Cordillera help to define the Precambrian crustal structure of western North America and possible reconstructions of the Late Pro terozoic supercontinent Rodinia. New geologic mapping and U-Pb dating in the infrastructure of the Priest River metamorphic complex, norther n Idaho, documents the first Archean basement (2651 +/- 20 Ma) north o f the Snake River Plain in the North American Cordillera. The Archean rocks are exposed in the core of an antiform and mantled by a metaquar tzite that may represent the nonconformity between basement and the ov erlying Hauser Lake gneiss, which is correlated with the Prichard Form ation of the Belt Supergroup. A structurally higher sheet of augen gne iss interleaved with the Hauser Lake gneiss yields a U-Pb zircon cryst allization age somewhat greater than 1577 Ma. The slivers of augen gne iss were tectonically interleaved with the surrounding Hauser Lake gne iss near the base of the Spokane dome mylonite zone, which arches acro ss this part of the Priest River complex. We conclude that the Spokane dome mylonite zone lies above the Archean basement-cover contact and that it was, in part, equivalent to the basal decollement of the Rocky Mountain fold and thrust belt. New U-Pb dates on metamorphic monazite and xenotime reveal peak metamorphism at ca. 72 Ma, compatible with m ovement along the Spokane dome mylonite zone at that time. The Archean basement could be interpreted as the western extension of the Hearne province, or a new Archean basement terrane separated from the Hearne province by an Early Proterozoic suture. The unique assemblage of 2.65 Ga basement, similar to 1.58 Ga felsic intrusive rocks, and the Middl e Proterozoic Belt Supergroup can be used as a piercing point for the identification of the conjugate margin to Laurentia. Our new dating su pports previous correlations of Australia's Gawler craton (2.55-2.65 G a) and its 1590 Ma plutons with the Priest River complex basement gnei sses. The Priest River complex basement may be a piece of eastern Aust ralia stranded during rifting of the supercontinent Rodina in the Late Proterozoic.