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Abstract

High–precision U–Pb TIMS, SHRIMP, and LA–ICPMS dating of magmatic and detrital zircons from the core of the Clearwater complex, northern Idaho, U.S.A., provide new ages and new tectonic interpretations for potential Precambrian basement rocks in this part of the Cordillera. The Boehls Butte anorthosite, which is exposed in lens–like masses within the core of the Clearwater complex, crystallized at 1787 ± 2 Ma. Amphibolites, which are intercalated with the anorthosite, crystallized during distinctly different magmatic episodes around 1587 Ma, 1467 Ma, and 1453 Ma. These dates better define the age of Precambrian basement in this region and document a new exposure of 1580 Ma igneous rocks along the western edge of the North American craton. Surrounding the anorthosite are metasedimentary rocks (Boehls Butte Formation) that have been interpreted as predating the anorthosite and the Mesoproterozoic BeltPurcell Supergroup. Detrital zircons from these metasedimentary rocks yield age populations that are predominantly Paleoproterozoic with some Archean grains. The youngest concordant 207Pb/206Pb ages are between 1597 and 1761 Ma, well after crystallization of the anorthosite. On this basis, we conclude that most of the rocks once assigned to the Boehls Butte Formation are better correlated with the lower part of the Belt–Purcell Supergroup. The only part of the Boehls Butte Formation that remains potential basement is the Al–Mg– rich schist that borders the masses of anorthosite. We propose that the anorthosite and bordering Al–Mg schists are displaced tectonic slivers that were juxtaposed against the metasedimentary rocks by shear zones that predate peak metamorphism. This zone of shear may be related to the basal decollement for the Rocky Mountain fold–and–thrust belt.

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