Abstract

Knowledge of the Precambrian history and crustal architecture of western North America is limited by the restricted extent of outcropping basement due to Phanerozoic sedimentary cover and magmatic disruption. Inherited zircon components in the magmatic rocks of the Idaho batholith provide a window into this largely obscured crust and are used here to constrain the architecture of the Precambrian basement of the northern U.S. Cordillera. The Cretaceous–Paleocene Idaho batholith is divided into a southern Atlanta lobe and northern Bitterroot lobe, and U-Pb ages of the inherited zircon components vary systematically with geographic location both between and within the lobes. The southern half of the Atlanta lobe of the batholith is dominated by two sharp age peaks at ca. 2.55 Ga and 670 Ma, with the Archean peak more prevalent in the southernmost samples. Since both age peaks can be correlated with known local bedrock exposures and xenoliths (in the case of the Archean), they are interpreted as representing the ages of subsurface igneous or metaigneous rocks. Late Jurassic and Early Cretaceous inherited zircons are also observed in a few samples, suggesting that localized magmatism of this age occurred in the area prior to batholith formation. The northern half of the Atlanta lobe yields a wide range of Proterozoic inherited ages broadly similar to the detrital zircon age spectra of local and regional Neoproterozoic (Windermere) metasedimentary rocks. Small plutons between the lobes of the batholith show a single inherited zircon peak at 1.38 Ga, identical to the age of orthogneisses that they intrude. Inherited zircons from the Bitterroot lobe yield a heterogeneous age distribution between 1.90 and 1.35 Ga, similar to the detrital zircon age spectra of much of the Belt Supergroup.

The extent of Archean inheritance in the southern Atlanta lobe suggests that Archean lithosphere extends as far west as the continental margin and further north than previously thought, making it a potentially important piercing point in reconstructions of the Rodinian and Columbian supercontinents. Further, in situ Hf isotopic analyses of the Archean cores provide evidence for previously existing pre-3.5 Ga crustal components in this Archean terrane, suggesting a tie to the neighboring Wyoming Province to the east.

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