Abstract

U-Pb ages of detrital zircons from metasedimentary rocks of the Sawtooth metamorphic complex (SMC) are key for identifying the stratigraphic age of metamorphosed strata within the Idaho batholith region of the Cordilleran orogen. The SMC is an exposure of medium- to high-grade metasedimentary rocks surrounded by the Idaho and Sawtooth batholiths. U-Pb ages of detrital zircons from SMC quartzites and quartzofeldspathic gneisses yield two distinctive age spectra consisting of primary 2900–2510 Ma and 1990–1760 Ma zircons for one group and 1870–1670 Ma, 1490–1330 Ma, and 1220–1020 Ma zircons for the other group. The suite of samples also yields a small number of zircons with concordant Cambrian and Neoproterozoic ages. Statistical and visual comparisons of age spectra with detrital-zircon data from Proterozoic and Paleozoic strata deposited on the western passive margin of Laurentia suggest that the SMC rocks were deposited in the Cambrian and Middle Ordovician. The identification of lower Paleozoic shelf strata in the SMC, along with similar sections recently identified in the Stibnite and Gospel Peaks inliers of the Idaho batholith, suggest that the Cordilleran passive margin sequence was continuous along the western margin of Laurentia, including the area occupied by the Idaho batholith.

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