PALEOPROTEROZOIC DETRITAL-ZIRCON AGE POPULATION AND ND ISOTOPE SIGNATURE IN ROCKS OF THE MESOPROTEROZOIC LEMHI GROUP, SALMON RIVER AND LEMHI RANGES, EAST-CENTRAL IDAHO AND SNOWSLIP FORMATION, SW MONTANA

Similar provenance of Mesoproterozoic strata in Idaho's Salmon River Mountains with the Missoula Group of the Belt Supergroup in southwest Montana has been proposed on the basis of similar detrital zircon age distribution patterns (Link et al., 2007). We test this hypothesis by analyzing new detrital zircon and bulk rock Nd isotope samples from the Lemhi Group and correlative strata. Zircon U-Pb ages obtained at the Arizona LaserChron facility support the similar provenance hypothesis. Age spectra from Mesoproterozoic sedimentary rocks of the Lemhi Group, east-central Idaho, are characterized by a dominant population of Paleoproterozoic (~1730 Ma) zircons (~70 % of 100 grain samples) in several samples. The Inyo Creek and Big Creek formations of the Lemhi Group, as well as the overlying Swauger Formation have statistically similar age spectra that meet the K-S test. The West Fork Formation of the Lemhi Range has a grain-population peak at 1760 Ma, but has a wider spread of ages; this spectra is statistically distinct from the Inyo Creek, Big Creek, and Swauger formations. These new results support correlations proposed by previous workers between the Apple Creek, Gunsight, and Swauger formations in the Lemhi Range with the Yellowjacket and Hoodoo formations from the Salmon River Mountains. Our detrital zircon age spectrum from the Snowslip Formation of the lower Missoula Group, Belt Supergroup also contains a peak at 1760 Ma, but includes more (14%) Archean zircons than do the Lemhi and Salmon River Mountain samples.

New bulk rock Nd isotope analyses on argillites and siltites from the Big Creek Formation and the Gunsight Formation of the Lemhi Range, and the Yellowjacket Formation of the Salmon River Mountains yield ε_Nd(i) of +0.1, -1.2, and -1.6 respectively, consistent with derivation from dominantly Paleoproterozoic sources. In contrast, an ε_Nd(i) value of -5.6 was found for the West Fork Formation, suggesting a significant Archean-age component, despite the paucity of Archean detrital zircons.

With the exception of the West Fork Formation sample with a more negative initial ε_Nd value and the Snowslip Formation with a larger detrital zircon Archean component, the similarity of detrital zircon age spectra and Nd isotopic compositions is striking and may suggest the dominant source of detritus was identical in all groups.