PROVENANCE OF THE UPPER BELT SUPERGROUP AND LEMHI GROUP: CONSTRAINTS FROM HF AND ND ISOTOPES

U-Pb ages on detrital-zircon grain populations from 12 samples of Mesoproterozoic Missoula and Lemhi Groups of western Montana and east-central Idaho yield unimodal spectral peaks derived from a major 1660 to 1780 Ma magmatic source. Six samples are statistically identical and combined yield an age peak of 1734 Ma on 330 zircon grains. Smaller syn-Belt (1470-1400 Ma) and Archean populations are present in some samples. Hf isotopes from 1675 to 1775 Ma Missoula Group detrital zircons have a wide range in εHf(i) (+9 to -12); with a similar range for Lemhi Group strata ( εHf(i) between +8 and -7). Thus although the ages of the zircons are unimodal, the Hf isotopes show major variance. Syn-Belt (1470-1400 Ma) grains from the Bonner Formation, Missoula Group have a wide range in εHf(i) values between +5 and -9, incompatible with direct derivation from A-type magmatic sources in the Mojave and Yavapai terranes. Finally, new bulk rock Nd isotopes from Lemhi Group argillites which show unimodal detrital zircon spectra yield a range in εNd(i) between +1.1 and -5.4, also incompatible with direct derivation from the Mojave and Yavapai terranes.

The new Hf isotope data and the bulk-rock Nd data support an isotopically heterogeneous 1660-1780 Ma source crystallized from mixed Paleoproterozoic juvenile mantle and Archean crust. Several km of south-derived Missoula and Lemhi Group coarse-grained feldspathic strata suggest a proximal source to the southwest. This provenance is best explained by a now-covered southwestern part of the Selway Terrane or a rifted fragment of Laurentia, rather than the Mojave-Yavapai-Mazatzal terranes far to the south.