Paleogeographic implications of non–North American sediment in the Mesoproterozoic upper Belt Supergroup and Lemhi Group, Idaho and Montana, USA

Abstract

A non-North American provenance for the lower Belt Supergroup of North America has been used to support various pre-Rodinian paleogeographic reconstructions. Unlike the lower Belt Supergroup, most upper Belt Supergroup provenance studies have inferred Laurentian sediment sources. We test this hypothesis by analyzing U-Pb and Lu-Hf isotopes on detrital zircons, and whole-rock Nd isotopes from the Missoula (upper Belt Supergroup) and Lemhi Groups, and comparing to possible Laurentian sources. Detrital zircons from 11 sandstones analyzed show dominant ages between 1680 and 1820 Ma. These zircons are predominantly magmatic in paragenesis. Belt Supergroup–aged (1400–1470 Ma) and 2400–2700 Ma populations represent minor components. Lu-Hf isotopic analyses for 1675–1780 Ma Missoula Group and Lemhi Group detrital zircons range from eHf(i) +9 to –12 and +8 and –7, respectively. Belt Supergroup–aged grains from the Bonner Formation, Missoula Group, have eHf(i) values between +5 and –9, exceeding coeval ranges from the Mojave and Yavapai terranes [eHf(i) between +5 and 0]. Whole-rock Nd isotopes from Lemhi Group argillites yield a range in eNd(1400) between +1.1 and –5.9. Immature feldspathic sediment, nearly unimodal detrital zircon spectra, and dissimilar Belt Supergroup–aged zircon Hf signatures suggest that distal portions of the Yavapai and Mojave terranes intruded by A-type magmas were not the source for the Missoula and Lemhi Groups. Instead, a slightly modified Mesoproterozoic proto-SWEAT (southwestern United States and East Antarctica) model can best account for the sedimentologic and isotopic characteristics of the Missoula and Lemhi Groups. An alternative model with a source from southeastern Siberia and the Okhotsk Massif is less preferred.