Abstract
During the Mesoproterozoic, western North America was connected to another continental mass of uncertain identity as part of the supercontinent Nuna. The ca. 1.4 Ga Belt Supergroup provides essential clues to the identity of this other continent, and here we present new detrital mineral age and geochemistry data for the Belt Supergroup to better understand its provenance and explore the implications for the configuration of Nuna. Detrital zircon age, trace element compositions, and Hf isotope compositions in the members of the Prichard Formation and formations of the Ravalli Group, together representing the lower portion of Belt stratigraphy, are remarkably uniform, with a main age peak at 1.61 Ga and juvenile Hf isotope compositions. Locally abundant detrital monazite in the Ravalli Group have age peaks at 1.67 Ga, 1.60 Ga, and 1.50 Ga. The Missoula Group and Lemhi subbasin units, representing the upper units of the Belt Supergroup, show a main zircon age population between 1.8 Ga and 1.7 Ga, with variable but generally juvenile Hf isotope compositions, and a minor population between 1.45 Ga and 1.40 Ga, also with juvenile Hf isotope compositions. Unusually high U content in zircon from the Garnet Range Formation of the Missoula Group may provide a new tool for correlation with the Lemhi subbasin. Both detrital monazite and rutile were recovered from several Missoula and Lemhi samples, with monazite recording metamorphic events at 2.43 Ga, 1.78 Ga, 1.73 Ga, and 1.68 Ga and rutile interpreted to record slow cooling from conditions ranging from amphibolite to granulite facies.
Results for the Prichard Formation and Ravalli Group are consistent with a modified Southwest U.S.−East Antarctic (SWEAT) Nuna model whereby the South Australia Craton is adjacent to the Belt basin, providing most Prichard and Ravalli detritus, while the North Australia Craton is linked to northwest Canada, and East Antarctica is linked to southwest Laurentia. Tasmania likely was positioned between the Belt basin and the South Australia Craton based on the similar detrital zircon age and Hf isotope compositions in the Rocky Cape Group, the presence of contemporaneous mafic magmatism in the lower parts of both sedimentary sequences, and comparably aged post-depositional metamorphic mineral growth, and this connection likely persisted until at least 1.1 Ga. Results for the Missoula Group and Lemhi strata are consistent with sediment derived from the Mojave and Yavapai terranes in the southwest United States, with very limited local input from the Wyoming province.