Proterozoic mafic magmatic rocks exposed along the western side of North America, or western Laurentia, from Kimberley, British Columbia, through to northwestern Wyoming have been previously divided into two large igneous provinces: the ca. 1460 Ma Moyie-Purcell and the ca. 780 Ma Gunbarrel large igneous provinces. New geochemical analysis from this study demonstrates that there are additional intraplate mafic magmatic rocks present. Distinguishable by variable normalized rare earth element patterns combined with differing slopes on a binary Ti versus V plot, there are 17 identifiable geochemical signatures in the 307 whole-rock and trace-element analyses from this study. Only seven of these signatures can be linked to the ca. 1460 Ma Moyie-Purcell large igneous province, and one signature to the 780 Ma Gunbarrel large igneous province. This study has identified two groups of intrusions with distinct geochemical signatures previously linked with the ca. 1460 Ma Moyie-Purcell large igneous province but now recognized to be separate events, a single unique geochemical signature with a U-Pb age correlative with the Moyie-Purcell large igneous province and seven other heretofore unidentified signatures interpreted to belong to additional undated events.

The Neihart Quartzite and LaHood Formation are the lowermost units exposed in the Helena embayment, which forms the eastern and southeastern margins of the Belt Basin. Ages of detrital zircons from the Neihart Quartzite (quartz arenite) and a range of lithologies in the LaHood Formation (conglomerates to arkoses to siltstones) show that these units do not share a common provenance. The dominant provenance is Paleoarchean for the LaHood Formation and Paleoproterozoic for the Neihart Quartzite. Provenance is further constrained by the geochemistry and U-Pb ages of zircons from cobbles from the classic LaHood conglomerate in Jefferson Canyon (Tobacco Root Mountains), ages of Paleoproterozoic crystalline basement in the Beaverhead-Tendoy Mountains (1.8–2.45 Ga), and elemental and Sm-Nd isotopic data for select samples of both sedimentary rocks and crystalline basement within the basin. These data show a pronounced lack of detritus from abundant, proximal Neoarchean (2.7–2.9 Ga) and Paleoproterozoic (1.9–2.5 Ga) crystalline basement exposed in Laramide uplifts and the soles of Sevier-style thrust faults within and near the basin. Analyses of detrital mineral assemblages in the Lower Belt Supergroup units clearly indicate that the finer-grained portions of the LaHood Formation were not locally derived, based on abundant white mica in sections overlying tonalite-trondhjemite-granodiorite (TTG) basement and lack of amphibole in units overlying hornblende tonalites. Significant fractionation also exists between sand- and cobble-size components in conglomerate of the LaHood Formation in terms of elemental abundances, isotopic compositions, and the U-Pb ages of zircons. Stratigraphically, the differences in the ages of the youngest zircons in all LaHood Formation samples and the Neihart Quartzite (1.71 Ga, Neihart; 1.78 Ga, LaHood) do not refute any proposed stratigraphic correlations. Nonetheless, age spectra of detrital zircons from the Neihart Quartzite, all LaHood lithologies, and previously published data for the Newland Formation show distinctions of provenance and an apparent lack of interaction among the sediment-supply systems of these three formations. This contrast suggests that distinct, likely fault-bounded, sedimentologically restricted subbasins characterized the initial stages of development of the eastern Belt Basin along the Perry line (southeastern margin of the Helena embayment), in the manner of a modern, but partially submerged, Basin and Range topography. The time of development of this topography is not clear, but it may have been related to the collapse phase of the Great Falls orogeny at ca. 1.7 Ga for the Helena embayment. The primary, north-south–trending Belt Basin also developed subsequent to the Great Falls orogeny along the western paleomargin of the newly amalgamated Wyoming–Medicine Hat–Hearne craton.