Five early Proterozoic volcanic terranes of the following ages occur in the southwestern United States: 1760-1800 Ma, 1730-1740 Ma, 1720 Ma, 1680-1700 Ma, and 1650 Ma. Volcanics are typically bimodal in composition and, for the most part, represent submarine eruptions. In terms of the least mobile incompatible element distributions, basalts from the 1760-1800 Ma and 1720 Ma terranes are similar to basalts in evolved oceanic arcs and associated incipiently opened back-arc basins. Basalts from the other three terranes resemble basalts from continental-margin arcs and associated back-arc basins. Andesites are important volcanic components at only two localities and they are similar chemically to andesites from evolved oceanic and continental-margin arcs. Felsic volcanics define two major geochemical populations. Those from the 1760-1800 Ma and 1720 Ma terranes, which have low HFSE and heavy REE contents, are similar in composition to felsic volcanics from modern arc systems while those from the other three terranes, which have relatively high HFSE and heavy REE contents, resemble felsic volcanics from rifts or back-arc basins in or near continental crust. The 1760-1800 Ma terrane appears to reflect a succession of oceanic arcs accreted to the Archaean craton from the southeast and the 1730-1740 Ma terrane, an incipient back-arc basin formed within the 1760-1800 Ma terrane. The 1720 Ma terrane is interpreted as an evolved back-arc basin in which depleted mantle is tapped. The 1680-1700 Ma and 1650 Ma terranes are remnants of an extensive continental-margin arc system and associated back-arc basins. Bimodal volcanism may reflect differences in preservation of arc and back-arc successions.