Early Proterozoic supracrustal rocks exposed in west-central Colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments. Three successions are recognized. The Dubois succession (1770-1760 Ma) is comprised chiefly of amphibolites and felsic volcanics. The amphibolites exhibit low concentrations of light REE and HFSE and are broadly similar in composition to tholeiites from immature island arcs and, in some respects, to NMORB. The Cochetopa succession (1745-1730 Ma) includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible-element enrichment similar to basalts from evolved island-arc systems and associated back-arc basins. The Black Canyon succession is coeval with the Cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite. These amphibolites exhibit significant incompatible-element enrichment similar to basalts from continental-margin arc systems. Geochemical modeling indicates that the amphibolites evolved by closed-system fractional crystallization of depleted (for the Dubois succession) to variably enriched parental basalts (for the Cochetopa and Black Canyon successions), chiefly at shallow depths. Incompatible-element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted (1770-1760 Ma) to variable but slightly enriched (1745-1730 Ma). A provisional model calls upon the Dubois succession forming in an island-arc system followed by collision with the Archean Wyoming Province at ∼ 1750 Ma. The Cochetopa succession was deposited in an intra-arc basin developed within a new continental-margin arc, and the Black Canyon succession formed in a back-arc basin associated with this arc.