The Belcher Islands of eastern Hudson Bay expose a continuous sequence of Proterozoic continental and shallow-marine sediments, and volcanics. The latter are represented by two sequences of continental basalts, the older Eskimo Formation and the younger Flaherty Formation. The lavas of both formations are tholeiitic basalts (MgO, $<$ 9 wt%) which, in each formation, can be divided into two groups based on their high field strength element concentrations. Both groups of Eskimo lavas have anomalously low Nb concentrations and fractionated rare earth element (REE) patterns. The Flaherty lavas have relatively flatter REE profiles and higher Nb contents. The chemical variations observed in most of the Eskimo lavas are consistent with an assimilation/fractional crystallization process involving a lower continental crust contaminant. The major and most trace element trends of the Flaherty lavas are attributed to gabbroic closed-system crystal fractionation of two parental liquids which are interpreted to represent varying degrees of partial melting of a common mantle source region. Variations in ratios of REE, however, require that the Flaherty lavas evolved under open-system conditions, possibly in a replenished magma chamber or by selective contamination.