Major and trace element data are presented for 2.2 Ga Proterozoic diabase sills from across the Nipissing magmatic province of Ontario. In situ differentiation of the magma coupled with assimilation of Huronian Supergroup roof sediments is responsible for the variation in composition between quartz diabase and granophyric diabase seen within many of the differentiated intrusions. Uniform trace element and isotope ratio signatures, such as La/Sm (2.8 – 3.7) and εNdCHUR (−2.7 to −5.9) characterize chilled margins and undifferentiated quartz diabases. These chemical signatures suggest the existence of a single magma source that was parental to intrusions throughout the magmatic province; this magma has higher La/Sm and lower Ti/Y than primitive mantle and is displaced towards the composition of shales. Most chilled diabases and quartz diabases have a similar Mg# (0.64 and 0.60) and Ni content (98 and 127 ppm), and it is argued that the magma differentiated at depth and was emplaced as a uniform low-Mg magma. The Wanapitei intrusion and Kukagami Lake sill are an exception in that although the quartz diabase has La/Sm similar to the Nipissing magma type, which suggests that they came from the same source, the Mg# (0.68–0.71) and Ni content (130–141 ppm) are higher, which may suggest that they are either slightly more primitive examples of the normal Nipissing magma or that cumulus hypersthene has been resorbed. The light rare earth element enriched signature of the Nipissing magmas was perhaps introduced from the continental crust as the magma migrated from the mantle to the surface, but a remarkably constant and large amount (>20%) of crustal contamination would be required. An addition of 1 –3% shale to the source of a transitional mid-ocean ridge basalt type magma can broadly reproduce the compositional features of the Nipissing magma type. The source characteristics were perhaps imparted during subduction accompanying the terminal Kenoran orogeny.