Keweenawan basalt analyses having high Al2O3 and CaO contents fall on a linear trend on a CaO versus A12O3 plot. Using plagioclase and augite compositions from the gabbroic anorthosites of the Duluth complex, it is possible to explain the basalt trend by separation of gabbroic anorthosites from a melt rich in A12O3 and CaO. Calculations of the amount of solidification indicate that as much as 60 percent solid material separated before other gabbroic rock types crystallized from the melt. Trends of MgO, FeO, and K2O suggest that another fractionation may have preceded the separation of gabbroic anorthosite. An important conclusion from these arguments is that the multiple intrusions of the Duluth complex represent the crystalline material in magma chambers from which the Keweenawan flows of Lake Superior's North Shore were derived.