Differentiation in Magmas of the Keweenawan Series

The Greenstone flow, a thick, basaltic flow of the Keweenawan series of Michigan, offers a unique opportunity for the study of differentiation in basaltic magma. Differentiation trends in the magma of the Greenstone flow during crystallization have been computed according to two hypotheses as to the manner in which the flow solidified. It was found that the ratio of iron oxides to magnesia increased in the magma during crystallization and that there was a slight to moderate absolute enrichment of the magma in Fe, Ti, Mn, Cu, P, Si, Na, and K until the flow was nearly solid. A residual magma was produced that was rich in silica and alkalies and deficient in the remaining constituents. Magma trends in the lavas of the Keweenawan series and in genetically related intrusives have also been studied and compared with those found in the Greenstone flow.