We show that a norite at the contact between a mafic diorite intrusion and a pelitic granulite gneiss formed via a reaction similar to that first postulated by N.L. Bowen in 1928, i.e., clinopyroxene (Cpx) + sillimanite = anorthite (An) + orthopyroxene (Opx). Major element, compatible trace element, and isotope chemistry are consistent with a mixed pelite-diorite origin for the norite. The norite consists of bytownite (An71) and Opx with minor accessory apatite and oxide minerals. The bytownite contains abundant, optically continuous quartz inclusions. The bytownite and the Opx are interpreted as having formed by reaction between pelite and diorite at the contact. The norite feldspar is notably calcic compared to plagioclase in the local diorite and pelite. Except for a slight Al enrichment, the Opx in the norite has major element chemistry similar to that of the Opx in the local diorite. However, norite Opx and plagioclase (Pl) are significantly enriched in Ga. The Ga and major element compositions of the Opx and bytownite are decoupled from the bulk composition of the norite, suggesting reaction rather than bulk compositional control on phase chemistry. Ga enrichment requires input from an aluminous phase. The presence of cocrystallizing quartz (Qz) and bytownite in the norite suggests that the aluminum source is cordierite in the gneiss via the reaction 5 Pl (An40) + 5 Cpx + 2.5 cordierite = 10 Pl (An72) + 10 Opx + 2.5 Qz. Enrichments in Y, Sc, and heavy rare earth elements in the Opx are consistent with garnet involvement. However, these elements exist in sufficient quantities in the precursor Cpx to account for abundances in most of the Opx. Although one can postulate a mixing origin for the norite on the basis of whole-rock chemistry, it is only through the analysis of minerals that the decoupling of bulk and phase chemistry can be identified and the phases involved in the assimilation reaction be constrained. The documentation of this classic reaction highlights the importance of reactive processes during assimilation.