The Precambrian rocks of northern Michigan, Wisconsin, and Minnesota have undergone widespread regional metamorphism, with thermal zoning recognizable by progressive mineralogic changes in their on formations and associated rocks. Oxygen isotopic compositions of magnetite, hematite, quartz, and calcite from some of the metamorphosed iron formations have been measured. Mineral pairs such as quartz-hematite and calcite-hematite have isotopic differences that bear consistent relation to inferred thermal gradients in the lower metamorphic zones. By relating the observed isotopic fractionation to the experimentally derived fractionation curve for CaCO₃-H₂O, it is possible to estimate that apparent temperatures of last...