Geochronology of the midcontinent region, United States: 4. Eastern area

Radiometric ages and geological and geophysical data demonstrate that the Precambrian rocks of Iowa, Illinois, Indiana, Ohio, and adjacent areas have had a complex history of metamorphism, plutonism, and volcanism. Iowa is underlain by a plutonic complex of gneiss, schist, and granite that probably dates from the Penokean orogeny (1600–1800 m.y. ago). The complex was intruded by gabbroic rocks having a minimum age of 1300 m.y. A belt of Keweenawan igneous and sedimentary rocks trending southwest across Iowa is delimited on the basis of gravity and magnetic anomalies. Ages determined for micas from nearby rocks reflect this igneous activity at approximately 1100 m.y. A terrane of granite and rhyolite extends from eastern Iowa to western Ohio, where it is terminated by metamorphic rocks which are the subsurface extension in Michigan and Ohio of the Grenville province of the Canadian shield. Granites in eastern Iowa, northern Illinois, and northwestern Indiana were emplaced 1350 m.y. or more ago. Rhyolite and trachyte in western Ohio are assigned to 1260 m.y. Granite in Indiana and granite and rhyolite in Illinois may belong to this period. Gravity and magnetic anomalies suggest that the Precambrian basement of the Michigan basin may be composed of Keweenawan igneous rocks. Micrographie granite in Indiana is tentatively assigned to the middle Keweenawan, approximately 1100 m.y. old. Mica ages for rocks from the basement in Ohio are similar to those for the Grenville province of Canada.