CHARACTERIZATION OF CRYSTALLINE ROCKS IN THE LAKE SUPERIOR REGION, USA: IMPLICATIONS FOR NUCLEAR WASTE ISOLATION

The Lake Superior region (Wisconsin, the Upper Peninsula of Michigan, and Minnesota) contains 41 Precambrian crystalline (medium- to coarse-grained igneous and high-grade mstamorphic) rock complexes comprising 64 individual but related rock bodies with known surface exposures. Each complex has a map area greater than 78 km2. About 54% of the rock complexes have areas of up to 500 km2, 15% fall between 500 km2 and 1000 km2, 19% lie between 1000 km2 and 2500 km2, and 12% are over 2500 km2. Crystalline rocks of the region vary widely in composition, but they are predominantly granitic. Repeated, thermo-tectonic events have produced early Archean gneisses, migmatites, and amphibolites with highly tectonized fabrics that impart a heterogeneous and anisotropic character to the rocks. Late Archean rocks are usually but not invariably gneissose and mignatitic. Proterozoic rocks of the region include synorogenic (foliated) granitic rucks, anorogenic (non-foliated) granites, and the layered gabbro-anorthosite-troctolite intrusives of the rift-related Keweenawan igneous activity. Compared with the Archean rocks of the region, the Proterozoic bodies generally lack highly tectonized fabrics and have more definable contacts where visible. Anorogenic intrusions are relatively homogeneous and isotropic. On the basis of observed geologic characteristics, postorogenic and anorogenic crystalline rock bodies located away from recognized tectonic systems have attributes that make them relatively<br>more desirable as a possible site for a nuclear waste repository in the region. This study was conducted at Argonne National Laboratory under the sponsorship of the U.S. Department of Energy through the Office of<br>Crystalline Repository Development at Battelle Memorial Institute, Columbus, Ohio.