ABSTRACT Outcrops within the broad expanse of the Minnesota River Valley in southwestern Minnesota mark the southernmost exposures of the Archean Superior Province of the Canadian Shield. Despite their relatively restricted exposure, the Meso- to Paleoarchean gneisses in the Minnesota River Valley have received considerable attention due to both their antiquity and their complexity. The rocks exposed include the migmatitic Morton and Montevideo granitic gneisses, schistose to gneissic amphibolite, metagabbro, and paragneiss. The units have undergone upper amphibolite to granulite facies metamorphism, multiple periods of folding, and intrusion by a weakly foliated Neoarchean granitic unit (the Sacred Heart Granite) and Paleoproterozoic mafic dikes and adamellite granite. Classic geochronologic studies of the Minnesota River Valley gneiss terrane from the 1960s through the 1970s used K-Ar, Rb-Sr, and U-Pb zircon isotopic techniques to establish the antiquity of the gneisses and general aspects of the geologic history of the terrane. However, more recent U-Pb SHRIMP (sensitive high-resolution ion microprobe) zircon geochronology has considerably refined our understanding of the complex history of the gneiss terrane. These studies indicate that the oldest units in the Minnesota River Valley terrane crystallized ca. 3500 Ma, but the rocks subsequently saw new zircon growth associated with events at ca. 3440, 3385, 3140, and locally 3080 Ma. The Archean history of the terrane culminated with high-grade metamorphism ca. 2619 Ma and intrusion of the Sacred Heart Granite at 2604 Ma. In addition to visiting classic outcrops of the Morton and Montevideo Gneiss, this field trip includes stops at each of the major gneissic rock units in the Minnesota River Valley. We will examine field relationships that are the basis for both our general understanding of the deformation and metamorphic history of the gneiss terrane and the sampling strategies for our recent geochronologic and ongoing isotopic studies.

In recent years, a rapidly expanding database, especially in sensitive high-resolution ion microprobe (SHRIMP) geochronology, has led to significant advances in understanding of the Precambrian tectonic evolution of the Grenville Province, including its Adirondack outlier, and the Mesoproterozoic inliers of the Appalachians. Based upon this information, we review the geochronology and tectonic evolution of these regions and significant similarities and differences between them. Isotopic data, including Pb isotopic mapping, suggest that a complex belt of marginal arcs and orogens existed from Labrador through the Adirondacks, the midcontinent, and into the southwest during the interval ca. 1.8–1.3 Ga. Other data indicate that Mesoproterozoic inliers of the Appalachians, extending from Vermont to at least as far south as the New Jersey Highlands, are, in part, similar in composition and age to rocks in the southwestern Grenville Province. Mesoproterozoic inliers of the Appalachian Blue Ridge likewise contain some lithologies similar to northern terranes but exhibit Nd and Pb isotopic characteristics suggesting non-Laurentian, and perhaps Amazonian, affinities. Models invoking an oblique collision of eastern Laurentia with Amazonia are consistent with paleomagnetic results, and collision is inferred to have begun at ca. 1.2 Ga. The collision resulted in both the ca. 1190–1140 Ma Shawinigan orogeny and the ca. 1090–980 Ma Grenvillian orogeny, which are well represented in the Appalachians. Several investigators have proposed that some Amazonian Mesoproterozoic crust may have been tectonically transferred to Laurentia at ca. 1.2 Ga. Data that potentially support or contradict this model are presented.

The St. Francois Mountains are the principal exposure of an extensive terrane of Precambrian silicic volcanic and epizonal intrusive rocks that extends from central Wisconsin and northern Ohio at least to the Texas Panhandle. Most of the volcanic rocks exposed in the St. Francois Mountains are ash-flow tuff intruded by rocks of granitic composition that have petrographic features indicative of shallow crystallization. U-Pb measurements on suites of cogenetic zircons from most of these rocks yield ages of about 1,500 m.y., but zircons from the Munger Granite Porphyry are apparently only 1,400 m.y. old. Rb-Sr whole-rock isochrons from the plutonic and volcanic rocks yield ages close to 1,300 m.y. Calculated Rb-Sr model ages for individual samples are inversely related to Rb/Sr and are directly related to Sr concentration. The data suggest that Rb-Sr ages have been lowered by Sr loss. It is concluded that the U-Pb ages of the zircon suites are the best estimates of the time of crystallization of the rock bodies and that the Rb-Sr ages are related to an event in the subsequent history of the rocks during which Sr was lost. This event was most likely a period of extensive hydrothermal alteration, possibly the one that caused emplacement of iron ore bodies. The petrographic similarity of rocks in the larger, mostly buried terrane to those of the St. Francois Mountains suggests that Rb-Sr ages of these rocks may also be too low if the rocks experienced similar subsequent alteration. The rocks of the St. Francois Mountains are similar in age to numerous anorogenic plutons in Wisconsin, the Rocky Mountains, and elsewhere in the Southwest.