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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.

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