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Geology and geochronology of Paleoarchean gneisses in the Minnesota River Valley

By
R.L. Bauer
R.L. Bauer
Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA
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M.E. Bickford
M.E. Bickford
Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA
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A.M. Satkoski
A.M. Satkoski
Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA
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D.L. Southwick
D.L. Southwick
Minnesota Geological Survey, University of Minnesota, St. Paul, Minnesota 55114, USA
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S.D. Samson
S.D. Samson
Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA
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Published:
January 01, 2011

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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Contents

GSA Field Guide

Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America

James D. Miller
James D. Miller
Department of Geological Sciences and Precambrian Research Center University of Minnesota-Duluth Duluth, Minnesota 55812 USA
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George J. Hudak
George J. Hudak
Precambrian Research Center Natural Resources Research Institute 5013 Miller Trunk Highway Duluth, Minnesota 55811 USA
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Chad Wittkop
Chad Wittkop
Department of Chemistry and Geology Minnesota State University Mankato, Minnesota 56001 USA
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Patrick I. McLaughlin
Patrick I. McLaughlin
Wisconsin Geological and Natural History Survey Madison, Wisconsin 53705 USA
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Geological Society of America
Volume
24
ISBN electronic:
9780813756240
Publication date:
January 01, 2011

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