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Explanatory text to accompany geologic map of southern Tobacco Root Mountains, Madison County, Montana

By
Charles J. Vitaliano
Charles J. Vitaliano
Department of Geology, Indiana University, Bloomington, Indiana 47405, USA
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H. Robert Burger, III
H. Robert Burger, III
Department of Geology, Smith College, Northampton, Massachusetts 01060, USA
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William S. Cordua
William S. Cordua
Department of Plant & Earth Science, University of Wisconsin, River Falls, Wisconsin 54022, USA
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Thomas B. Hanley
Thomas B. Hanley
Department of Earth Science, Columbus College, Columbus, Georgia 31907, USA
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David F. Hess
David F. Hess
Department of Geology, Western Illinois University, Macomb, Illinois 61445, USA
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Forrest K. Root
Forrest K. Root
Ensearch, P.O. Box 4815, Midland, Texas 79701, USA
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Published:
January 01, 2004

The core of the Tobacco Root Mountains, a domal uplift of southwestern Montana, is composed of regionally metamorphosed Archean rocks that have been intruded by diabase dikes of late Precambrian age and by igneous rocks of late Mesozoic and Cenozoic age. The Archean rocks, originally a stratiform sequence of epiclastic, volcaniclastic, and carbonate sedimentary rocks, have been metamorphosed to amphibolite-granulite rank with attendant migmatization. The distribution of the metamorphic rocks and the later intrusions is shown on the map.

In this report to accompany the map, the nature of the crystalline rocks is described briefly. Two thermal events affected the area in Precambrian time, one about 2.7 b.y. B.P. and the second at about 1.6 b.y. B.P. A third event may have begun in very late Precambrian time, but is most strongly represented by changes of late Mesozoic age.

Because of the structural complexities, correlation of the metamorphic rocks in the Tobacco Root Mountains with Precambrian sequences elsewhere in southwestern Montana is extremely difficult.

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Contents

GSA Special Papers

Precambrian Geology of the Tobacco Root Mountains, Montana

John B. Brady
John B. Brady
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H. Robert Burger
H. Robert Burger
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John T. Cheney
John T. Cheney
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Tekla A. Harms
Tekla A. Harms
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Geological Society of America
Volume
377
ISBN print:
9780813723778
Publication date:
January 01, 2004

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