Skip to Main Content
Book Chapter

Geology and geochemistry of the Spuhler Peak Metamorphic Suite

By
H. Robert Burger
H. Robert Burger
1
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
Search for other works by this author on:
William H. Peck
William H. Peck
2
Department of Geology, Colgate University, Hamilton, New York 13346, USA
Search for other works by this author on:
Kathleen E. Johnson
Kathleen E. Johnson
3
Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148, USA
11
Present address: P.O. Box 8352, Monterey, California 93940, USA, volcan@mac.com.
Search for other works by this author on:
Kara A. Tierney
Kara A. Tierney
4
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
Chris J. Poulsen
Chris J. Poulsen
5
Carleton College, Northfield, Minnesota 55057, USA
Search for other works by this author on:
Pamela Cady
Pamela Cady
6
Colorado College, Colorado Springs, Colorado 80903, USA
Search for other works by this author on:
Josh Lowell
Josh Lowell
6
Colorado College, Colorado Springs, Colorado 80903, USA
Search for other works by this author on:
William A. MacFarlane
William A. MacFarlane
6
Colorado College, Colorado Springs, Colorado 80903, USA
Search for other works by this author on:
M. Jennifer Sincock
M. Jennifer Sincock
7
Franklin & Marshall College, Lancaster, Pennsylvania 17604-3003, USA
Search for other works by this author on:
LeAndra L. Archuleta
LeAndra L. Archuleta
8
Pomona College, Claremont, California 91711-6339, USA
Search for other works by this author on:
Ann Pufall
Ann Pufall
9
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
Search for other works by this author on:
M. Jason Cox
M. Jason Cox
10
Washington & Lee University, Lexington, Virginia 24450, USA
Search for other works by this author on:
Published:
January 01, 2004

The Spuhler Peak Metamorphic Suite consists of a thick sequence of Archean metamorphic rocks, dominated by amphibolite, orthoamphibole-garnet gneiss, and hornblende gneiss and containing minor quantities of quartzite, aluminous schist, quartzofeldspathic gneiss, and meta-ultramafic rocks. Although the Spuhler Peak Metamorphic Suite represents less than 3% of Precambrian exposures in the Tobacco Root Mountains, it has been recognized as a unique suite in the region, due in large part to the unusually abundant amphibolite and the spectacular appearance of the orthoamphibole-garnet gneisses.

An analysis of mafic rocks within the Spuhler Peak Metamorphic Suite, using classification diagrams, tectonic discrimination diagrams, and rare earth element plots, provides results consistent with primary igneous processes and pre-metamorphic hydrothermal alteration. Spuhler Peak Metamorphic Suite amphibolites compare well to typical Archean tholeiitic basalts with arc-related affinities. The major element geochemistry of the Spuhler Peak Metamorphic Suite orthoamphibole-garnet gneisses is within the range of the suite's amphibolites, excepting CaO and MgO contents. This supports the hypothesis that the amphibolites represent metamorphosed equivalents to the unaltered orthoamphibole-garnet gneiss protolith. When the effects of hydrothermal alteration are taken into account, the geochemistry of the Spuhler Peak Metamorphic Suite orthoamphibole-garnet gneisses is consistent with a volcanic protolith, genetically related to the protolith of its amphibolites. Outcropping within the suite are a number of small bodies of meta-ultramafic rocks, but these rocks are chemically distinct from the suite. We believe these bodies possibly are tectonically emplaced and not directly related to the Spuhler Peak Metamorphic Suite protolith. This protolith, therefore, is interpreted as largely mafic volcanic rocks (90% of the total package) deposited in a marine environment and intercalated with minor amounts of sedimentary material.

The Spuhler Peak Metamorphic Suite is distinct from the other Archean suites in the Tobacco Root Mountains—the Indian Creek Metamorphic Suite, which consists of quartzofeldspathic gneisses and metasedimentary rocks including marble, and the Pony–Middle Mountain Metamorphic Suite, which is composed primarily of quartzofeldspathic gneisses with subsidiary hornblende gneiss. Field relationships and radiometric dating demonstrate that the Spuhler Peak Metamorphic Suite was brought into fault contact with the Indian Creek and Pony–Middle Mountain Metamorphic Suites during the Big Sky orogeny, a major tectonothermal event at 1780–1720 Ma.

You do not currently have access to this article.

Figures & Tables

Contents

GSA Special Papers

Precambrian Geology of the Tobacco Root Mountains, Montana

John B. Brady
John B. Brady
Search for other works by this author on:
H. Robert Burger
H. Robert Burger
Search for other works by this author on:
John T. Cheney
John T. Cheney
Search for other works by this author on:
Tekla A. Harms
Tekla A. Harms
Search for other works by this author on:
Geological Society of America
Volume
377
ISBN print:
9780813723778
Publication date:
January 01, 2004

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal