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Precambrian meta-ultramafic rocks from the Tobacco Root Mountains, Montana

By
Kathleen E. Johnson
Kathleen E. Johnson
Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148, USAPresent address: P.O. Box 8352, Monterey, California 93940, USA, volcan@mac.com.
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John B. Brady
John B. Brady
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
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William A. MacFarlane
William A. MacFarlane
Colorado College, Colorado Springs, Colorado 80903, USA
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Rebecca B. Thomas
Rebecca B. Thomas
Williams College, Williamstown, Massachusetts 01267, USA
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Chris J. Poulsen
Chris J. Poulsen
Carleton College, Northfield, Minnesota 55057, USA
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M. Jennifer Sincock
M. Jennifer Sincock
Franklin & Marshall College, Lancaster, Pennsylvania 17604-3003, USA
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Published:
January 01, 2004

Meta-ultramafic rocks occur as small (2 to 100 m long), podiform bodies in all three major Precambrian rock suites of the Tobacco Root Mountains of southwest Montana. Most samples consist of a randomly oriented, coarse-grained assemblage of orthopyroxene, olivine, and magnesiohornblende ± spinel, partially replaced by a fine-grained assemblage that may include anthophyllite, talc, cummingtonite, magnesiohornblende, chlorite, serpentine, and/or magnetite. Blackwall reaction zones of anthophyllite, actinolite, chlorite, and/or biotite surround several of the meta-ultramafic bodies. The absence of clinopyroxene with orthopyroxene limits the metamorphic pressure-temperature history of these rocks to temperatures below ∼800 °C. The presence of anthophyllite with olivine indicates that these rocks passed through 650–700 °C at pressures below 0.6 GPa. These constraints are consistent with the detailed pressure-temperature path determined for the surrounding upper-amphibolite to granulite facies metamorphic rocks. Whole-rock chemical analyses of the meta-ultramafic rocks show them to be rich in SiO2 (44–54 wt%) and poor in MgO (21–34 wt%) relative to mantle peridotites and typical Alpine-type ultramafic rocks. Rare earth element concentrations are all enriched relative to chondritic values and the light rare earth elements are especially enriched (10–30 times), inconsistent with either an upper-mantle or a komatiitic origin. All samples have similar TiO2/Zr ratios, suggesting that they have a common or related origin, and that TiO2 and Zr were conserved in the processes that led to the observed chemical variations. Together, the chemical data point to a protolith that was an ultramafic cumulate rich in orthopyroxene, and therefore probably formed in a continental setting from a basaltic magma enriched in silica. One possible time of origin is a magmatic event during the continental rifting at 2.06 Ga that led to the intrusion of a suite of mafic dikes.

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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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