Skip to Main Content
Book Chapter

Proterozoic metamorphism of the Tobacco Root Mountains, Montana

By
John T. Cheney
John T. Cheney
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
John B. Brady
John B. Brady
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
Search for other works by this author on:
Kara A. Tierney
Kara A. Tierney
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
Kathleen A. DeGraff
Kathleen A. DeGraff
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
Heidi K. Mohlman
Heidi K. Mohlman
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
Jessica D. Frisch
Jessica D. Frisch
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
Christine E. Hatch
Christine E. Hatch
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
Michael L. Steiner
Michael L. Steiner
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
Sarah K. Carmichael
Sarah K. Carmichael
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
Search for other works by this author on:
Robin G.M. Fisher
Robin G.M. Fisher
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
Search for other works by this author on:
Carrie B. Tuit
Carrie B. Tuit
Beloit College, Beloit, Wisconsin 53511, USA
Search for other works by this author on:
Kurt J. Steffen
Kurt J. Steffen
Carleton College, Northfield, Minnesota 55057, USA
Search for other works by this author on:
Pamela Cady
Pamela Cady
Colorado College, Colorado Springs, Colorado 80903, USA
Search for other works by this author on:
Josh Lowell
Josh Lowell
Colorado College, Colorado Springs, Colorado 80903, USA
Search for other works by this author on:
LeAndra L. Archuleta
LeAndra L. Archuleta
Pomona College, Claremont, California 91711-6339, USA
Search for other works by this author on:
Jillian Hirst
Jillian Hirst
Pomona College, Claremont, California 91711-6339, USA
Search for other works by this author on:
Karl W. Wegmann
Karl W. Wegmann
Whitman College, Walla Walla, Washington 99362, USA
Search for other works by this author on:
Brian Monteleone
Brian Monteleone
College of Wooster, Wooster, Ohio 44691-2363, USA
Search for other works by this author on:
Published:
January 01, 2004

Textures and mineral assemblages of metamorphic rocks of the Tobacco Root Mountains are consistent with metamorphism of all rocks during the Big Sky orogeny (1.77 Ga) at relatively high pressure (P >1.0 GPa) followed by differential reequilibration on a clockwise P-T path at lower pressures (0.6–0.8 GPa). The highest pressures are documented by coarse-grained kyanite and orthopyroxene in aluminous orthoamphibolites, which require P ≥ 1.0 GPa. Other higher-pressure mineral assemblages of note include kyanite + orthoamphibole and kyanite + K-feldspar. Abundant textural evidence for partial melting in pelitic and basaltic rocks includes leucosomes, very large (several cm across) porphyroblasts of garnet, and an absence of primary (foliation-defining) muscovite. Partial to complete overprinting of the coarse-textured, high-pressure assemblages by lower-pressure assemblages and textures occurred across the Tobacco Root Mountains, especially where assisted by deformation and the availability of water. In aluminous rocks, sillimanite bundles typically replace kyanite, and garnet may be rimmed by cordierite + orthopyroxene symplectite or, in quartz-absent rocks, sapphirine + spinel + cordierite symplectite. Orthoamphibolites with partial pseudomorphs of garnet by cordierite are common. Garnet necklaces surround orthopyroxene in orthopyroxene-plagioclase gneisses, whereas orthopyroxene + plagioclase pseudomorphs of garnet occur in nearby hornblende amphibolites. These features appear to require nearly isobaric cooling at pressures near 0.8 GPa, followed by nearly isothermal decompression at temperatures near 700 °C. The resulting P-T path is believed to be the result of tectonic denudation late in the orogenic cycle. Quartz-plagioclase-garnet-hornblende amphibolites occur throughout the Tobacco Root Mountains. Near-rim mineral compositions from these rocks have been used to calculate Ts of 650–750 °C at Ps of 0.7–0.9 GPa across the terrane. There is no systematic variation in calculated P and T between units nor geographically within units; differences appear to reflect variations in thermometer closure possibly due to the availability of water during cooling. Field relations involving metamorphosed mafic dikes, as well as geochronological data from monazite and zircon, demonstrate that some rocks were first metamorphosed at high temperatures and pressures at 2.45 Ga. However, we have not identified mineral assemblages that can be assigned unequivocally to this earlier event.

You do not currently have access to this article.
Don't already have an account? Register

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

A comprehensive resource of eBooks for researchers in the Earth Sciences

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal