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General geology and tectonic setting of the Tobacco Root Mountains

By
H. Robert Burger
H. Robert Burger
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
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Published:
January 01, 2004

The Tobacco Root Mountains are located near the northwestern margin of the Archean Wyoming province. This position places the Archean rocks of the Tobacco Root Mountains approximately midway between the Great Falls tectonic zone, which many believe marks the collisional boundary between the Hearne and Wyoming provinces, and the Madison mylonite zone, 3-km-wide shear zone that is believed to represent major displacement during collision of these two Archean provinces.

The Archean of the Tobacco Root Mountains is dominated by quartzofeldspathic orthogneisses but contains substantial and unequivocal metasedimentary rocks that facilitate detailed structural mapping, an allochthonous sequence of metamorphosed mafic and ultramafic volcanic rocks, and mafic dikes and sills that crosscut early fabrics but also are metamorphosed and deformed. The unique location and compositional diversity of these Archean rocks supports new insights into the timing and tectono-thermal effect of the collision of the Wyoming province with the Hearne province.

Geochronologic and geothermobarometric results support a collision between 1.78 and 1.72 Ga that produced a metamorphic episode at 5–6 kbar and 650–750 °C and involved partial melting. A sequence dominated by tholeiitic and komatiitic basalts and possible komatiites was brought into tectonic contact with the relatively autochthonous orthogneisses and metasedimentary rocks. The intensity of this tectonothermal event strongly reworked all Archean rocks, so that the dominant folds, fabrics, and mineralogy now observed in Tobacco Root rocks date from this time.

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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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