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The Beartooth Mountains consist of a Precambrian crystalline core uplifted during the Laramide orogeny. This core has been studied by several groups under the direction of Arie Poldervaart to determine the origin and geologic history of the rocks. All published and unpublished work is summarized herein as well as the evolution of ideas of origin of the granitic rocks. Rock types include: microclinerich and microcline-poor granitic gneiss, migmatite, biotite gneiss, para- and orthoamphibolite, ultramafic and mafic igneous rocks, and various quartzites and schists.

Facies indicators point to cordierite-amphibolite facies of Abukuma-type metamorphism for relict metasedimentary rocks that have escaped metasomatism. K and Na metasomatism is indicated by: replacement of plagioclase by albite and microcline, alteration of amphibolite to biotite gneiss, mantles of mica around ultramafic pods, progressive variation between quartzite and granite, and growth of orbicular granite.

All rocks were deformed by passive folding along north- or northeast-trending axes that plunge from 0° to 40° toward the south. Individual fold axes are traceable for more than 5 miles. Penetrative fabric elements show monoclinic symmetry on a mesoscopic scale. Fabric relations show that folding was contemporaneous with metamorphism. One large folded fold has been found that may indicate a two-phase deformational history.

Consistent radiogenic ages indicate metamorphism 2750 m.y. ago. However, a complex history is indicated for similar rocks along the base of the Stillwater Mafic Complex. Radiogenic ages indicate this intrusion was emplaced between 3200 and 3800 m.y. ago, but biotite and plagioclase ages within the complex are 2660 m.y. The intrusive has altered gneisses and amphibolites which are thus older than the complex. It appears that a polymetamorphic history is possible for rocks of the Beartooth Range.

Earlier theories postulated an event 2750 m.y. ago that statically metamorphosed and grantized a folded sedimentary series. It is now believed that folding was contemporaneous with this event. Accordant contacts formerly considered indicative of sedimentary layering are explained as being produced during flow of rocks at varying temperatures under high confining pressures in the presence of a fluid phase.

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