Deformational History of Archean Metasedimentary Rocks of the Beartooth Mountains in the Vicinity of the Mineral Hill Mine, Jardine, Montana

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Archean metasedimentary rocks of the South Snowy Block of the Beartooth Mountains, in the vicinity of Jardine, Montana, consist predominantly of schistose rocks (interpreted as metamorphosed turbidities) with rare iron formation. These rocks are intruded by Precambrian granitic stocks and minor mafic dikes and sills. Evidence for three phases of folding and late-stage kinking is found within the metasedimentary rocks, whereas rocks of the Crevice Mountain stock (2700 Ma) are unaffected by any of these events. The first folding event involved the development of isoclinal, recumbent folds of varying scale. F₁ fold hinges are rare, most commonly observed underground in Mineral Hill. An S₁ schistosity has developed axial planar to these folds. This schistosity, which is subparallel to bedding, is very well developed and ubiquitous in the metasedimentary rocks of the Jardine region. Large scale F₁ folds are inferred from the pattern of occurrence of iron formation within drill cores and because of the ubiquitous nature of the bedding-subparallel S₁ schistosity. Two later phases of folding are also recognized. F₂ folds are nearly upright with gently to moderately plunging fold hinges. An S₂ crenulation cleavage exists, axial planar to the F₂ folds. F₃ folds are also nearly upright, with gently to moderately plunging hinges, and an S₃ crenulation cleavage, though weakly developed, is recognized, axial planar to F₃ folds. The F₂ and F₃ fold axial surfaces are nearly orthogonal. This geometry results in widespread development of dome and basin (Ramsay type 1) fold interference patterns involving F₂ and F₃ folds, and developed on S₁ surfaces. These are observed in outcrop, and are inferred on the larger scale from mine tunnel exposure and drill core data. Rare, late-stage kink bands affect the S₁ schistosity. Temperature and pressure conditions during deformation, as revealed by calculations from microprobe analyses, suggest that the peak of metamorphism occurred at a temperature of about 560°C and a pressure of 2.9 kb. Thin section observations indicate that the metamorphic peak accompanied the formation of S₁ schistosity. Structural, metamorphic, and geochemical data are consistent with the hypothesis that the metasedimentary rocks of the Jardine region are allochthonous and constitute one of a number of tectonostratigraphic terranes in the western Beartooth Mountains that were juxtaposed tectonically against the western margin of an Archean continent during a Late Archean colli-sional event. The ductile deformational events in the Jardine region likely predate the terrane emplacement. Uplift of the Beartooth Mountains occurred during late Cretaceous time (Laramide). A number of steeply dipping dip-slip faults and shear zones, probably associated with this uplift, cut the folded metasedimentary rocks.