Abstract

The Cathedral Peak area encompasses 35 square miles in the Precambrian core of the Bear-tooth Mountains in Montana, and is bounded by the Stillwater complex on the northeast. The major rock type is granitic gneiss consisting of quartz-feldspar rocks with a wide range in ratio of plagioclase to microcline and having a color index less than 20. Included in the gneiss are small, generally conformable lenses of biotite schist, amphibolite, and other metamorphic rocks, all having assemblages typical of amphibolite facies. Field relationships suggest that the granitic gneiss was formed by metasomatism. A thick aureole of contact metamorphosed rocks, the inner part graded to pyroxene-hornfels facies, underlies the Stillwater complex. Igneous and metamorphic rocks are altered locally to assemblages of greenschist facies along fault zones.

Foliation and compositional layering of metasedimentary rocks (presumably representing original bedding) and foliation of granitic gneiss all show monoclinic symmetry. The macroscopic fabric and outcrop pattern indicate a series of open folds with axes trending north-northwest. Directions of mineral elongation and minor fold axes parallel the B-axis of the major fold system. In the southern part of the area, fold axes plunge 10°–15° N., but the plunge increases to 40 degrees near the base of the Stillwater complex. West of the Cathedral Peak area, the plunge is 10°–15° S.

Postulated major events of geologic history are: (1) deposition of a sedimentary sequence, (2) formation of open folds, (3) regional metamorphism, granitization, and pegmatite formation, (4) emplacement of Stillwater complex, (5) regional metamorphism and pegmatite formation about 2700 m.y. ago, (6) intrusion of late Precambrian dolerite dikes, and (7) Laramide orogeny.

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