GEOLOGIC EVOLUTION OF THE BEARTOOTH MOUNTAINS, MONTANA AND WYOMING

Abstract

The Beartooth Mountains form an elongated range with longer axis trending northwest and consist of a core of granitic gneiss flanked by migmatites and metasediments. The Quad Creek area is astride the northeast boundary of granitic gneiss and migmatites and metasediments. The area is 7 square miles in extent and is occupied by a large syncline with axis striking north-northeast and plunging 10°–30°S.-SW. Detailed field studies indicate the following geologic history. (1) Original deposition of an Archean sedimentary sequence. (2) Emplacement of metagabbro and ultramafic intrusions, followed by folding; fold axes strike north-northeast. (3) Regional metamorphism and granitization, resulting in a core of granitic gneiss and mantle of migmatites and metasediments with boundaries trending northwest. The last expression of granitization was the production of pegmatites; a few metanorite intrusions were emplaced before pegmatite formation. (4) Emplacement of a metabasaltic dike swarm, younger than the pegmatites but probably within the same plutonic cycle, and striking mainly northwest. (5) Emplacement of a younger Precambrian dolerite dike swarm which has the same dominant strike as the older dike swarm. (6) Peneplanation and deposition of Paleozoic sediments. (7) Laramide uplift and thrusting, and emplacement of felsic porphyries early in this cycle. Laramide structures are controlled by basement structures. The dominant northwest trend was established in the Archean cycle of regional metamorphism and granitization, yet the direction of the oldest foldings is unique. Field and laboratory studies indicate in situ formation of granitic gneiss. Fold axes pass continuously and without deflection from the mantle of metasediments and migmatites across the boundary zone into the core of granitic gneiss, although the folds intersect the boundary zone at 40°–50°. The boundary zone consists of interdigitating tongues of migmatites and granitic gneiss, and these rock types grade into one another along and across strike. In the boundary zone more resistant rock types persist at definite horizons, continuous with skialiths of similar rocks in granitic gneiss. Foliation in granitic gneiss and banding in migmatites are parallel throughout to bedding in metasediments. Growth phenomena shown by zircons of different rocks also indicate autochthonous formation of granitic gneiss. Mineral assemblages of resisters of para-amphibolite, ultramafic rocks, biotite schists, and banded ironstones indicate metamorphism in sillimanite-almandine subfacies of amphibolite facies; temperatures probably were 500°–600° C. Subsequent increase in water concentrations can be traced in seemingly regressive changes in mineral assemblages. This culminated in metasomatic changes which produced granitic gneisses from pre-existing rocks. The writers conclude that granitization was effected by migrating alkaline aqueous solutions during a prolonged Archean cycle of thermal activity. Twenty-three chemical analyses are given, and chemical variation during granitization is discussed.