Abstract

Twenty well-defined zones of schistose rocks occur within a thick amphibolite body in the southern Bighorn Mountains, Wyoming. The zones are 2 to 12 m thick and as much as 300 m long. Many of the zones are parallel to the strike of the amphibolite body. However, a significant number are oriented across its trend and at an angle to the strike of its foliation. Locally, the strike of amphibolite foliation is deflected near the schist zones, and blocks of amphibolite occur isolated within one of the zones.

Adjacent to the schist, the amphibolite is typically a massive or layered, granoblastic aggregate of hornblende, andesine, some quartz, and minor accessories. The schist bodies consist of tremolite or magnesian actinolite, chlorite, talc, and local soclic oligoclase. Chemical analyses of schist and adjacent amphibolite indicate that the schist contains smaller amounts of TiO2, A12O3, CaO, Na2O, K2O, and greater amounts of MgO and H2O.

Representing greenschist facies conditions, the schist formed by local retrograde metamorphism along shear zones superimposed on the amphibolite, which itself evolved during amphibolite-facies regional metamorphism. Retrograde metamorphism was facilitated by means of shearing stress and magnesium-rich aqueous fluids.

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