ABSTRACT

Kyanite and sillimanite occur in the Precambrian schist, gneisses, and pegmatites of the Cherry Creek metamorphic rocks south of Ennis, Montana. The metamorphie rocks formed as a result of the regional metamorphism of a thick sequence of sedimentary rocks, mostly limestones and shales. The metamorphic rocks are about 3,050 feet thick and consist of interlaminated dolomite-marble, dolomitic marble, kyanite- and silliamanite-bearing schists and gneisses, amphibolites, and gneisses and schists, undivided. The rocks are tightly folded, and a large isoclinal syncline, which plunges steeply to the east(?), repeats the section in the map area.

Three types of pegmatites occur in the map area, and each displays a preference for certain host rocks: kyanite pegmatites occur in kyanite schist or gneiss; microline-quartz-muscovite-tourmaline pegmatites occur in marble and in kyanite schist; and microline-quartz pegmatites are most prominent in quartz-feldspar gneiss.

The greatest concentration of kyanite pegmatites occurs along the crest of folds in the kyanite schist. The kyanite pegmatites apparently formed by processes of metamorphic differentiation, in which aluminum ions were reconstituted in and/or added to a quartz segragation and subsequently grew on kyanite nuclei already present in the quartz.

Kyanite is the stable phase in the kyanite schist and is metastable in the kyanite pegmatites, as indicated by the inversion kyanite-sillimanite. The inversion is probably due to increased temperature and reduced pressure. Petrographic evidence indicates that the PT changes may have been produced by shearing of the pegmatite during its formation.