Abstract

In the northern Tobacco Root Mountains, Pony metamorphic rocks overlie Cherry Creek metamorphic rocks; they are parts of the same depositional unit and have been subjected to the same sequence of metamorphic processes.

Early in pre-Beltian time, a thick sequence of geosynclinal rocks was isoclinally folded about northeast-trending axes under high-grade metamorphic conditions; recumbent isoclinal folds were produced, accompanied by metasomatic feldspathization and metamorphic differentiation. Basalt sills and dikes were injected into the metamorphic rocks, cutting isoclinal folds and migmatitic patterns.

The entire rock complex, including the basalts, was subjected to horizontal-shearing deformation under granulite-facies conditions and then under amphibolite-facies conditions.

Later in pre-Beltian time in a period of concentric folding the pre-existing, approximately horizontal schistosity was deformed into large, north-plunging folds.

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