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Metamorphism at the Contact of the Cable Stock, Montana

A study of metamorphic rocks is integrated with the areal geology in the vicinity of the Cable stock, a granodiorite intrusive surrounded by Paleozoic carbonate rocks. Detailed geology of the zones of most intense metamorphism at the intrusive contact is described.

Isochemical thermal metamorphism was followed by metasomatic addition of material and accompanied by various stages of alteration. Silicic magnesian limestones altered to diopside-grossularite marble by the introduction of iron. The addition of iron, little magnesia, and alumina altered the marble to a diopside-grandite tactite. Additional iron was deposited as magnetite and accompanied widespread dedolomitization and alteration. Rocks near the intrusive contact were replaced by epidote, mica, and scapolite.

Areas of intense metamorphism occur at the granodiorite contact. Metamorphism is localized in areas of intense folding, fracturing, and shearing.

The basic border of the stock indicates assimilation of calcareous rocks by the granodiorite; the transfer of calcium across the contact resulted in the development of pargasite, diopside, and scapolite. Deformation of the sediments was preceded and, in part, accompanied by forceful intrusion of the granodiorite.

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Metamorphism at the Contact of the Cable Stock, Montana