Precambrian geology of the Cottonwood Creek area, Montana and Wyoming

Includes bibliographical references.Includes illustrations and maps.Precambrian rock units mapped are the: 1) metamorphic complex, 2) Hellroaring Mountain quartz monzonite stock, 3) leuco quartz-tonalite sills and 4) diabase dikes and sills. The metamorphic complex is the oldest unit and con­sists of eastward-dipping quartz and mica-rich metasedi­ments. These include sparse metaconglomerate and amphibole schist. Plagioclase is common in all these rocks but potassium feldspar is absent. The earliest decipherable event (Dj) involved metamorphism and deformation of the sedimentary pile which produced the dominant N-S trending fabric. The deformation produced probable large-scale isoclinal folds with axial-plane mica foliation together with amphibolite grade metamorphism. Folding and meta­morphism were complex and polyphase in nature. The Hellroaring Mountain quartz monzonite stock intruded the metamorphic complex a minimum of 2,800 m.y. ago. The stock is mainly concordant and has shouldered aside the metamorphic complex. Thermal effects of the intrusion are confined to within several meters of the stock. Leuco quartz-tonalite and diabase probably intruded after the Hellroaring stock. Shearing accompanied by mild metamorphism is evident in the diabase and may represent a later Precambrian event.M.A. (Master of Arts