Casella, Clarence J.
M.S. (Master of Science)
Department of Geology
Geology--Montana; Geology; Stratigraphic--Precambrian; Geology--Wyoming
The thesis area is located along the southwest margin of the Beartooth Mountains. Major Precambrian units from oldest to youngest are: 1) metamorphic complex, 2) granodiorite gneiss, 3) Hellroaring Mountain quartz monzonite, and 4) metagabbro dike. Minor units include igneous breccia and aplite and pegmatite dikes. The metamorphic complex consists of quartz-biotite schist with intercalated biotite quartzite, garnetiferous biotite schist, garnetiferous amphibole schist, and quartz pebble metaconglomerate. The granodiorite gneiss sill and the quartz monzonite stock are intrusive into the complex while the metagabbro dike is found only in the quartz monzonite. Apophyses of granodiorite gneiss in the metamorphic complex are deformed while dikes of the Hellroaring Mountain stock are not. This indicates that deformation occurred after the intrusion of the granodioritegneiss but prior to the intrusion of the Hellroaring Mountain stock. Two metamorphic events are recognized prior to the intrusion of the Hellroaring Mountain stock. The first was of almandine-amphibolite grade. Associated with this metamorphism is the deformation that formed the regional north-northeast trending structure in the metamorphic complex. The second metamorphism was of epidote-amphibolite grade. Associated with this metamorphism is the deformation that produced a second, cross-cutting biotite foliation. There is some evidence of a third very low-grade metamorphism that affected the area. The intrusion of the quartz monzonite stock shouldered aside the metamorphic complex rotating its strike parallel to the contact and locally changing dips from easterly to westerly. Contacts are concordant at structurally low levels of the stock and discordant at upper levels. Structural and mineralogical criteria indicate that the stock intruded as a crystal mush and crystallized in the mesozone. Resurgence occurred after the margins were crystallized producing cataclastic textures. Subsolidus recrystallization affected alkali feldspars and quartz, but volatile content was not sufficient to allow complete recrystallization and many primary textures are preserved.
Eble, Edward, "Precambrian geology of the Hellroaring Mountain area, Southwest Beartooth Mountains, Montana and Wyoming" (1976). Graduate Research Theses & Dissertations. 4464.
iv, 77 pages
Northern Illinois University
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