Casella, Clarence J.
M.S. (Master of Science)
Department of Geology
Sheep Mountain is four miles north of Cooke City, Montana. The predominant Prccambrian rock type in the area is granitic gneiss. The oldest rocks are interlayered and occasionally isoclinally folded pelitic schists and meta-ironstones. These are found as large inclusions, up to 500 meters long, in the granitic gneiss. Tabular xenoliths of lineated amphibolite, which themselves contain schist inclusions and smaller xenoliths of foliated amphibolite, are also found in the granitic gneiss. The granitic rocks are the product of predominantly synkinematic, magmatic injection. Occasional, yet consistent, diking and agmatitic relations indicate the following sequence of intrusion, from the oldest to youngest: l) hornblende quartz diorite; 2)tonalitic gray granitic rocik; and 3) grandioritic-adamellitic pink granitic rock. Mesoscopic folds indicate an episode of isoclinal folding (F1) of metasedimentary rocks, and a later episode (F2) in which the pervasive granitic gneiss foliation is axial plane. F2 macroscopic structure is not conclusively defined. An apparently folded, postgranite, metadiabase dike may testify to a third orogenic event (F3). Electron microanalysis of cordierite-almandine pairs indicates 630°C and 6.2 kb as possible temperature and pressure of metamorphism. Structural and petrographic similarities suggest that the Sheep Mountain area is in the core of an Archean batholith whose country rocks crop out in Yellowstone Park, thirty miles to the west. The sequence of Precambrian events is: l) metamorphism and isoclinal folding of a package of Archean sedimentary rocks; 2) metasedimentary rocks intruded by mafic dikes; 3) intrusion of granitic magmas serially from hornblende quartz-diorite to adamellite and amphibolitization of mafic rocks; A) refolding of metasedimentary rocks and formation of folds and lineations in the granitic rocks by plastic or viscous flow, largely concomitant with intrusion; 5) emplacement of diabase dikes; 6) regional metamorphism and deformation; and 7) emplacement of the latest diabase dikes.
Davis, Harold G., "The Precambrian geology of the Sheep Mountain area, Beartooth Mountains, Montana" (1979). Graduate Research Theses & Dissertations. 6098.
x, 120 pages
Northern Illinois University
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