Casella, Clarence J.
M.S. (Master of Science)
Department of Geology
Batholiths; Geology; Structural; Granite--Inclusions; Beartooth Mountains (Mont. and Wyo.)
Recent studies indicate that the 2,700 m.y. old granites, granodiorites, and biotite tonalites of the Beartooth Mountains are not metasomatized metasediments but instead comprise a svntectonic batholith that invades 3,000 m.y. old schistose metasedments. Within these 3,000 m.y. metasediments are small, massive, coarse-grained quartz-hornblende diorite intrusions that retain igneous textures. These are identical to inclusions, formerly called orthoamphibolites, found in the granitic rocks. These vary from fist sized up to the largest reported, 65 km^. These huge inclusions (?) can comprise up to 20% of the granitic terrain. Sparse metagabbros, metanorites and ultramafic inclusions are also found. Diorites were commonly combined together with medium-grained layered amphibolites that have a granoblastic texture, and a ,common origin was proposed. However, bulk chemical and modal analysis together with mineral chemistry indicates that although a serial chemical relationship is possible, the layered amphibolites are more basaltic and the diorites range from basaltic to andesitic. The larger bodies of layered amphibolite are commonly interlavered with metasediments and are thought to have been dikes or lava flows. The data also permit speculation that the metagabbros, metanorites and diorites are the remnants of an earlier large differentiated mafic body that was a fore-runner to the granitic rocks. They were later enveloped by granitic magma. The clarification and redefinition of the mafic rocks opens the way for speculation about their relationship with more granitic rocks. Mafic rocks are commonly the earliest rocks found associated with the emplacement of large granitic batholiths. Thus, the Beartooth Batholith is very like granitic batholiths the world over in space and time.
Fengler, Timothy A., "A petrological investigation of older mafic rocks in the Beartooth Mountains, Wyoming, and Montana" (1983). Graduate Research Theses & Dissertations. 461.
vii, 136 pages
Northern Illinois University
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