Explanatory text to accompany geologic map of southern Tobacco Root Mountains, Madison County, Montana
Charles J. Vitaliano, H. Robert Burger, III, William S. Cordua, Thomas B. Hanley, David F. Hess, Forrest K. Root, 2004. "Explanatory text to accompany geologic map of southern Tobacco Root Mountains, Madison County, Montana", Precambrian Geology of the Tobacco Root Mountains, Montana, John B. Brady, H. Robert Burger, John T. Cheney, Tekla A. Harms
Download citation file:
The core of the Tobacco Root Mountains, a domal uplift of southwestern Montana, is composed of regionally metamorphosed Archean rocks that have been intruded by diabase dikes of late Precambrian age and by igneous rocks of late Mesozoic and Cenozoic age. The Archean rocks, originally a stratiform sequence of epiclastic, volcaniclastic, and carbonate sedimentary rocks, have been metamorphosed to amphibolite-granulite rank with attendant migmatization. The distribution of the metamorphic rocks and the later intrusions is shown on the map.
In this report to accompany the map, the nature of the crystalline rocks is described briefly. Two thermal events affected the area in Precambrian time, one about 2.7 b.y. B.P. and the second at about 1.6 b.y. B.P. A third event may have begun in very late Precambrian time, but is most strongly represented by changes of late Mesozoic age.
Because of the structural complexities, correlation of the metamorphic rocks in the Tobacco Root Mountains with Precambrian sequences elsewhere in southwestern Montana is extremely difficult.