Volcanism and Plutonism at Shallow Crustal Levels: The Elkhorn Mountains Volcanics and the Boulder Batholith, Southwestern Montana
Route
Start point
Helena, MT
End point
Butte, MT
Roads
U.S. 287/12, I-15, I-90, U.S. 2, Montana 393; other county, gravel, and Forest Service roads
Total distance
50.5 miles
Geology
Summary
The Upper Cretaceous Elkhorn Mountains Volcanics (EMV) and Boulder batholith of southwestern Montana provide an example of a large-volume, epizonal, volcanic-plutonic complex whose deep level of erosion has exposed the cogenetic intrusive rocks while preserving sizeable portions of the volcanic field. Such a volcanic-plutonic association provides a unique opportunity for evaluation of many aspects of the evolution of a shallow-crustal magmatic system, such as geochemical relations of both the volcanic and plutonic rocks and the nature of intrusive-extrusive relationships at the present level of exposure.
Key Lithologic Features
- Elkhorn Mountains volcanics
- Boulder batholith
- Boulder Quartz Monzonite
- Kokoruda Ranch Complex (early mafic rocks of the Boulder Batholith)
- Granodiorite of Rader Creek
- aplite-alaskite bodies and dikes cutting Boulder Creek Monzonite
- Climax Gulch pluton
Structures
- rim of volcanic caldera and associated ring dike
- joints
- contact between the Boulder Quartz Monzonite and the Elkhorn Mountains volcanics
- contact between the Boulder Quartz Monzonite and the Climax Gulch pluton
Landforms
Other Features
- Chicago Mine (breccia pipe deposit)
Reference
Rutland, C., Smedes, H.W., Tilling, R.I., and Greenwood, W.R., 1989, Volcanism and plutonism at shallow crustal levels: The Elkhorn Mountains volcanics and the Boulder batholith, southwestern Montana,
in Hyndman, D.W., ed.,
Cordilleran volcanism, plutonism, and magma generation at various crustal levels, Montana and Idaho : 28th International Geological Congress, Field Trip Guidebook T337: Washington, D.C., American Geophysical Union, p. 16-31.
Availability
Order from the American Geophysical Union Bookstore. Go to the
AGU On Line Book Catalog for ordering information.
