Volcanism and Plutonism at Shallow Crustal Levels: The Elkhorn Mountains Volcanics and the Boulder Batholith, Southwestern Montana

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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

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