Geology of the Highwood Mountains, Montana: A Survey of Magma Types and Sources

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Route

Start point

Shonkin Ranch, MT

End point

Haystack Butte, near Geraldine, MT

Roads

Shonkin-Geraldine road, Upper Shonkin Creek road, Coalmine Coulee road, Geraldine-Geyser road, unnamed gravel road

Total distance

at least 14.25 miles (more if either of the two side trips are followed)

Geology

Summary

The Highwood Mountains represent a dissected Eocene volcanic and intrusive complex composed of latites and highly potassic, mafic to felsic igneous rocks. Along with the Bearpaw Mountains and Eagle Buttes, they form a northeast-trending belt within the Montana Alkalic Province, and were all emplaced between 54 and 50 Ma. In the Highwoods an older suite of quartz-normative latites is overlain and intruded by a younger suite composed mostly of mafic phonolite flows and dikes, minette and trachyte dikes, and shonkinite-mafic syenite stocks. This trip examines a variety of dikes, sills, and stocks composed of these unusual rock types.

Key Lithologic Features

  • Tertiary conglomerate
  • latite breccia
  • mafic phonolite and minette dikes and sills, some containing xenoliths
  • minette chonoliths in the Coalmine Coulee area
  • the "Headed dike": an unusual inverted teartrop shaped dike with a fergusite upper portion rich in pseudoleucite, transitioning down to olivine shonkinite with stringers of syenite
  • Haystack Butte: small stock composed of monticellite peridotite with adjacent metasomatized sandstone

Structures

Landforms

Other Features

Reference

McCallum, I.S., O'Brien, H.E., and Irving, A.J., 1989, Geology of the Highwood Mountains, Montana: a survey of magma types and sources, in Hearn, B.C., Jr., Dudas, F.O., Eggler, D.H., Hyndman, D.W., O'Brien, H.E., McCallum, I.S., Irving, A.J., and Berg, R.B., Montana High-Potassium Igneous Province : 28th International Geological Congress, Field Trip Guidebook T346: Washington, D.C., American Geophysical Union, p. 23-35.

Availability

Order from the American Geophysical Union Bookstore. Go to the AGU On Line Book Catalog for ordering information.