Field Guide; Little Rocky Mountains

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Route

Start point

Havre, Montana

End point

Chinook, Montana

Roads

U.S. 2, U.S. 87

Total distance

135.3 miles

Geology

Summary

The Bearpaw Mountains, the eroded remains of an early Eocene volcanic complex, is the largest eruptive center in the central Montana alkalic province. The volcanic and less-voluminous plutonic rocks of the mountains form a roughly circular, isolated mountain group that rises up to 1,400 meters above the surrounding plains and covers an area of approximately 2,500 km2. In general, the volcanic rocks, dominantly shonkinitic flows and breccias, lie in two roughly equal fields which are separated by an E-trending, anticlinal arch composed of sedimentary rocks about 12 kilometers wide and 65 kilometers long. This central arch is a composite uplift formed by multiple emplacement of shallow plutons. Bounded by normal faults to the north and south, the central arch is intruded by numerous shonkinite and latite dikes, plugs, laccoliths and stocks. The field trip provides a view of important rock types including biotite pyroxenite, shonkinite intrusions and flows, and tinguaite (a pseudoleucite colored green by fine needles of aegirine in the groundmass), a latite intrusion and a breccia pipe. Field relations between shonkinite and latite and the role of magma mixing in producing a complete compositional gradation between shonkinite and latite will be observed. The complete gradation can be seen in the field if enough occurrences are examined.

Key Lithologic Features

  • Rocky Boy stock
  • pseudoleucite-sodalite tinguaite dike
  • shonkinite lava flows
  • latite stock

Structures

Landforms

Other Features

Reference

Tureck-Schwartz, K., and Hyndman, D.W., 1991, Field Guide; Bearpaw Mountains, in Baker, D.W., and Berg, R.B., eds., Guidebook of the central Montana alkalic province: Geology, ore deposits and origin , Montana Bureau of Mines and Geology Special Publication 100, p. 195-200.

Availability

Out of print; can be photocopied for a fee. For more information, see this publication's record at the MBMG .