Smoky Butte Lamproite, Montana

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Route

Start point

Lewistown, MT

End point

Billings, MT

Roads

Montana 87, Montana 200, unnamed ranch roads, Montana 244

Total distance

274 miles

Geology

Summary

Smoky Butte is a composite pluglike mass, one of several in a 3-km-long zone of dikes and plugs of lamproite. K-Ar age is 27 +/- 3 Ma on phlogopite in a sample from the quarry on Smoky Butte and is the youngest igneous age in eastern Montana. The wall rocks are the Tullock Member of the Paleocene Fort Union Formation, immediately above the Tertiary-Cretaceous boundary. Dikes are discontinuous and show en echelon offsets probably related to intrusion mechanics rather than to faulting. Pockets of lamproite breccia and heterolithic breccia, containing vesicular lamproite fragments (locally pumaceous) and slightly metamorphosed fragments of Fort Union rocks, occur along the dikes and make up part or all of some of the irregular igneous bodies along the zone. Some dike margins show ropy pahoehoelike texture. Welded agglutinates are locally interlayered with bedded tuffs and occur as blocks wtihin some of the heterolithic breccia bodies.

Key Lithologic Features

  • Smoky Butte lamproite dikes and plugs, including chill zones, breccias, and xenoliths

Structures

Landforms

Other Features

  • Cretaceous-Tertiary boundary
  • armalcolite collecting locality

Reference

Hearn, B.C., Jr., 1989, Bearpaw Mountains, Montana, 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. 75-78.

Availability

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