Plutonism at Deep Crustal Levels: The Idaho Batholith, Montana and Idaho

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Route

Start point

Missoula, Montana

End point

Lowell, Idaho

Roads

U.S. 12

Total distance

188 miles

Geology

Summary

The Idaho batholith field trip traverses a well exposed cross-section of the northern Idaho batholith, briefly examines the broad aspects of this deep-seated granitoid batholith and its regionally metamorphosed country rocks, and considers the role of the synplutonic mafic magmas from the mantle in providing heat for melting of continental crustal rocks to form the more felsic main-phase units of the batholith.

Key Lithologic Features

  • low- to high-grade regional metamorphic rocks of the eastern border zone
  • sheet-like intrusions of the contact zone of the granitoid rocks
  • main-phase granodiorite to granite of the batholith
  • high- to medium-grade regional metamorphic rocks and sheet-like intrusions of the western border zone
  • injection migmatites of the border zone rocks
  • emplacement-related structures of the batholith
  • early western border zone tonalites
  • fine-grained synplutonic dikes of basaltic andesite to andesite which cut the batholith
  • small granitoid dikes which cut the dikes of basaltic andesite
  • complex mixing relationships between the basaltic andesite magmas and the granitoid magmas of the batholith
  • sub-horizontal granitoid to pegmatite sheets which cut the batholith

Structures

  • Lolo Creek fault

Landforms

Other Features

Reference

Hyndman, D.W., and Foster, D.A., 1989, Plutonism at deep crustal levels: the Idaho batholith, Montana and Idaho, 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. 3-15.

Availability

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