Field Guide to an Archean Transect, Eastern Beartooth Mountains, Montana-Wyoming
Route
Start point
Red Lodge, Montana
End point
Beartooth Lake
Roads (and Trails)
U.S. 212; pack trail into Rock Creek drainage
Total distance
52.7 miles
Geology
Summary
The eastern Beartooth Mountains of Montana and Wyoming contain a record of crustal evolution that spans almost 1000 Ma and culminates in a major episode of crustal growth 2700-2800 Ma. The earlier record is sparse and complex as a result of extensive magmatism and intense metamorphism associated with Late Archean activity. In general, however, it appears that continental material was present in this area by roughly 3600 Ma, and that a stable continental shelf accumulated quartzites, iron-formation, and lesser amounts of pelitic to psammitic units interspersed with small volumes of mafic to silicic volcanic rocks. This cycle of accumulation was apparently terminated by an episode of granulite facies metamorphism 3300-3400 Ma, perhaps as a result of continent-continent collision....About 2800-2900 Ma, a second major cycle of crustal growth began that bears some resemblance to those associated with modern continent-ocean subduction zones....The first igneous rocks produced during this cycle were andesitic or dioritic rocks, both coarse and fine grained, that were subsequently metamorphosed to amphibolite facies....This interval is restricted by the presence of a granodioritic series (Long Lake granodiorite) that was intruded late in the kinematic cycle... (2779 Ma)...and marks the lower limit for the last major episode of regional metamorphism. The last and most volumetrically important rock produced during the cycle was the Long Lake granite. This unit composes approximately 80-90% of the eastern portion of the range and engulfs all older rock types. It appears to have been intruded about 2740 Ma....This excursion will attempt to view the evidence of these two major cycles in four main stops: (1) Hellroaring Plateau, (2) Lower Quad Creek, (3) Upper Quad Creek, and (4) Long Lake. Late Archean mafic dikes (at Beartooth Lake) will also be examined.
Key Lithologic Features
- Orthoquartzites, iron-formations, pelites, mafic to intermediate volcanic rocks, quartzofeldspathic rocks, migmatites, and intercalated ultramafic rocks at the Hellroaring Plateau
- Orthoquartzites, iron-formation, gneisses (tonalite-trondhjemite-granite), and Late Archean granitoids at Lower Quad Creek
- Orthoquartzites, iron-formation, mafic amphibolites, pelites, mafic-ultramafic agmatite, and quartzofeldspathic gneisses (trodhjemitic and granitic varieties) at Upper Quad Creek
- Long Lake granite (with inclusions of andesitic amphibolite and granodiorite), Long Lake granodiorite, and a late mafic dike at Long Lake
- Late Archean mafic dikes at Beartooth Lake
Structures
Landforms
Other Features
Reference
Mueller, P.A., Wooden, J.L., 1988, Field guide to an Archean transect, eastern Beartooth Mountains, Montana-Wyoming,
in Lewis, S.E., and Berg, R.B., eds.,
Precambrian and Mesozoic plate margins: Montana, Idaho and Wyoming, with field guides for the 8th International Conference on Basement Tectonics : Montana Bureau of Mines and Geology Special Publication 96, p. 131-140.
Availability
Order from the Montana Bureau of Mines and Geology. Search for 'this publication's record at the MBMG for ordering information.
