Garnetiferous gneiss from the Precambrian of Montana, USA.

Pale pink = K-feldspar Gray glassy = quartz Extremely dark green = probably chlorite Red = garnet

The Beartooth Mountains are the most scenic set of mountains in America that are not in a national park (see photos elsewhere in this album). The Beartooth Highway between the town of Red Lodge, Montana and Yellowstone National Park, Wyoming traverses the Beartooth Mountains and provided ready access to numerous rock exposures. The rocks in the Beartooths are principally Archean-aged, high-grade metamorphic rocks. Lithologies I have observed in this area include gneiss, migmatite, quartzite, amphibolite, serpentinite, chromitite, meta-iron formation, garnet-biotite metamorphite, plus dikes of diabase and alaskite granite.

The rock shown above is gneiss, a high-grade, foliated metamorphic rock. It has potassium feldspar, quartz, and garnet. The foliation consists of subtle bands of very dark greenish material - probably chlorite. This gneiss formed as a result of at least two metamorphic events during the Archean. At about 3.2 Ga, the protolith was subjected to granulite facies metamorphism, followed by upper amphibolite facies metamorphism at about 2.8 Ga.

Locality: Quad Creek section (= locality of Mueller et al., 1987) - roadcut on southern side of Beartooth Highway (Rt. 212), immediately west of Quad Creek, between milepost 50 and milepost 51, southern side of Rock Creek Canyon, just north of the eroded edge of Beartooth Plateau, southwest of the town of Red Lodge, Beartooth Mountains, southern Montana, USA (45° 01' 41.27" North & 109° 24' 55.31" West)

Some info. from:

Mueller et al. (1987) - Geological Society of America Centennial Field Guide, Rocky Mountain Section, pp. 75-76.