stripes
Occurrence number:
174-00-072
Country:
United States
Region:
Wyoming
Location:
Longitude: -104.27, Latitude: 44.28
Carbonatite:
No
The Black Buttes are an irregularly-shaped intrusive body, probably formed chiefly by several laccoliths of nepheline syenite, with a later subsidiary area of porphyritic syenite. The nepheline syenite contains abundant phenocrysts of alkali feldspar, zoned aegirine-augite and subordinate sodalite or nosean in a groundmass of the same minerals, melanite, occasional brown amphibole and analcime. Another variety of nepheline syenite contains nepheline phenocrysts. Aegirine-augite syenite also occurs, and feldspathic rocks described as bostonites form several hills in the western part of the complex. Igneous breccias are also present.
References:
DARTON, N.H. 1905. Sundance Folio: Wyoming-South Dakota. Geologic Atlas of the United States: 127
Location: