Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Black Buttes

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

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith