Phonolitic Intrusive Rocks

Gray to greenish-gray sills, laccoliths, and small stocks of phonolite, trachyphonolite, and trachyandesite. Contains phenocrysts of andesine, feldspathoids, aegirine-augite, biotite, and sphene in a finely crystalline plagioclase-biotote-feldsphoid groundmass.
State South Dakota
Name Phonolitic Intrusive Rocks
Geologic age Eocene to Paleocene
Lithologic constituents
Igneous > Hypabyssal > Felsic-hypabyssal > Hypabyssal-felsic-alkaline (Dike or sill, Laccolith, Stock or pipe)Phonolite, trachyphonolite
Igneous > Hypabyssal > Felsic-hypabyssal > Hypabyssal-trachyte (Dike or sill, Laccolith, Stock or pipe)trachyandesite

Martin, J.E., Sawyer, J.F., Fahrenbach, M.D., Tomhave, D.W., and Schulz, L.D., 2004, Geologic Map of South Dakota: South Dakota Geological Survey, GM 10, scale 1:500,000.

NGMDB product
Counties Lawrence