Is youngest igneous rock unit (of eight) of the Chico sill complex mapped in western part of Raton-Clayton volcanic field, Colfax Co, NM in Las Vegas-Raton basin. Three varieties of intrusive phonolite recognized: green phonolite, silver-gray phonolite, and ocellar analcine phonolite. Green and silver-gray varieties seem to represent gradational series from green phonolite having little alkali feldspar but abundant acmite, to green phonolite having twinned and platy alkali feldspar phenocrysts and much acmite, to silver-gray having platy and twinned feldspar phenocrysts and sparse acmite, and finally to nonporphyritic silver-gray, to silver-gray having curved, pearly alkali feldspar phenocrysts and very little acmite. Distinction between green and silver-gray phonolite based chiefly on weathering characteristics displayed in hand specimens and thin sections, but chemically they appear part of single gradational series. Petrography and petrochemistry described. K-Ar ages suggest the phonolites were intruded between 26 and 20 m.y. ago. Age is Oligocene and Miocene.

Source: GNU records (USGS DDS-6; Denver GNULEX).