Abstract
A very small intrusion of a dark soda-minette rock occurs intermixed with a leuco-phase along Squaw Creek in the northern Black Hills. It consists of phenocrysts of pyroxene, phlogopite, and carbonate pseudomorphs of olivine(?), the latter surrounded by phlogopite, in a groundmass of aegirine, alkali feldspar, and secondary carbonate. Pyroxene phenocrysts are largely replaced by ferroan dolomite. Quartz xenocrysts show replacement rims of alkali feldspar and an inner rim of hematite. Chemically the rock is relatively low in silica and ferrous oxide, but relatively high in magnesia, lime, and potash, with carbon dioxide as the main volatile component. The rock probably originated as a differentiate of the alkaline magmas that produced the Tertiary phonolites in the northern Black Hills. Carbonate pseudomorphs of olivine(?), plus phenocrysts of pyroxene and phlogopite, suggest an origin in the lower crust or upper mantle. Crystallization of the phenocrysts most likely began at great depth and followed the sequence of olivine, phlogopite, and pyroxene, whereas the aegirine and feldspar groundmass crystallized after injection into the lower Paleozoic section.
