Reference Work Entry


Part of the series Encyclopedia of Earth Science pp 576-577

Ultrabasic igneous rocks

  • D. R. Bowes

Ultrabasic is a general term, probably first used by J.W. Judd in 1881, applied to igneous rocks containing little or no feldspar and characterized essentially by the abundance of one or more common mafic minerals such as olivine, pyroxene, and amphibole. Chemically, the rocks contain <45% SiO2 and the term ultrabasic is used along with basic, intermediate, and acid (ic) in a chemical classification of igneous rocks as distinct from ultramafic, mafic, and felsic which refer to a mineralogical color index. However, the term ultrabasic commonly is used incorrectly as synonymous with ultramafic to include such rocks as ultramafic pyroxenites that contain as much as 55% SiO2.

Daly (1933) recognized three clans of ultrabasic rocks—(1) gabbroic and their extrusive equivalents, (2) feldspathoidal, and (3) ultramafic. For the first clan, the subdivision is not a natural one becaus ...

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