NEPHELINITIC AND BASANITIC ROCKS

Abstract

Basanites (which mineralogically should be characterized by an abundance of plagioclase and only a little feldspathoid) and nephelinitic rocks (which normally lack modal plagioclase but should contain nepheline) are not easily distinguished in the field, and both are commonly confused with basalts. Since the mineralogical distinction sometimes cannot be applied, as in the case when no modal felsic minerals are apparent in the groundmass, a chemical distinction between nephelinitic and basanitic rocks becomes desirable. The best solution is provided by the use of the CIPW norm with basanites recognized as having > 5% normative ab and < 20% normative ne, melanephelinites as having < 5% normative ab and < 20% normative ne, and nephelinites as having > 20% normative ne. It is proposed that melanephelinites defined in this manner can be further divided and that rocks formerly termed olivine nephelinite should now be called olivine melanephelinite, and that the pyroxene-phyric, olivine-poor melanephelinites should now be termed pyroxene melanephelinite.