Nephelinitic and Basanitic Rocks |
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| Authors: | le BAS M J |
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| Institution: | Department of Geology, University of Leicester Leicester LE1 7RH, UK |
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| Abstract: | Basanites (which mineralogically should be characterized byan abundance of plagioclase and only a little feldspathoid)and nephelinitic rocks (which normally lack modal plagioclasebut should contain nepheline) are not easily distinguished inthe field, and both are commonly confused with basalts. Sincethe mineralogical distinction sometimes cannot be applied, asin the case when no modal felsic minerals are apparent in thegroundmass, a chemical distinction between nephelinitic andbasanitic rocks becomes desirable. The best solution is providedby the use of the CIPW norm with basanites recognized as having> 5% normative ab and < 20% normative ne, melanephelinitesas having < 5% normative ab and < 20% normative ne, andnephelinites as having > 20% normative ne. It is proposedthat melanephelinites defined in this manner can be furtherdivided and that rocks formerly termed olivine nephelinite shouldnow be called olivine melanephelinite, and that the pyroxene-phyric,olivine-poor melanephelinites should now be termed pyroxenemelanephelinite. |
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