Nephelinite is a fine-grained oraphaniticigneous rock made up almost entirely ofnepheline andclinopyroxene (varietyaugite). Ifolivine is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemblebasalt in hand specimen. However,basalt consists mostly of clinopyroxene (augite) and calcicplagioclase.
Basalt,alkali basalt,basanite, tephritic nephelinite, and nephelinite differ partly in the relative proportions of plagioclase and nepheline. Alkali basalt may contain minor nepheline and does contain nepheline in its CIPWnormative mineralogy. A critical ratio in the classification of these rocks is the ratio nepheline/(nepheline plus plagioclase).Basanite has a value of this ratio between 0.1 and 0.6 and also contains more than 10% olivine. Tephritic nephelinite has a value between 0.6 and 0.9. Nephelinite has a value greater than 0.9. Le Maitre (2002) defines and discusses these and other criteria in the classification of igneous rocks.
Nephelinite is an example of a silica-undersaturated igneous rock. The degree of silica saturation can be evaluated withnormative mineralogy calculated from chemical analyses, or with actual mineralogy for completely crystallized igneous rocks with equilibrated assemblages. Silica-oversaturated rocks containquartz (or another silicapolymorph). Silica-undersaturated mafic igneous rocks contain magnesian olivine but not magnesian orthopyroxene, and/or afeldspathoid. Silica-saturated igneous rocks fall in between these two classes.
Silica-undersaturated, mafic igneous rocks are much less abundant than silica-saturated and oversaturated basalts. Genesis of the less common mafic rocks such as nephelinite is usually ascribed to more than one of the following three causes:
Nephelinites and similar rocks typically contain relatively high concentrations of elements such as the light rare earths, as consistent with a low degree of melting of mantleperidotite at depths sufficient to stabilizegarnet. Nephelinites are also associated withcarbonatite in some occurrences, consistent with source rocks relatively rich in carbon dioxide.
Nephelinite is found on ocean islands such asOʻahu, although the rock type is very rare in theHawaiian Islands. It is found in a variety of continental settings. An example is theHamada nephelinite lava flow in southwestJapan which occurred in the lateMiocene age. Nephelinite is also associated with the highly alkalic volcanism of theOl Doinyo Lengaivolcanic field inTanzania.Nyiragongo, another African volcano known for its semipermanent lava lake activity, erupts lava made ofmelilite nephelinite. The unusual chemical makeup of thisigneous rock may be a factor in the unusual fluidity of its lavas.
Olivine nephelinite flows also occur in theWells Gray-Clearwater volcanic field in east-centralBritish Columbia and atVolcano Mountain in centralYukon Territory. Melilite olivine nephelinite intrusives ofCretaceous age are found in the area aroundUvalde, Texas.
Flett, John Smith (1911)."Nephelinites" . InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 19 (11th ed.). Cambridge University Press. p. 384.
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