Phonolite

Phonolite

Photographed by Michael P. Klimetz

[Analcime-Bearing]

Alberta

CANADA

Sanidine Phenocryst

Phonolite is a rare igneous, volcanic (extrusive) rock of intermediate composition, with aphanitic to porphyritic texture. Phonolite is unusual in that it forms from a highly silica undersaturated melt by low degrees of partial melting (less than 10%) of highly aluminous lower crustal rocks such as tonalite, monzonite and metamorphic rocks. Melting of such rocks to a very low degree promotes the liberation of aluminium, potassium, sodium and calcium via melting of feldspar, with some involvement of mafic minerals. The melt formed is silica undersaturated (that is, quartz is absent from the melts or solidified rocks), with feldspathoid species dominating over feldspar species in the melt. Phonolite occurrences are associated with certain geological processes and tectonic events lead to the melting of appropriate precursor lithologies. These include intracontinental hotspot volcanism, such as may form above mantle plumes covered by thick continental crust. A-type granites and alkaline igneous provinces are usually associated with phonolites. Phonolites may also be produced by low degree partial melting of underplates of granitic material in collisional orogenic belts. Phonolites are products of low degree partial melts, are silica undersaturated, and have feldspathoids in their normative mineralogy. Mineral assemblages in phonolite occurrences are usually feldspathoids (nepheline, sodalite, hauyne, leucite and analcite) and alkali feldspar (sanidine, anorthoclase or orthoclase), and rare sodic plagioclase. Biotite, sodium rich amphiboles and pyroxenes along with iron rich olivine are common minor minerals. Accessory phases include titanite, apatite, corundum, zircon, magnetite and ilmenite. Phonolite is a fine-grained equivalent of nepheline syenite.