Genesis
Phonolite is unusual in that it forms from magma with a relatively low silica content, generated by low degrees of partial melting (less than 10%) of highly aluminous rocks of the lower crust 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. Because the rock is silica undersaturated, it is free of quartz or other silica crystals, and is dominated by low-silica feldspathoid minerals more than feldspar minerals.
A few geological processes and tectonic events can melt the necessary precursor rocks to form phonolite. 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 found alongside phonolites. Phonolites may also be produced by low degree partial melting of underplates of granitic material in collisional orogenic belts.
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