Phonolite

Aegirine phonolite. Dark prismatic minerals are aegirine phenocrysts.

Phonolite is a rare igneous, volcanic (extrusive) rock of intermediate (between felsic and mafic) composition, with aphanitic to porphyritic texture.

The name phonolite comes from the Greek meaning (more or less) "sounding stone" because of the metallic sound it produces if an unfractured plate is hit, hence the English name clinkstone.

Genesis

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 (ie; quartz is absent from the melts or solidified rocks), with feldspathoid species dominating over feldspar species in the melt.

Phonolite occurrences are associated with a few geological processes and tectonic events, which can 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.

Mineralogy

Phonolites, as they are products of low degree partial melts, are silica undersaturated, and have feldspathoids in their normative mineralogy.

Mineral assemblages in phonolite occurrences are usually abundant 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.^[1] Phonolites are silica under-saturated, as illustrated by the position of phonolite in the TAS classification and QAPF diagrams.

Phonolite is a fine-grained equivalent of nepheline syenite, and the genesis of such magmas is discussed in the treatment of that rock type.

Occurrence

Nepheline syenites and phonolites occur widely distributed throughout the world^[2] in Canada, Norway, Greenland, Sweden, the Ural Mountains, the Pyrenees, Italy, Brazil,the Transvaal region, and Magnet Cove igneous complex of Arkansas, as well as on oceanic islands (eg; Canary Islands^[3]).

Nepheline-normative rocks occur in close association with the Bushveld Igneous Complex, possibly formed from partial melting of the wall rocks adjacent to that large ultramafic layered intrusion.

Examples

• Devil's Tower, a striking example of columnar jointed phonolite.
• Dunedin, New Zealand^[4]
• Hoodoo Mountain, northwestern British Columbia, Canada.
• Jebel Nefusa, Libya^[5]
• Teide, a stratovolcano on the island of Tenerife.^[6]
• Mont Gerbier de Jonc South East France

Economic importance

Phonolites can be of interest as dimension stone or as aggregate for gravels.

Rarely, economically mineralised phonolite-nepheline syenite alkaline complexes can be associated with rare earth mineralisation, uranium mineralisation and phosphates, such as at Phalaborwa, South Africa.

Phonolite tuff was used as a source of flint for adze heads and such by prehistoric man from Hohentwiel and Hegau, Germany.^[7]

See also

• Normative mineralogy
• Fractional crystallisation

References

1. ^ Blatt, Harvey and Robert J. Tracy, Petrology, Freeman, 2nd ed. 1996, p. 52, ISBN 0-7167-2438-3
2. ^ Woolley, A.R., 1995. Alkaline rocks and carbonatites of the world., Geological Society of London.
3. ^ S. E. Bryana, R. A. F. Cas, and J. Mart, 1998. Lithic breccias in intermediate volume phonolitic ignimbrites, Tenerife (Canary Islands): constraints on pyroclastic flow depositional processes, Journal of Volcanology and Geothermal Research, v81, May 1998, Pages 269-296.
4. ^ Marshall, Patrick, 'The occurrence of a mineral hitherto unknown in the phonolites of Dunedin, New Zealand', 1929.
5. ^ Bausch, W.M., The central part of the Jebel Nefusa volcano (Libya) survey map, age relationship and preliminary results, International Journal of Earth Sciences, 1978, pp 389-400 ISSN 1437-3262
6. ^ Ablay, G.J. et al. (1998) Basanite–Phonolite Lineages of the Teide–Pico Viejo Volcanic Complex, Tenerife, Canary Islands, Journal of Petrology, volume 39 number 5 pages 905-936 PDF - retrieved 2010-02-24
7. ^ Affolter, J., 2002 Provenance des silex préhistoriques du Jura et des régions limitrophes., Archéologie neuchâteloise, 28

• See Woolley, A.R., Alkaline rocks and carbonatites of the world