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Aegirine phonolite. Dark prismatic minerals are aegirine phenocrysts.

Phonolite is a rare extrusive volcanic rock of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grain) to porphyritic (mixed fine- and coarse-grain).

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.

Contents

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.

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

Outcrop of phonolite at Beemerville Complex, New Jersey

Nepheline syenites and phonolites occur widely distributed throughout the world[2] in Canada, Norway, Greenland, Sweden, the Ural Mountains, the Pyrenees, Italy, Eifel in Germany, Brazil, the Transvaal region, the Magnet Cove igneous complex of Arkansas, the Beemerville Complex of New Jersey,[3] as well as on oceanic islands such as the Canary Islands.[4]

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

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.[9]

See also

References

  1. ^ Blatt, Harvey and Robert J. Tracy, Petrology, Freeman, 2nd ed. 1996, p. 52, ISBN 0-7167-2438-3 [Amazon-US | Amazon-UK]
  2. ^ Woolley, A.R., 1995. Alkaline rocks and carbonatites of the world., Geological Society of London.
  3. ^ Eby, G. N., 2012, The Beemerville alkaline complex, northern New Jersey, in Harper, J. A., ed., Journey along the Taconic unconformity, northeastern Pennsylvania, New Jersey, and southeastern New York: Guidebook, 77th Annual Field Conference of Pennsylvania Geologists, Shawnee on Delaware, PA, p. 85-91.
  4. ^ 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.
  5. ^ Bassett, W. A. (1961-10). "Potassium-Argon Age of Devils Tower, Wyoming". Science 134 (3487): 1373–1373. Bibcode:1961Sci...134.1373B. doi:10.1126/science.134.3487.1373. ISSN 0036-8075.
  6. ^ Marshall, Patrick, 'The occurrence of a mineral hitherto unknown in the phonolites of Dunedin, New Zealand', 1929.
  7. ^ 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
  8. ^ 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
  9. ^ Affolter, J., 2002 Provenance des silex préhistoriques du Jura et des régions limitrophes., Archéologie neuchâteloise, 28