Shoshonite

{{short description|Potassium-rich variety of basaltic trachyandesite}} thumb|File:Shoshonite lava flows on South Table Mountain, Colorado

'''Shoshonite''' is a type of igneous rock. More specifically, it is a potassium-rich variety of basaltic trachyandesite,<ref name="LeMaitre2002">{{cite book | title=Igneous Rocks — A Classification and Glossary of Terms | publisher=Cambridge University Press | author=Le Maitre, R.W. (editor) | year=2002 | location=Cambridge | page=141 | isbn=0-521-66215-X| edition=2nd }}</ref> composed of olivine, augite and plagioclase phenocrysts in a groundmass with calcic plagioclase and sanidine and some dark-colored volcanic glass. Shoshonite gives its name to the shoshonite series and grades into absarokite with the loss of plagioclase phenocrysts and into banakite with an increase in sanidine.<ref>Gest, D. E. and A. R. McBirney, ''Genetic relations of shoshonitic and absarokitic magmas, Absaroka Mountains, Wyoming,'' Journal of Volcanology and Geothermal Research, Vol 6; issues 1-2, Sept 1979. pp 85-104</ref> Shoshonite was named by Iddings in 1895 for the Shoshone River in Wyoming.<ref>[http://www.websters-online-dictionary.org/sh/shoshonite.html Shoshonite: Webster's Online Dictionary] {{webarchive|url=https://web.archive.org/web/20091127105515/http://www.websters-online-dictionary.org/sh/shoshonite.html |date=2009-11-27 }}</ref> Textural and mineralogical features of potash-rich rocks of the absarokite-shoshonite-banakite series strongly suggest that most of the large crystals and aggregates are not true phenocrysts as previously thought but are xenocrysts and microxenoliths, suggesting a hybrid origin involving assimilation of gabbro by high-temperature syenitic magma.<ref>Prostka, Harold J., ''Hybrid Origin of the Absarokite-Shoshonite-Banakite Series, Absaroka Volcanic Field, Wyoming,'' 1973 GSA Bulletin February, 1973 v. 84 no. 2 p. 697-702 [http://bulletin.geoscienceworld.org/cgi/content/abstract/84/2/697 abstract]</ref>

==Chemical characteristics==

Igneous rocks with shoshonitic chemical characteristics must be:<ref name="morrison80">Morrison, Gregg, 1980, ''Characteristics and tectonic settings of shoshonite rock association,'' Lithos, 13, 97-108</ref> #Near-saturated in silica; #Low iron enrichment; #High total alkalies (Na₂O + K₂O > 5%); #High K₂O/Na₂O; #Steep positive slope for K₂O versus SiO₂ at low SiO₂; #Enrichment in P, Rb, Sr, Ba, Pb, light rare earth elements; #Low TiO₂; #High but variable Al₂O₃; #High Fe₂O₃/FeO.

==Tectonic settings and examples==

Shoshonitic rocks tend to be associated with calc-alkaline island-arc subduction volcanism, but the K-rich shoshonites are generally younger and above deeper, steeper parts or the Benioff zone.<ref name="morrison80"/><ref name="mullerandgroves2000"/>

Volcanic rocks of the absarokite-shoshonite-banakite series described from Yellowstone Park by Iddings and the similar ciminite-toscanite series described from western Italy by Washington are associated with leucite-bearing rocks, potassium-rich trachytes and andesitic rocks. Similar associations are described from several other regions including Indonesia and the East African Rift.<ref>Joplin, Germaine A., ''The shoshonite association: A review,'' Journal of the Geological Society of Australia, v. 15, #2, 1968, pp 275-294 DOI:10.1080/00167616808728699</ref>

In the Aeolian Arc in the southern Tyrrhenian Sea (between the Eurasian and African tectonic plates), volcanism has changed between calc-alkaline to high-K calc-alkaline to shoshonitic with the last one million years, possibly due to the progressive steepening of the Benioff zone, which is inclined at 50-60°.<ref name="morrison80"/> An example of shoshonite lava in this region is the Capo Secco lava shield near Vulcano.<ref name="Peccerillo2017">{{cite book | title=Cenozoic Volcanism in the Tyrrhenian Sea Region | publisher=Springer | author=Peccerillo, Angelo | year=2017 | page=239 | isbn=978-3-319-42489-7| edition=2nd }}</ref> Late Cretaceous Puerto Rican volcanism is interpreted to have occurred in a similar tectonic setting.<ref name="morrison80"/>

In places, shoshonitic and high-potassium calc-alkaline magmatism is associated with world-class hydrothermal gold and copper-gold mineralization. Examples include:<ref name="mullerandgroves2000">Müller D., Groves D.I. (2019) Potassic igneous rocks and associated gold-copper mineralization (5th ed.). Mineral Resource Reviews. Springer-Verlag Heidelberg, 398 pp</ref> :Ladolam gold mine, Lihir Island, Papua New Guinea; :Bingham copper-gold mine, Utah; :Grasberg copper-gold mine, Indonesia; :Oyu Tolgoi copper-gold mine, Mongolia.

==References== {{reflist}} {{Rock type}}

Category:Volcanic rocks