Shoshonite

Shoshonite is a type of igneous rock. More specifically, it is a potassium-rich variety of basaltic trachyandesite,[1] 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.[2] Shoshonite was named by Iddings in 1895 for the Shoshone River in Wyoming.[3]

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.[4]

Chemical characteristics

Igneous rocks with shoshonitic chemical characteristics must be:[5]

1. Near-saturated in silica;
2. Low iron enrichment;
3. High total alkalies (Na₂O + K₂O > 5%);
4. High K₂O/Na₂O;
5. Steep positive slope for K₂O versus SiO₂ at low SiO₂;
6. Enrichment in P, Rb, Sr, Ba, Pb, light rare earth elements;
7. Low TiO₂;
8. High but variable Al₂O₃;
9. 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.[5][6]

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.[7]

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°.[5] An example of shoshonite lava in this region is the Capo Secco lava shield near Vulcano.[8] Late Cretaceous Puerto Rican volcanism is interpreted to have occurred in a similar tectonic setting.[5]

In places, shoshonitic and high-potassium calc-alkaline magmatism is associated with world-class hydrothermal gold and copper-gold mineralization. Examples include:[6]

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

1. Le Maitre, R.W. (editor) (2002). Igneous Rocks — A Classification and Glossary of Terms (2nd edition). Cambridge: Cambridge University Press. p. 141. ISBN 0-521-66215-X.CS1 maint: extra text: authors list (link)
2. 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
3. Shoshonite: Webster's Online Dictionary Archived 2009-11-27 at the Wayback Machine
4. 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 abstract
5. Morrison, Gregg, 1980, Characteristics and tectonic settings of shoshonite rock association, Lithos, 13, 97-108
6. Müller D., Groves D.I. (2019) Potassic igneous rocks and associated gold-copper mineralization (5th ed.). Mineral Resource Reviews. Springer-Verlag Heidelberg, 398 pp
7. 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
8. Peccerillo, Angelo (2017). Cenozoic Volcanism in the Tyrrhenian Sea Region (2nd edition). Springer. p. 239. ISBN 978-3-319-42489-7.