Shoshonite

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Shoshonite is a basaltic rock, properly a potassic trachyandesite, 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 bannakite with an increase in sanidine.[1] Shoshonite was named by Iddings in 1895 for the Shoshone River in Wyoming.[2]

Textural and mineralogical features of potash-rich basaltic 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.[3]

Chemical characteristics

Igneous rocks with shoshonitic chemical characteristics must be:[4]

  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.[4]

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.[5]

In the Eolian 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, with is inclined at 50-60°. Late Cretaceous Puerto Rican volcanism is interpreted to have occurred in a similar tectonic setting.[4]

References

  1. ↑ 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
  2. ↑ Shoshonite: Webster's Online Dictionary
  3. ↑ 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
  4. ↑ 4.0 4.1 4.2 Morrison, Gregg, 1980, Characteristics and tectonic settings of shoshonitie rock association, Lithos, 13, 97-108
  5. ↑ 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
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