Shoshonite

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. Shoshonite was named by Iddings in 1895 for the Shoshone River in Wyoming.

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.

Igneous rocks with shoshonitic chemical characteristics must be:

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.

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.

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