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]
Igneous rocks with shoshonitic chemical characteristics must be:[5]
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]
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.[6]
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.[7] 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:[8]
Ladolam gold mine, Lihir Island, Papua New Guinea;
Bingham copper-gold mine, Utah;
Grasberg copper-gold mine, Indonesia;
Oyu Tolgoi copper-gold mine, Mongolia.