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Shoshonite is a type ofigneous rock. More specifically, it is apotassium-rich variety ofbasaltictrachyandesite,[1] composed ofolivine,augite andplagioclasephenocrysts in agroundmass with calcic plagioclase andsanidine and some dark-coloredvolcanic glass. Shoshonite gives its name to the shoshonite series and grades intoabsarokite with the loss of plagioclase phenocrysts and intobanakite with an increase in sanidine.[2] Shoshonite was named by Iddings in 1895 for theShoshone River inWyoming.[3]Textural and mineralogical features of potash-rich rocks of the absarokite-shoshonite-banakite series strongly suggest that most of the large crystals andaggregates are not true phenocrysts as previously thought but arexenocrysts andmicroxenoliths, suggesting a hybrid origin involving assimilation ofgabbro by high-temperaturesyeniticmagma.[4]
Igneous rocks with shoshonitic chemical characteristics must be:[5]
Shoshonitic rocks tend to be associated withcalc-alkalineisland-arcsubductionvolcanism, but the K-rich shoshonites are generally younger and above deeper, steeper parts or theBenioff zone.[5][6]
Volcanic rocks of the absarokite-shoshonite-banakite series described fromYellowstone Park by Iddings and the similar ciminite-toscanite series described from westernItaly by Washington are associated withleucite-bearing rocks, potassium-richtrachytes and andesitic rocks. Similar associations are described from several other regions including Indonesia and theEast African Rift.[7]
In theAeolian Arc in the southernTyrrhenian Sea (between the Eurasian and Africantectonic plates), volcanism has changed betweencalc-alkaline to high-K calc-alkaline to shoshonitic with the last one million years, possibly due to the progressive steepening of theBenioff zone, which is inclined at 50-60°.[5] An example of shoshonite lava in this region is the Capo Secco lava shield nearVulcano.[8] LateCretaceousPuerto Rican volcanism is interpreted to have occurred in a similartectonic 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]
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