Characteristics and tectonic setting of the shoshonite rock association

A review of the major occurrences of shoshonitic rocks suggests there is a group that is near-silica saturated, K-rich and has low iron enrichment that cannot be unambigously classified as part of calc-alkaline or alkali-basalt associations. This group is here referred to as the shoshonite rock association. The shoshonite rock association is characterised by: hypersthene-olivine normative basalts, low iron enrichment, high Na₂O + K₂O, high content of light ion lithophile elements, high but variable Al₂O₃, high Fe₂O₃/FeO and low TiO₂. Mineralogical characteristics include: coexisting plagioclase and sanidine in the groundmass, K-feldspar rims on plagioclase phenocrysts, plagioclase An_50?85 Ab_40?15 Or_10?0 and low TiO₂ content and lack of iron enrichment in clinopyroxene. Shoshonitic rocks on continental margins are younger, stratigraphically higher and more distant from the oceanic trench than the high-K calc-alkaline or calc-alkaline suites, but there is a complete gradation between the suites. A similar zonation occurs in some island arcs. In other island arcs there is no spatial zonation of the suites but successively more K-rich lavas are produced above an ever steepening subduction zone. Steepening leads to ‘failure’ or flipping of the subduction zone and uplift and block faulting within the arc. Shoshonitic rocks are most commonly associated with this phase of island are development.