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boninite
Boninite is a mafic extrusive rock high in both magnesium and silica, formed in fore-arc environments, typically during the early stages of subduction. The rock is named for its occurrence in the Izu-Bonin arc south of Japan. It is characterized by extreme depletion in incompatible trace elements that are not fluid mobile (e.g., the heavy rare earth elements plus Nb, Ta, Hf) but variable enrichment in the fluid mobile elements (e.g., Rb, Ba, K). They are found almost exclusively in the fore-arc of primitive island arcs (that is, closer to the ocean trench) and in ophiolite complexes thought to represent former fore-arc settings.
Boninite is considered to be a ''primitive'' andesite derived from melting of metasomatised mantle.
Similar Archean intrusive rocks, called sanukitoids, have been reported in the rocks of several early cratons.
== Petrology ==
Boninite typically consists of phenocrysts of pyroxenes and olivine in a crystallite-rich glassy matrix.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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