Adakites: some variations on a theme

Abstract

Adakites were proposed over a decade ago to be products of the melting of young subducted oceanic crust. In fact, several new localities have been discovered since the original work documented approximately ten localities in modern arcs (e. g., southwestern Japan, Trans Mexican Volcanic Belt, etc.). But work over the past ten years has also shown that adakites can be generated by other processes during subduction (e. g., along the edge of tears in the subducting slab, remnant slabs left in the upper mantle, etc.). In addition, adakites appear to be associated with a suite of rocks including high-Mg andesites resulting from either adakite interaction with the mantle (Adak-type) or melting of the mantle during adakite interaction (Piip-type), niobium enriched arc basalts (NEAB) that are believed to be derived from the partial melting of a mantle metasomatized extensively by adakites, and possibly boninites (several researchers have found an adakite component in boninites). A new rock suite, the adakite metasomatic volcanic series, has been proposed to account for the various associations. In addition, a large number of NEAB have been found to contain ultramafic mantle xenoliths with clear evidence of reaction between ultradepleted mantle and adakites.