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 metasoma
Adakites are geochemically distinct volcanic and plutonic granitoid rocks found in intraoceanic island arc settings where relatively young, hot oceanic lithosphere has been subducted and melted (termed "slab melting"), and in continental arcs, such as the Andes, where melting has taken place at the base of tectonically- or magmatically-thickened lower crust (termed "lower crustal melting"). In both settings, the distinctive geochemical signature of adakitic granitoids is attributed to an origin by partial melting of a variably-hydrated metabasaltic protolith at sufficient depths for garnet to be stable within the residual crystalline assemblage (i.e., residues of garnet-amphibolite and/or eclogite). Once generated, "pristine" or "parental" adakite melts may have their composition subsequently modified by processes of assimilation (of either mantle or continental material) and crystal fractionation during transport to and emplacement in the middle-upper crust. Late Mesozoic (early-mid Cretaceous, similar to160
Andean adakites are common. The high-pressure source mineralogy signaled by Andean adakite trace element characteristics originates in three ways. Listed in order of relative importance, the ways are tectonic thickening of Andean crust, subduction-erosion of forearc crust, and subduction of young oceanic crust. Over the past 30 Ma adakites representing each mode of origin have erupted in the Chilean-Argentine Andes. Occurrence is matched with specific tectonic conditions and events in the rapidly changing tectonics of the Chilean-Argentine Andean margin. Adakite occurrences in Andean margins of the past promise to be excellent tectonic indicators, as well as guides to mineralization.