Adakite

A term for a series of andesites, dacites and sodic rhyolites of unusual chemical composition erupted in some continent- based and island arc settings. They are characterized by high values of Sr, high Sr/Y and La/Yb ratios and negative Nb, Ti and Zr anomalies; the more mafic rocks of the series may contain high contents of transition metals such as Cr and Ni. They usually contain phenocrysts of plagioclase, amphibole, mica, very rare orthopyroxene, but no clinopyroxene; titanomagnetite, apatite, zircon and titanite are common.

The word adakite introduced by Defant and Drummond (1990) refers to intermediate to acidic plutonic and volcanic rocks which appear to be products of direct partial melting of subducted oceanic lithosphere at high pressure in which garnet is a stable residual phase and plagioclase is absent. According to Peacock et al. (1994) the necessary geodynamic condition which allows for the formation of adakitic magmas is the subduction of young and hot oceanic plates. More recently, other authors have proposed that adakitic rocks can be generated in the presence of asthenospheric window, partial melting of thickened lower crust, high pressure fractionation of garnet and amphibole or delaminated mafic lower crust. However, key adakitic geochemical features can be produced in normal asthenosphere derived arc magmas by melting-assimilation-storage-homogenization (MASH) and assimilation-fractional-crystallization (AFC) processes which have been well documented particularly in the Central Andes.