An adakitic pluton on Kyushu island, southwest Japan arc

An adakitic pluton is designated the Shiraishino granodiorite ca. 15 km(2) in outcrop area on Kyushu Island, southwestern Japanese arc. Sr, Y, and rare earth element concentrations and crystallization characteristics of the pluton are similar to those of adakites. Geochemical and tectonic considerations show that the pluton was derived from partial melting of a subducted slab with minor contamination of high Sr-87/(86) Sr material and subsequent fractional crystallization of plagioclase + hornblende. Compiled Sr and Y data indicate that most Cretaceous granitic rocks of southwestem Japan, generated by partial melting of mafic lower crust. Adakitic plutons derived by slab melting occur rarely and have older ages. It is concluded that plutonism in the southwestem Japanese arc during Cretaceous time was began with the emplacement of small adakitic intrusions and progressed to more abandant granitic rocks of the type derived from the mafic lower crust. (C) 2003 Elsevier Ltd. All rights reserved.