Origin and evolution of alkali basaltic magma in the Okinawa Trough

The contents of major element cromposition of the phenocrysts and the matrix glass as well as the spinel inclusions and the melt inclusion in the phenocrysts of the basalt dredged from Station 133 of the Okinawa Trough are determined by electron microprobe.The results show that the basalt is a dorgalite consisting of phenocrysts of bytownite, chrysolite, clinopyroxene and magnetite as well as labradorite microcrystal,matrix glass and a few bits of broken vein quartz.Glassy melt inclusion and chromohercynite or chrompleonaste exist in bytownite and chrysolite.The formation of the spinels is related to partial melting of mantle.The melt inclusions stand for a primary alkali dorgalitic magma,whose composition corresponds to olivine gabbro.The basaltic magma was generated from partial melting of spinel-lherzolite of the upper mantle and evolved in a process of "alkali dorgalitic magma→trachybasaltic magma→basaltic trachytic magma→trachytic magma".Assimilation and hybridization of crustal material may exist during magma upwelling in every evolutionary stage.