Glass inclusions in volcanic rocks in the Okinawa Trough back-arc basin:constraints on magma genesis and evolution

The major elemnt compositions of glass inclusions in plagioclase and pyroxene phenocrysts of basalt and pumice in the Okinawa Trough back-arc basin are determined by electron microprobe.The results indicate that basalt and pumice are cognate and respectively represent the proluots at early stages of mgmtism and at late stage of crystal fractionation.The initial magrma in the trough is rich in H₂O.The variation of H₂O content in magma may play an important role in the magma evolution.Plagioclase is the mineral crystallized throughout the whole magrmatic process and accumulates in the zoned magma chamber.From these features it can he inferred that the initial magma in the Okinawa Trough,whose opening began in recent years,is serious ly affected by fluid or other materials carried by subducting slab and the geocbemical feature of volcanic rocks is in some degree similar to that of lavas in island-arc environments.