Sr, Nd isotope geochemistry of volcanic rock series and its geological significance in the middle Okinawa Trough

摘要

There exists extensive basic-acidic volcanic rock series in themiddle section of the Okinawa Trough. Different types of thesevolcanic rocks have their own average strontium ratios Of 0.704 749,0.705 062, 0.708 771, 0.704 840 and 0.720 301 with average143~Nd/~144Nd ratios of 0.512 820, 0.512 673, 0.512 413, 0.512 729and 0.512 034. These ratios of Sr and Nd isotopes all fall on atheoretic hyperbolic curve of mixing between two end-members of MORBand rhyolitic magma. So we infer that these different kinds ofvolcanic rocks in the middle Okinawa Trough are the erupted productin different stages of formation and evolution of the trough crust.MORB magma, which had suffered assimilation, mixed with theearly-formed crust-derived rhyolitic partial melt mass at differentratios; then, these mixed magma erupted and formed volcanic rocktypes of the trough. This study indicates that the Okinawa Trough iscoming into a stage of submarine spreading from the stage ofcontinental rift.