冲绳海槽南部橄揽拉斑玄武岩研究

STUDY ON TIIE OLIVINE THOLEIITE OF THE SOUTHERN OKINAWA TROUGH

1995年夏季航次冲绳海槽地质、地球物理调查对海底岩石实施拖网取样，在与那国海底洼地西南边线采集到新鲜玄武岩样品。对岩石样品进行了薄片鉴定、岩石化学分析、稀土和微量元素分析以及银同位素分析等。结果表明，岩石为橄榄拉斑玄武岩，岩石微量元素和稀土元素分布具有地慢柱洋中脊玄武岩的特点；成岩熔浆来自上地慢，分异程度较低，混合作用较弱，结晶液线温度为1150－1200℃。

Dredging of submarine rocks was catried out during the geological survey of the Okinawa Trough in the summer of 1995. Rock samples of fresh basalt in Place were collected from the sorthwest margin of the Yonaguni Depression,the eariest spreading part of a backarc basin in the southern Okinawa Trough. The sampling site was located at 24°54'-24°55' N and 123°11'-123°12' E. For trhe specimens, thin sechon examinahon, rare earth element and ndcroelement analysis, stronhum isotope assaying and petrochemistry stUdy were undertaken. The restilts show that the rock samples represent olivine tholeiite, whose distribuhon pattems in terms of rare earth element and ndcroelement charaterishcs are similar to those of P-MORB. The lavabelongs to a low silica undersamfated type, deriving from the upper manlle, around 43km below the surfase. In that depth, there is lower differenhation and slight hybridiahon, the minera begins crystalizing at temperatures ranging betWeen 1150℃and 1200℃.The studes suggest that the Olivine tholeiite start to solidify while the melted uPper manhe material rose rapidly and was exposed to the seawater or loose submarine sediments. It is signified that the seafloor spreading developed in the okinawa Trough at that time, or was a nonconhnenta crust. However, the Okinawa Trough is an important boundary between the petrologic tectonic environments under any circumstances.