冲绳海槽南部橄揽拉斑玄武岩研究

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℃。

STUDY ON THE OLIVINE THOLEIITE OF THE SOUTHERN OKINAWA TROUGH

Dredging of submarine rocks was carried 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 southwest margin of the Yonaguni Depression, the earliest spreading part of a back-arc basin in the southern Okinawa Trough. The sampling site was located at 24°54' - 24°55' N and 123°11' - 123°12' E. For the specimens, thin section examination, rare earth element and microelement analysis, strontium isotope assaying and petrochemistry study were undertaken. The results show that the rock samples represent olivine tholeiite, whose distribution patterns in terms of rare earth element and microelement characteristics are similar to those of P-MORB. The lava belongs to a low silica, undersaturated type, deriving from the upper mantle, around 43km below the surface. In that depth, there is lower differentiation and slight hybridization, the mineral begins crystallizing at temperatures ranging between 1150°C and 1200°C.

The studies suggest that the Olivine tholeiite start to solidify while the melted upper mantle material rose rapidly and was exposed to the seawater or loose submarine sediments. It is signified that the sea-floor spreading developed in the Okinawa Trough at that time, or was a non-continental crust. However, the Okinawa Trough is an important boundary between the petrologic tectonic environments under any circumstances.