琉球列島久米島の鮮新統宇江城岳層からピクライト玄武岩の発見

抄録

Picrite basalts containing MgO up to 16 wt.% were found in the lowest part of the Pliocene Uegusukudake Formation on the north coast of Kumejima island ; in the close proximity northwest of the island lies an active back-arc basin, Okinawa Trough. The high Fo (92.2) of olivine and Cr₂O₃ (56.5%) of spinel in the picrite basalts indicate the existence of a magma with >13% MgO and >0.15% Cr₂O₃. The Kumejima picrite basalts are tholeiitic arc picrites having relatively high SiO₂ and low incompatible element abundances.They may have formed accompanying the opening of the Okinawa Trough.