Determination and Tectonic Significance of Volcanic Rock Series in the Okinawa Trough

Guo Kun; Zhai Shikui; Yu Zenghui; Cai Zongwei; Zhang Xia

doi:10.3799/dqkx.2016.524

Abstract：The Okinawa Trough is a young back-arc basin at its early back-arc spreading stage,and is a natural laboratory for understanding the formation and evolution of back-arc basins and crust-mantle interaction processes under this tectonic setting. However,with the gradually deepening of research work,the rock series of volcanic rocks from the Okinawa Trough are be-coming an important and urgent need to solve problem.Based on the geochemical compositions of some pumices determined re-cently by authors and the petrologic and geochemical data of volcanic lavas from the Okinawa Trough in literatures,the rock series of volcanic rocks from the Okinawa Trough have been determined in detail.The tectonic significance of volcanic rocks and the genesis relationship between pumices and basalts have also been discussed.The magmatism in the Okinawa Trough is bi-modal,being comprised dominantly of basic basalts and acidic rhyolites,accompanied by minor intermediate rocks.The basalts are mainly olivine tholeiitic basalts,which belong to subalkalic series,and the pumices are mostly rhyodacite or rhyolite,which belong to subalkalic series too.According to the geochemical compositions,basalts are not only similar to those from the mid-ocean ridge but also from the island arc tectonic environment.They are different from those occurring at mid-ocean ridge sprea-ding center or mature back-arc basins,but with distinct geochemical features associated with distinct tectonic environment their occurred,a back-arc basin at early spreading stage.The acid pumice,a widely distributed rock type in Okinawa Trough,is characterized by the geochemical compositions that are typical for volcanic rocks from island arcs.The magma of intermediate-acidic pumices,with same sources as associated basalts,may be formed by fractional crystallization of basalt magma.