Elemental Compositions of Clinopyroxenes from Southern Okinawa Trough Basalt:Implications for Magmatism and Structural Environment in an Initial Back-Arc Basin  

作　　者：GUO Kun ZHAI Shikui FAN Di ZHANG Xia ZHANG Yu ZHU Zhimin 

机构地区：[1]Laoshan Laboratory,Qingdao 266237,China [2]Key Laboratory of Submarine Geosciences and Prospecting Techniques,Ministry of Education,College of Marine Geosciences,Ocean University of China,Qingdao 266100,China [3]Key Laboratory of Submarine Geosciences&Second Institute of Oceanography,Ministry of Natural Resources,Hangzhou 310012,China

出　　处：《Journal of Ocean University of China》2024年第3期643-653,共11页中国海洋大学学报（英文版）

基　　金：supported by the National Natural Science Foundation of China(No.42276085);the National Basic Research Program of China(No.2013CB429702).

摘　　要：The major and trace elemental compositions of clinopyroxene from basalt were used to characterize the nature of the primitive magma and structural environment beneath the southern Okinawa Trough(SOT),which is an initial back-arc basin at a continental margin.The clinopyroxenes in the basalt were augite with variable Mg^(#)contents(73.37-78.22).The regular variations in major oxide contents(i.e.,CaO,FeO,TiO_(2),and Cr)with decreasing Mg#implied that the clinopyroxenes evolved from being enriched in Mg,Ca,and Cr to being enriched in Fe and Ti.The clinopyroxenes had relatively low rare earth element concentrations(7.51×10^(-6) to 12.68×10^(-6))and negative Eu anomalies(δEu=0.67-0.95).The Kd_(cpx) values of clinopyroxenes(0.2-0.26),which were used to examine whether the clinopyroxene was equilibrated with its host basalt,demonstrate that these clinopyroxene phenocrysts were not captured crystals but were instead produced by crystallization differentiation of the magma.The calculated clinopyroxene crystallization temperatures showed a narrow range of 990-1061℃,and their crystallization pressures ranged from 2.0 to 3.2 kbar.The geochemistry features of these clinopyroxenes indicated that the parent magma belonged to the subalkaline tholeiitic magma series and suggested that the magma experienced crystallization differentiation of olivine,plagioclase,and clinopyroxene,where the crystallization of plagioclase occurred earlier than that of clinopyroxene.Combined with geophysical data,this research on primitive magma and its crystallization differentiation from clinopyroxene indicates that the SOT is in the stage of‘seafloor spreading’and that basaltic rocks produced from tholeiitic magma represent the generation of oceanic crust.

关 键 词：trace elements CLINOPYROXENES southern Okinawa Trough seafloor spreading back-arc basin 

分 类 号：P736[天文地球—海洋地质]