首页|Elemental Compositions of Clinopyroxenes from Southern Okinawa Trough Basalt:Implications for Magmatism and Structural Environment in an Initial Back-Arc Basin
Elemental Compositions of Clinopyroxenes from Southern Okinawa Trough Basalt:Implications for Magmatism and Structural Environment in an Initial Back-Arc Basin
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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,TiO2,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(8Eu=0.67-0.95).The Kdcpx 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 3.2kbar.The geochemis-try 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 plagio-clase occurred earlier than that of clinopyroxene.Combined with geophysical data,this research on primitive magma and its crystal-lization 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.
Key Laboratory of Submarine Geosciences and Prospecting Techniques,Ministry of Education,College of Marine Geosciences,Ocean University of China,Qingdao 266100,China
Key Laboratory of Submarine Geosciences & Second Institute of Oceanography,Ministry of Natural Resources,Hangzhou 310012,China
National Natural science Foundation of ChinaNational Basic Research Program of China
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