Evolution Process Of Magma In The Okinawa Trough And Its Subduction Composition Characteristics:Melt Inclusion Indication

As an active back-arc basin,the Okinawa Trough is located in the southeastern region of the East China Sea shelf and is strongly influenced by the subduction of the Philippine Sea Plate.Major element,trace element and Sr-NdPb isotopic composition data are presented for volcanic rocks from the Iheya Ridge,the middle Okinawa Trough.The Iheya Ridge rocks record large variations in major elements and range from basalts to rhyolites.Similar trace element distribution characteristics together with small variations in Sr-Nd-Pb isotopic ratios,demonstrate that the Iheya Ridge rocks are derived from a similar magma source.The fractional crystallization of olivine,clinopyroxene,plagioclase,and amphibole,as well as accessory minerals,can reasonably explain the compositional variations of these Iheya Ridge rocks.The simulations suggest that approximately 60%and 75%fractionation of an evolved basaltic magma can produce trace element compositions similar to those of the intermediate rocks and acid rocks,respectively.The analysis of their Sr-Nd-Pb isotopic content ratios suggest that the source of the rocks from the Iheya Ridge is close to the depleted mantle but extends to the enriched mantle II,indicating that the mantle source of these rocks is a mixture between the depleted mantle and enriched mantle II end members.The simulations show that the source of the Iheya Ridge volcanic rocks can be best interpreted as the result of the mixing of approximately 0.8%–2.0%subduction sediment components and 98.0%–99.2%mantlederived melts.In the study of the genesis of basaltic magma and the effects of subduction components on magma evolution,we focused on the analysis of melt inclusions in the middle Okinawa Trough and southern Okinawa Trough.The chemical composition of the olivine in basalt from middle Okinawa Trough suggests that the lithology of the basalt source may be pyroxenite.The major element compositions of silicate melt inclusions in the middle Okinawa Trough trachyandesite show a scattered variation trend of medium?K to shoshonitic series and suggest that a K?rich magma source may exist in the Okinawa Trough.Two groups with distinct element distribution patterns observed in silicate melt inclusions of the middle Okinawa Trough trachyandesite may represent magmas with different evolutionary histories.Compared with fractional crystallization and partial melting processes,magma mixing may serve as a reasonable process for the production of trachyandesite in the middle Okinawa Trough.Fractional crystallization has played an important role in basaltic magma evolution in the middle Okinawa Trough and sourthern Okinawa Trough.Simulations conducted using MELTS show that the fractional crystallization evolution trends observed correspond well to the presence of a magma chamber at 300 MPa with an H₂O content level of 0.2 wt%in the middle Okinawa Trough and to the presence of a magma chamber at 300 MPa with an H₂O content level of 0.4 wt%in the sourthern Okinawa Trough.Ratios of incompatible trace elements?e.g.,Th/Ce,Th/Sm,Ba/Th,and Th/Nb?;positive Pb anomalies;negative Nb,Ta,and Ti anomalies;and the existence of organic hydrocarbons?CnHm?in the silicate melt inclusions can be attributed to the involvement of subducted sediment from the subducting slab.Modelling results suggest that more primitive magma represented by silicate melt inclusions from the middle Okinawa Trough has formed through the melting of less than 6%of a metasomatized depleted mantle with contributions of 1.2%sediment melt and 0.8%sediment fluid.In contrast,more primitive magma from the sourthern Okinawa Trough formed through the melting of 4–10%of the depleted mantle with contributions of 3%sediment melt and 1%sediment fluid.The different features of the silicate melt inclusions together with different tectonic settings of the middle Okinawa Trough and sourthern Okinawa Trough have formed in response to the westward subduction of the Philippine Sea Plate.