Petrological And Geochemical Records Of Fluid Activity During The Seafloor Subduction

Subduction zone metamorphism?SZM?has important significance for both arc magmatism and mantle compositional heterogeneity.Experimental studies and studies on natural rock have proved that the geochemical behavior of elements during the process of seafloor subduction is complex and controlled by many factors.In this paper,we studied the rocks rich in veins from the southwest Tianshan to discuss the influence of fluids on geochemical processes during the seafloor subduction.Based on petrological characteristics,the sample can be divided into three sections:the eclogite part,the blueschist part and the vein.The vein is mainly composed of omphacite,besides,some carbonate minerals and apatite is also contained,indicating the sample has experienced alteration by fluids modification of CO₂,P and halogen-rich elements under the eclogite facies.The Lu-Hf isochron age of the sample is 332±24 Ma,which is consistent with the peak metamorphic age of the southwestern Tianshan by using SHRIMP U-Pb dating reported in previous studies.The partitioning patterns of trace elements for the three rock sections,such as Th-U-Nb-Ta-Zr-Hf and rare earth elements,are similar to ocean-island basalt?OIB?,indicating that the original rock composition is similar to OIB.The K,Rb,Sr and Ba of the three parts are variabley depleted compared with OIB,and the content of these elements in blueschists is significantly higher than that in eclogites,indicating the mobility of these elements.In addition,Rb,Sr and Ba are significantly correlated with K.Considering K is the major element for muscovite,this correlation reflects the control of the presence of muscovite for these elements.All these lines of petrological and geochemical evidence reflect that the sample has undergone two stages of metamorphism,i.e.,the eclogitization process and the rehydration overprints with different degrees.At the first stage,water-soluble elements are lost.At the second stage,the external fluids cause rehydration,and lead to the formation of abundant water-bearing minerals in the blueschist part such as muscovite,which can preserve these elements.Therefore,if muscovite is stable,the characteristically high contents of water soluble elements of the island arc magma cannot be attributed to the simple metamorphic dehydration of the subducting seafloor.