Various fluids and complex geochemical processes in the subduction channel: Constraints from the ultrahigh pressure metamorphic belt of Southwestern Tianshan, China

The knowledge of fluids derived from the subducting slab is important to constrain the mass transfer in sub-duction zones, which is important to understand arc magma characteristics. The varying scales of veins in subduction-zone metamorphosed rocks reflect different scales of previous fluid flow. Metamorphic rocks from (ultra-) high pressure (UHP) metamorphic belt of Southwestern Tianshan orogen, China, developed various large-scale veins, which indicate complex mass transfer processes in subduction zones. To further understand the complex geochemical processes and constrain the slab-released fluids in subduction zones, we investigated two rock blocks (13TS-I and 13TS-VI) in this study, which developed different types of veins. The glaucophane-rich breccias and common presence of garnet and omphacite in the vein of rock block 13TS-I reflect a significant dehydration process, while the rock block 13TS-VI with garnet-dominated selvage and epidote-dominated veins reflect a strong fluid-rock interaction process. Although both of these rock blocks were affected by external fluids, they have experienced different metamorphic processes. Compared to the Sr-Nd isotope mixing curve with different potential fluid endmembers, the external fluids for 13TS-I and 13TS-VI under eclogite-facies meta-morphic conditions are derived from pelagic sediments and altered oceanic crust, respectively. The different metamorphic processes (e.g., dehydration, fluid-rock interactions) and different sources for fluids (e.g., pelagic sediments or altered oceanic crust) lead to the development of various veins in subduction-zone metamorphic rocks, which reflect complex geochemical processes in subduction zones.