Fluid Activity and Tectonic Evolution in the Northern Qilian High-pressure Metamorphic Belt

The Northern Qilian high-pressure metamorphic belt has experienced multiple deformation-metamorphism, which consists of at least four stages.In 550.8-526 Ma, eclogites were formed. High temperature and pressure caused the escape of a large quantity ofgas-liquid fluids from rocks while silicate melt was generated. In the late stage, small amounts of CO2 and H2Oinfiltrating along fractures were introduced.In the formation of glaucophane schist (447-362 Ma), devolatilization reactions were dominated during thesubduction-uplift stage of the paleoplate.In the uplift-exhumation stage (400-380 Ma) the increase of internal space of fractures in the rocks favoured fluidinfiltration and concentration. These fluids participated in hydration reactions in the retro-metamorphism. The fluidsparticipating in the mineral reactions have the compositions of CaCl2-NaCl-H2O.In subsequent thrusting (＜380 Ma), the metamorphic terrain was uplifted to the shallower crust and ductile-shearingdeformation took place, which caused mainly dehydration reactions of minerals. In a near-surface environment themetamorphic terrain experienced brittle deformation, forming many accompanying fractures. Immiscible CO2 andlow-salinity aqueous fluids occurred in these secondary microfractures and were trapped and sealed.The thermodynamic conditions of different deformation-metamorphic stages of the metamorphic terrain werecalculated and the corresponding P-T-t path was deduced, showing that the metamorphic terrain has experienced aclockwise path indicated by T- and P-rising, and T- and P-falling processes. This reveals that the subduction zone hasundergone multiple tectono-dynamic processes, i.e. initial deep burial, subsequent quick uplift and near-surfacetectonism.