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

Abstract  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 of gas‐liquid fluids from rocks while silicate melt was generated. In the late stage, small amounts of CO 2 and H 2 O infiltrating along fractures were introduced. In the formation of glaucophane schist (447‐362 Ma), devolatilization reactions were dominated during the subduction‐uplift stage of the paleoplate. In the uplift‐exhumation stage (400‐380 Ma) the increase of internal space of fractures in the rocks favoured fluid infiltration and concentration. These fluids participated in hydration reactions in the retro‐metamorphism. The fluids participating in the mineral reactions have the compositions of CaCl 2 ‐NaCl‐H 2 O. In subsequent thrusting (<380 Ma), the metamorphic terrain was uplifted to the shallower crust and ductile‐shearing deformation took place, which caused mainly dehydration reactions of minerals. In a near‐surface environment the metamorphic terrain experienced brittle deformation, forming many accompanying fractures. Immiscible CO 2 and low‐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 were calculated and the corresponding P‐T‐t path was deduced, showing that the metamorphic terrain has experienced a clockwise path indicated by T ‐ and P ‐rising, and T ‐ and P ‐falling processes. This reveals that the subduction zone has undergone multiple tectono‐dynamic processes, i.e. initial deep burial, subsequent quick uplift and near‐surface tectonism.