CHANGES IN DEFORMATION MECHANISM DURING EXHUMATION OF UHP ECLOGITE ARE RELATED TO THE PRESENCE OR ABSENCE OF SUPERCRITICAL FLUID OR MELT, SULU BELT, CHINA

Characterizing the deformation mechanisms of natural ultrahigh-pressure (UHP) eclogites is essential to understand exhumation dynamics in collisional belts, particularly to determine the effect of supercritical fluid or melt. Here, for the first time, we document the microstructures, CPOs and inherited structural water contents of minerals in three UHP foliated eclogites with different mineral assemblages, to explore links between their rheological behavior and the presence or absence fluid or melt. The eclogites are from the same rootless tight-isoclinal fold nose that formed under UHP conditions. Results show that bimineralic eclogite preserves the lowest inherited structural water content in omphacite and garnet. The abundance of intracrystalline misorientation and subgrain boundaries indicates dislocation creep was the dominant deformation mechanism with dynamic recrystallization by subgrain rotation. Supercritical fluid was present in Ky-bearing quartz eclogite during the early stage of exhumation. Omphacite and garnet preserve the highest amount of inherited structural water. Strong shape preferred orientation, straight grain boundaries with well-developed multi-junction structures, and the absence of intracrystalline misorientation or subgrain boundaries show that this eclogite deformed dominantly by grain boundary sliding. The Ph-Zo-bearing eclogite has the highest mode of hydrous minerals but preserves lower structural water than Ky-bearing eclogite, supporting decompression dehydration of NAMs (e.g., Omp). Supercritical fluid was present during early exhumation, whereas Omp+Ph-breakdown melting occurred during the late stage of exhumation. Omphacite and garnet show elongated shapes associated with compositional zoning and absence of intracrystalline misorientation, indicating dissolution-precipitation creep dominated deformation. The eclogites record different omphacite CPOs: L-, SL-, and S-types, respectively. There is no prominent garnet CPO in any of them. We conclude that the dominant deformation mechanism changed from dislocation to diffusion creep with the presence of supercritical fluid or melt during exhumation. Partial melting is significant in reducing the rheological strength of eclogite, potentially enhancing exhumation.