High‐Pressure Fluid‐Rock Interaction and Mass Transfer During Exhumation of Deeply Subducted Rocks: Insights From an Eclogite‐Vein System in the Ultrahigh‐Pressure Terrane of the Dabie Shan, China

A petrological study was performed on an eclogite‐vein system from the Dabie ultrahigh‐pressure terrane by focusing on high‐pressure (HP) fluid‐rock interaction and mass transfer during exhumation. The veins comprise mainly garnet and quartz and resulted from a HP leaching process along fractures in the eclogite during exhumation. Relict peak garnet, phengite, and apatite in this rock system were altered by HP fluids forming characteristic compositional zonings. Zoned garnet in the eclogite records an isothermal exhumation from 3.3 GPa and 665–680 °C. Partial dissolution of garnet and a back incorporation of Mn‐heavy rare earth element (REE)‐Y increased the content of these elements in the altered garnet zone in the veins. Altered phengite is characterized by a Ba‐rich rim. Fluid‐aided alteration reduced the light REE (LREE) contents and produced oriented monazite, calcite, and pyrrhotite inclusions within apatite, reflecting mobilization and redistribution of LREEs, CO 3 2− , Fe, and S between these minerals. The active HP fluid is proposed to have been a Si‐Al‐Na‐Ba‐rich aqueous fluid, as indicated by sodic plagioclase rims around various eclogitic minerals and by the altered phengite composition. Mass balance calculations reveal that LREEs, middle REEs, and Zr‐Hf‐Th‐U were added to and Li, large ion lithophile elements, and Ti‐Nb were removed from the quartz‐garnet veins, requiring a high fluid flux. These findings shed new light on the nature and behavior of HP fluids and highlight their key role in resetting mineral chemistry and mass transfer in subduction zone metamorphic environments.