Metamorphic P-T trajectory and multi-stage fluid events of vein-bearing UHP eclogites from the Dabie terrane: insights from compositional zonations of key minerals

Abstract

The multi-stage metamorphic evolution and high-pressure (HP) to ultra-high-pressure (UHP) fluid events of vein-bearing eclogites from the Ganghe in the Dabie UHP terrane have been established based on a detailed study of compositional zonations in garnet, omphacite, and epidote. Comparison of mineral assemblages and zoned garnet compositions with the predictions of phase equilibrium modelling results in six stages of metamorphism in eclogites: (i) prograde low-temperature (LT)-HP eclogite-facies metamorphism (2.0–2.3 GPa, <540°C); (ii) peak UHP lawsonite eclogite-facies metamorphism (~3.1 GPa, ~650°C); (iii) decompressional breakdown of UHP lawsonite during initial exhumation (2.8–3.0 GPa, 650–680°C), as we have previously reported; (iv) middle-temperature (MT) and HP eclogite-facies metamorphism (~2.6 GPa, ~720°C); (v) retrograde eclogite-facies metamorphism (1.5–1.7 GPa, 610–670°C); and (vi) amphibolite-facies metamorphism (<1.1 GPa, <600°C). A heating event with decompression is confirmed from metamorphic stages (iii) to (iv), indicating that the Ganghe eclogites underwent a relatively sluggish exhumation during the stages from the UHP to HP conditions. During prograde metamorphic stages (i) and (ii), fluid activities in the eclogites are limited because lawsonite growth consumed large amounts of early-stage hydrous minerals and potential free water. Dehydration reactions during metamorphic stage (iii) triggered a significant liberation of UHP fluid. Intense fluid-eclogite interaction and dissolution-precipitation processes during metamorphic stages (iii) to (iv) led to the occurrence of patchy zones in epidote cores and the systematic compositional change of epidote and omphacite cores near the vein. A late-stage infiltration of HP fluid into eclogites occurred in metamorphic stage (v), leading to the final-stage growth (mineral rims) of garnet, omphacite, and epidote. This study presents detailed mineral zonation patterns and mineral compositions of the vein-hosting eclogites to link the complicated metamorphic P-T evolution with episodic multi-stage HP-UHP fluid events in deeply subducted continental crust.