Stable isotope characteristics of eclogites from the ultra-high-pressure metamorphic terrain, east-central China

A stable isotope study was made for the ultra-high-pressure (UHP) metamorphic rocks in the Dabie and Sulu terrains, east-central China. δ18O values of minerals in eclogites cover a wide range, from −4 to +6‰. The O-isotope fractionations between minerals (i.e. omphacite, phengite and/or amphibole) and garnet in eclogites are mostly not in isotopic equilibrium. The large range of O-isotope composition of garnet may approximate the δ18O variations of eclogites during UHP metamorphism and must have been largely inherited from their pre-subduction basaltic precursors which might have interacted with meteoric waters under variable temperature conditions and water/rock ratios before UHP metamorphism. The O-isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration reactions. The fluids required for these hydration reactions must have low or even negative O-isotope compositions. It is postulated that such fluids were either originated from meteoric deep-infiltration or were derived from low-δ18O (para)gneisses.