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) meta morphic rocks in the Dabie and Sulu terrains, east-central China. delt a(18)O values of minerals in eclogites cover a wide range, from -4 to +6 parts per thousand. 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 comp osition of garnet may approximate the delta(18)O variations of eclogit es during UHP metamorphism and must have been largely inherited from t heir pre-subduction basaltic precursors which might have interacted wi th meteoric waters under variable temperature conditions and water/roc k ratios before UHP metamorphism. The O-isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration r eactions. The fluids required for these hydration reactions must have low or even negative O-isotope compositions. It is postulated that suc h fluids were either originated from meteoric deep-infiltration or wer e derived from low-delta(18)O (para)gneisses.