Mineral oxygen isotope and hydroxyl content changes in ultrahigh-pressure eclogite–gneiss contacts from Chinese Continental Scientific Drilling Project cores Z-F. ZHAO, B. CHEN, Y-F. ZHENG, R-X. CHEN & Y-B. WU CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China (zfzhao@ustc.edu.cn) ABSTRACT A combined study of mineral O isotopes and hydroxyl contents was carried out for the contacts between ultrahigh-pressure eclogite and gneiss from main hole of the Chinese Continental Scientific Drilling Project in the Sulu orogen. While there is a large d 18 O variation from )8.3 to 7.3& for all minerals, different styles of mineral-pair fractionation occur at the boundaries of different lithologies. Both equilibrium and disequilibrium O isotope fractionations are observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some samples of eclogite. This suggests that both eclogite and gneiss acquired their negative d 18 O values by meteoric-hydrothermal alteration of their protoliths at high temperatures before subduction, and that fluid-assisted O isotope exchange did take place across the boundary of different lithologies at local scales during amphibolite- facies retrogression. Fourier Transform Infrared Spectroscopy analysis yielded H 2 O concentrations of 50 to 1144 p.p.m. (by weight) for garnet and 139 to 751 p.p.m. for omphacite. The state of equilibrium or disequilibrium O isotope fractionations between omphacite and garnet are correlated with variations in their water content at local scales, indicating that the internally derived fluid plays a critical role in retrograde metamorphism during exhumation. The retrograde metamorphism results in mineral reactions and O isotope disequilibria between some of the minerals, but the fluid for retrogression was derived from the decompression exsolution of structural hydroxyl and thus internally buffered in the O isotope composition. A quantitative estimate suggests that a hand specimen (3 · 6 · 9 cm) of eclogite composed of 70% garnet and 30% omphacite can release 0.316 g water by the decompression exsolution of structural hydroxyl, which can form 14.4 g amphibole during exhumation. This provides sufficient amounts of water for the amphibolite-facies retrogression. Key words: Chinese Continental Scientific Drilling; eclogite; fluid; gneiss; hydroxyl; O isotope. INTRODUCTION The Dabie-Sulu orogenic belt in east-central China is well known for the largest exposure of Triassic ultrahigh-pressure (UHP) metamorphic rocks in the world (e.g. Cong, 1996; Liou et al., 1996; Carswell & Compagnoni, 2003; Chopin, 2003; Rumble et al., 2003; Zheng et al., 2003). Discovery of coesite, dia- mond and extreme 18 O-depletion in eclogites from this exciting region has contributed much to our understanding of continental subduction to mantle depths and its subsequent exhumation (e.g. Okay et al., 1989; Xu et al., 1992; Yui et al., 1995; Zheng et al., 1996). Since the discovery of a negative O isotope anomaly in UHP eclogite at Qinglongshan in the Sulu orogen (Yui et al., 1995; Zheng et al., 1996), a large number of studies has been made on stable isotope geochemistry of UHP metamorphic rocks, which has been successfully used to characterize chemical geodynamics and fluid regime during con- tinental subduction and exhumation (Zheng et al., 2003, and references therein). The anomalously low d 18 O values of )11 to )2& in the UHP eclogite and gneiss indicate both intensive and extensive O isotope exchange of meteoric water with their protoliths be- fore Triassic UHP metamorphism (Yui et al., 1995, 1997; Zheng et al., 1996, 1998, 1999, 2003, 2004; Baker et al., 1997; Rumble & Yui, 1998; Fu et al., 1999; Rumble et al., 2000, 2002; Xiao et al., 2002). Very limited fluid mobility is demonstrated to occur during UHP metamorphism by the Triassic subduc- tion of continental crust to mantle depths (Rumble et al., 2000; Fu et al., 2001; Zheng et al., 2003). On the basis of a comprehensive study on O isotopes in UHP metamorphic minerals from the Dabie-Sulu orogenic belt, Zheng et al. (2003) demonstrated that continent–continent collision is characterized by fast subduction and fast exhumation of cold slabs with short-lived residence for UHP metamorphism at mantle depths. Stable isotope studies are thus a J. metamorphic Geol., 2007, 25, 165–186 doi:10.1111/j.1525-1314.2007.00693.x Ó 2007 Blackwell Publishing Ltd 165