OXYGEN AND HYDROGEN ISOTOPE GEOCHEMISTRY OF ULTRAHIGH-PRESSURE ECLOGITES FROM THE DABIE MOUNTAINS AND THE SULU TERRANE

The oxygen and hydrogen isotope compositions of mineral separates have been determined for ultrahigh-pressure (UHP) eclogites from Shuanghe in the eastern Dabie Mountains and from Donghai in the western Sulu te rrane, East China. The results show a large variation in delta(18)O va lues of garnet and omphacite (-2.6 to +7.0% for Shuanghe and -10.4 to +4.8% for Donghai) but a small range in phengite delta D value (-104 t o -73%). Oxygen isotope equilibrium has been preserved between the ecl ogite minerals and thus records the metamorphic temperatures of 550-73 0 degrees C for the Shuanghe eclogites and 650-750 degrees C for the D onghai eclogites. These not only demonstrate that the UHP rocks acquir ed the unusual delta(18)O values prior to eclogite-facies metamorphism by interaction with O-18-depleted fluids, but also precludes the infi ltration of external fluids during exhumation as the cause for the O-1 8 depletion in the eclogites. Ancient meteoric water is assumed to exc hange oxygen isotopes with the eclogite precursors on the continental crust prior to plate subduction. The extremely low delta(18)O values ( -10 to -9%) and delta D values (-104 to -100%) for the Qinglonoshan ec logite may represent the oxygen and hydrogen isotope compositions of a ncient meteoric water at some earlier time than subduction. The surviv al of the oxygen and hydrogen isotopic signature of meteoric water in the UHP eclogites indicates that the eclogites resided at mantle depth s only for a short time, otherwise the extremely O-18-depleted eclogit es would be re-equilibrated isotopically with the mantle due to diffus ion and recrystallization. This suggests restricted fluid mobility and limited crust-mantle interaction during the UHP metamorphism. The con sistency of oxygen isotope temperatures between different mineral pair s in this study suggests relatively rapid cooling and ascent for the U HP eclogites in the early stage of their exhumation. However, there ar e differential exchanges of oxygen and hydrogen isotopes in hydroxyl-b earing minerals (and rutile) with retrograde fluid during exhumation, which has not only resulted in lower oxygen isotope temperatures for m ineral pairs containing zoisite and rutile, but also disequilibrium an d reversed hydrogen isotope fractionations between phengite, amphibole and zoisite. (C) 1998 Elsevier Science B.V.