The oxygen and hydrogen isotope compositions of different gneisses associated with eclogites at Shuanghe in the Dabie Mountains

Abstract

Meteoric-hydrothermal water-rock interactions have been suggested for the origin of unusually low 6180 valies in ultrahigh pressure (UHP) eclogites from Shuanghe in the eastern Dabie Mountains. In order to constrain the importance of fluids in the development of the isotopic anomaly, the oxygen and hydrogen isotope compositions of mineral separates were determined for two types of gneisses (biotite gneiss and granitic gneiss) associated with the eclogites. The results show significant differences in oxygen isotope composition between biotite gneiss and granitic gneiss. The UHP biotite gneiss has a large 6180 range of -4.7 to +10.3 %O similar to the associated eclogites, while the 6'*0 values of the non-UHP granitic gneiss vary only from -4.2 to +0.8 %o. Oxygen isotopic equlibrium has been achieved and preserved among various minerals in these gneisses. Oxygen isotopic geothermometry based on quartzphengite and quartz-garnet fractionations yields 425 to 635 % and 620 to 765 'C for biotite gneiss, respectively; and those for granitic gneiss 395 to 540 C and 560 to 620 %, respectively. These isotopic temperatures are close to the retrograde metamorphic temperatures of amphibolite facies. Thus the protoliths of both gneisses have the 6"0 values as low as -5 to -4 Yw. Hydrogen isotope ratios were determined for epidotel zoisite (-37 to -64 %) and biotite (-78 to -109 O h ) from both gneisses. The low SD values for the micas are consistent with an origin of ancient meteoric water. Because no significant shift in the oxygen isotope compositions of the gneisses was controlled by the retrograde exchange processes of oxygen isotopes between the minerals during cooling of the rocks and by the net-transfer reactions between the minerals during metamorphism, the "0-depletion of both gneisses is interpreted to result from isotopic exchange of fluid with the rocks before amphibolite facies metamorphism. According to application of fluid flow model on stable isotopes and the investigation of the associated eclogites, it is suggested that the correlative "0depletion of the biotite gneiss and associated eclogites is due to the infiltration of meteoric water into their precursor rocks prior to the ultrahigh-pressure metamorphism. Therefore, the biotite gneiss and associated eclogites experienced the same geodynamic processes and thus were part of a single tectonic entity throughout the course of subduction. ultrahigh-pressure metamorphism and exhumation. They are thus foreign relative to the granitic gneiss.