Stable Isotope Geochemistry of High to Ultrahigh Pressure Metamorphic Rocks in the Dabie Mountains

Abstract

The identification of coesite and micro-diamond in the metamorphic rocks of the Dabie and S U ~ U terranes implies that the crustal materials canbe subducted to mantle depths of a t least 100 km (Okay et al., 1989; Wang et al., 1989; Xu et al., 1992). Stable isotope analyses of high to ultrahigh pressure (UHP) metamorphic rocks can be used to determine whether isotopic compositions are largely inherited from pre-metamorphic protoliths or, alternatively, if compositions have been modified by the infiltration of external fluids. Previous investigations show that minerals in eclogites from Qinglongshan in the western S U ~ U have extreme negative 6I8O values of -10 to -8%, whereas eclogites from Shuanghe in the eastern Dabie have 6180 values of -3 to +7%0 (Yui et al., 1995; Zheng et al., 1996, 1998). Preservation of oxygen isotope equilibrium between the eclogite minerals suggests that the eclogite protoliths underwent hydrothermal alteration by fluids of meteoric origin prior to subduction. Limited crust-mantle interaction has been inferred between the '*O-depleted eclogites and the mantle when the slab containing the eclogite protoliths was subducted to mantle depths, implying a short residence time of the eclogite-containing slab in the mantle (Zheng et al., 1998). Concordant oxygen isotope temperatures are obtained from different mineral pairs, suggesting great cooling rates of the eclogite slab at the initial stage of exhumation. In this study, the carbon, oxygen and hydrogen isotope compositions of mineral separates have been determined for marble, eclogites, and two types of gneiss (biotite gneiss and granitic gneiss) from the Dabie Mountains. The marble, eclogites and biotite gneiss from the eastern Dabie contain coesite and thus belong to the UHP slab, whereas there is no mineralogical evidence for the marble and eclogite from the western Dabie and the granitic gneiss from the eastern Dabie for having experienced the UHP metamorphism. The results place constraints on the sedimentary environment of marble protolith, the scale of fluid-rock interaction during plate subduction, and tectonic relationship between UHP and non-UHP units.