Fluid/rock interaction and mass transfer in continental subduction zones: constraints from trace elements and isotopes (Li, B, O, Sr, Nd, Pb) in UHP rocks from the Chinese Continental Scientific Drilling Program, Sulu, East China

Yilin Xiao 0 1 2 Jochen Hoefs 0 1 2 Zhenhui Hou 0 1 2 Klaus Simon 0 1 2 Zeming Zhang 0 1 2 0 Z. Zhang Institute of Geology, Chinese Academy of Geological Sciences , 100037 Beijing, China 1 Y. Xiao J. Hoefs K. Simon Geowissenschaftliches Zentrum der Universitat Gottingen , Goldschmidtstrasse 1, 37077 Gottingen, Germany 2 Y. Xiao (&) Z. Hou CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China , 230026 Hefei, China In order to better understand the role of fluids during subduction and subsequent exhumation, we have investigated whole-rock and mineral chemistry (major and trace elements) and Li, B as well as O, Sr, Nd, Pb isotopes on selected continuous drill-core profiles through contrasting lithological boundaries from the Chinese Continental Scientific Drilling Program (CCSD) in Sulu, China. Four carefully selected sample sets have been chosen to investigate geochemical changes as a result of fluid mobilization during dehydration, peak metamorphism, and exhumation of deeply subducted continental crust. Our data reveal that while O and Sr-Nd-Pb isotopic compositions remain more or less unchanged, significant Li and/or B isotope fractionations occur between different lithologies that are in close contact during various metamorphic stages. Samples that are supposed to represent prograde dehydration as indicated by veins formed at high pressures (HP) are characterized by element patterns of highly fluid-mobile elements in the veins that are complementary to those of the host eclogite. A second sample set represents a UHP metamorphic crustal eclogite that is separated from a garnet peridotite by a thin transitional interface. Garnet peridotite and eclogite are characterized by a [10% difference in MgO, which, together with the presence of abundant hydroxyl-bearing minerals and compositionally different clinopyroxene grains demonstrate that both rocks have been derived from different sources that have been tectonically juxtaposed during subduction, and that hydrous silicate-rich fluids have been added from the subducting slab to the mantle. Two additional sample sets, comprising retrograde amphibolite and relatively fresh eclogite, demonstrate that besides external fluids, internal fluids can be responsible for the formation of amphibolite. Li and B concentrations and isotopic compositions point to losses and isotopic fractionation during progressive dehydration. On the other hand, fluids with isotopically heavier Li and B are added during retrogression. On a small scale, mantle-derived rocks may be significantly metasomatized by fluids derived from the subducted slab. Our study indicates that during high-grade metamorphism, Li and B may show different patterns of enrichment and of isotopic fractionation. - It is well known that subduction of altered basaltic oceanic crust and overlying sediments is characterized by significant releases of aqueous fluids from metamorphosing slabs (Bebout 2007; Forneris and Holloway 2003; Jarrard 2003; Peacock 1990). Since subducted oceanic material rarely returns back to the surface, inferences about the fluid inventory during subduction relies on indirect evidence, such as the analysis of arc rocks. In contrast, deeply subducted continental crust is usually brought back to the surface due to its lower density. However, only limited attention has been paid so far to the role of fluids during subduction and exhumation of continental crust (e.g., Zheng 2009). In comparison with hydrated oceanic crust, subducted continental crust is drier, commonly older and colder, less dense, and enriched in incompatible elements. Nevertheless, subducted continental materials must have been subjected to dehydration, releasing fluids from the subducting slab. Substantial amounts of H2O may have been transported to depths of more than 100200 km by hydrous minerals during UHP metamorphism (Poli and Schmidt 1995; Thompson 1992). Volatiles released by decomposition of hydrous phases may enter the structure of anhydrous mantle minerals (Su et al. 2004; Xia et al. 2005). Lithium (Li), boron (B) and oxygen (O) isotopes are excellent tracers for fluid/rock interactions during downgoing and uplift processes of subducted slabs (e. g., Elliott et al. 2006; Ionov and Seitz 2009; Leeman et al. 2004; Magna et al. 2006; Moriguti and Nakamura 1998; Moriguti et al. 2004; Teng et al. 2007; Tomascak 2004). Early Li and B isotope analyses have highlighted the behavior of these elements under hydrothermal and low-temperature conditions, with the heavier isotopes (7Li and 11B) becoming enriched in fluid phases (Chan et al. 1992; Hervig et al. 2002; Hoefs and Sywall 1997; Palmer and Swihart 1996; Rudnick et al. 2004; Seyfried et al. 1998). Recently, experimental work documented the partitioning and isotopic fractionation of Li and B between fluids and minerals at higher temperatures (e.g., Wunder et al. 2005, 2006; Meyer et al. 2008). Interestingly, at high temperatures, B isotopes vary regularly with the degree of B enrichment, whereas Li isotopes do not correlate in such a general way (e.g., Elliott et al. 2004; Tomascak et al. 2002). Although extremely low d7Li values in HP eclogites have been reported, the cause for the depletion of Li isotopes is unclear (Agostini et al. 2008; Marschall et al. 2007a; Simons et al. 2010; Zack et al. 2003). Thus, apparently isotopes of the two elements may behave differently at high-temperature conditions. At mantle conditions, Li can be incorporated into magnesian silicates while B is dominantly fluid controlled (Bebout et al. 1993; Seyfried et al. 1998). Consequently, the Li and B systematics may provide a record of fluid/rock and crust/ mantle interactions in deep subduction zones. The DabieSulu UHP metamorphic belt, which is the largest known UHP terrain, is a suitable region to study the fluid regime of subducted crustal material, because of its abundant veins that formed during subduction (Castelli et al. 1998; Franz et al. 2001) and exhumation (Wu et al. 2009; Zheng et al. 2007). Furthermore, variable extents of fluid/melt metasomatism are evident at contacts between different lithological units (Zhao et al. 2007). In this paper, we outline the petrology, bulk-rock and mineral geochemistry, and isotope systematics of four carefully selected sample sets collected from drill cores of the Chinese Continental Scientific Drilling program (CCSD), representing different stages of fluid/rock interaction. We investigate elemental and isotopic fractionations, during (i) prograde metamorphism, (ii) peak metamorphic conditions, and (iii) retrograde exhumation. Of special relevance in our study are investigations into Li and B concentrations and isotopic compositions. Geological background and samples The CCSD has been carried out in the DabieSulu metamorphic belt at Donghai with a final depth of 5,158 m for the main hole (Xu 2007). Orthogneiss, paragnei (...truncated)