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Abstract: The Late Jurassic Jingshan granite located at the south-eastern margin of the North China Craton contains abundant garnets which can be subdivided into three types based on texture and composition: (i) euhedral garnet in mafic biotite and garnet rich enclave (Grt I), (ii) coarse-grained garnet (Grt II) in the host granite, and (iii) small euhedral garnet in aplite (Grt III). In general, Grt I has higher FeO, CaO and lower MnO contents than Grt II. Grt III has higher Mn, but lower Ca contents than others. Grt I has lower MREE and HREE contents than Grt II. Grt III has prominent and distinctly negative Eu anomaly as well as higher MREE composition compared to the others. Systematic variations in oxygen isotope compositions are observed among the three garnet types, with δ18O values of <3.8‰ in most of Grt I, 3.8–4.7‰ in most Grt II (for inclusion-free garnets), and typically >4.7‰ in Grt III. Some of the Grt II and Grt III display two distinct zonings with cores having similar major and trace element compositions to Grt I. Cathodoluminescence (CL) images revealed that the zircons from different garnet-bearing samples possess fine-scale oscillatory zoned magmatic rims with inherited cores. In situ zircon U–Pb dating and trace element analyses show that the dark-luminescent magmatic rims all have Jurassic concordia ages (∼160Ma) and similar trace element patterns. Most of the inherited cores also display similar Triassic ages of 210–236Ma, which is similar to the ages of ultrahigh pressure (UHP) metamorphic rocks of the Dabie–Sulu orogen (230Ma). In addition, Jurassic concordia ages were also found in a zircon inclusion in Grt I, implying that the Grt I was formed shortly before the main magmatic event. The age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events. Based on the zircon U–Pb ages from different garnet-bearing samples, the major element, trace element, oxygen isotope, and zoning textures of the three kinds of garnet we suggest that Grt I may be peritectic garnet, whereas Grt II and III are probably the results of magmatic dissolution–precipitation processes and re-equilibration of garnets with changing magmatic conditions during melting, differentiation, crystallization, and cooling within the granite. We conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the Jingshan granites are from Dabie–Sulu orogen representing the South China Craton. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The Dabie Shan ultrahigh-pressure (UHP) metamorphic terrane is located in the eastern part of the east–west-striking Qinling–Dabie orogenic belt in China. A major mylonitized contact zone of 200–300 m width divides Dabie Shan into the South Dabie Terrane (SDT) and the North Dabie Complex (NDC). Combined investigation of major and trace element geochemistry, fluid inclusions, and oxygen and hydrogen isotopes constrains the fluid history during the metamorphic evolution of the two metamorphic belts, which differ in their fluid and metamorphic evolution. Fluid inclusions in rocks from the SDT are mainly aqueous with varying salinities, whereas those from the NDC are dominated by CO2. Low δ18O values in the SDT rocks (−2·8 to 8·6‰) indicate meteoric water–rock interactions before UHP metamorphism, whereas rocks from the NDC show ‘normal’ δ18O values (6·7–9·0‰) with no obvious meteoric water–rock signature. Whole-rock rare earth element (REE) contents correlate with oxygen isotope compositions: samples from the SDT have higher REE contents and lower δ18O values, whereas samples from the NDC have lower REE contents and higher δ18O values. During retrograde metamorphism fluids with different hydrogen isotope compositions interacted with the rocks from the SDT. [ABSTRACT FROM PUBLISHER]