| The study of deeply subducted continental is forefront subject to advance the plate tectonic theory. It has been realized recently that sufficient attention must be paid to processes of the subduction and exhumation of continental and oceanic crust, and intra-slab fluid flow and chemical changes. HP-UHP metamorphic rocks, including both continental and residual oceanic protolith, have been well recognized to occur in the Western Dabie orogen in the east of China and the North Qaidam orogen in the west of China, which is a wonderful target to investigate exhumation and fluid-rock interaction of subducted slab during the continental collision. By taking advantage of in situ zircon U-Pb age, trace element, Lu-Hf isotope and O isotope composition were carried out. Available results from this study can be summarized as follows:(1) There are two types of HP-eclogites in the Huwan shear zone. U-Pb age of residued magmatic zircon in an eclogite constrain its protolith formation at411±4Ma. The zircon in this sample displays εHf(t) values up to+14.4. The positive εHf(t) value suggests that the protolith was derived from a relatively depleted mantle source, most likely Late Silurian Paleotethyan oceanic crust. Whereas a granitic gneiss and the other eclogite yield protolith U-Pb ages of738±6and700±14Ma, respectively, which are both the Neoproterozoic basement rocks of the Yangtze Block. The zircon in the eclogite has εHf(t) values of-1.0to+7.4and TDMI ages of1294to966Ma, implying prompt reworking of juvenile crust during its protolith formation. Metamorphic zircon in both eclogite samples displays low Th/U ratios, trace element concentrations, relatively flat HREE patterns, weak negative Eu anomalies, and low176Lu/177Hf ratios. All these features suggest that both the oceanic and continental crustal rocks in the Huwan shear zone have experienced Carboniferous eclogite facies metamorphism at ca.310Ma. The HP continental rocks in the Huwan shear zone might have played a key role in the exhumation and preservation of the oceanic rocks through buoyancy-driven uplift.(2) Magmatic zircons in the amphibolite yielded protolith U-Pb age of1806±63Ma. Metamorphic zircons in a quartz-rich leucosome and the amphibolite gave weighted mean206Pb/238U ages of241±1Ma and243±4Ma, respectively. These metamorphic zircons contain mineral inclusions of garnet and omphacite, and are characterized by relatively flat HREE patterns with slight negative Eu anomalies. This indicates that they formed under eclogite facies conditions. The zircons in the amphibolite show a negative εHf(t) value of-16.3, suggesting that the protolith was derived from reworking of the ancient continental crust. The U-Pb age of ca.243Ma is consistent with eclogite-facies metamorphic ages of the juvenile oceanic crust in the western Dabie orogen, suggesting that the initiation of continental subduction for high-pressure metamorphism of the Dabie orogen can be traced back to ca.243Ma. The western Dabie HP-UHP slices might suffer from multi-slice or differential subduction and exhumation during the Triassic.(3) The first record of coesite is reported as an inclusion in a metamorphic zircon, which provides unambiguous evidence for the UHP metamorphism of the Xitieshan terrane. Combined with previous results, the HP-UHP metamorphism of the Xitieshan terrane may have lasted460-440Ma with the peak UHP metamorphism at441±9Ma. A compilation of the reported geochronological data reveals that all four terranes of the North Qaidam orogen might have experienced coeval UHP metamorphism during the early Paleozoic(420-450Ma), and thus may have suffered a coherent subduction, UHP metamorphism, and exhumation cycle. U-Pb age of zircon in an serpentinised harzburgite from Dulan terrane is448±9Ma. It consists with the metamorphic zircon U-Pb age of Ky-eclogite (455±5Ma). Both the oceanic-and continental-type rocks in the North Qaidam belt share the same episode HP-UHP metamorphism, suggesting a model of continuous processes from oceanic to continental subduction. Buoyancy of low density serpentinites and continental rocks likely contributed to the exhumation of higher density mafic oceanic rocks.(4) In situ zircon U-Pb age, trace element, and O-Hf isotope were carried out of difference quartz veins from the Xinxian UHP unit of the west Dabie orogen. The LT-UHP rocks of the west Dabie might experiment two episodes of fluid activity during exhumation stage. The zircons of quartz vein hosting by granitic gnesiss and eclogite have206Pb/238U ages for214.5±3.1Ma and213.8±2.4Ma, respectively. It suggests the second episode fluid flow occurred at ca.215Ma, responding to the stage from HP eclogite-facies to amphibolite-facies during exhumation. The zircons have consistent O isotope compositions with the zircons from their hosting rocks, but variable in Hf isotope. The zircon of quartz vein within gneiss display low εHf(t) value of-5.92. Whereas the zircon of quartz vein within eclogite have εHf(t) value of-0.97. The fluid might be aqueous fluid derived from the decomposition of hydrous minerals of hosting rocks. The pervasive fluid flow results in HP-eclogite-to amphibolite-facies retrogression.(5) Integrated study of in situ U-Pb age, trace element, and O-Hf isotope for zircons from a suite of metamorphic rocks including a quartz vein, a host eclogite, and a surrounding granitic gneiss in the Xitieshan UHP terrane of the North Qaidam orogen decipher: Oscillatory zoning zircons from the quartz vein show euhedral shape, relatively high contents of U, Li, Th, Nb, Ta and REEs, with a formation age of442±6Ma consistent with the timing of UHP metamorphism. This demonstrates that the zircons grew very probably from a channelized supercritical fluid close to the peak of UHP metamorphism. In particular, the oscillatory zoning zircons have similar Lu-Hf isotope compositions to zircons from the surrounding gneiss, but variable O isotope compositions between the host eclogite and the surrounding gneiss. Therefore, it is implied that the fluid from the surrounded gneisses was a supercritical fluid with high transport ability for Lu and Hf, while the fluid from the host eclogites was an aqueous fluid without significantly transport such elements. In contrast, both of them have a great ability to carry O. Both eclogite and gneiss from Xitieshan terrane suffered pervasive partial melting during exhumation to the HP granulite-facies regime. Most hydrous melt derived from country gneiss at this stage. The hydours melt can escape the surrounding granitic gneiss for a long distance.(6) Oceanic-and continental rocks (or serpentinised peridotite) can share coupled subduction-exhumation cycle. It suggests that the low density continental rocks and serpentinised peridotites might have played a key role in the exhumation and preservation of the higher density oceanic rocks through buoyancy-driven uplift. The fluids in the LT/UHP terrane mainly derived from multistage aqueous fluids pulse during deeply subductionof continental crust. Occurrence of aqueous fluids can enhance retrograde metamorphism of UHP rocks. Supercritical fluids and hydrous melts can be found in the MT/UHP terrane. The supercritical fluid formed at the initial exhumation stage of UHP rocks and hydours melt formed at HP granulite-facies regime. The supercritical fluid that incorporates these two types of fluid is fed to the overlying mantle wedge from the subducting slab. This process not only transport elements and water from the subducting slab to the mantle wedge, but also triggers the melting of the mantle wedge, which leads to the generation of island arc magmatism with the typical geochemical signatures. |
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