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Now showing items 1 - 14 of 14

  • The transport of water in subduction zones

    Zheng YongFei   Chen RenXu   Xu Zheng   Zhang ShaoBing  

    The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low (a parts per thousand currency sign5A degrees C/km), lawsonite may carry water into great depths of a parts per thousand currency sign300 km. In the hot subduction zone where the geothermal gradient is high (> 25A degrees C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of < 80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of a parts per thousand currency sign60-80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80-160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.
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  • Growth and reworking of cratonic lithosphere RID C-4781-2008

    Zheng YongFei   Wu FuYuan  

    To study the thinning of cratonic lithosphere in North China has been the hot subject of basic research in the fields of solid earth science in China. This paper presents an overview on the formation and evolution of continental crust, and outlines the mechanisms of forming the lithospheric mantle. It is suggested that the thinning of cratonic lithosphere principally proceeds in two ways, one by subduction erosion (e.g., North China), and the other by a combination of subduction erosion and underplating degistion (e.g., Yangtze).
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  • 25 years of continental deep subduction RID C-4781-2008

    Zheng YongFei  

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  • Formation and evolution of precambrian continental crust in South China RID C-4781-2008

    Zheng YongFei   Zhang ShaoBing  

    The occurrence of zircons with U-Pb ages of similar to 3.8 Ga and Hf model ages of similar to 4.0 Ga in South China suggests the existence of the Hadean crustal remnants in South China. Furthermore, a detrital zircon with a U-Pb age as old as 4.1 Ga has been found in Tibet. This is the oldest zircon so far reported in China. These results imply that continental crust was more widespread than previously thought in the late Hadean, but its majority was efficiently reworked into Archean continental crust. On the basis of available zircon U-Pb age and Hf isotope data, it appears that the growth of continental crust in South China started since the early Archean, but a stable cratonic block through reworking did not occur until the Paleoproterozoic. Thus the operation of some form of plate tectonics may occur in China continents since Eoarchean. The initial destruction of the South China craton was caused by intensive magmatic activity in association with the assembly and breakup of the supercontinent Rodinia during the Neoproterozoic. However, most of the Archean and Paleoproterozoic crustal materials in South China do not occur as surface rocks, but exist as sporadic crustal remnants. Nevertheless, the occurrence of Neoproterozoic magmatism is still a signature to distinguish South China from North China.
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  • Continental subduction channel processes: Plate interface interaction during continental collision

    Zheng YongFei   Zhao ZiFu   Chen YiXiang  

    The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth's surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of peridotite, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust- and mantle-derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism.
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  • Stable Isotope Geochemistry of High to Ultrahigh Pressure Metamorphic Rocks in the Dabie Mountains

    Zheng Yongfei   Fu Bin   Gong Bing  

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  • Oxygen isotope fractionation in TiO2polymorphs and application to geothermometry of eclogites

    Zheng Yongfei  

    Oxygen isotope fractionation in TiO_2 polymorphs has been calculated by the modi-fied increment method. The results suggest that rutile is enriched in ~(18)O relative to brookite but depleted in ~(18)O relative to anatase. Due to the same crystal str%ucture, oxygen isotope partitioning in the TiO_2 polymorphs is determined by the cation-oxygen interatomic distances. The theoretical calibrations involving rutile are in fair agreement with known experimental measurements and empirical estimates. Application of the theoretical quartz-rutile calibration to geothermometry of natural eclogite assemblages indicates the preservation of isotopic equilibrium at high temperatures. The isotopic temperatures calculated are only slightly lower than the non-isotopic temperatures, indicating the slow rates of exchange for oxygen diffusion in rutile. The kinetics of exchange for oxygen diffusion in rutile is accordingly estimated by reconciling the differences between the isotopic and the non-isotopic temperatures. The rates of exchange for oxygen diffusion in rutile should be smaller than those for hornblende, but may be equal to or greater than those for diopside.
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  • Oxygen isotope fractionation in TiO2polymorphs and application to geothermometry of eclogites

    Zheng Yongfei  

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  • Sulfur isotope fractionation in magmatic systems: Models of Rayleigh distillation and selective flux

    Zheng Yongfei  

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  • Sulfur isotope fractionation in magmatic systems: Models of Rayleigh distillation and selective flux

    Zheng Yongfei  

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  • A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt RID C-4781-2008

    Zheng YongFei  

    The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonics theory in the twenty-first century. The Dabie-Sulu orogenic belt in China crops out the largest lithotectonic unit containing ultrahigh-pressure metamorphic rocks in the world. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in the Dabie-Sulu rocks. By taking these rocks as the natural laboratory, earth scientists have made seminal contributions to understanding of ultrahigh-pressure metamorphism and continental collision. This paper outlines twelve aspects of outstanding progress, including spatial distribution of the UHP metamorphic rocks, timing of the UHP metamorphism, timescale of the UHP metamorphism, the protolith nature of deeply subducted continental crust, subduction erosion and crustal detachment during continental collision, the possible depths of continental subduction, fluid activity in the continental deep-subduction zone, partial melting during continental collision, element mobility in continental deep-subduction zone, recycling of subducted continental crust, geodynamic mechanism of postcollisional magmatism, and lithospheric architecture of collision orogen. Some intriguing questions and directions are also proposed for future studies.
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  • Water contents and hydrogen isotopes in nominally anhydrous minerals from UHP metamorphic rocks in the Dabie-Sulu orogenic belt

    GONG Bing   CHEN RenXu   ZHENG YongFei  

    A continuous flow method,by a combination of thermal conversion elemental analyzer(TC/EA)with isotope ratio mass spectrometry(MS),was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure(UHP)metamorphic rocks in the Dabie-Sulu orogenic belt.By using the developed step-heating technique,we have studied H2O concentration and H isotope composition of the different forms of water(structural OH and molecular H2O)in garnet.The quantitative measurements of H2O concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical outcrops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust.Therefore,by decompression dehydration at the contact between eclogite and gneiss,the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss.The measured maximum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500and 3500 ppm,respectively,under the peak UHP metamorphic conditions.
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  • Remelting of subducted continental lithosphere: Petrogenesis of Mesozoic magmatic rocks in the Dabie-Sulu orogenic belt RID C-4781-2008

    Zhao ZiFu   Zheng YongFei  

    The Dabie-Sulu orogenic belt was formed by the Triassic continental collision between the South China Block and the North China Block. There is a large area of Mesozoic magmatic rocks along this orogenic belt, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. The Late Triassic alkaline rocks and the Late Jurassic granitoids only crop out in the eastern part of the Sulu orogen, whereas the Early Cretaceous magmatic rocks occur as massive granitoids, sporadic intermediate-mafic intrusive and volcanic rocks throughout the Dabie-Sulu orogenic belt. Despite the different ages for their emplacement, the Mesozoic magmatic rocks are all characterized not only by enrichment of LREE and LILE but depletion of HFSE, but also by high initial Sr isotope ratios, low E > (Nd)(t) values and low radiogeneic Pb isotope compositions. Some zircons from the Jurassic and Cretaceous granitoids contain inherited magmatic cores with Neoprotozoic and Triassic U-Pb ages. Most of the Cretaceous mafic rocks have zircon delta (18)O values and whole-rock delta (13)C values lower than those for the normal mantle. A systematic comparison with adjacent UHP metaigneous rocks shows that the Mesozoic granitoids and mafic rocks have elemental and isotopic features similar to the UHP metagranite and metabasite, respectively. This indicates that these magmatic and metamorphic rocks share the diagnostic features of lithospheric source that has tectonic affinity to the northern edge of the South China Block. Their precursors underwent the UHP metamorphism and the post-collisional anatexis, respectively at different times and depths. Therefore, the Mesozoic magmatic rocks were derived from anatexis of the subducted continental lithosphere itself beneath the collision-thickened orogen; the geodynamic mechanism of the post-collisional magmatisms is tectonic collapse of orogenic roots in response to lithospheric extension.
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  • “Calculation of oxygen isotope fractionation in anhydrous silicate minerals.” Geochimica et Cosmochimica Acta: Erratum To Yong-Fei Zheng (1993) 57, 1079–1091

    Yong-Fei Zheng  

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