| Geographically,the northeast(NE)Asian continental margin includes the northeast China,the Russia Far East,the Korean Peninsula,and the Japan island arc.The NE Asian continental margin has been influenced by the subduction of the western Pacific Plate,and experienced a transformation from an active continental margin setting to a trench-arc-basin system during Cenozoic.Widespread Cenozoic potassic basalts,sodic basalts and high-Mg andesites occurred at the eastern margin of NE Asia.Thus,NE Asian continental margin has become a natural laboratory for exploring the transformation of tectonic nature and the crust-mantle interaction.However,the timing,mechanism and deep mantle process of the transformation of tectonic nature in the NE Asian continental margin have been a controversial issue.In addition,the material cycle within subduction zones directly affects the surface ecosystems and climate change,and is closely related to the Earth’s livability,which has always been the forefront and focus of Earth science research.Recent studies on the material cycle within subduction zones have mainly focused on different material-derived fluids/melts metasomatizing the overlying mantle in two-dimensional,and the evolutionary mechanism of recycled plate material.However,the researches on the three-dimensional variations of material cycle,and fluid evolution of the subduction zone during the transformation of tectonic nature are insufficient.To answer these scientific questions,this thesis studies the petrology,geochronology,major-and trace-element geochemistry,Sr-Nd-Pb-Hf-Mg-Zn isotopic geochemistry of the Cenozoic basalts widely occurred in the Songnen Massif,Jiamusi Massif,Khanka Massif,Sikhote-Alin Orogenic belt and Japan Arc.93 basalts from different locations and ages are selected according to their spatio-temporal framework.Based on the spatio-temporal variations of geochemistry of the Cenozoic basalts and geophysical research in the NE Asian continental margin,this thesis determines the timing and mechanism of transformation from an active continental margin setting to a trench-arc-basin system,identifies the spatial variations in carbon input from the Pacific Plate and the essential factor controlling these spatial variations.This thesis also reveals the influence of mantle corner flows on the evolution of subduction zone fluids through time during the transformation of tectonic nature.The main achievements are as follows:1.Determining the time and mechanism for transformation from an active continental margin setting to a trench-arc-basin system in the NE Asian continental marginThe Cenozoic basaltic volcanism in the NE Asian continental margin can be subdivided into Middle Eocene(52-45 Ma),Oligocene to Early Miocene(34-20 Ma),Middle Miocene(14-10 Ma),and Late Miocene to Pliocene(5-2 Ma)based on field geological relationship and the latest Ar-Ar and K-Ar dating results.The Middle Eocene(52-45 Ma)basalts in NE China and Russia Far East exhibit arc-like geochemical features,implying that their primary magma could be derived from partial melting of a mantle wedge that had been modified by slab-derived fluids.In contrast,the Oligocene-Early Miocene(34-20 Ma)basalts in NE China display the ocean island basalt(OIB)-like geochemical features,suggesting that their primary magma could be originated from partial melting of an asthenospheric mantle that had been modified by slab-derived melts.However,the coeval basalts in Hokkaido exhibit arc-like geochemical features,indicating that their primary magma could be derived from partial melting of a mantle wedge that had been modified by slab-derived fluids.Additionally,the Middle Miocene(14-10 Ma)basalts in NE China and Russia Far East can be subdivided into two types:one type of basalts have a weak arc-like geochemical signature,implying that their primary magma could be originated from remelting of lithospheric mantle that had been modified by slab-derived fluids,and the other type of basalts have OIB-like geochemical features,suggesting that their primary magma could be derived from partial melting of an asthenospheric mantle that had been modified by slab-derived melts.The coeval basalts in Hokkaido are arc-like geochemical features,implying that their primary magma could be originated from partial melting of a mantle wedge that had been modified by slab-derived fluids.The Late Miocene-Pliocene(5-2 Ma)basalts in NE China and Russia Far East exhibit OIB-like geochemical features,indicating that their primary magma could be derived from partial melting of an asthenospheric mantle modified by slab-derived melts,whereas the coeval basalts in Hokkaido display arc-like geochemical features,implying that their primary magma be originated from partial melting of a lithospheric mantle modified by slab-derived fluids or an asthenospheric mantle modified by slab-derived supercritical fluids.The spatio-temporal variations of geochemistry of the Cenozoic basalts in the NE Asian continental margin shows:1)the arc-like magmatic activity is located on the eastern part of NE Asian continental margin(near the trench),whereas OIB-like magmatic activities occur on the western part of NE Asian continental margin(intraplate)in every stage;2)the arc-like and OIB-like magmatic activities exhibit twice eastward shifts,from 52 Ma to 33 Ma and from 33 Ma to 20 Ma,indicating the episodic back-arc extensions in early Cenozoic and the formation of the trench-arc-basin system in NE Asia continental margin.Combined with temporal variation of geochemistry of the Cenozoic basalts and previous studies,it is concluded that the opening of the back-arc basin(Sea of Japan)happened at ca.20 Ma.The temporal evolution of geochemistry of basalts from different spatial positions on the NE Asian continental margin indicate the episodic back-arc extensions on the NE Asian continental margin is controlled by the eastward asthenospheric mantle flow.Combined with Cenozoic structural evolution history,it is concluded that the first eastward shift(from 52 Ma to 33 Ma)can be attributed to eastward movement of asthenospheric mantle triggered by the slab rollback and the interaction between the slab and the 660 km discontinuity,and that the second eastward shift(from 33 Ma to 20 Ma)is attributed to the trench retreat caused by further eastward movement of asthenospheric mantle.2.Discovering the spatial variations of carbon input in the Pacific subducted plate,revealing the essential factor controlling these spatial variationsThe spatial variations of the geochemistry of the coeval basalts effectively reveals the spatial variation of the recycled components derived from the subducted plate.In order to identify the spatial variation of carbon input in the Pacific plate,the Mg-Zn isotopes of the Middle Miocene(14-5 Ma)basalts are analyzed,These Middle Miocene basalts are distributed parallel to the subduction zone in the NE Asian continental margin.The samples can be subdivided into three groups according to their spatial position and Mg-Zn isotopic compositions,i.e.,northern,central,and southern groups.The northern samples have extremely heavy δ66Zn(0.40‰ to 0.62‰)and lightδ26Mg(-0.48‰ to-0.37‰)compositions,i.e.,a coupling of Mg and Zn isotopic compositions.The central samples have δ66Zn values that overlap with those of global MORBs(0.25‰ to 0.31‰),and have slightly lighter δ26Mg(-0.35‰ to-0.19‰)compositions than those of normal mantle,i.e.,a slight decoupling of Mg and Zn isotopic compositions.In contrast,the southern samples have low δ26Mg(-0.56‰to0.02‰)and normal δ66Zn values(0.26‰ to 0.40‰),i.e.,a decoupling of Mg and Zn isotopic compositions.The spatial variation of Mg-Zn isotopes of these basalts indicates spatial heterogeneity in carbon input of the subducted Pacific Plate.The northern samples with coupled Mg-Zn isotopic characteristics indicate that the significant carbonate dissolution happened in the shallow mantle wedge of the northern subducted Pacific Plate,with only a small fraction of carbonate surviving into the deep mantle.In contrast,the southern samples show a decoupled Mg-Zn isotopic signature,implying that undissolved carbonates could be retained in the slab and transported to great depths and long distances beneath the mantle wedge,which is also supported by the wide occurrence of wehrlite xenoliths in the corresponding intraplate basalts.Geophysical studies have revealed that the surface temperature of the Pacific slab gradually decreased from north to south.At the sub-arc depth,the subducted angle of the northern subducted Pacific Plate is moderate,and the surface temperature of the plate is hot.In contrast,the southern plate has high dip angles,and relatively low surface temperature.Therefore,it is concluded that the spatial heterogeneity in carbon input of the subducted Pacific Plate can be driven by the three-dimensional thermal structure of the Pacific subduction zone.3.Identifying the evolution of subduction zone fluids during the transformation of tectonic nature in the NE Asian continental margin,proposing that the thermal structural variation induced by mantle corner flows is the driving factor for evolution of the fluidsHow do subduction zone fluids evolve accordingly during the evolution process from arc to back-arc in the same region?To answer this issue,the Mg-Zn isotopes are analyzed for the Hokkaido basalts with different time,which cover the entire transformation period of tectonic nature(34-3 Ma)in the NE Asian continental margin.The samples fall into three groups according to their Mg-Zn isotopic compositions:early stage(34-22 Ma),middle-stage(13 Ma),and late stage(10-3 Ma).The early stage samples have δ66Zn values(0.25‰ to 0.32‰)that overlap with those of global MORBs(0.25‰ to 0.31‰),and have slightly heavier δ26Mg(-0.18‰ to-0.09‰)compositions than those of normal mantle(-0.32‰ to-0.18‰),i.e.,a slightly decoupling of Mg and Zn isotopic compositions,indicating that the metasomatic material of the mantle source is sediment-derived fluids.The middle-stage samples haveδ66Zn values(0.26‰ to 0.32‰)that overlap with those of global MORBs,and have heavier δ26Mg(-0.05‰ to 0.01‰)compositions than those of normal mantle,i.e.,a decoupling of Mg and Zn isotopic compositions,indicating that the metasomatic material of the mantle source is plate serpentinite-derived fluids.In contrast,the late stage samples have extremely heavy δ66Zn(0.28‰to 0.42‰)and light δ26Mg(-0.37‰to-0.20‰)compositions,i.e.,a coupling of Mg and Zn isotopic compositions,indicating that the metasomatic material of the mantle source is carbonatite-derived supercritical fluids.The temporal variation of Mg-Zn isotopic compositions of these basalts indicates that the subduction zone fluids experienced an evolution process from sediment-derived fluids in early stage(arc stage)to plate serpentinite-derived fluids in middle-stage,and then to carbonatite-derived supercritical fluids in late stage(back-arc stage).Then,what is the driving factor of the evolution of subduction zone fluids mentioned above?Numerical simulations and geophysical studies indicate that the mantle corner flows can dominate the surface temperature structure of subducted plates,thereby affecting the recycled materials of the plates.In the early stage of the evolution from arc to back-arc in Hokkaido(34-13 Ma),the asthenospheric mantle upwelling resulted in the formation of initial mantle corner flow,further causing rapid heating up in the sub-arc mantle,which also led to a transition from subducted plate-derived fluids(dominated by sediment components or AOC)to plate serpentinite-derived fluids.During the evolution from arc to back-arc in Hokkaido(13-10 Ma),the continuous expansion of the back-arc basin had led to an increasing distance between the center of back-arc expansion and the sub-arc mantle,meanwhile,the flow path of the corner flow became wider.The corner flow,that undergoes the melting in back-arc and then reaches the sub-arc,causes a decrease in the temperature of sub-arc mantle compared to the initial stage,which also reduces the contribution of plate serpentinite in the subducted plate fluid.After the completion of the back-arc extension(10-3 Ma),the corner flow,formed by the asthenospheric mantle upwelling,causes significant warming of the sub-arc mantle again,further leading to the generation of carbonatite-derived supercritical fluid.Taken together,it is concluded that the thermal structural variation induced by mantle corner flows is the driving factor controlling the evolution of subduction zone fluids during the transformation of tectonic nature. |
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