| Cenozoic basic magmatic events are one of the key geological processes in eastern China,which are of great significance for understanding regional tectonic evolution and its dynamic process.Subei Basin is located in Sulu orogenic belt in the north,Zhangbaling uplift belt in the west,Pacific subduction belt in the east,and close to the tectonic boundary between North China plate and South China plate.It is a Cenozoic basin with the most special tectonic position in eastern China.Cenozoic diabases and basalts are widely developed in the basin,which records abundant information of regional basic magmatic events in this period.In this work,Gaoyou Sag,which has the strongest magmatic activities in Subei Basin,is selected as the research area.The representative basic rock core samples are selected to carry out zircon U-Pb geochronology and rock geochemistry analysis,which provides important references for further understanding of the Cenozoic basic magmatic events in eastern China.In this work,detailed core samples and microscopic observations were carried out first,and the petrographic characteristics of basic magmatic rocks in Gaoyou Sag were clarified.Then,the LA-ICP MS zircon U-Pb geochronological analysis was used to determine the temporal and spatial evolution characteristics of magmatic activities in Gaoyou Sag,combining with the characteristics of magmatic emplacement horizon and spatial distribution.On this basis,through the analysis of major elements,trace elements and rare earth elements,the geochemical characteristics of the rocks are expounded,and the magmatic processes such as partial melting and magma mixing/dyeing are analyzed.Combined with whole-rock Sr-Nd-Pb isotope analysis,the characteristics of magmatic source area and magmatic genesis mechanism are discussed.Finally,the deep dynamic process and regional dynamic mechanism of magmatic are discussed with the analysis of regional tectonic background.Through the above-mentioned series of research works,the main achievements are as follows:1.The basic rocks in Gaoyou Sag mainly developed in the deep depression zone in the middle of the sag and migrated upward along the slope zone.The main development position of magma corresponds to the strongest extensional structure and fault deposit in the basin,indicating the coupling relationship of magma-structure-sedimentation under extensional background.Diabase deposits are often characterized by superposition of multi-stage emplacement underlay,and there are two peak periods of magmatic emplacement activity,which are 58.0Ma and 42.5Ma respectively.It indicates that the former age is Wubao magmatic event and the latter age is Sanduo magmatic event,among which Sanduo magmatic activity is the strongest,coincides with the strong extension deformation and sedimentary fault time of the basin.2.The geochemical analysis of the whole rock shows that the magmatic rocks in Gaoyou Sag belong to the series of quasi-aluminum,sodium-rich,magnesium-rich and alkaline-subalkali magma.All samples have similar trace element cobwebs and REE distribution curves,indicating that magmatic rocks in Gaoyou Sag have the same magma source area.Rocks have lower LREE/HREE,SI,and higher Mg~#,K/Rb,Ce/Pb,Nb/U,DI,which indicates that magma in Gaoyou Sag has lower evolution degree.The analysis of the magmatic process shows that this set of magma has undergone a remarkable process of magma mixing or contamination,but it does not have typical characteristics of separation and crystallization and partial melting in different degrees.Therefore,the magma may have originated from lithospheric mantle with roughly the same degree of melting,and has undergone different degrees of mixing/contamination processes in the crust.3.Petrography and rock geochemical analysis show that the magma has the mineral assemblage characteristics of mantle-derived magma,and relatively deficient Sr-Nd-Pb isotopic composition.Magma end-member analysis shows that the magma mainly comes from the partial melting of the new lithospheric mantle in Cenozoic,and there are early EM I or EM II enriched mantle end-members left;The magma composition has the trend of orogenic belt magma,which is consistent with its structural characteristics in the foreland deformation zone of Indosinian orogenic belt.The magma has the trend of subduction melt/fluid metasomatism,which indicates the melting/fluid precipitation process of oceanic crust under the subduction background of the Pacific plate.On the whole,the magma contains EM I-enriched mantle end-member components and lithosphere information pro-North China,which is consistent with the crocodile mouth model of collision wedging in Dabie-Sulu orogenic belt.4.A series of discrimination diagrams of basic magma tectonic environment show that the basic magma in Gaoyou Sag has the characteristics of continental intraplate basalt,or continental rift basalt and back-arc basalt,which is consistent with the regional dynamic background of back-arc extension in Cenozoic.The samples contains some geochemical characteristics of MORB,indicating that they originated from the magmatic properties of the new lithosphere mantle/asthenosphere under extensional background.Comprehensive analysis shows that the basic magma in Gaoyou Sag is the product of decompression and melting of new lithosphere mantle,and its regional dynamic mechanism is back-arc tension under the background of subduction of Pacific plate to Eurasian plate.The strong tensile and thinning zone in the plate not only developed the strong extension deformation,but also accompanied by the strong fault-depression basin-forming process,and the depression-melting process under the strong tension occurred in the basin,which resulted in the formation of basic magmatic activity.5.By comparing with the peak magmatic activities in Bohai Bay Basin and Zhejiang province,and combining with the regional dynamic background of magmatic activities,it is considered that the temporal and spatial evolution of Cenozoic basic magma in eastern China is mainly controlled by the subduction-rewinding direction of the Pacific plate.With the trench retreat and arc-land separation caused by the rolling back of subduction plates in the Pacific Ocean,the continental extension gradually migrated to the southeast over time,which led to the magmatic activity center gradually migrating from northwest to southeast. |
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