Tectonic evolution of convergent plate margins and its geological effects

Oceanic lithosphere is generated at divergent plate boundaries and disappears at convergent plate boundaries.Seafloor spreading and plate subduction together constitute the physical coupling and mass conservation relationships to the movement of lithospheres on Earth.Subduction zones are a key site for the transfer of both matter and energy at converging plate boundaries,and their study has been the hot spot and frontier of Earth system science since the development of plate tectonics theory.As far as the dynamic regime and geothermal gradient of convergent plate margins are concerned,they have different properties in different stages of the subduction zone evolution.In general,the early low-angle subduction leads to compressional tectonism dominated by low geothermal gradients at the plate interface,and the late high-angle subduction results in extensional tectonism dominated by high geothermal gradients at the plate interface and its hanging wall.Active rifts are produced along suture zones through not only slab rollback or slab breakoff in the terminal stage of oceanic subduction but also foundering and thinning of the lithosphere in the post-subduction stage.Due to the differences and changes in the geometric and thermobaric structures of convergent plate margins,a series of changes in the type of metamorphism and magmatism can occur in active and fossil subduction zones.Dehydration and melting of the subducting oceanic crust are prominent at subarc depths,giving rise to fluids that dissolve different concentrations of fluid-mobile incompatible elements.The subduction zone fluids at subarc depths would chemically react with the overlying mantle wedge peridotite,generating metasomatites as the mantle sources of mafic magmas in oceanic and continental arcs.However,these metasomatites did not partially melt immediately upon the fluid metasomatism to trigger arc magmatism,and they did not melt until they were heated by asthenospheric convection due to rollback of the subducting slab.Therefore,recognition of the changes in the dynamic regime and geothermal gradient of subduction zones in different stages of plate convergence not only provides insights into geodynamic mechanisms of the tectonic evolution from subduction zones to orogenic belts,but also places constraints on the formation and evolution of different types of metamorphic and magmatic rocks within the advanced framework of plate tectonics.

Mesozoic mafic magmatism in North China:Implications for thinning and destruction of cratonic lithosphere

The North China Craton(NCC) has been thinned from >200 km to <100 km in its eastern part. The ancient subcontinental lithospheric mantle(SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction.This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series,manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts(OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast,mafic igneous rocks emplaced before and after this age exhibit both island arc basalts(IAB)-like trace element distribution patterrs and enriched Sr-Nd isotope compositions.This difference indicates a geochemical mutation in the SCLM of North China at^121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite notonly with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at^144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative ε_(Nd)(t) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled astheno spheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying astheno spheric mantle peridotite to generate the ultramafic metasomatites that show positive ε_(Nd)(t) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at^121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by moder seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.

Post-collisional mafic magmatism:Record of lithospheric mantle evolution in continental orogenic belt

In order to better understand the role of post-collisional mafic magmatism at convergent plate boundaries in revealing the earth’s evolution,this paper has systematically summarized the research history of post-collisional mafic magmatism,different types of collision and their influence on the nature of orogenic mantle,the concept and implication of post-collisional magmatism,and the relationship between post-collisional mafic magmatism and orogenic mantle evolution and mineralization.Post-collisional mafic igneous rocks are not only the direct records for studying the nature and evolution of orogenic mantle,but also the important carriers for regional mineralization.However,the type and quantity of the crustal materials involved in modifying the overlying lithospheric mantle during collisional orogeny,the process and mechanism of such modification,and the major control factors and mechanism of mafic magmatism-related mineralization during the post-collisional period are the main contents and direction of future researches in this field.Therefore,the study of post-collisional mafic magmatism is of significant implications for developing the theory of plate tectonics.