Study On Origin Of The Tianchi Volcano, Changbaishan

Tianchi volcano has a unique geological setting, located far from the Pacific subduction trench and underlaid by the deeply subducted paleo-Pacific slab at around 660 km. In terms of geodynamic origin, the Tianchi volcano is distinctly different from arc volcano, which is generated by mantle partial melting enhanced by subduction slab dehydration at shallow depth. The origin of the Tianchi volcano is described by many hypotheses, such as plume, back-arc extension and small scale mantle flow. The morphology of the stagnant Pacific slab in the transition zone can be observed clearly, with the development of high resolution tomography technology. Therefore, deep subducting slab dehydration is considered as the main mechanism to explain far field intercontinental volcanism/magmatism in northeast of China region. The origin mechanism of partial melting due to deep subducting slab dehydration is commonly researched by numerical modeling and tomography speculation. However, the direct evidence, especially the lithological one is still lacking or deficient. This study is divided into geodynamics numerical modeling and lithological analysis. A plausible explanation of the origin of the Tianchi volcano is presented, according to the result of geodynamics numerical modeling and rock lithological analysis result. A 2D thermo-mechanical coupled numerical subduction model is implemented to simulate the entire process, which including subducting slab hydration, subducting, dehysration(deep dehydration), partial melting, melt upwelling and intracontineantal magmatism. The research content of the geodynamics mainly includes the process of deep dehydration and back-arc extension. The aim of the former is to explain that the water can be transit to deep the transition zone and eventually released at the depth of about 600 km, and the whole geodynamic process of deep dehydration induced overlying mantle wedge partial melting and intracontinental magma chamber and magmatism due to partial melt upwelling. The latter is used to explain that subduction can form a broad back-arc basin(width is wider than 1000 km), so the front edge of the continental plate can be far away the subduction boundary/trench about 1000 km. Lithological analysis is implemented by field investigation, laboratory geochemistry analysis and age determination and compared with the result of geodynamic modeling to further research into the origin of the Tianchi volcano magmatism. The following conclusions can be drawn by this research:(1)Numerical modeling results show that the subducting slab has the ability to carry water to ~660 km transition zone surface and released. More water is released from the slab, and thus more deep partial melts are generated. The deep partial melts gather into a mass and upwelling. The plumes from deep slab dehydration-induced magmatism are emplaced and form magma chamber under far intercontinental plate, and then induce intracontinental magmatism.(2)Slab deep dehydration and deep mantle wedge partial melting is a very complex geodynamic process, with many influence factors. The solidus curve of the mantle is the most important influence factor, which directly controls the occurrence of the mantle wedge partial melting. A large number of partial melting generation needs a long evolution time, so the subduction temporal duration is another important influence factor. In our experiments, slab detachments are observed from the surface flat part of the subducting slab to 660 km transition zone. The detachment of the slab controls slab deep dehydration, deep mantle wedge melting and the location of the melting and melt upwelling. The variation of the temperature-pressure condition of the mantle wedge is unconspicuous, thus has little influence on the slab deep dehydration.(3)Four major subduction stages, which controlling back-arc extension are revealed by numerical modeling. Three patterns of back-arc extension process can be recognized from our experiments. Our numerical indicate that slab with proper geophysical parameters is capable of forming ~1000 km back-arc basin. Systematical discussion of the influence factors implies that the slab with medium age(120 Ma), suitable slab coupling force and slow convergence velocity can generate a broad back-arc extension zone.(4)The basalt of Erdaobaihe river of the Changbai mountain‘s characteristics of the major elements, trace elements and rare earth elements show that basalt of Erdaobaihe river of the Changbai mountain is derived from olivine, pyroxene and basic plagioclase crystallization differentiation under mantle condition. The magma source of the basaltic lavas is mostly close to primitive mantle. The trachybasalt and basaltic trachyandesite is generated by crystallization differentiation. The basalt retains partial melting trend, and the generation of the basaltic magma is related to the partial melting of subducted slab. The magma is experienced assimilation and contamination by the crustal materials during upwelling process.It is the first time that combining the geodynamic numerical modeling and lithological analysis to research the origin of the Tianchi volcano, which is far away from the subduction trench. A plausible explain of the origin of the Tianchi volcano is presented based on eodynamic numerical modeling and lithological analysis. The result of our research will provide(1)important reference for the research on the geodynamic mechanism of the plate subduction and magmatism,(2)basic data for the research on the evolution mechanism of the Cenozoic volcanic rocks and magma,(3)new gist for the origin of the regional basalt, character and origin of the magma,(4)quantitative reference data for further volcanic study, such as the activity, eruption mechanism and prediction methods,(5)quantitative and visualize deep geodynamic background for the origin of intracontinental volcano in eastern Asian,(6)reference date for volcanic disaster monitoring and prevention, and has important pratical significance.