腾冲火山起源的地球物理和地球化学研究进展

The origin of the Tengchong volcano:A review of geophysical and geochemical studies

腾冲新生代火山位于印度板块与欧亚板块碰撞边界上.该区域构造活动强烈,火山具有潜在的喷发性,研究腾冲火山起源对于认识板块俯冲过程、火山活动规律具有重要意义.本文总结了近年来腾冲火山起源的最新进展,包括地球物理和地球化学的新成果,探讨了火山岩浆来源和火山形成的深部动力学机制.这些研究发现腾冲火山的形成主要与板块俯冲有关,早期俯冲形成的残余大洋板片和现今俯冲的印度板块都可能是交代物质的来源,大洋板片在深部释放融流体形成富集软流圈地幔和岩石圈地幔.后期岩石圈的伸展作用可能诱导了富集软流圈地幔的部分熔融,导致岩浆物质喷出地表.根据^(87)Sr/^(86)Sr与SiO_(2)的相关性,得到腾冲玄武岩遭受到地壳混染作用不明显,而安山岩和英安岩遭受地壳混染作用明显.地球物理成像显示腾冲火山下方地壳中有不同尺度的岩浆囊,其中上地壳有若干小岩浆囊,在中下地壳有大岩浆囊.地震成像显示地壳中的低速体向下延伸至上地幔,很可能反映地壳中的岩浆囊有地幔热物质的持续供给.

The Tengchong Cenozoic volcano is located near the collision boundary between the Indian and Eurasian plates.The tectonic activity in this area is strong,and the volcano has a potential for eruption.Studying the origin of the Tengchong volcano is important for understanding plate subduction processes and the mechanism of volcano eruption.This review first summarizes the latest researches on the origin of the Tengchong volcano,including new advances in geophysics and geochemistry,and then discusses the magma source and deep dynamic mechanism.These studies have found that the genesis of Tengchong volcanism is mainly related to plate subduction.The subducted residual oceanic plate and the currently subducting Indian plate may both contribute to the source of metasomatic materials.The oceanic plate released melt and fluid to form metasomatized asthenospheric and lithospheric mantles,and Tengchong magma originated from the mixing of lithospheric and asthenosphere mantle melts.The metasomatized lithospheric mantle is characterized by low^(3)He/^(4)He(<7.0 Ra)and Nb/La(average of 0.36)ratios as well as highSr/Sr(average of 0.707928)and La/Yb(average of 22.0)ratios.In contrast,the enriched asthenospheric mantle is characterized by high^(3)He/^(4)He(>7.0 Ra)and Nb/La(average of 0.52)ratios with relatively lowSr/Sr(average of 0.706708)and La/Yb(average of 17.0)ratios.The increasing trend of^(3)He/^(4)He ratios and decreasing trend of^(87)Sr/^(86)Srratios over time suggest that the contribution of the metasomatized lithospheric mantle decreased relative to that of the enriched asthenosphere after the late Pleistocene,reflecting the progressive lithospheric extension and thinning.Tengchong basalts do not show clear correlation between^(87)Sr/^(86)Srratio and SiO_(2) content,whereas basaltic andesite,andesite,and dacite display positive correlation between^(87)Sr/^(86)Srratio and SiOcontent.This phenomenon indicates that Tengchong basalt suffered limited crustal contamination,while basaltic andesite,andesite,and dacite underwent obvious crustal contamination.Moreover,seismic imaging along with magnetotelluric and geothermal studies revealed several magma chambers(low-speed anomalies)in the crust beneath the Tengchong volcano.However,the size and depth of the magma chambers vary between different studies.The low-velocity bodies in the crust extend downward to the upper mantle,which may indicate that the magma chambers in the crust are fed by hot materials from the upper mantle.However,a consensus on the origin of hot materials has yet to be reached.Seismic tomography studies suggest that hot materials could be generated from the dehydration of the stagnant slab in the mantle transition zone,tearing of the subducted plate,or slab rollback.Differences in seismic imaging results may be caused by the limited observation data,data imaging methods,and the multi-solution of geophysical inversion.The utilization of more data and advanced geophysical methods will help obtain high-resolution imaging results to further constrain the origin of the Tengchong volcano.