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

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 ³He/⁴He (< 7.0 Ra) and Nb/La (average of 0.36) ratios as well as high ⁸⁷Sr/⁸⁶Sr (average of 0.707928) and La/Yb (average of 22.0) ratios. In contrast, the enriched asthenospheric mantle is characterized by high ³He/⁴He (> 7.0 Ra) and Nb/La (average of 0.52) ratios with relatively low ⁸⁷Sr/⁸⁶Sr (average of 0.706708) and La/Yb (average of 17.0) ratios. The increasing trend of ³He/⁴He ratios and decreasing trend of ⁸⁷Sr/⁸⁶Sr ratios 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 ⁸⁷Sr/⁸⁶Sr ratio and SiO₂ content, whereas basaltic andesite, andesite, and dacite display positive correlation between ⁸⁷Sr/⁸⁶Sr ratio and SiO₂ content. 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.