Lithospheric thinning beneath the Tengchong volcanic field, Southern China: Insight from Cenozoic calc-alkaline basalts

Abstract

The Tengchong Cenozoic volcanic field lies in SE margin of the Tibetan Plateau. The basalts of the Tengchong field exhibit evident spatial-temporal variations, but consensus on their meaning has not been reached yet. In this study, we collected basalts from western, central and eastern areas in the Tengchong volcanic field and measured the whole-rock and olivine major and trace elements of basalts. Tengchong basalts exhibit remarkable chemical and isotopic diversity, showing a strong correlation with eruption locations and ages. Specifically, basalts in the western and eastern areas (formed at 7.2–2.8 Ma) are characterized by high 87Sr/86Sr and low 3He/4He ratios, while those in the central area (formed at 0.6–0.02 Ma) feature low 87Sr/86Sr and high 3He/4He ratios. Based on the temperature- and pressure-dependent elemental partition coefficients, this phenomenon is interpreted as mainly caused by the difference in lithospheric thickness among these areas. On the one hand, the estimated primary magmas in the eastern and western areas show higher SiO2, Na2O, (La/Sm)N, Hf/Lu and Ba/Zr ratios than those in the central area. On the other hand, the Ni contents in olivine phenocrysts are higher in the western and eastern areas than in the central area. As different amounts of extension result in different degrees of decompression of the asthenosphere, finally influencing the compositional variation of magmas, these results indicate that the lithosphere in the eastern and western areas is thicker than that in the central area. In addition, basalts erupted in the eastern and western areas are older than those in the central area, suggesting lithospheric thinning. We propose that lithospheric extension due to slab rollback may have caused lithospheric thinning. In addition, according to the different deformation modes of the crust and lithospheric mantle, our study supports mantle-crust decoupling south of ∼26°N in SE margin of the Tibetan Plateau.