Binary mixing of lithospheric mantle and asthenosphere beneath Tengchong volcano, SE Tibet: evidence from noble gas isotopic signatures

Abstract

ABSTRACT The Miocene–Quaternary Tengchong volcanic field in SE Tibet was generated after the main stage of continental collision between India and Eurasia. Consensus on the origin of Tengchong volcanism has not yet been achieved. In this study, we analysed the He-Ar isotopic compositions of olivine and pyroxene phenocrysts, whole-rock major and trace elements and Sr-Nd-Pb isotopic compositions. The 3He/4He ratios range from 4.1 to 8.2 Ra (Ra = 1.4 × 10−6), overlapping the values of mid-ocean ridge basalt (7.0–9.0 Ra) and subcontinental lithospheric mantle (SCLM, 5.2–7.0 Ra). The low 3He/4He (< 7.0 Ra) basalts have high 87Sr/86Sr (average of 0.707928) and La/Yb (average of 22.0) ratios and low Nb/La (average of 0.36) ratios, whereas the high 3He/4He (> 7.0 Ra) basalts exhibit relatively low 87Sr/86Sr (average of 0.706708) and La/Yb (average of 17.0) ratios and relatively high Nb/La (average of 0.52) ratios. These observations indicate that the primitive magmas originated from a mixture of metasomatized SCLM and enriched asthenospheric mantle. The SCLM was likely metasomatized by the subducted Neo-Tethyan oceanic plate, while the asthenospheric mantle was enriched by the subducting Indian oceanic plate. The increasing trend of 3He/4He ratios and decreasing trend of 87Sr/86Sr ratios over time suggest that the contribution of the metasomatized SCLM decreased after the late Pleistocene relative to that of the enriched asthenosphere, reflecting progressive lithospheric extension and thinning. Our results reveal that magmatic He isotopes can be used to constrain deep dynamic processes.