Quaternary volcanism in southeastern Tibetan Plateau: A record of Neo-Tethyan oceanic slab stagnant in the mantle transition zone

Abstract

Tibetan lateral mantle flow could help to decipher the material movement mechanisms within global plate convergence zones. However, the front edge of this mantle flow is unclear. We conducted petrological, geochronological, mineralogical, geochemical, and Sr-Nd-Pb isotopic investigations of Quaternary intracontinental alkali basalts from southwestern Yunnan (south of 27°N) to determine the petrogenesis of the Quaternary alkali basalts in the southeastern Tibetan Plateau in particular and to trace the recent Tibetan lateral mantle flow. Alkali basalts in the region are mainly basanite and trachybasalt that erupted during the Pleistocene epoch. They possess highly incompatible elemental and radiogenic Sr-Nd-Pb isotopic compositions similar to those of the oceanic-island basalts, consistent with melts derived from asthenospheric mantle with a low degree of partial melting. Calculated magma water contents of regional alkali basalts range from 1.32 ± 0.48 wt% to 2.23 ± 0.18 wt%, which corresponds to water content of their mantle source comprising 269 ppm to 3591 ppm, which is significantly higher than that of the normal upper mantle (i.e., 50−250 ppm). Quantitative trace-element modeling and dramatic variations in oceanic crust−sensitive indicators such as Eu/Eu*, Sr/Sr*, Ce/Pb, (Nb/Th)N-PM, and (Ta/U)N-PM indicate variable contributions of upper and lower oceanic crust to magma sources. Systematic examinations of petrological, geochemical, and geophysical evidence reveal that the temporary small-volume Quaternary volcanism in the southeastern Tibetan Plateau is unrelated to Tibetan southeastward mantle flow but is primarily attributed to stagnant Neo-Tethyan slab in the mantle transition zone. Our study offers a distinctive perspective for reconciling the geochemical features of intracontinental alkali basalts and highlights the potential role of alkali basalts in tracing the front edge of recent Tibetan lateral mantle flow.