Asthenospheric flow and lithospheric erosion driving the outward growth of the northeastern Tibetan Plateau

Abstract

The northeastern (NE) Tibetan Plateau is extruding eastward at a rapid rate (∼15 mm a⁻¹), but the role of the upper mantle in this process remains unclear. Early-Miocene primary melilitites from the leading edge of the extruding plateau provide critical insights into the upper mantle dynamics. Geochemical and Sr–Nd–Pb–Os isotopic data, supported by experimental melt comparisons, reveal that these melilitites originate from a hybrid source of CO₂-bearing mantle source, probably dominated by peridotite and pyroxenite/wehrlite lithologies. This is consistent with carbonate minerals found in mantle xenoliths (peridotite + pyroxenite) entrained within the melilitites. Geothermobarometric calculations indicate magma generation at 116–135 km depth, below the lithosphere-asthenosphere boundary (∼112 km) constrained by xenoliths and seismic data. Isotopic data trace the carbon origin to a carbonated lithosphere associated with the 150-km-thick eastern tectonic blocks. Recent seismological studies suggest that eastward-flowing asthenosphere beneath the northeastern Tibetan Plateau is actively eroding the thicker lithosphere (150–200 km) of the eastern blocks. We propose that this asthenospheric flow not only thins the lithosphere but also mobilizes carbon from the eastern carbonated lithosphere into generating the melilitite, likely via edge-driven convection. The resulting melilitite compositions is therefore a petrological record of these dynamic processes. This study highlights the critical role of upper mantle processes—astenospheric flow and lithospheric erosion—in driving the eastward extrusion of the Tibetan Plateau. It also underscores the importance of carbon mobilization in understanding mantle carbon cycling during continental collision.