Recycling of sub-continental lithosphere during early ocean spreading revealed by Triassic trachytes in the central Tibetan Plateau

Abstract

Delamination of sub-continental lithosphere fragments and their descent into the asthenospheric mantle should be a common process following the rupture of continental lithosphere and the initiation of an oceanic basin. However, evidence for sub-continental lithospheric mantle recycling in the source of oceanic intra-plate non-plume volcanism remains unclear. This study reports newly identified ca. 240 Ma trachytes in the Duoma ophiolite complex within the Bangong-Nujiang suture zone, central Tibet, which shed new light on sub-continental lithospheric mantle recycling. These trachytes have high and variable SiO2 (59.5–67.3 wt%) along with low MgO, and Cr contents and this combined with significant depletions in Eu, Ba, Sr, and Ti, suggests extensive fractional crystallization. Furthermore, they display positive Nb-Ta anomalies, along with moderately depleted Nd (εNd(t) from +3.3 to +4.5) and Hf isotopes (εHf(t) from +6.1 to +8.0), along with high 207Pb/204Pb values (15.50 to 15.59), and zircon δ18O values (5.3 ± 0.3 ‰). These data indicate that an enriched mantle endmember, possibly sub-continental lithosphere along with asthenosphere played a role in the generation of these trachytic magmas. We propose that fragments of sub-continental lithospheric mantle were trapped by upwelling asthenosphere during the initiation of oceanic spreading. This resulted in the generation of alkaline magmas in an oceanic intraplate setting. This study provides the first evidence for recycling of continental lithosphere in extinct ocean basins, and the process may be more widespread in the geological record than previously recognized.