Recycling of eroded crust by mélange diapirs evidenced from the Early Cretaceous arc magmatism in southern Tibet

Abstract

Crustal material recycling in a subduction factory increases the diversity and complexity of arc magma. However, the exact origin and mechanism of its contribution to arc magma are not fully understood. Here, we present new data on the Molang basaltic rocks from the southern Lhasa terrane to gain an in-depth-understanding of the mechanisms of crustal recycling. The zircon UPb ages indicate that these rocks were emplaced at 128 Ma. They display characteristics ranging from forearc basalts to tholeiitic arc basalts with depleted mantle isotope compositions (εNd(t) = +4.58 to +4.86) and zircon ɛHf(t) values (+7.3 to +8.7). The uniform isotopic composition (SrNd) and fractionation of trace elements show that crustal materials could have been added to the magma source in the form of bulk sediment. Therefore, we have proposed an argument supporting the mélange diapirs model. The age spectra of the inherited zircons are consistent with those of the detrital zircons from the subduction hanging wall. This implies that the inherited zircons may be unmolten residues of eroded forearc crust that survived erosion and subduction from the slab in well-mixed mélange diapirs that passed through a comparatively hot mantle wedge and occur as key evidence for crustal recycling in the arc magma. Based on a synthesis of earlier research, we suggest that the Molang basaltic rocks were derived from the partial melting of the mélange diapirs in a steep subduction setting during a new subduction initiation event involving the Neo-Tethyan oceanic slab.