Recycling of subducted Indian continental crust constrained by late Cretaceous mafic dykes in Central Lhasa block of the Tibetan plateau

Abstract

The question of whether subducted continental crust can be recycled into post-collisional magmatism in continental collisional zones remains controversial. Post-collisional mantle-derived ultrapotassic rocks are widespread in western part of central Lhasa block (WCL) of the Tibet-Himalaya orogen and show arc-type trace-element signatures and very enriched SrNd isotopes, which have been explained by the recycling of subducted Indian continental crust. However, due to the lack of late Cretaceous mafic magmatism in the WCL, the nature of the ancient subcontinental lithospheric mantle (SCLM) beneath the WCL has not been well constrained. It is therefore still unclear whether the enriched components of these ultrapotassic rocks were derived from subducted Indian continental crust or inherited from the ancient SCLM. Here we report the mafic dykes from the TangraYumco area in the WCL, which formed at ∼90 Ma (whole-rock 40Ar39Ar, and zircon and titanite UPb ages). The diabase-porphyrites and diorite-porphyrites are similar to intraplate Nb-enriched basalts and magnesium adakitic rocks, respectively. This rock association reveals the lithospheric foundering process. Furthermore, the diabase-porphyrites generated by the interaction between the ancient SCLM and asthenosphere provide important constraints on the nature of the ancient SCLM, which shows distinct Nd isotopes to the post-collisional ultrapotassic rocks. In addition, the NW Lhasa block was in a rear-arc environment since the late Cretaceous and thus this SCLM was not significantly influenced by the ongoing northward subduction of the Indus-YarlungZangbu Neo-Tethys ocean. Finally, we propose that the post-collisional ultrapotassic rocks cannot be sourced from the ancient SCLM, but instead were derived from a relatively depleted mantle (e.g., juvenile lithospheric mantle) metasomatized by subducted Indian continental crust. Thus, this study confirms the recycling of subducted continental materials in continental collisional orogens.