Miocene high Sr/Y magmatism, south Tibet: Product of partial melting of subducted Indian continental crust and its tectonic implication

Abstract

Post-collisional (26–10 Ma) high Sr/Y and HREE-depleted intrusive rocks occur in an E–W trending belt paralleling the Yarlung–Zangbo suture in south Tibet. Their petrogenesis has been a subject of debate. Here, we present chronological, geochemical and Sr–Nd–Hf isotopic data for two newly discovered large-volume high Sr/Y granitoids (the Pagu granodiorite and the Nanmuqie granite) and three high Sr/Y porphyries intruding the granitoids in the Lhasa terrane. LA-ICP-MS zircon U–Pb dating for the Pagu granodiorite and Nanmuqie granite yielded identical magma crystallization ages of 14.0–14.4 Ma, which is indistinguishable from their associated high Sr/Y porphyries (14.2–14.6 Ma). The granitoid was intruded at middle-crust depth, whereas the porphyry was intruded at upper-crust depth, indicating that the Lhasa terrane has experienced a rapid crustal uplift during the high Sr/Y magma emplacement. Zircon Hf isotopic and whole-rock Sr–Nd isotopic compositions for these granitoids and porphyries suggest that their magmas were dominantly derived from partial melting of crustal materials. The granitoids and porphyries have ε Hf ( t ) values overlapping with the mafic granulites in the Himalayan terrane (Indian plate). Their Sr–Nd isotopic compositions show two-endmember mixing between the Himalayan mafic lower crust and the ultrapotassic lavas/the Lhasa lower crust. We suggest that the high Sr/Y magmas in the Lhasa terrane could be derived from partial melting of subducted Indian mafic lower crust with incorporation of the ultrapotassic lava and/or the Lhasa lower crust components. Our study suggests a new model for the high Sr/Y magma generation in the Lhasa terrane and provides a line of geochemical evidence that the Indian continental crust was subducted beneath the southern Lhasa terrane in the early–middle Miocene.