Zircon U-Pb and Lu-Hf isotopes and geochemistry of granitoids in central Tibet: Bringing the missing Early Jurassic subduction events to light

Abstract

Jurassic magmatic and metamorphic rocks in central Tibet are considered to record the subduction history of the Meso-Tethys Ocean (MTO) and have therefore attracted considerable research interest. However, Early–Middle Jurassic island arc magmatism has been reported only from the Amdo and Basu areas in the eastern section of central Tibet. In this paper, we report newly discovered Early–Middle Jurassic granitoids from Lhaguotso in the middle section of central Tibet and from Shiquanhe in the western section of central Tibet. Zircon U–Pb dating and whole-rock geochemical and zircon Lu–Hf isotope analyses were conducted on three groups of samples: plagiogranites from Lhaguotso (LG, 185–181 Ma); granodiorites from Shiquanhe (SGD, 184–183 Ma); and amphibole tonalites from Shiquanhe (SAT, 170 Ma). The three groups of samples are differentially enriched in Mg, Fe, and Ca. In addition, the samples show high SiO2 contents and Mg# values, enrichment in large-ion lithophile elements, and depletion in high-field-strength elements. Combining these characteristics with their differing Th, Ba, and rare earth element contents and Hf isotopic compositions, we suggest that the LG were formed by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids, whereas the SGD and SAT were formed by interaction between subducted sediment melts and the overlying depleted mantle. On the basis of our new data in combination with previous results, we propose that a broad oceanic island arc system existed in the MTO during the Early–Middle Jurassic. Differential influence of ancient microcontinent materials or crust-derived sediment led this intra-oceanic island arc to exhibit clearly different magmatic responses in its eastern and western regions.