Over 15 km high-δ18O juvenile lower crust in central Tibet formed by high-flux magmatic arc underplating

Arc magma underplating is a fundamental process in building the continental crust, with zircon Hf–O isotopes widely used to trace its timing and material contributions. A key challenge to this approach is determining whether—and to what extent—high-δ18O zircon can archive crustal growth. By integrating bulk-rock trace element proxies and Nd isotopes, zircon Hf–O isotopes and xenocryst ages, and thermodynamic modeling, this study demonstrates that postcollision Eocene lavas in central Tibet were derived from partially molten mid–lower continental crust column composed of Early Cretaceous mafic arc underplates across depths between 35 km and 65 km. These lavas host zircon with supra-mantle δ18O values (>6.5‰) overlapping with those of olivine and zircon from nearby Early Cretaceous mafic–ultramafic arc roots, indicating that the mid–lower crust consists of high-δ18O materials of a mantle-derived origin. Combining our results with regional Early Cretaceous magmatic records and estimates of crustal shortening from the Early Cretaceous to the Eocene, we infer that this high-δ18O crust initially exceeded 15 km in thickness and formed through magmatic underplating during a brief phase of high-flux arc magmatism. Our findings suggest that the current zircon Hf–O isotope paradigm may oversimplify the timing and material contributions of continental crustal growth on both regional and global scales.