A zircon petrochronologic perspective on raising the central Tibetan crust

Abstract

Being the most commonly dated accessory mineral, the geochemical makeup of zircon has increasingly been recognized as a promising indicator of the parent magma composition, providing a means to reconstruct the paleo-crustal thickness of orogenic belts where whole-rock records are limited. However, the robustness of zircon approaches in paleo-crustal thickness reconstruction remains controversial. Here, we report on UPb age, trace element and Hf isotope data for zircon in sedimentary rocks of central Tibet, aiming to reconstruct the central Tibetan paleo-crustal thickness evolution from Meso-Tethyan subduction to the India–Asia collision, and test the results by comparing against geological evidence. The consistent variations of different pressure-sensitive geochemical proxies over time, e.g., (La/Yb)_N, (Sm/Yb)_N, MREE/HREE, Eu anomaly, and Y values, indicate that pressure is the first-order control on zircon compositional changes. Using southern Tibet as a reference and comparing the results from various geochemical proxies, we find that zircon Eu geochemical proxy yields relatively reasonable estimates of paleo-crustal thickness for this orogen, which is thereby employed to explore the evolution of paleo-crustal thickness in central Tibet. A history of multi-stage thickening crust separated by episodes of thinning for central Tibet is revealed by zircon multi-proxy approach, and supported by multiple lines of independent evidence, including whole-rock geochemical proxies, structural geology, low-temperature thermochronology, and paleo-altimetry. We propose that a thick crust similar to the current one was likely formed in central Tibet by the Late Eocene (∼40–35 Ma), predominantly due to crustal shortening as a consequence of the India–Asia collision. Our study strengthens the applicability of zircon petrochronologic data in delineating the paleo-crustal thickness evolution of specific regions when supported by rigorous and comprehensive regional geological evidence.