The timing of crustal thickening constrained by metamorphic zircon U-Pb-Hf and trace element signatures in the Lüliang Complex, Trans-North China orogen

Abstract

Understanding the onset timing of crustal thickening is fundamental to correctly interpreting the geodynamic evolution of orogenic events, but it is still controversial in the Trans-North China Orogen, North China Craton. Here, we investigate a suite of high-grade rocks from the Lüliang Complex by integrating zircon mineragraphy, mineral inclusion, trace element, U-Pb age, and Lu-Hf isotope with garnet trace element data to track zircon formation events and confidently constrain the onset timing of crustal thickening caused by the collision of the Eastern and Western Blocks. Three zircon age groups were identified from the meta-basites and meta-granite: 2.15–2.05 Ga, 1.95–1.92 Ga, and 1.89–1.84 Ga, of which the oldest group represents protolith formation time and the two younger groups are metamorphic zircons. Metamorphic zircons occur close to Zr-bearing minerals ilmenite, hornblende, biotite, orthopyroxene, or chlorite, and have mineral inclusion of plagioclase and biotite, and lower Ti-in-zircon temperatures than the peak metamorphic temperatures, suggesting that they most likely grow during the post-peak stage. They are also characterized by low Th, Nb + Ta, and REE + Y contents, low Th/U and, high Lu/GdN ratios, indicating the effect of aqueous solutions and absence of garnet on their growth. Therefore, the meta-basites formed at the surface of the upper plate of the subduction zone were transported to ca. 30 km depth by crustal thickening related to the collision of the Eastern and Western Blocks before 1.95–1.92 Ga, much earlier than previously suggested.