U-Pb and Hf isotopic study of detrital zircons from the Yejishan group of the Lüliang Complex: constraints on the timing of collision between the Eastern and Western Blocks, North China Craton

The Trans-North China Orogen is a recently recognized continent-continent collisional belt along which the discrete Eastern and Western Blocks amalgamated to form the coherent basement of the North China Craton. However, there is a hot debate on the timing of the collision between the two blocks, with one school of thought arguing the collision occurred at 2.5Ga, whereas others believe it took place at ~1.85Ga. U-Pb ages and Hf isotopic data of detrital zircons from foreland basins in the Trans-North China Orogen can provide rigorous constraints on this controversial issue. One of such foreland basins is represented by the Yejishan Group in the Lüliang Complex in the middle sector of the western margin of the Trans-North China Orogen. The group consists of conglomerates, quartz wackes, sandstones and minor limestones, most of which contain large amounts of detrital zircons. CL images reveal that nearly all detrital zircons from the Yejishan Group possess oscillatory zoning, with Th/U ratios of 0.2-2.34, typical of an igneous origin. The ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons range between 3.24 and 1.81Ga, with most having Paleoproterozoic ages, of which the youngest group ages are around 1.87-1.81Ga. This indicates that the foreland basin in which the Yejishan Group was deposited must have formed not earlier than 1.87-1.81Ga, thus supporting the collision between the Eastern and Western Blocks at ~1.85Ga, not at ~2.5Ga.