Zircons in mantle xenoliths record the Triassic Yangtze-North China continental collision

The Jurassic Xinyang diatremes intrude the southern edge of the North China Craton, near the Triassic collisional boundary with the Yangtze Craton. Zircon grains extracted from three mantle xenoliths in the diatremes reflect a long and complex lithospheric history, with zircon growth and/or recrystallisation at ca 3.2 Ga, 2.3-2.4 Ga and mainly at 210-240 Ma. The U-Pb and Hf-isotope systematics of three grains indicate growth also occurred at some time between 665 and 1400 Ma. The Triassic zircons show a wide range of ¹⁷⁶Hf/¹⁷⁷Hf, indicating the mixing of Hf derived from juvenile sources with ancient unradiogenic Hf stored in the lithospheric mantle. The zircon growth events recorded in the xenoliths probably reflect the periodic addition of Si, Zr and Hf to the subcontinental lithosphere mantle by metasomatic fluids and/or melts. These peridotitic zircons have trace-element affinities with zircons from granitoids with SiO₂ contents < 65% (pre-Triassic zircons) to 70-75% (Triassic zircons). We suggest that the Triassic zircons formed during the subduction and collision of the Yangtze Craton beneath the southern edge of the North China Craton and record metasomatism by fluids/melts released from the subducted Yangtze continental crust.