LA-ICP-MS U-Pb zircon geochronology of the Yushulazi Formation in the North China Craton

The Paleoproterozoic Jiao-Liao-Ji orogen in the North China Craton is composed mainly of the Liaohe Group (and its equivalents) and the Liaoji Granites, which are uncomformably overlain by a sedimentary succession, named Yushulazi Formation. The Yushulazi Formation has long been considered as a Proterozoic assemblage, but the age of the formation has never been precisely constrained in terms of modern geochronology. Cathodoluminescence images reveal that most detrital zircon grains from this formation are characterized by concentrically-oscillatory zoning, with comparatively low luminescence and high Th/U ratios, suggesting an igneous origin; whereas minor other grains have structureless overgrowth or recrystallization rims that are characterized by low Th/U ratio, indicating a metamorphic origin. The results of LA-ICP-MS U-Pb zircon analyses show that the detrital zircon grains have concordant U–Pb ages ranging from ca.1.05-1.3 1Ga and 1.87-2.51 Ga, with an inherited zircon core giving a concordant age of 3.35 Ga. The U-Pb age range of 1.87-2.51 Ga is in good accordance with that of the Paleoproterozoic Liaohe Group [1] and the basement rocks of the North China Craton. However, zircon ages of 1.05-1.31 Ga are, for the first time, reported for the Precambrian basement of the Eastern Block of the North China Craton. The recognition of the Mesoproterozoic zircons from the Yushulazi Formation indicate that (1) the Yushulazi Formation is not part of the Paleoproterozoic Liaohe Group, as some workers previously suggested; (2) and there existed local Grevillian-aged magmatism in the Eastern Block of the North China Craton, which may have been related to the assembly of the Meso-Neoproterozoic supercontinent Ronidia. Acknowledgements This study was financially supported by RGC projects (7055/03P, 9048/03P and 7058/04P) and National Nature Science Foundation of China (Grants 40472098 and 40002015). Reference [1] Luo Y., Sun M., Zhao G.C., Li S.Z., Xu P., Ye K., Xia X.P., 2004. Precam. Res. 134, 349-371