LA-ICP-MS zircon U-Pb and Hf isotopic studies of the Wutai Group and their geological significance.

Zircon LA-ICP-MS U-Pb and Hf isotopic studies of four typical samples from the Jingangku Formation in the Wutai Group were carried out in order to investigate the geochronology of the Wutai Group and the crustal evolution of the North China Craton (NCC) from Neoarchean to Paleoproterozoic. Zircon U-Pb isotopic data reveal that the intrusion of gneissic granite into the Jingangku Formation took place at 2548Ma, so the top boundary age of the Wutai Group is about 2.5Ga; the protolith age of the biotite quartz schist is 2663Ma, the protolith age of the gneissic quartz diorite is 2636Ma, so the bottom boundary of the Wutai Group is about 2.7Ga. The geological age is 2.5~2.7Ga, so the Wutai Group was formed in Neoarchean. Hf isotopic data reveal that both the biotite quartz schist and the gneissic quartz diorite have the average two-stage Hf model age (tDM2) of 2.8Ga, which suggests that ~2.8Ga was an important period of crustal growth in the Wutai Mountain area and even in the whole NCC. The average tDM2 of the gneissic granite is 2.57Ga, which is very close to zircon U-Pb age representing the timing of zircon growth, suggesting that ~2.5Ga was another important period of crustal growth in the NCC. The crust grew in episodic form, and both ~2.8Ga and ~2.5Ga represented the crustal growth in the NCC.