Hf isotopes of the 3.8 Ga zircons in eastern Hebei Province, China: Implications for early crustal evolution of the North China Craton

摘要

Zircon U-Pb dating indicates that the fuchsite quartzite in eastern Hebei Province was derived from weathering and erosion of the 3.6-3.8 Ga granitic rocks. In-situ zircon Hf analyses show that the Lu-Hf isotopic system remained closed during later thermal disturbances. Zircons with concordant ages have Hf isotopic model ages of about 3.8 Ga, suggesting a recycling of this ancient crest. The approx 3,8 Ga zircons have similar Hf isotopic compositions to those of chondrite, indicating that their source rocks (granitic rocks) were derived from partial melting of the juvenile crust which originated from a mantle without significant crust-mantle differentiation. Therefore, it is proposed that there was no large-scale crustal growth before approx 3.8 Ga in eastern Hebei Province. Considering zircon Hf isotopic data from other areas, it is concluded that the most ancient crust in the North China Carton probably formed at about 4.0 Ga, and the possibility to find crust older than 4.0 Ga is very limited.