Zircon U-Pb and Hf isotopic constraints on the Early Archean crustal evolution in Anshan of the North China Craton

The North China Craton is one of the few places in the world where >3.8 Ga crustal material exists, since zircons of this age have been found in the Baijiafen and Dongshan gneisses from Anshan in northeastern China. However, it has been questioned whether any 3.8 Ga rock exists in the area, since various younger zircons exist within single rock samples, and zircons with age of ∼3.8 Ga are few and can be interpreted as inherited in origin. A study of zircons using combined cathodoluminescence imaging and SHRIMP U-Pb dating indicates that the Baijiafen and Dongshan gneisses record several stages of granitic magmatism. The oldest magmatic event is recorded by a zircon with an age of 3887 ± 5 Ma, with a subsequent magmatic event at 3.8 Ga (3808 ± 24, 3798 ± 30, 3802 ± 11 and 3799 ± 6 Ma), confirming the existence of 3.8 Ga materials in the area. The next magmatic event took place at ∼3.6-3.7 Ga. However, all samples contain younger zircons with ages of 3.1-3.3 Ga, although they contain a few zircon grains with ages of ∼3.6-3.8 Ga. The 3.1-3.3 Ga zircons show typical igneous oscillatory zoning and do not show any evidence that were produced by metamorphism, indicating that these samples were emplaced at 3.3 and 3.1 Ga, respectively and the zircons with older ages are interpreted as inherited in origin. The exposure of 3.8 Ga rock is therefore much smaller than previously thought. In situ zircon Hf isotopic analyses indicate that these granitic rocks were derived from juvenile crust with age peaks of crustal growth at ∼3.4, 3.6 and 3.9 Ga, there is no evidence for existence of crustal material older than 4.0 Ga.