New U-Pb and Hf isotopic data confirm Anshan as the oldest preserved segment of the North China Craton

Abstract

The Anshan area in the northeastern part of the North China Craton contains the oldest known rocks in China. Rocks with ages of ∼3.8 Ga are present in three trondhjemite-dominated gneiss complexes, referred to as the Baijiafen, Dongshan and Shengoushi complexes. The oldest material identified in these complexes has weighted mean 207Pb/206Pb ages of 3800 ± 5 Ma, 3794 ± 4 Maand 3777 ± 13 Ma, respectively. In addition, younger geological events at 3680 ± 19 Ma, 3620 ± 23 Ma, and 3573 ± 21 Ma have been identified, recording the emplacement of several generations of trondhjemite. A biotite schist of possible supracrustal origin contains a zircon population with a weighted mean 207pb/206Pb age of 3723 ± 17 Ma and is cut by a 3620 Ma trondhjemite, implying formation prior to ∼3.62 Ga. TDM(Hf) model ages for the zircons range from 3256 to 4264 Ma, but are mostly ∼3.9 Ga, indicating that significantly older crust was not present in the source region of the trondhjemitic magmas.