SHRIMP U-Pb zircon dating and geochemistry of the 3.8–3.1 Ga Hujiamiao Complex in Anshan (North China Craton) and the significance of the trondhjemites for early crustal genesis

Abstract

Anshan is located in the northeastern North China Craton and has a long-term geological history back to 3.8 Ga. Four complexes with ages of 3.8–3.1 Ga have been identified previously, namely the Baijiafen, Dongshan, Shengousi and Guodishan complexes. Being similar to the neighbouring Beijiafen Complex, the newly-discovered Hujiamiao Complex extends in a northwest-southeast direction and shows tectonic contacts on one side with the Chengtaigou granite and on the other with the Chengtaigou supracrustal rocks. In the Hujiamiao Complex, trondhjemitic rocks of 3789 Ma, 3786 Ma, 3698 Ma, 3617 Ma, 3566 Ma, 3504 Ma and 3106 Ma have been identified. More work is required to establish if a gabbro also has a formation age of 3.8 Ga. The trondhjemitic rocks show similar compositional features in being high in SiO2 (72.37–76.71 %) and Na2O (5.00–6.47 %) and low in K2O (0.60–1.95 %), ∑REE contents (32.83–91.83 ppm) and (La/Yb)n ratios (11.09–48.64). They have whole-rock εNd(t) values of 0.35–4.43, tDM(Nd) of 3.74–3.46 Ga and tCC(Nd) ages of 3.74–3.43 Ga, and in situ magmatic zircon εHf(t) values of −12.3 to 1.56, tDM(Hf) ages of 4.2 to 3.5 Ga and tCC(Hf) ages of 4.6 to 3.7 Ga. Combined with previous work, it is concluded that all the Anshan area 3.8–3.3 Ga complexes are the relics of a larger complex which was once more widely distributed in the Anshan area. The trondhjemitic rather than tonalitic character of the Anshan Eoarchean rocks shows diversity in the formation and evolution of the earliest continental crust with a range of T/P for crustal anatexis.