Discovery of 2.45 Ga trondhjemitic gneiss in Eastern Hebei, North China Craton: A constraint on Precambrian crustal evolution

Abstract. The early Paleoproterozoic era (2.45–2.20 Ga), known as the Tectono-Magmatic Lull (TML), is characterized by a decline in global magmatic activity. This study first identifies ca. 2.45 Ga trondhjemitic gneiss (2446 ± 15 Ma) in Eastern Hebei of the Eastern Block, North China Craton. These rocks exhibit adakitic geochemical characteristics, marked by high SiO₂, Al₂O₃, and Sr contents, with low MgO, Y, and Yb contents. Their low MgO, Cr, and Ni contents, along with slightly high zircon δ¹⁸O (5.96–6.53 ‰) and positive ε_Hf(t) (3.3–4.9) values, indicate that they originated from partial melting of a juvenile thickened lower crust. All samples show low concentrations of Y, Yb, Ti, Nb, and Ta, coupled with their high (La/Yb)_N and Nb/Ta ratios, suggesting that they formed at a high-pressure condition, with garnet and rutile as residues. In combination with our new data and published zircon U-Pb ages in the region, we have identified multiple stages of magmatism (3.84–3.64 Ga, 3.53–3.22 Ga, 3.12–2.80 Ga, and 2.61–2.45 Ga) and metamorphism (3.50–3.23 Ga, 3.18–2.80 Ga, ~2.50 Ga, ~2.45 Ga, ~1.82 Ga) in Eastern Hebei. Based on a compilation of these magmatic zircon U-Pb ages and Hf isotope data, Eoarchean to early Paleoproterozoic crustal evolution processes in Eastern Hebei is established. The Eoarchean is dominated by Hadean crustal reworking, and the Paleoarchean is primarily characterized by crustal reworking with a minor contribution of crustal growth. Both crustal growth and reworking occurred during Mesoarchean time, with the proportion of crustal growth increasing from the Paleoarchean to the Mesoarchean. The late Neoarchean represents a major period of crustal growth with minor crustal reworking. The ca. 2.45 Ga trondhjemitic gneiss discovered in this study was probably a continuation of the late Neoarchean magmatism and the crustal growth persisted into this period.