Neoarchean-Paleoproterozoic magmatic arc evolution in the Wutai-Hengshan-Fuping area, North China Craton: New perspectives from zircon U–Pb ages and Hf isotopic data

Abstract

The Wutai granite-greenstone belt, which formed in the Neoarchean-Paleoproterozoic, with the adjoining high-grade metamorphic Hengshan-Fuping region, record important transitions in crustal and tectonic evolution, although these evolutionary processes have still been in dispute. In this study, new U–Pb and Lu–Hf LA-ICP-MS zircon analyses were obtained from eight metamorphosed magmatic rocks in the Wutai-Hengshan-Fuping area in North China Craton (NCC). The results reveal that most of Wutai granitoids yield emplacement ages of 2.56–2.52 Ga, with corresponding εHf(t2) of +4.7 to +8.3. The tonalitic gneiss from Dongshandi intrusion, however, gives a new age of 2482 Ma and εHf(t2) of +2.5 to +4.6, which are similar with Yixingzhai Pluton marked by ages of 2501–2499 Ma and εHf(t2) of +3.2 to +4.4. Also, ∼2.7 Ga and ∼2.5 Ga continental crust generation are further revealed. But ∼2.5 Ga crustal growth is more complicated due to the involvement of old crustal components, which can be identified by variable εHf(t2) values ranging from −1.8 to +7.5. Thus, based on the distinctions of Hf isotopes, compositional and spatial features, two consecutive groups of 2.56–2.52 Ga and 2.52–2.48 Ga are recognized and a two-step tectonic evolution model for the Wutai-Hengshan-Fuping area is proposed. During 2.56–2.52 Ga, intra-oceanic subduction led to the formation of Wutai oceanic island arc, accompanied mainly by the development of Wutai granitoids (εHf(t2) of +4.7 to +8.3) and coeval volcanics (predominant arc-tholeiitic chemical affinity), corresponding to the juvenile crustal growth. Between 2.52 Ga and 2.48 Ga, with intervening oceanic crust between intra-oceanic arc and continent consumed, the Wutai intra-oceanic arc then collided and was obducted onto the western edge of the Hengshan-Fuping continental block. Large amounts of TTG gneisses produced in both the Wutai and Hengshan-Fuping (εHf(t) of −1.8 to +7.5), along with calc-alkaline volcanism in Wutai, possibly in response to the thickening through arc accretion and commencing the continental arc. This proposed two-step tectonic evolution model, combined with a late Neoarchean (2.6–2.5 Ga) intra-oceanic arc system along the northwestern of the Eastern Block (EB), probably provides a better explanation for the fundamental geodynamic mechanism for continental arc development at the Proterozoic dawn.