First boron isotopes in the southern Jilin TTG series uncover a Neoarchean oceanic arc in the eastern North China Craton

Abstract

The Neoarchean evolution of the eastern North China Craton (NCC) is still controversial. This study presents the first B isotopes, together with zircon U-Pb-Hf isotopes and whole-rock geochemical analyses, for the TTG and dioritic series in the Baishan area of the southern Jilin region. LA-ICP-MS zircon U-Pb results uncover the Neoarchean magmatic activities, including granodioritic gneisses (2648 ± 10 Ma and 2622 ± 8 Ma), and quartz dioritic gneiss (2539 ± 7 Ma). The 2.65–2.60 Ga TTG series exhibit intermediate calc-alkaline characteristics, with relatively lower Th/La ratios (0.11–0.41) and positive zircon ε_Hf(t) values (+3.73 to +7.93), suggesting that the TTG series were likely derived from partial melting of mafic lower crust. By contrast, the 2.54 Ga dioritic series show positive Zr, Hf and Eu anomalies, with relatively lower Nb/Zr ratios (0.013–0.028) and ε_Hf(t) values (+2.00 to +5.49), indicating that they were possibly produced by mixing of the mantle-derived magma and crustal melts. Importantly, the 2.65–2.60 Ga TTG series are characterized by positive whole-rock δ¹¹B values of +4.11 to +15.08 ‰, resembling the Izu-Bonin-Mariana oceanic arc and South Sandwich Island arc volcanic rocks. The formation of these TTG rocks is attributed to ¹¹B-rich fluids released by subducted oceanic slab and subsequent metasomatism of the subarc mantle wedge. Unlike the oceanic arc TTG series, the 2.54 Ga dioritic series exhibit lighter whole-rock δ¹¹B values of −4.23 to −4.50 ‰, reflecting an arc-continental collision induced by slab breakoff and mantle-derived magma upwelling. Integrated with previous studies, it suggests that the subduction-collision process in the eastern NCC resulted from the co-evolution of oceanic arc and continental margin arc.