Late Neoarchean to early Paleoproterozoic magmatism and its tectonic setting in Eastern Hebei, North China Craton

The late Neoarchean to early Paleoproterozoic era represents a key period for the formation and evolution of the Earth’s early continental crust. It is characterized by a transition from intense global magmatism and significant crustal growth to dramatically reduced magmatism with increasing crustal reworking. The tonalite-trondhjemite-granodiorite (TTG) and dioritic rocks constitute the dominant component of the early continental crust, and their petrogenesis and tectonic setting provide critical insights into the formation mechanism of the continental crust. In this contribution, we present a systematic geochronological, geochemical, and zircon Hf-O isotopic study for the late Neoarchean and early Paleoproterozoic TTG and dioritic gneiss in Eastern Hebei, North China Craton.

Zircon U-Pb dating shows that the late Neoarchean TTG and dioritic gneiss rocks have protolith crystallization ages of 2.55–2.52 Ga and metamorphic ages of 2.51–2.47 Ga and ~2.45 Ga. The early Paleoproterozoic trondhjemitic gneiss yields protolith crystallization ages of 2.45 Ga and inherited zircon ages of 2.55 Ga. Accordingly, a prolonged tectonothermal events (2.55–2.45 Ga) occurred in Eastern Hebei.

The late Neoarchean TTG gneiss samples shows geochemical affinities akin to adakitic rocks, characterized by high concentrations of SiO₂, Al₂O₃, and Sr, low concentrations of MgO, Y and Yb, as well as high Sr/Y and (La/Yb)_N ratios. They can be divided into two groups. The first group has low MgO, Cr, and Ni contents, coupled with the positive zircon ε_Hf(t) (3.0 to 7.3) and δ¹⁸O values of 5.36–6.59 ‰, indicating that it was derived from partial melting of juvenile thickened lower crust. The second group contains high MgO, Cr, and Ni contents, which are more consistent with the TTG rocks derived from partial melting of subducted oceanic crust. A majority of positive zircon ε_Hf(t) values (0.2–5.2) and lower δ¹⁸O values (5.58–6.54 ‰) suggest its magmatic source of juvenile crust. Notably, some zircons display negative ε_Hf(t) values (-2.6 to -0.2) and higher δ¹⁸O values (7.19–7.47 ‰), indicating the involvement of ancient crust materials. The dioritic gneiss has moderate SiO₂ con­tent, high CaO, MgO, Cr, Ni, Sr, and Ba contents, as well as high K₂O/Na₂O and (La/Yb)_N ratios, akin to the Archean sanukitoids. They are enriched in large-ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), combined with their positive zircon ɛ_Hf(t) values (4.0–6.4) and slightly higher δ¹⁸O values (5.99–6.42 ‰), suggesting a metasomatized mantle source. In addition, they show large variations in (Hf/Sm)_N, high Nb/Ta and Zr/Hf ratios, and low Nb/Zr ratios, implying that both subducted fluids and slab-derived melts are metasomatic agents. The 2.45 Ga trondjemitic gneiss shows similar geochemical and zircon Hf-O isotopic characteristics to the above mentioned first group TTG gneiss, further supporting that it was a continuation of the late Neoarchean magmatism.

Based on the diverse sources of the studied 2.55–2.45 Ga granitoid gneisses and regional geological data, we propose that the vertical mantle plume coexisted with the horizontal slab subduction in Eastern Hebei in the late Neoarchean to early Paleoproterozoic.

The research was funded by the National Key Research and Development Program of China (2024YFF0808000).