Petrogenesis of 3.3–3.1 Ga granitoids in the Anshan continental nucleus, North China Craton: Implication for multi-stage magmas underplating processes

Abstract

Archean tonalite-trondhjemite-granodiorite (TTG) suite and K-rich granitoids provide direct evidence of the generation and differentiation of the Archean continental crust. Within the North China Craton (NCC), the Anshan area serves as an exceptional natural laboratory, preserving a geological record spanning from 3.8 to 2.5 Ga. Here, we conducted comprehensive geochemical, geochronological, and zircon Hf–O isotopic analyses for the Paleoarchean–Mesoarchean granitoids from the Anshan region. Zircon U–Pb dating suggest that these granitoids formed during 3.3–3.1 Ga. The ca. 3.3 Ga monzogranitic gneisses exhibit high SiO₂ and K₂O, low Mg# values, negative Eu anomalies, unradiogenic Hf isotopes (i.e., ε_Hf(t) values = -5.16–+0.08, T_DM² age = 4264–3729 Ma), and “mantle-like” δ¹⁸O values (up to + 5.64 ‰). Geochemical features divide them into two distinct groups, both are recycling products of pre-existing Eoarchean TTGs. Group 1 and Group 2 originated from 10 %–20 % and 20 %–30 % partial melting of pre-existing Eoarchean TTGs at depths of 1.25–1.75 GPa and 0.5–0.75 GPa, respectively. The ca. 3.3 Ga trondhjemite gneisses show high SiO₂, low Mg# values, low Sr/Y and (La/Yb)_N ratios. Their ε_Hf(t) values range from + 0.91 to + 1.10, with T_DM² ages of 3682–3665 Ma. In contrast, the ca. 3.3 Ga monzonitic gneisses have lower SiO₂ contents, higher Al₂O₃, total alkali (Na₂O + K₂O), and Sr/Y and (La/Yb)_N ratios. Their ε_Hf(t) values range from −2.69 to + 0.50, with T_DM² ages of 4040–3750 Ma. The ca. 3.1 Ga trondhjemite gneisses display higher Sr/Y and (La/Yb)_N ratios, enriched LREEs and depleted HREEs. Magmatic zircons yield δ¹⁸O values of 3.87–6.52 ‰ and ε_Hf(t) values of −2.39 to + 0.95, with T_DM² ages of 3881–3550 Ma. The geochemical and Hf-O isotope characteristics suggest that both the 3.3–3.1 Ga trondhjemite gneisses and the ca. 3.3 Ga monzonitic gneisses originated from partial melting of mafic rocks at variable depths. During 3.3–3.1 Ga, the NCC experienced intensive crustal growth and rework/recycling. Two magmatic events (ca. 3.3 Ga and ca. 3.1 Ga) and subsequent tectono-thermal events were induced by multi-stage magmatic underplating of mantle upwelling within a plume-related tectonic setting.