Integrated constraints on the Paleoproterozoic Changyi BIF, eastern North China Craton: Implications for the Great Oxidation Event and metamorphism

The Paleoproterozoic banded iron formation (BIF) at Changyi in the eastern North China Craton is a rare example of a Superior-type BIF in China. However, its genesis, depositional age, and tectonic setting remain poorly constrained and highly debated. In this study, we integrated zircon U-Pb geochronology, whole-rock geochemistry, Fe-O isotopes, and P-T phase equilibrium modeling to systematically constrain the formation age, provenance, and depositional environment of the Changyi BIF. Detrital zircon U-Pb geochronology reveals a provenance derived from both Archean cratonic basement (~2.9 Ga and ~2.7 Ga) and Paleoproterozoic magmatic materials (~2.45 Ga and ~2.2 Ga). The detrital zircon age spectrum shows a distinct peak at ~2.45 Ga, providing clear evidence for a major magmatic event of this age in the source region, while the youngest zircon population (2243 Ma) constrains the maximum depositional age of the BIF. The ores are dominated by SiO₂ and total Fe₂O₃ (TFe₂O₃). The increasing contents of Al₂O₃ and TiO₂ from Type I to Type III ores indicate progressively enhanced detrital input. The REY distribution patterns are characterized by LREE depletion, positive La, Eu, and Y anomalies, and low ΣREY. Mixing model results indicate that the ore-forming materials were mainly derived from seawater, with minor contributions from high-temperature hydrothermal fluids and continental solutes. Whole-rock δ¹⁸O values range from 8.8‰ to 10.9‰, with reconstructed original δ¹⁸O values between 8.8‰ and 12.25‰. The δ¹⁸O values of different ore types are jointly controlled by mineral proportions and the intensity of metamorphic/fluid processes, with Type III ore exhibiting the lowest δ¹⁸O values, reflecting fluid-rock exchange and isotopic resetting. However, large-scale isotopic homogenization did not occur regionally, and most ores largely preserve their primary oxygen isotope signatures. Iron isotope analysis showed that ore δ⁵⁶Fe values ranged from -0.44‰ to +1.17‰, with predominantly positive values and the highest values in Type III ore. The δ⁵⁶Fe values covaried with Fe³⁺/(Fe²⁺ + Fe³⁺) ratios among different ore types, reflecting fluctuations in redox conditions during deposition and the geochemical response to oceanic oxidation events, providing robust geochemical evidence for the local manifestation of the Great Oxidation Event (GOE) within the depositional basin. Phase equilibrium modeling indicated that the Changyi BIF experienced peak amphibolite-facies metamorphism at ~785 ℃/6.2 kbar, recording an incomplete prograde P-T path indicative of rapid burial and crustal thickening. Collectively, our results support a genetic model in which the Changyi BIF was deposited in a Paleoproterozoic back-arc basin along an active continental margin, demonstrating that such settings can provide transiently stable depositional environments for Superior-type BIF formation. The deposit systematically documents geochemical responses to both global (GOE) and regional (metamorphism) events, providing new constraints on the genesis and paleoenvironmental evolution of BIFs in the eastern North China Craton.