Genesis of Archean to Paleoproterozoic banded iron formations in the North China Craton: Geological and paleoenvironmental implications

The source characteristics and depositional environments of banded iron formations (BIFs) can provide key information on the chemical composition, redox environment, and tectonic evolution of early Precambrian paleo-oceans. Large quantities of Neoarchean to Paleoproterozoic BIFs are exposed in the North China Craton (NCC). This study focuses on petrography, major and trace element geochemistry, zircon U-Pb geochronology, and Sm-Nd isotopic compositions of a set of Neoarchean BIF-bearing sedimentary rocks preserved in the Zanhuang Massif of the NCC, including comparison with Archean to Paleoproterozoic BIFs throughout the NCC. This study provides constraints for deciphering the paleo-ocean chemical composition, redox environment and relationship between the geological-environments co-evolution and the formation of the BIFs in the NCC. The Zanhuang BIFs are mainly composed of quartz and magnetite, with small amounts of grunerite, actinolite (ferroactinolite) and garnet in the Fe-rich bands, and are classified as Algoma-type. Zircon U-Pb ages of the metasedimentary rocks interbedded with the BIFs indicate that the Zanhuang BIFs formed at ∼2.51 Ga and were subjected to metamorphism at ∼2.47–2.48 Ga. Most BIFs have low contents of Al2O3, TiO2 and high field strength elements Zr, Hf, Th and U, indicating that they are pure chemical sedimentary rocks. The BIFs are characterized by depleted light rare earth elements (LREEs) and positive La and Y anomalies, which are similar to the geochemical characteristics of modern seawater. The absence of Ce anomalies and the low (Pr/Yb)SN ratios indicate an anoxic state of the ocean. The Zanhuang BIFs precipitated from a mixture of minor high-temperature hydrothermal fluids and seawater mixed with continent-derived freshwater (∼10–20%) or other additional fluids such as ocean crust-derived fluxes indicated by Eu anomalies and Y/Ho and Sm/Yb ratios. The variable initial εNd(t) values of Zanhuang BIFs suggest complex and dynamic input of continental crustal fluxes. The statistical comparison with other BIFs shows that the peak formation ages of the Archean to Paleoproterozoic BIFs in the NCC is ∼2.54 Ga. Most of the BIFs in the NCC are Algoma-type, while a few Paleoproterozoic BIFs are Superior-type. A mixture of <0.1%–1% of modern seafloor high-temperature hydrothermal and seawater can account for the REE + Y characteristics of the BIFs, while εNd(t) values suggest possible contributions from continental sources. The Eu anomalies indicate that ∼2.7 Ga and ∼ 2.55 Ga are the two peaks of submarine hydrothermal and magmatic activities of the NCC, which correspond to crustal growth and subduction-collision events in the NCC, respectively. The Ce anomalies and Fe isotopes indicate the transition from an anoxic ocean with possible regional oxygen oases before 2.5 Ga to a redox-stratified ocean during the GOE. Most Neoarchean BIFs of the NCC were formed in island arc-related tectonic settings, while the Paleoproterozoic BIFs likely reflect environments of rifting to subduction in the NCC.