Age and genesis of the Neoarchean Algoma-type banded iron formations from the Dengfeng greenstone belt, southern North China Craton: Geochronological, geochemical and Sm–Nd isotopic constraints

Abstract

Banded iron formations (BIFs) are important for understanding the composition and redox condition of paleo-oceans, as well as the evolution of the Precambrian crust. The Neoarchean Dengfeng greenstone belt (ca. 2.54–2.50 Ga) in the southern North China Craton (NCC) consists of metamorphosed igneous and sedimentary assemblages including some BIFs. Here, we present mineralogical, geochronological, geochemical and Sm–Nd isotopic data of the Dengfeng BIFs for the first time to constrain their age, origins and source characteristics, and then discuss their paleo-environmental implications. The BIFs are mainly associated with metabasaltic rocks and quartz-rich schists, and can be classified as Algoma-type BIFs. Zircon U-Pb dating of quartz-rich schists interbedded with the BIFs, coupled with ages of wall-rock metabasites, demonstrate that the Dengfeng BIFs formed at ~2.53–2.51 Ga. Most BIF samples have high contents of SiO2 and Fe2O3T, and extremely low contents of Al2O3, TiO2, and high-field strength elements (e.g., Zr, Hf, Th, U), indicating that they are detrital-free deposits. In the Post-Archean Australia Shale normalized multi-element diagram, the detritus-free BIF samples are characterized by depleted light rare earth elements, positive La and Y anomalies, and suprachondritic Y/Ho ratios, resembling geochemical features of modern seawater. The absence of negative Ce anomaly indicates an anoxic condition. Positive Eu anomalies and relatively high concentrations of transition metal elements suggest that high-temperature hydrothermal fluids contributed to the source of BIFs. Isotopically, all BIF samples have positive εNd(t) values (+0.12 to + 4.53), consistent with the range of those (+0.10 to + 4.31) of ca. 2.66–2.50 Ga Dengfeng metabasites and TTG gneisses, suggesting input of possible juvenile crustal fluxes. The elemental and isotopic characteristics of the Dengfeng BIFs collectively suggest that they precipitated from anoxic seawater with various input of hydrothermal fluids and oceanic crust-derived fluxes.