Depositional age and source material of banded iron formations in the Western Shandong Province, North China Craton: Constraints from geochemistry, zircon UPb geochronology and SmNd isotope composition

As an important iron ore concentration area in the North China Craton (NCC), the Western Shandong Province (WSP) is an ideal area to study the formation and evolution of banded iron formation (BIF). This paper performs a comparative study on the BIF deposits of Dong'e Shanzhuang (SZ), Yiyuan Hanwang (HW), Yishui Yangzhuang (YZ), Linyi Lanling (LL), and Jining Yandian (YD), from different BIF-hosted units and depths. Zircon UPb dating revealed that the depositional age of the WSP BIFs here was constrained to the Late Neoarchean (2.56–2.50 Ga), consistent with most of the BIF depositional age (2.55–2.52 Ga) in the NCC. The majority of BIFs have undergone greenschist- to amphibolite-facies metamorphism and associated magmatism that began at ∼2.49 Ga. The BIFs are characterized by depleted light rare earth elements (LREEs) and positive Y and negative Ce anomalies, indicating that BIFs precipitated from a variable mixing of seawater and minor high-T hydrothermal fluids. The other additional rare earth and yttrium (REY) sources such as fluvial inputs and (or) oceanic crust-derived fluxes are probably involved during the formation of BIFs. However, most supracrustal rocks and metagranitoids show a right-tilting trend in REY pattern, which is similar to that of the ocean island basalts (OIB) pattern, while the REY pattern of amphibolite is flat, similar to that of normal mid-ocean ridge basalts (N-MORB) pattern. The protoliths of supracrustal rocks in the Late Neoarchean reflect an evolution process from komatiite-tholeiite to calc-alkaline basalt-andesite-dacite, and BIFs were possibly formed in a volcanic-arc setting. The YZ BIF is the purest chemical sediment closest to seawater in the WSP, which can be used as a favourable archive for tracing ancient seawater. The variable initial ε_Nd(t) values of the BIFs and associated rocks were determined as +0.85 − +6.96, mostly between the depleted mantle and the chondritic uniform reservoir, indicating the seawater was largely affected by the depleted mantle material, but cannot excluded the contributions from continental crust. The Nd model age of different rocks of WSP BIFs was 2.80–2.58 Ga, corresponding to a large-scale continental crustal growth age of the NCC. The apparent isochron age fitted to the SmNd isotope pseudo-isochron of 2685 ± 89 Ma represents a “paleo-isochron”, possibly corresponding to the age of the older ∼2.70 Ga continental crust growth event.