Geochemical Characteristics And Geodynamic Implications Of The Xiaolaihe Banded Iron Formation And Its Wall Rock In Qingyuan Area Of Liaoning,China

Early Precambrian banded iron formations（BIFs）are the most important source of iron in China,accounting for more than 60%of the national iron reserves,among which the late Neoarchean banded iron formation in North China Craton is the most typical.Here we combine petrological,geochemical and isotopic data of the Xiaolaihe BIF and its wall rock in Qingyuan,Liaoning Province,North China Craton,in order to constrain their provenance and tectonic setting.The Xiaolaihe BIF is composed of magnetite,quartz and silicates（e.g.,ferrohypersthene）,and is characterized by variable Si O₂ and total iron(Fe O_tot)contents.The Xiaolaihe BIF preserved in the Shipengzi Formation is commonly found to be interbedded with metamorphosed volcanic rocks,and minor metamorphosed clastic sedimentary rocks.The chemical characteristics of the clastic sedimentary rocks associated with the Xiaolaihe BIF are mainly controlled by complex source compositions（tonalite and trondhjemite,and minor mafic rocks）within an island arc setting,although they have undergone low–intermediate source weathering,which is consistent with provenance of the BIF.The higher Al₂O₃ contents（>1.0 wt%）of subordinate amounts of Xiaolaihe BIFs indicate some terrigenous input during deposition.Other detrital-free Xiaolaihe BIF samples have low abundances of Al₂O₃,high field strength elements（HFSEs）,and transition metals,normalized rare earth elements（REEs）patterns enriched in heavy REEs and depleted in light REEs,and positive Eu anomalies,which is reflective of seawater.Modeling reveals that the REE patterns of the Xiaolaihe BIF well match that of mixture from the high-temperature hydrothermal fluids and ambient seawater with the ratio of 1:100,suggesting that heat and mass from the seafloor hydrothermal activity contributed to the Xiaolaihe BIF.The Xiaolaihe BIF lacks obvious negative Ce anomaly and presents low Mn content,suggesting that the seawater p H was higher than 5 when the BIF was precipitated,and the paleomarine environment as a whole was in a continuous anoxic state,rather than before the great oxidation event.Our new data,combined with recent studies on geology,geochemistry,isotope and geochronology of the BIF,reveals that the Xiaolaihe BIF is classified as Algoma-type one deposited in a back-arc basin within an oceanic island arc system.