Stratiform Siderites And Ankerite Concretions From The Mesoproterozoic Xiamaling Formation In North China:Genesis And Environmental Implications

It has been commonly thought that the large-scaled iron formations in Precambrian were mainly concentrated in two periods of 2.5¹.8 Ga and 0.8 Ga,but are rare during1.8⁰.9 Ga.However,our study shows that substantial Fe-rich sedimentary rocks are present in the Mesoproterozoic Xiamaling Formation?1.4¹.35 Ga?in North China Platform,with their size may be comparable to the typical iron formations formed during the period of 2.5¹.8 Ga.In order to reveal the potenial mechanism of Fe-rich sediments,their iron source and environmental implications.A thorough study on the sedimentology,petrography and geochemistry were conducted on the Xiamaling Formation of the North China Platform.Facies analysis and mineralogical studies indicate that Fe-rich sedimentary rocks in the Xiamaling Formation were mainly concentrated in Member I and Member II.Siderite concretions are largely concentrated in Member I,layers siderites in lower part of Member II,and ankerite concretions enriched in middle part of Member II.Well preserved wrapping structures,ptygmatic curves,and differential compaction structures seen in the siderite and ankerite concretions likely indicate that these concretions are of early diagenetic origin.The significant negative?¹³C values of siderites?-12.35‰to-24.44‰?and of ankerites?-5.6‰to-8.2‰?suggest that microbialy organic degradation have provided abundant bicarbonate for the formation of Fe-rich sediments during early diagenesis;The positive cerium?Ce?anomalies?siderites:1.14¹.35;ankerites:1.27¹.37?,low Y/Ho ratios?siderites:18.81²6.00;ankerites:20.35²6.67?,enrichment of MREE and the presence of?m-scale hematite inclusions in siderite grains likely suggest that Mn-Fe oxyhydroxides were the major precursor minerals of siderites and ankerites,and dissimilatory iron reduction?DIR?during early diagenesis may have played a critical role in anoxic bottom water and/or porewater,during which Fe³⁺was reduced to Fe²⁺and combined with bicarbonate to form siderite or ankerite.The presence of syndepositional hyalophane concretions in the Xiamaling Formation suggests that there was hydrothermal activity during the Xiamaling deposition,which may gave served as an important source of Fe-rich deposits.The positive correlation of europium?Eu?anomalies and Fe contents in siderites and ankerites further indicated that iron mainly sourced from hydrothermal solution,with an additional suplyment from terrestrial input.There exist significant differences between siderites and ankerites in?⁵⁶Fe values,with?⁵⁶Fe values and Eu anomalies positively correlated,likely suggest that the hydrothermal solution with relatively heavy iron isotope composition was the main iron source when siderites formed,while the continental DIR recycled iron with relatively light iron isotope composition played a more important role for the formation of ankertite concretions.The production of siderites and ankerites during the Xiamaling deposition is of important paleoenvironmental significance.Due to requirement of an active iron-redox cycle in the overlying seawater column.Facies analysis indicates that the Xiamaling siderites were formed in shallow water environments above the storm wave base and these ankerite concretions formed below the storm wave base.The redoxcline decreased during the deposition of the lower part to the middle part of Member II.The weakening of hydrothermal activity may be an important causes for the increased surface seawater oxidation during the period of Member II deposition.The formation of Fe-rich sediments in Member II of the Xiamaling Formation may have been facilitated by enhanced hydrothermal activity that probably related to the rifting prior to the final breakup of the North China Craton from the Columbian supercontinent.