Terminal Paleoproterozoic Ooidal Ironstone from North China: A Sedimentary Response to the Initial Breakup of Columbia Supercontinent
As archives of atmosphere oxygenation and ocean chemistry,ironstones and iron formations have been used for tracing the processes of tectonics,magmatism,and microbe interactions.Precambrian iron formations have been extensively studied in past decades,whereas non-cherty ironstones of the period have been rarely explored in terms of their origin,paleoenvironment and tectonic background.In this paper,micro-fabrics,mineral and chemical composition of the ironstone from the Chuanlinggou Formation (1.65-1.64 Ga) are studied using scanning electronic microscopy (SEM),X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) techniques,with emphases on their depositional environment and potential links to the breakup of Columbia supercontinent.The study shows that ooids in the ironstone consist mainly of hematite and minor amount of kaolinite,with rare terrestrial detritus and very low Al2 O3 content.Ooidal cortices are characterized by alternating dense and porous laminae formed of hematite micro-flakes that arranged in different patterns.A marked negative correlation between Fe and Al contents in ooidal cortices suggests that ooid ironstones are of a Fe-source from anoxic and Fe-rich deep seawaters,rather than terrestrial weathering as suggested previously.The ironstones concentrated in the intervals of sediment starvation were caused by rapid transgression and low detrital input,and formed in subtidal dysoxic environment near to the chemocline.Magmatism,rapid basement subsidence and swift marine transgression accompanied with the initial breakup of the Columbia supercontinent at the time were likely the major geologic factors facilitating deposition of the ironstone.Thus the Chuanlinggou ironstone may have signified major environment and tectonic changes of the North China Platform responding to the breakup of the Columbia supercontinent.
ironstonemicro-fabricspaleo-environmentsLate PaleoproterozoicNorth China Platformsedimentology
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