Terminal Paleoproterozoic 〇oidal 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 trac- ing the processes of tectonics, magmatism, and microbe interactions.Precambrian iron formations have been extensively studied inpast decades,  whereas non-cherty ironstones of the period have been rarely exp1ored in terms of their origin,  paleo- environment and tectonic background.In this paper, micro-fabrics, minera1 and chemica1 composition of the ironstone from the Chuanlinggou Formation(1.65-1.64Ga) are studied using scanning electronic microscopy(SEM), X-ray diffraction(XRD) and energy dispersive spectroscopy(EDS) techniques, with emphases on their depositiona1 environment and potentia1 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 terrestria1 detritus and very1ow A1203 content.0oida1 cortices are characterized by alternating dense and porouslaminae formed of hematite micro-flakes that arranged in different patterns.A marked negative correlation between Fe and A1 contents in ooida1 cortices suggests that ooid ironstones are of a Fe-source from anoxic and Fe-rich deep seawa- ters, rather than terrestria1 weathering as suggested previously. The ironstones concentrated in the intervals of sediment starva- tion were caused by rapid transgression and1ow detrita1 input, and formed in subtida1 dysoxic environment near to the chemo- cline.Magmatism, rapid basement subsidence and swift marine transgression accompanied with the initia1 breakup of the Co- 1umbia supercontinent at the time werelikely the major geo1ogic factors facilitating deposition of the ironstone. Thus the Chuan- 1inggou ironstone may have signi「ied major environment and tectonic changes o「 the North China Plat「orm responding to the breakup of the Columbia supercontine