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Rare earth element and yttrium compositions of the Paleoproterozoic Yuanjiacun BIF in the Lüliang area and their implications for the Great Oxidation Event(GOE)

引用

Science China Earth Sciences 2014年第11期57卷 2469-2485页

作者： WANG ChangLe ZHANG LianChang LAN CaiYun DAI YanPei Key Laboratory of Mineral Resources Institute of Geology and Geophysics Chinese Academy of Sciences University of Chinese Academy of Sciences Guangzhou Institute of Geochemistry Chinese Academy of Sciences

In China, most Precambrian banded iron formations （BIFs） are situated in the North China Craton. The Yuanjiacun iron depos- it, located in the Ltlliang area, is arguably the most representative Superior-type BIF. This iron deposit is coherent with the sedimentary rock succession of the Yuanjiacun Formation in the lower Lliliang Group, and was interpreted to be deposited at 2.3-2.1 Ga, based on ages of overlying and underlying volcanic strata. This age overlaps with the time range of the Great Oxidation Event （GOE, 2.4-2.2 Ga）. The Yuanjiacun BIF consists mainly of subhedral-xenomorphic magnetite and quartz and rarely other minerals with a lower degree of metamorphism, from greenschist to lower amphibolite facies. The geochemical characteristics of this BIF are similar to those of Superior-type BIFs. Prominent positive La, Y, and Eu anomalies normalized by the Post Archean Australian Shale （PAAS） indicate that the primary chemical precipitate is a result of solutions that repre- sent mixtures of seawater and high-T hydrothermal fluids. The contamination from crustal detritus found is negligible based on low abundances of Al2O3 and TiO2 （〈0.5%） and of trace elements such as Th, Hf, Zr, and Sc （〈1.5 ppm）, as well as the lack of co-variations between Al2O3 and TiO2. In particular, the Yuanjiacun BIF samples do not display significant negative Ce anom- alies like those of the Archean iron formations, but rather, the Yuanjiacun BIF samples exhibit prominent positive Ce anoma- lies, low Y/Ho ratios, and high light to heavy REE （（Pr/Yb）sN） ratios, which are essentially consistent with the late Paleoprote- rozoic （〈2.0 Ga） BIFs around the world. These characteristics of the Yuanjiacun BIF samples imply that the ancient ocean （2.3-2.1 Ga） was redox-stratified from oxic shallow water to deeper anoxic water. The specific redox conditions of the ancient ocean may be related to the GOE, which gave rise to the oxidation of Ce and Mn in the upper water, and to the presence of a Mn

关键词： Yuanjiacun iron deposit geochemistry of BIFs Great Oxidation Event (GOE) Mn hydroxide shuttle precipitationmechanism

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