鞍本太古代条带状铁建造(BIF)的稀土及微量元素特征

REE AND TRACE ELEMENT GEOCHEMISTRY OF THE ARCHEAN ANSHAN-BENXI BIF, LIAONING,NORTHEAST CHINA

本区围岩和BIF具有完全相同的地球化学性质。BIF有低S、CO_2和Al_2O_3,特别低的TiO_2和Zr含量,其REE特征是低的∑REE含量和强的正Eu异常。推测鞍本地区的BIF属于晚太古代陆壳盆地内的化学沉积变质矿床。在沉积过程中,有极少量泥质组分加入,成矿物质与火山活动无明显关系。

The majority of BIF in the Anshan-Benxi area, Northeast China are well developed iron oxide formations composed of quartz and magnetite with minor amounts of Ca-Mg-Fe silicates. Generally there is a close-association between amphibolites and BIF, but metasediments are more abundant in the EW-trending ore zone near Anshan City. Although the proportions of various rock types and their metamorphic grade differ from one mine to another, the major and trace element geochemistry of each rock type shows no difference. Amphibolites have higher LREE and LIL but lower HFS contents, implying that they are of sub-continental man-tle origin. All BIF ores from different mines are very poor in S, CO2 and Al2O3, especially TiO2 and Zr. The REE patterns in BIF ores from the major deposits exhibit strong positive Eu anomalies and very low total REE contents. As viewed from the abundance of metase-dimenis in the EW-trending ore zone, it is suggested that there would be depositional periods (Late Archean and Early Proterozoic) for Anshan BIF. Buta small number of chlorite schists in the EW-trending zone have the same chemical composition as .the amphibolites, and some amphibolites have been partly converted to chlorite schists through retrogressive metamorphism. Based on the mineral assemblages of BIF and their geochemistry, it is considered that all BIF in the area studied were deposited during the Late Archean in a continental basin.