冀东柞栏杖子BIF地球化学特征、形成时代及其地质意义

冀东地区柞栏杖子BIF出露于绿片岩相—低角闪岩相朱杖子岩群变质岩中。铁矿石主要由石英和磁铁矿组成,还含有少量透闪石和黑云母。主量元素主要为Si O2、Fe2O3和Fe O,其次为少量的Ca O和Mg O。较低的Al2O3含量、极低的Ti O2含量和高场强元素(HFSE)暗示,铁矿石中陆源碎屑物质含量很低。铁矿石的稀土元素含量较低,稀土元素配分模式特征为轻稀土元素相对亏损、重稀土元素相对富集。较明显的Eu正异常、轻微的Y正异常及较高的Y/Ho值的稀土元素特征,类似于高温热液和海水的混合热液,暗示成矿物质主要来自于海水和高温热液。对柞栏杖子BIF矿体夹层黑云斜长变粒岩进行SHRIMP锆石U-Pb定年,207Pb/206Pb年龄加权平均值为2572±8Ma(MSWD=5.8),可代表柞栏杖子BIF的形成年龄。综合前人研究,认为冀东地区变质级别不同的BIF物质来源相同、形成年代相近,BIF的变质可能和2500Ma左右华北克拉通东部陆块发生的地幔岩浆底侵事件有关。

Formation age, geochemical characteristics and geological significance of the Zhalanzhangzi BIF in eastern Hebei Province

Eastern Hebei Province is a main BIF concentration area in China. However, previous studies of BIF in this area which had undergone low-grade metamorphism were relatively weak. Zhalanzhangzi BIF occurs in the Zhuzhangzi Group which has generally suffered from green schist to low amphibolite facies metamorphism. The iron ore predominantly consists of magnetite and quartz, with minor tremolite and biotite. The main chemical constituents of iron ores are SiO2, Fe2O3 and FeO. Low content of Al2O3 and extremely low content of TiO2 as well as High Field Strength Element (HFSE) indicate that the contribution of continental detritus was insignificant. The concentration of REE in iron ores is low, and Post Archean Australia Shale (PAAS)-normalized rare earth elements profiles for the iron ores display depletion of light REE relative to high REE. All samples have distinct positive anomalies of Eu and slightly positive anomalies of Y, the ratio of Y/Ho in iron ores is high, and the typical characteristics of the REY(REE+Y) profiles resemble those of the mixture of high-temperature hydrothermal fluid and seawater, suggesting that the ore material might have been derived from submarine high temperature hydrothermal fluids and sea water. SHRIMP zircon U-Pb dating was conducted for the biotite-plagioclase granulites interlayered with orebodies in the Zhalanzhangzi BIF which are mainly prismatic grains with visible internal zonation. The mean Th/U ratio of 17 zircon samples is 1.02, indicating that they are magmatic zircons. Ten concordantly distributed data with a weighted mean 207Pb/206Pb age of 2572±8 Ma (MSWD=5.8) indicates the formation age of the Zhalanzhangzi BIF. A comparison with previous studies of medium or high-grade metamorphic BIFs shows that BIF underwent different grades of metamorphism that originated from the same sources and formed nearly simultaneously in eastern Hebei Province, and metamorphism of BIF was probably connected with mantle plume event in Eastern Block of the North China Craton at 2500Ma.