新太古代清原绿岩带下甸子BIF铁矿地质特征及含黄铁矿条带BIF的成因探讨  

Geological features and genesis of the Neoarchean pyritebearing Xiadianzi BIF, Qingyuan greenstone belt

作　　者：彭自栋 张连昌[1,2] 王长乐[1,2] 佟小雪 南景博[3,4] 

机构地区：[1]中国科学院矿产资源研究重点实验室,中国科学院地质与地球物理研究所,北京100029 [2]中国科学院地球科学研究院,北京100029 [3]中国科学院大学,北京100049 [4]中国科学院深海科学与工程研究所深海科学部,三亚572000

出　　处：《岩石学报》2018年第2期398-426,共29页Acta Petrologica Sinica

基　　金：本文受国家自然科学基金项目(41572076、41602085)和”973”项目(2012CB416601)联合资助.致谢在SIMS锆石u.Pb年代学、原位氧同位素分析和微量元素分析过程中得到了李献华研究员及凌潇潇、李文君和高炳宇工程师的帮助,在此表示衷心的感谢.同时感谢中国科学院地质与地球物理研究所郑梦天博士在写作过程中就相关问题的讨论.此外,中国地质科学院矿产资源研究所李厚民老师和另一位匿名审稿人的评审对文章的改善具有重要意义,在此表示感谢.

摘　　要：新太古代清原绿岩带位于华北克拉通北缘,该绿岩带中发育独特的VMS和BIF(Algoma型)组合,其中下甸子铁矿是此类BIF的典型代表。下甸子BIF赋存于绿岩带南天门组下部,围岩及夹层为斜长角闪岩及少量石榴云母片岩。矿体夹层斜长角闪岩中锆石的SIMS年代学分析获得了2497.8±7.4Ma的变质年龄,而原位氧同位素分析显示变质锆石的δ18O值为5.3‰~6.2‰,与现代地幔基本一致,表明在变质过程中其锆石的氧同位素体系保持稳定。BIF矿石类型主要有硅酸盐型和含黄铁矿条带型两种,前者的矿物组合为石英、磁铁矿和铁阳起石,而后者的矿物组合为石英、磁铁矿、阳起石、黄铁矿和少量方解石。大部分矿石的Al_2O_3、TiO_2和HFSE(如,Zr、Hf、Th、U等)含量极低,说明其未受到碎屑物质混染,PAAS标准化后,两类矿石稀土元素显示与海水类似的特征,即La、Y的正异常和LREE相对于HREE的亏损;同时,显著的正Eu异常指示成矿过程中有海底高温热液组分的参与;此外,所有样品均无明显的Ce异常,表明其沉淀自还原的海水中。通过与华北地区其他Algoma型BIF对比发现,下甸子两类矿石均具有较高的CaO/(CaO+MgO)值以及接近球粒陨石的Y/Ho值,表明其可能沉淀环境与海底热液喷口较近,且热液组分(以高温热液为主,可能有少量低温热液)的贡献较大。相比于硅酸盐型矿石,含黄铁矿条带型矿石的HREE含量较低、Eu正异常和LREE含量偏高,这可能与其沉淀过程中海底的局部热液的脉动式活动有关,其中黄铁矿条带可能为热液喷流沉积成因。BIF围岩斜长角闪岩的地球化学特征分析显示,其原岩玄武质岩石的岩浆可能来自亏损地幔,但在上升过程中受到了少量陆壳物质的混染,结合前人对清原绿岩带表壳岩系和TTG的年代学及地球化学研究,推测下甸子BIF可能形成于晚太古代洋陆俯冲过程中的火山弧或弧后盆地环境中。The Neoarchean Qingyuan greenstone belt （QGB） is located in the northern margin of the North China Craton （NCC）, despite close geological similarities with other greenstone belts in the NCC, the QGB is characterized by hosting the unique Volcanogenic Massive Sulfide （VMS） and Banded Iron Formation （BIF） （Algoma type） paragenetic assemblage, among which the Xiadianzi iron ore is a typical of these BIF. The Xiadianzi BIF is located in the bottom of the Nantianmen Formation of the QGB, the iron bearing lithofacies are amphibolites with minor garnet-mica schists. SIMS zircon U-Pb analyses on interbeded amphibolites suggest that the metamorphic age of the Xiadianzi BIF is 2497.8±7.4Ma, in suit oxygen isotopic analysis shows the metamorphic zircon yield δ18O values of 5.3‰~6.2‰, which is similar to the value of the modern mantle, this indicate that the oxygen isotope of these zircons remain stable during the metamorphism. Ore types of the Xiadianzi BIF are mainly silicate type and pyrite-bands bearing type, most of the BIFs have low Al2O3, TiO2 and HFSE （for example Zr, Hf, Th, U） content, indicate they was no significant terrigenous input. In PAAS-normalized REE diagrams the two type ores both display distinctively seawater-like REE＋Y profiles, characterized by positive La and Y anomalies and HREE enrichment relative to LREE. Consistently positive Eu anomalies are also observed, which are typically from high-T hydrothermal fluids. In addition, there was no Ce anomalies recoded in the Xiadianzi BIF indicate they might deposited in anoxic water. Comparing with other BIFs in the NCC, all the samples from Xiadianzi have predominantly higher CaO/（CaO＋MgO） value and lower Y/Ho value, which might indicates the deposition took place close to the hydrothermal vents and the hydrothermal （mostly high-T fluid and perhaps some low-T fluid） has higher contribution to those BIFs. Furthermore, the pyrite-bands bearing type ore has higher LREE content and lower HREE content when compared with

关 键 词：下甸子 BIF 地质特征 矿石成因 清原绿岩带 新太古代 

分 类 号：P597.3[天文地球—地球化学] P618.31[天文地球—地质学]