山西五台绿岩带柏枝岩组条带状铁建造(BIFs)成因及其环境意义  

The Origin and Environmental Significance of Banded Iron Formations in the Baizhiyan Formation of Wutai Greenstone Belt,Shaanxi Province

作　　者：孙迪 李秋根[1] 陈隽璐[2] 王宗起[3] 高山松 YASIN Rahim 胡鹏月 SUN Di;LI Qiugen;CHEN Junlu;WANG Zongqi;GAO Shansong;YASIN Rahim;HU Pengyue(Key Laboratory of Orogenic Belts and Crustal Evolution,Ministry of Education,School of Earth and Space Sciences,Peking University,Beijing 100871,China;Xi'an Center of China Geological Survey,Xi'an 710054,Shaanxi,China;Institute of Mineral Resources, Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]造山带与地壳演化教育部重点实验室,北京大学地球与空间科学学院,北京100871 [2]中国地质调查局西安地质调查中心,陕西西安710054 [3]中国地质科学院矿产资源研究所,北京100037

出　　处：《西北地质》2021年第4期16-41,共26页Northwestern Geology

基　　金：国家自然科学基金项目“华北克拉通古元古代吕梁群火山沉积作用与构造环境”(41372193);国家自然科学基金项目“华北克拉通东部太古宙壳幔作用与地壳生长方式”(41530207)。

摘　　要：五台绿岩带BIFs主要分布在金岗库组、文溪组和柏枝岩组中,其中以柏枝岩组BIFs铁矿最具工业规模和开采价值。对柏枝岩组8个BIFs矿区及出露点的12件铁矿石和1件变质火山岩样品进行了岩相学、地质年代学和地球化学分析。研究表明,柏枝岩组BIFs自东向西含铁矿物由磁铁矿逐渐转变为碳酸铁矿物(铁白云石或菱铁矿),对应的CaO含量和FeO/Fe_(2)O_(3)值有明显增加的趋势。LAICPMS锆石UPb定年结果显示,BIFs赋存的变质火山岩(石英绿泥钠长片岩)原岩形成于(2526±14)Ma,代表了柏枝岩组BIFs的沉积年龄。高Al_(2)O_(3)、TiO_(2)、HFSE和TREY含量及其之间呈正相关性暗示BIFs中有微量陆源碎屑物质混染,并且根据La/Sc,Th/Sc和Zr/Sc值认为这些物质主要来源于地壳长英质碎屑。尽管如此,大部分BIFs显示出一致的La正异常、Y正异常、LREE亏损、HREE富集及高于球粒陨石的Y/Ho值的特征,与现代海水REY特征相一致。并且BIFs还显示出强烈的正Eu异常,表明有高温热液流体的参与。根据改进的Sm/Yb vs.Y/Ho和Eu/Sm vs.Sm/Yb元素混合计算模型可知,柏枝岩组BIFs的成矿物质来源于大量海水和少量海底高温热液(0.1%~1%)的混合,并在沉积过程中混入少量地壳长英质碎屑组分(<0.1%)。相比西部富碳酸盐BIFs,东部和中部富磁铁矿BIFs受到地壳长英质碎屑的贡献比例更大。此外,根据五台绿岩带新太古代—古元古代“二阶段”构造演化模型,柏枝岩组BIFs形成于第一阶段的末期。并且,依据无Ce异常、正Eu异常、较低的Th/U值和极高的Fe/Mn值,认为沉积时水体整体处于缺氧状态,但其水体是不均一的,出现了细微的氧化还原分层。西部富碳酸盐BIFs因沉积于深部的还原水体中而具有更高的正Eu异常,而中部和东部富磁铁矿BIFs沉积于相对较浅的弱还原水体中而具有较低的Eu异常。The Wutai greenstone belt preserves abundant BIFs resources,which distributes in the formations of Jingangku,Wenxi and Baizhiyan.Among them,the BIFs of Baizhiyan Formation have the largest industrial scale and commercial value.In this study,the authors present detailed petrographic and geochemical analyses for twelve iron ores from eight mines or ore occurrences,and LAICPMS UPb zircon dating for one metamorphic volcanic rock(quartz chlorite albite schist)interbedded with BIFs.The results show that the petrography of BIFs from the eastern to western part has changed,with the iron-bearing mineral switching from magnetite to iron carbonate(ankerite or siderite)gradually,corresponding to the increasing tendency of CaO content and FeO/Fe_(2)O_(3) value.LAICPMS UPb zircon dating result suggest that the protolith of meta-volcanic rock formed in the(2526±14)Ma,which represents the deposition age of BIFs in the Baizhiyan Formation.High concentrations of Al_(2)O_(3),TiO_(2),HFSEs,TREY and their positive correlations indicate that there was a minor terrigenous input,and these detritus are probably crustal felsic rocks based on La/Sc,Th/Sc and Zr/Sc ratios.Even so,the majority of BIF samples display seawater-like REY profiles characterized by concordant positive La and Y anomalies,HREE enrichment relative to LREE and superchondritic Y/Ho ratios in PAAS-normalized REE diagrams.Consistently positive Eu anomalies are also observed,which suggests the participation of high-T hydrothermal fluids.According to the improved element mixing calculation model of Sm/Yb vs.Y/Ho and Eu/Sm vs.Sm/Yb,the source of BIFs in the Baizhiyan Formation are derived from mixtures of major seawater and minor high-T hydrothermal fluids(0.1%~1%),accompanied by minute contamination of crustal felsic clasts(<0.1%).Compared with carbonate-rich BIFs in the west,the magnetite-rich BIFs in the eastern and middle part received larger contribution from the terrigenous clast.In addition,based on the“two-step tectonic evolution model”for Neoarchean-paleoprote

关 键 词：条带状铁建造 主微量及稀土元素 物质来源 沉积环境 柏枝岩组 五台绿岩带 

分 类 号：P618.31[天文地球—矿床学]