岩石学报  2012, Vol. 28 Issue (11): 3574-3594   PDF    
引用本文
代堰锫, 张连昌, 王长乐, 刘利, 崔敏利, 朱明田, 相鹏. 2012. 辽宁本溪歪头山条带状铁矿的成因类型、形成时代及构造背景. 岩石学报, 28(11): 3574-3594  
DAI YP, ZHANG LC, WANG CL, LIU L, CUI ML, ZHU MT and XIANG P. 2012. Genetic type, formation age and tectonic setting of the Waitoushan banded iron formation, Benxi, Liaoning Province. Acta Petrologica Sinica, 28(11): 3574-3594(in Chinese with English abstract)   
辽宁本溪歪头山条带状铁矿的成因类型、形成时代及构造背景
代堰锫1,2, 张连昌1, 王长乐1,2, 刘利1,2, 崔敏利1, 朱明田1, 相鹏1,2     
1. 中国科学院地质与地球物理研究所，中国科学院矿产资源研究重点实验室，北京 100029;
2. 中国科学院研究生院，北京 100049
2012-07-15 收稿; 2012-09-18 改回.
基金项目: 本文受国家重点基础研究发展计划973项目(2012CB416601) 和中国科学院知识创新工程重要方向项目群(KZCX-2YW-Q04-07) 联合资助
第一作者简介: 代堰锫，男，1986年生，博士生，矿床学专业，E-mail: diyeplas@foxmail.com
通讯作者: 张连昌，男，博士，研究员，矿床地球化学专业，E-mail: lczhang@mail.iggcas.ac.cn
摘要: 辽宁鞍本地区位于华北克拉通东北缘，分布有诸多大型-特大型条带状铁矿床。本文对该区歪头山铁矿进行了岩石学、矿物学及年代学研究。歪头山铁建造以条带状铁矿石为主，兼含有少量的块状矿石，其顶底板围岩及矿体夹层主要为太古界鞍山群斜长角闪岩。元素地球化学分析表明，铁矿石富集重稀土[(La/Yb)PAAS=0.24~0.33]，具La正异常(La/La*=1.43~1.61)、Eu正异常(Eu/Eu*=2.40~4.54) 及Y正异常(Y/Y*=1.10~1.30)，Y/Ho值平均30.59，Sr/Ba值平均17.62，Ti/V值平均19.45，反映成矿物质可能来源于由海底火山活动带来的高温热液与海水的混合溶液。铁矿石无明显Ce负异常(Ce/Ce*=0.92~1.06)，暗示BIF沉积时海水处于缺氧环境。除Fe2O3T与SiO2外，铁矿石中其它氧化物含量均非常低，且贫Th、U、Zr等具有陆源性质的元素，表明大陆碎屑物质对BIF贡献极少。斜长角闪岩稀土元素配分型式近于平坦[(La/Yb)N=0.80~1.10]，无明显Ce异常(Ce/Ce*=0.95~0.99) 与Eu异常(Eu/Eu*=0.88~1.16)；其大离子亲石元素富集，高场强元素无明显亏损。地球化学分析表明，斜长角闪岩原岩可能为产于弧后盆地的玄武质火山岩。锆石形态与微量元素分析显示，斜长角闪岩中的锆石均属岩浆成因。SIMS锆石U-Pb定年显示斜长角闪岩原岩形成于2533±11Ma，代表了歪头山BIF的成矿年龄；在玄武质岩浆喷发过程中，还捕获了一组年龄为2610±5Ma的锆石。电子探针分析显示磁铁矿成分纯净(FeOT=92.04%~93.05%)，其标型组分特征暗示歪头山BIF属沉积变质型铁矿。综合分析认为，歪头山铁矿属Algoma型BIF，成矿与弧后盆地岩浆活动密切相关，指示了新太古代末华北克拉通普遍发育的一期BIF成矿事件。
关键词: 条带状铁建造     斜长角闪岩     新太古代     弧后盆地玄武岩     歪头山铁矿     鞍本地区    
Genetic type, formation age and tectonic setting of the Waitoushan banded iron formation, Benxi, Liaoning Province
DAI YanPei1,2, ZHANG LianChang1, WANG ChangLe1,2, LIU Li1,2, CUI MinLi1, ZHU MingTian1, XIANG Peng1,2     
1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. Graduate School of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Anshan-Benxi area in Liaoning Province, where plenty of large-superlarge banded iron formations are distributed, is located at the northeastern part of the North China craton. Petrology, mineralogy and geochronology of the Waitoushan iron deposit in Anshan-Benxi area are studied in this paper. Iron ores in the Waitoushan deposit are predominantly banded, with a scrap of massive. The Archean amphibolites in the Anshan Group were developed as main wall rocks and interlayer among the orebodies. Geochemical analyses show the iron ores are enriched in HREE [(La/Yb)PAAS=0.24~0.33], and characterized by positive La anomaly (La/La*=1.43~1.61), positive Eu anomaly (Eu/Eu*=2.40~4.54) and positive Y anomaly (Y/Y*=1.10~1.30) with Y/Ho=30.59, Sr/Ba=17.62, Ti/V=19.45, indicating the ore-forming materials were derived from the intermixture of high temperature hydrotherm and seawater. There is no distinct negative Ce anomaly (Ce/Ce*=0.92~1.06), imlying the seawater was under anoxic condition during the deposition of the Waitoushan BIF. Apart from Fe2O3T and SiO2, the contents of other oxides are rather low, with depletion in continental origin elements (such as Tu, U, Zr), implying the continental detrital materials contributed little to the metallogenic process of the Waitoushan BIF. The chondrite-normalized REE pattern of amphibolites is flat [(La/Yb)N=0.80~1.10] with no distinct Ce anomalies (Ce/Ce*=0.95~0.99) and Eu anomalies (Eu/Eu*=0.88~1.16), while the N-MORB-normalized trace element pattern shows enrichment in LILE without pronounced depletion in HFSE. Geochemical analyses show the protolith of amphibolites was probably basic volcanics originating in back-arc basin setting. Morphology and trace element analyses of zircon grains indicate the zircons selected from amphibolites were all magmatic. SIMS zircon U-Pb dating shows the protolith of amphibolites was formed at 2533±11Ma, standing for the formation age of the Waitoushan iron deposit. During the eruption of basaltic magma, a group of zircons with the age of 2610±5Ma were captured. Electron microprobe analysis shows the composition of magnetites is pure and single (FeOT=92.04%~93.05%), and the typomorphic studies indicate the Waitoushan deposit is attached to metamorphosed sedimentary iron deposit. To sum up, we propose the Waitoushan deposit belongs to the Algoma-type BIF, showing an affinity with the magmation in back-arc basin setting and representing a Neoarehean metallogenic event of BIF widely developed in the North China craton.
Key words: Banded iron formation     Amphibolite     Neoarehean     Back-arc basin basalt     Waitoushan iron deposit     Anshan-Benxi area    

前寒武纪条带状铁建造(banded iron formation，简称BIF) 是铁矿石的主要来源，全世界约有90%的铁产自BIF (Isley, 1995)。BIF多赋存于早前寒武古老变质岩系之中，通常指全铁含量大于15%，具有由富铁矿物(以磁铁矿为主) 和脉石矿物(以石英为主) 组成的条带状或条纹状构造的化学沉积岩(James, 1954, 1983)。根据BIF的形成时代及含矿建造，将其分为Algoma型及Superior型(Gross, 1980)。Algoma型主要产于太古宙，通常形成于岛弧、弧后盆地或克拉通内裂谷带中，与海底火山活动关系密切；Superior型主要产于早元古代，一般形成于浅海环境且与沉积作用密切相关，不含或含有极少量的火山岩，其沉积规模远大于Algoma型(Gross, 1980, 1983, 1996)。

BIF是早前寒武纪地球特殊环境的产物(Bekker et al., 2010)。据统计，全球BIF型铁矿主要分布于3.8~1.9Ga之间(Huston and Logan, 2004)，尤以2.7~2.0Ga最为发育(James, 1983; Isley, 1995; Klein, 2005)。目前国际上对于BIF的形成机制尚颇具争议，但多数学者认为BIF的沉积与全球重大地质事件有关，并将其与大氧化事件(Cloud, 1973; Catuneanu and Eriksson, 1999; Bekker et al., 2004; Huston et al., 2004; 赵振华, 2010)、地幔柱活动(Isley, 1995; Isley and Abbott, 1999; Abbott and Isley, 2001; Pirajno, 2004; Bekker et al., 2010)、地壳快速生长(Rasmussen et al., 2012)、板块俯冲(Dobson and Brodholt, 2005) 甚至陨石撞击事件(Glikson and Vickers, 2007; Slack and Cannon, 2009) 联系起来。Fe和Si的物质来源一直是BIF研究的核心问题。Holland (1973)根据亚铁在深海海水中具有较高的含量，认为铁质来自于海水。Jacobsen and Pimentel-Klose (1988)研究Hamersley与Michipicoten铁建造时发现Nd同位素呈正值，提出铁质主要源于地幔。Rao and Naqvi (1995)发现BIF具有明显的Eu正异常，认为铁质主要来自海底高温热液。近年来，国内外地质学家通过对BIF稀土元素及同位素的详细研究，普遍认为铁质来源于深海热液与海水的混合溶液(Khan et al., 1996; Bau and Dulski, 1996; Kato et al., 1998; Bolhar et al., 2005; 李志红等, 2008; 沈其韩等, 2009; 沈其韩等, 2011)。Hamade et al.(2003)利用Ge/Si比值判别出BIF中的硅质主要源于陆壳风化。目前，主流观点认为硅质主要来自海底热液(蒋少涌等, 1992; Steinhoefel et al., 2009; Heck et al., 2011)，同时也有为数不少的陆源硅质组分的加入(Heck et al., 2011)。对于含矿流体的运移及沉淀机制，主要有海底喷流模式(Goodwin, 1973; Lascelles, 2006)、上升洋流模式(Clout and Simonson, 2005) 及微生物成因(Konhauser and Ferris, 1996; Noffke et al., 2006; Craddock and Dauphas, 2011; Konhauser et al., 2011; Li et al., 2011) 等观点。几十年来，BIF相关科学问题是地球科学研究的热点与难点，许多关键问题迄今尚无定论。

在我国，对BIF的系统研究始于20世纪50年代(程裕淇, 1957)。半个多世纪以来，中国地质学家对国内的BIF特征主要认识如下：(1) BIF多分布于华北克拉通边缘及裂谷之中；(2)80%形成于新太古代，其次为早元古代；(3) 围岩以变质火山岩为主，兼含少量变质沉积岩，铁矿床多属于Algoma型；(4) 后期复杂的变质变形作用对BIF改造强烈；(5) 以贫矿为主，富矿少且多位于深部(Zhai and Windley, 1990; Zhai et al., 1990; 沈保丰等, 1994, 2006; Zhang et al., 2011b)。辽宁鞍山-本溪地区是我国最大的BIF型铁矿石资源基地，包括东西鞍山、大孤山、齐大山、南芬及歪头山等大型-特大型铁矿床。从建国初期至今，中国地质学家在该区进行了大量全面、深入的研究工作(周世泰, 1994)。由于BIF型铁矿形成时代久远，前人的工作又缺乏运用现代先进测试技术，致使对一些铁建造的矿床类型、成矿时代等基本问题认识不清。例如，乔广生等(1990)测定了齐大山-胡家庙和歪头山-北台硅铁建造中斜闪角长岩的全岩Sm-Nd年龄，分别为2729±245Ma和2724±102Ma，具有较大的误差。近年来，高精度U-Pb定年技术的迅速发展，为精确厘定BIF型铁矿床的形成时代提供了契机。本文选取鞍本地区的歪头山条带状铁矿，采用岩石地球化学及锆石U-Pb年代学等方法，拟对该矿床的含矿原岩建造、形成时代及成矿构造背景作出系统分析。

1 区域地质

鞍本地区位于华北克拉通东北缘，地处太子河-浑江凹陷西段，可再分为辽阳向斜、本溪向斜、歪头山隆起、鞍山隆起、南芬隆起等四级构造单元。区内基底地层为太古宙鞍山群、古元古代辽河群，盖层有震旦系、古生界、中生界及新生界。条带状铁建造赋存于太古宙中上鞍山群(图 1)。中鞍山群主要分布于本溪及北台一带，以斜长角闪岩、混合岩化片麻岩及黑云变粒岩为主，夹云母石英片岩、绿泥石英片岩及条带状铁矿层，原岩为基性-中酸性火山岩、火山碎屑岩，夹泥质-粉砂质沉积岩和硅铁质岩，变质程度为角闪岩相；上鞍山群主要分布于鞍山附近，主要为绢云石英千枚岩、绢云绿泥片岩、绿泥石英片岩，夹变粒岩、磁铁石英岩及薄层斜长角闪岩，原岩为泥质-粉质沉积岩，夹硅铁质岩及少量基性-中酸性火山岩，变质程度为绿片岩相(周世泰, 1994; 李士江和全贵喜, 2010)。值得注意的是，原认为是上下关系的表壳岩，很可能形成于同一时代，当属空间相变的产物(万渝生, 1993)。

图 1 辽宁鞍本地区条带状铁建造与太古宙地质体分布图(据沈其韩，1998；Wan et al., 2011修改) Fig. 1 The distribution of banded iron formations and Archean geological bodies in Anshan-Benxi area, Liaoning Province (modified after Shen, 1998; Wan et al., 2011)

区内断裂构造发育，主要分为四组：(1) NNE向断裂均分布于西部地区，以郯庐断裂为代表，限定了鞍本地区的西界；(2) NE向属压扭性断裂，以寒岭断裂为代表，带内主要为鞍山群变质岩及混合岩化岩石；(3) NW向以石桥子断裂为代表，此断裂形成较早，被寒岭断裂所切，为歪头山隆起的东部边界，带内夹有震旦系、寒武系；(4) EW向断裂是太子河深断裂在地表的表现，连续性差，多产于混合岩化岩石中，控制了鞍本地区的南界(周世泰, 1994)。郭洪方(1994)厘定了鞍山地区太古宙地壳的构造演化序列，认为存在过三期构造变形：(1) 第一期构造变形约发生于2900~2800Ma，在花岗岩中形成大量NE走向的片麻理，在表壳岩中形成紧闭同斜褶皱和密集的轴面片理；(2) 第二期构造变形约发生于2600Ma，形成NNW与EW向的韧性剪切带，含铁岩系以构造片岩的形式沿韧性剪切带就位；(3) 第三期构造变形发生于2000Ma左右，此时某些韧性剪切带构造层次变浅并遭受后期变形叠加。特殊的构造演化史，使鞍本地区BIF经历了强烈的叠加改造与复杂的内部变形(杨振升等, 1983; 张宝华等, 1986; 郑峻庆等, 1986; 徐仲元, 1991; 张瑞华和王守伦, 1994)。

鞍本地区太古宙地质体主要出露于鞍山、弓长岭、南芬、本溪和歪头山等地，约由30%的含铁岩系和70%的花岗质岩石组成，含铁岩系以较陡的倾角分布于花岗质岩石之中(万渝生, 1993)。花岗质岩石岩性主要包括位于歪头山附近的TTG岩系、分布于歪头山-北台一带的二长花岗岩及花岗闪长岩、大致呈弧形展布于鞍本地区的正长花岗岩，以及部分时代不清的太古宙花岗岩(图 1)。此外，鞍本地区还出露了时代大于2.8Ga的花岗质杂岩体，包括白家坟片麻岩(3812~3804Ma, Song et al., 1996; 3817~3140Ma, Wu et al., 2008)、陈台沟花岗岩(3306Ma, Song et al., 1996; 3369~3136Ma, Wu et al., 2008)、铁架山花岗岩(2964~2962Ma, Song et al., 1996)、东山片麻岩(3751~3269Ma, Wu et al., 2008) 以及东西鞍山花岗岩(3001~2994Ma, Song et al., 1996) 等。中国最老的锆石年龄纪录(~3.8Ga) 即产于鞍本地区(Liu et al., 1992; Wan et al., 2005a; Liu et al., 2008; Wan et al., 2012)。

2 矿床地质

歪头山铁矿出露地层主要是太古界鞍山群，由斜长角闪岩、黑云片麻岩、阳起石磁铁石英岩、阳起石片岩及阳起石英岩等组成，混合岩化作用比较普遍，变质程度为绿帘角闪岩相-角闪岩相(姚培慧, 1993; 蒋永年和修群业, 1989)，其地层柱状图如图 2b所示。

图 2 歪头山铁矿地质图(a)、地层柱状图(b) 和剖面图(c) (据冶金工业部鞍山冶金地质勘探公司，1983①；周世泰，1994修改) 图(a) 中：1-片岩; 2-阳起磁铁石英岩; 3-透闪石英岩, 云母石英岩; 4-阳起石英岩; 5-斜长角闪岩; 6-条带状混合片麻岩; 7-辉绿岩; 8-煌斑岩; 9-安山玢岩; 10-伟晶岩; 11-塑性断层; 12-断裂；图(c) 中：1-第四系; 2-片岩; 3-磁铁石英岩; 4-阳起石英岩; 5-斜长角闪岩; 6-带状混合片麻岩; 7-辉绿岩; 8-安山玢岩; 9-伟晶岩 Fig. 2 Geological map (a), stratigraphic column (b) and stratigraphic section (c) of the Waitoushan iron mine (modified after Zhou, 1994) In Fig. 2a: 1-schist; 2-actinolitic magnetite quartzite; 3-tremolite quartzite and micaceous quartzite; 4-actinolite quartzite; 5-amphibolite; 6-banded amphogneiss; 7-diabase; 8-lamprophyre; 9-andesitic porphyrite; 10-pegmatite; 11-ductile fault; 12-fracture; in Fig. 2c: 1-Quaternary; 2-schist; 3-magnetite quartzite; 4-actinolite quartzite; 5-amphibolite; 6-banded amphogneiss; 7-diabase; 8-andesitic porphyrite; 9-pegmatite

①冶金工业部鞍山冶金地质勘探公司. 1983.鞍本地区鞍山式铁矿地质(内部资料). 312-321

矿区主要存在两期褶皱构造，早期褶皱为一个向东倒转的大型向斜构造，轴向近南北，枢纽向南南东倾伏，形成于新太古代；晚期褶皱为近共轴叠加褶皱，多为中小型规模的紧闭-中等开阔褶皱，大部分受控于早期褶皱，局部可见其改造早期褶皱的现象。褶皱构造是矿区主要的控矿构造，铁矿体多赋存于向斜轴部(姚培慧, 1993; 张瑞华和王守伦, 1994; 李东林, 2003)。矿区发育三条韧性剪切带，最大的一条为NNE走向，宽20~30m，长约1000m (图 2a)。韧性剪切带内岩石变形强烈，呈密集板状、条带状和构造透镜体出现。张瑞华和王守伦(1994)认为矿区韧性剪切带同样具有控矿作用，可使矿体加厚及运移上升，亦可拉薄或拉断矿体，甚至造成矿体尖灭。矿区断裂多为压性断裂，走向40°~70°，倾向NW，倾角70°~80°，主要形成于燕山期(姚培慧, 1993; 李东林, 2003)。此外，矿区南端发育一系列NE向叠瓦式逆断层。

矿区混合岩以带状混合片麻岩为主，遍布整个矿区。由于遭受混合岩化作用，鞍山群变质岩呈残留体分布于混合岩中。岩浆岩主要为脉岩，包括伟晶岩、安山玢岩、辉绿岩及煌斑岩等(图 2a)。伟晶岩形成于混合岩化后期；辉绿岩和煌斑岩时代大致为燕山期，多分布于断裂两侧(姚培慧, 1993)。

矿体呈层状、似层状产出，走向近南北，倾向西，倾角20°~50°(图 2c)，大致可分为三层(图 2b)。由磁铁石英岩构成的铁矿体及其围岩呈向斜构造产出(图 2c)，一般在向斜核部和转折端矿体加厚，在轴部往往受挤压形成透镜状矿体。矿石类型主要为阳起磁铁石英岩，具条纹状、条带状和块状构造，细粒变晶结构，主要成分包括磁铁矿、石英、阳起石及透闪石等。围岩蚀变以绿泥石化、镁铁闪石化、黄铁矿化及黑云母化为主，在矿体两侧、褶皱转折端及断裂附近最为发育(姚培慧, 1993)。

3 样品采集与测试 3.1 岩(矿) 相学

本次工作以歪头山铁矿条带状矿石、块状矿石、斜长角闪岩及磁铁矿为主要研究对象。铁矿石采自矿区主矿体；斜长角闪岩采自矿体夹层，与矿体界线明显，且二者产状大致平行(图 3a)。条带状铁矿石为深灰色，具有明暗相间、宽0.8~3mm的条带，部分条带发生了弯曲变形(图 3c, d)。显微镜下观察，暗色条带主要为磁铁矿，粒径约50~500μm，半自形-他形结构，兼含少量石英、赤铁矿、阳起石及透闪石等；白色条带主要是石英，次为磁铁矿、阳起石、透闪石、绿泥石及黑云母(图 3e)。块状矿石为黑色，不具有条纹-条带状构造，主要成分为磁铁矿与石英，含有阳起石、透闪石及少量稀疏分布的黄铁矿等。磁铁矿粒径约50~300μm，半自形-他形结构，遍布整个视野(图 3f)。

图 3 歪头山矿区铁矿石与斜长角闪岩野外及镜下特征 (a)-产于铁矿体夹层的斜长角闪岩；(b)-斜长角闪岩主要由斜长石与角闪石组成，含少量磁铁矿(正交偏光)；(c)-条带状铁矿石；(d)-变形条带状铁矿石；(e)-条带状铁矿石由铁层与硅层组成(单偏光)；(f)-块状矿石中磁铁矿不具有条带状构造(反射光) Fig. 3 The field and microscopic characteristics of iron ores and amphibolites in the Waitoushan deposit (a)-amphibolites as the interlayer of orebodies; (b)-amphibolites mainly consisting of plagioclase and hornblende with a few of magnetites (cross-polarized light); (c)-banded iron ores; (d)-the deformed banded iron ores; (e)-banded iron ores consisting of iron layer and silicon layer (plane-polarized light); (f)-the magnetites in massive iron ores without banded structure (catoptric light)

斜长角闪岩为暗绿色，粒状变晶结构，块状构造或弱定向构造，主要成分为角闪石(40%~45%)、斜长石(40%~45%)，兼含石英( < 3%)、磁铁矿( < 5%)、黑云母( < 5%) 及绿泥石( < 2%)。斜长石多为板状，粒径0.1~0.8mm，发育聚片双晶；角闪石呈柱状，粒径0.1~0.5mm，多色性显著(图 3b)。黑云母与绿泥石往往分布于角闪石边部，为角闪石退变质产物。

3.2 分析方法

矿石主量元素测试在核工业北京地质研究院分析测试中心完成，矿石微量元素、围岩主微量元素分析由中国科学院地质与地球物理研究所矿产资源研究重点实验室完成。矿石主量元素采用Phillips PW 2404型X荧光光谱仪分析，RSD < 2%~3%；围岩主量元素使用XRF-1500型X荧光光谱仪测试，RSD=0.1%~1%；微量元素及稀土元素利用酸溶法制备样品，用ICP-MS (Element, Finnigan MAT) 测试，RSD < 2.5%。电子探针分析在中国科学院地质与地球物理研究所电子探针室JEOL JXA-8100型电子探针仪上完成，加速电压为15kV，加速电流20nA，束斑直径5μm。

锆石分选在河北区域地质调查院完成，于双目镜下将锆石粘到双面胶上并制成靶。透反射显微照相及阴极发光图象分析在中国科学院地质与地球物理研究所完成。锆石定年在中国科学院地质与地球物理研究所Cameca IMS-1280二次离子质谱仪上进行，详细分析方法见Li et al.(2009)，同位素比值及年龄误差均为1σ，数据结果处理采用Isoplot软件(Ludwig, 2001)。锆石微量元素分析在国家地质实验测试中心Thermo Element II等离子质谱仪上进行，采用He作为剥蚀物质的载气，束斑直径30μm，RSD < 5%。

4 分析结果 4.1 铁矿石

矿石主微量分析结果见表 1。条带状铁矿石SiO2含量为53.82%~57.14%，平均55.74%；MgO含量为2.20%~4.17%，平均3.21%；CaO含量为0.68%~2.09%，平均1.25%；Fe2O3T含量为38.52%~40.51%，平均39.31%；其他氧化物组分含量均很低。块状铁矿石SiO2含量为16.64%，Fe2O3T含量为72.94%，Al2O3含量为1.21%，MgO含量为4.32%，CaO含量为4.13%。在主量元素含量上，歪头山铁矿与鞍本BIF及Algoma型BIF相似(图 4a)。铁矿石的Fe2O3T与SiO2含量占89.58%~95.66%，表明只有极少碎屑物质的加入，且二者呈明显的反比例关系。Fe2O3T-(CaO+MgO)-SiO2图解显示，歪头山铁建造与鞍本BIF及Algoma型BIF接近；除块状矿石外，条带状矿石均落入世界条带状铁矿分布范围(图 4b)。

表 1 歪头山矿区铁矿石主量元素(wt%)、微量元素(×10-6) 分析结果 Table 1 Major (wt%) and trace (×10-6) element contents of iron ores in the Waitoushan deposit

图 4 铁矿石主量元素含量(a) 和主量元素图解(b, 底图据Lepp and Goldich, 1964; 沈其韩等, 2009) 鞍本BIF数据据周世泰(1994)，Algoma型BIF数据据Gross and Mcleod (1980) Fig. 4 Major element contents (a) and major element diagrams (b, the base map after Lepp and Goldich, 1964; Shen et al., 2009) of iron ores The data of Anshan-Benxi BIF and Algoma BIF after Zhou (1994) and Gross and Mcleod (1980), respectively

条带状铁矿石稀土总量(ΣREE+Y) 为18.72×10-6~22.51×10-6，平均为20.48×10-6；块状矿石稀土总量为20.92×10-6。用PAAS (Post Archean Australian Shale)(Mclennan, 1989) 对铁矿石稀土元素进行标准化，两种类型铁矿石稀土配分型式(图 5a) 相当吻合，均显示富集重稀土[(La/Yb)PAAS=0.24~0.33]，具有明显的La正异常(La/La*=1.43~1.61)、极强的Eu正异常(Eu/Eu*=2.40~4.54) 与显著的Y正异常(Y/Y*=1.10~1.30)，且无明显的Ce异常(Ce/Ce*=0.92~1.06)。现代海水的REE配分模式具有重稀土富集、La正异常及Y正异常的特征(Zhang et al., 1994; Alibo and Nozaki, 1999)，而Eu的正异常则是海底高温热液的特征(Danielson et al., 1992; Bau and Dulski, 1996; Douville et al., 1999)，表明歪头山BIF铁矿石中稀土元素可能来自于海水与深海热液的混合溶液。海水的Y/Ho比值约为44~74，且随着深度增加而减小，而陆地岩石与球粒陨石的Y/Ho值恒为26(Byrne and Lee, 1993; Nozaki et al., 1997; Bau and Dulski, 1999; Bolhar et al., 2004)。歪头山铁矿石Y/Ho值介于28.04~32.27，平均为30.59，与海水显示亲缘性。

图 5 铁矿石稀土元素PAAS标准化配分图(a, 标准化值据McLennan, 1989) 与微量元素原始地幔标准化蛛网图(b, 标准化值据Sun and McDonough, 1989) Fig. 5 PAAS-normalized REE (a, normalization values after McLennan, 1989) and primitive-mantle-normalized trace element patterns (b, normalization values after Sun and McDonough, 1989) of iron ores

除Sr元素外，铁矿石中其他微量元素含量均较低。Th、U、Zr等陆源性质的元素具有很低的含量，暗示极少有大陆碎屑沉积物的加入。在原始地幔标准化微量元素配分图上，两种类型铁矿石微量元素含量及配分型式相似，表明二者成矿物质来源可能相同。此外，铁矿石显示U、La、Pb、P、Eu正异常，Nb、Ta、Zr、Hf、Ti负异常(图 5b)。火山岩和海相沉积物的Sr/Ba值大于1，陆源沉积岩的Sr/Ba值小于1(沈其韩等, 2009)；歪头山铁矿石Sr/Ba值为8.17~26.79，平均为17.62，与火山岩和海相沉积物的Sr/Ba值一致。铁质页岩Ti/V值变化于1.33~10.90，火山建造为13~85(沈其韩等, 2009)；歪头山铁矿石Ti/V值在6.33~44.40之间，平均为19.45，与火山建造一致。Sr/Ba值与Ti/V值暗示成矿与火山作用有关。

4.2 斜长角闪岩

歪头山斜长角闪岩主微量元素分析结果见表 2。斜长角闪岩SiO2含量为46.06%~48.42%，平均46.85%；Al2O3含量为13.85%~14.96%，平均14.40%；MgO含量为6.72%~8.60%，平均7.73%；CaO含量为6.37%~9.78%，平均7.56%；Fe2O3T含量为13.68%~15.45%，平均14.79%；TiO2、MnO、K2O及P2O5含量很低。碱度δ((Na2O+K2O)2/(SiO2-43))=0.84~2.42，显示亚碱性玄武岩(δ < 3.3) 的特征；K2O含量很低，与岛弧拉斑玄武岩一致(Gill, 2010)。此外，TiO2-SiO2与(Al+Fe+Ti)-(Ca+Mg) 图解表明斜长角闪岩属正变质岩，原岩为基性火山岩(图 6a, b)。

表 2 斜长角闪岩主量元素(wt%)、微量元素(×10-6) 分析结果 Table 2 Major (wt%) and trace (×10-6) element contents of amphibolites

图 6 斜长角闪岩主微量元素原岩恢复图解(底图分别据Tarney, 1976; Moine and Roche, 1968; 赵振华, 1997; Winchester and Floyd, 1977) Fig. 6 Major and trace element diagrams for protolith reconstruction of amphibolites (the base map after Tarney, 1976; Moine and Roche, 1968; Zhao, 1997; Winchester and Floyd, 1977 respectively)

斜长角闪岩稀土元素总量(ΣREE) 为30.71×10-6~38.24×10-6，平均34.63×10-6。(La/Yb)N=0.80~1.10，平均0.93，表明轻重稀土不存在明显分馏。球粒陨石标准化稀土配分型式近于平坦(图 7a)，不具有明显的Ce异常(Ce/Ce*=0.95~0.99，平均0.97) 与Eu异常(Eu/Eu*=0.88~1.16，平均1.02)。

图 7 斜长角闪岩稀土元素球粒陨石标准化配分图(a, 标准化值据Taylor and McLennan, 1985) 与微量元素N-MORB标准化蛛网图(b, 标准化值据Sun and McDonough, 1989) Mariana ARCTH-Mariana岛弧拉斑玄武岩(Elliott et al., 1997); Scotia BABB-Scotia弧后盆地玄武岩(Fretzdorff et al., 2002) Fig. 7 Chondrite-normalized REE (a, normalization values after Taylor and McLennan, 1985) and N-MORB-normalized trace element patterns (b, normalization values after Sun and McDonough, 1989) of amphibolites Mariana ARCTH-Mariana arc tholeiite (Elliott et al., 1997); Scotia BABB-Scotia back-arc basin basalt (Fretzdorff et al., 2002)

斜长角闪岩微量元素含量几乎均低于上地壳的平均含量(Taylor and McLennan, 1985)。在微量元素N-MORB标准化蛛网图中，斜长角闪岩富集大离子亲石元素(如Rb、Ba、Sr、K)，其高场强元素(如Nb、Ta、Ti、Zr、Hf) 无明显亏损(图 7b)。稀土元素与高场强元素活动性很低，可有效识别变质岩原岩(赵振华, 1997)。La/Yb-ΣREE与Zr/TiO2-Nb/Y图解显示斜长角闪岩原岩为基性火山岩(图 6c, d)。

4.3 磁铁矿矿物学

歪头山铁矿石中磁铁矿电子探针分析结果列于表 3。磁铁矿成分以FeOT(用FeO表示的全铁含量) 为主，条带状铁矿石中的磁铁矿FeOT为92.64%~93.05%，平均92.85%；块状矿石中的磁铁矿FeOT为92.04%~92.16%，平均92.11%，较前者略低。两种类型磁铁矿其他成分含量大多低于0.1%，部分低于检测限，且平均成分未见明显差别(图 8)。

图 8 不同类型铁矿石中的磁铁矿主要成分平均含量蛛网图 Fig. 8 The average content of main constituent in magnetite of different iron ores

Annersten (1968)与Rumble (1973)认为，产于岩浆矿床中的磁铁矿TiO2、MgO、Al2O3、Cr2O3、NiO含量高；矽卡岩矿床中的磁铁矿TiO2、Cr2O3、NiO含量有所降低，但MgO、Al2O3仍较高；沉积变质型铁矿床中的磁铁矿则以“纯磁铁矿”为特征。Dupuis and Beaudoin (2011)统计了世界上不同成因类型矿床中磁铁矿组分特征，认为较之其他类型的磁铁矿，BIF型铁矿中的磁铁矿具有很低的TiO2、MnO、CaO、Al2O3含量。歪头山矿区两种类型磁铁矿FeOT含量接近于理论值93.09%(磁铁矿分子式Fe3O4换算为FeO)，且其他氧化物含量均非常低(表 3)，暗示歪头山铁矿属于沉积变质型铁矿。徐国风和邵洁涟(1979)、林师整(1982)、陈光远等(1984)总结了不同成因类型铁矿床中磁铁矿的标型组分特征，并将磁铁矿成因类型分为岩浆型、火山岩型、接触交代型、热液交代型及沉积变质型(表 4)。对比显示，歪头山矿区磁铁矿标型组分含量与沉积变质型磁铁矿颇为接近，暗示歪头山BIF属沉积变质型铁矿。

表 3 不同类型铁矿石中磁铁矿电子探针分析结果(wt%) Table 3 The composition of magnetite in different iron ores (wt%)

表 4 不同成因类型磁铁矿标型组分对比表(wt%) Table 4 Comparison of components in different magnetites (wt%)
4.4 锆石SIMS U-Pb定年

锆石选自歪头山铁矿体夹层中的斜长角闪岩。阴极发光图像显示，锆石颗粒棱角分明，多呈长柱状，长度为70~130μm，长宽比约为2:1(图 9)。锆石不具有清晰的振荡环带，可能是受到变质重结晶作用的影响。

图 9 歪头山斜长角闪岩锆石阴极发光图像 Fig. 9 Cathodoluminescence (CL) images of zircons selected from amphibolites in the Waitoushan deposit

17个分析点的207Pb/206Pb年龄明显分为两群，分别介于2619~2602Ma与2544~2521Ma (表 5、图 10a)。部分锆石存在不同程度的铅丢失，谐和图上具有两个上交点年龄(图 10c)：年龄大的一组共10个数据点，其上交点年龄为2610±5Ma (MSWD=0.51)，与207Pb/206Pb加权平均年龄2611±5Ma一致；年龄小的一组共7个数据点，其上交点年龄为2533±11Ma (MSWD=0.27)，亦与207Pb/206Pb加权平均年龄2530±7Ma吻合。

表 5 歪头山斜长角闪岩锆石U-Pb定年结果 Table 5 Zircon U-Pb dating results of amphibolites in the Waitoushan deposit

图 10 斜长角闪岩锆石207Pb/206Pb年龄明显分为两组(a)、不同年龄组锆石的Th/U比值不同(b) 和斜长角闪岩锆石U-Pb年龄(c) Fig. 10 Two groups of 207Pb/206Pb age of zircons selected from amphibolites (a), different Th/U ratios of zircons in different age groups (b) and zircon U-Pb age of amphibolites (c)
4.5 锆石微量元素

锆石测试点微量元素分析结果列于表 6。10个较大年龄点ΣREE=1601×10-6~4682×10-6，平均2951×10-6；7个较小年龄点ΣREE=1385×10-6~3380×10-6，平均1999×10-6；Y含量总体分布于2644×10-6~9440×10-6，平均5008×10-6。两组锆石ΣREE及Y含量均接近于基性岩中锆石稀土总量及Y含量(ΣREE=2780×10-6，Y含量为5184×10-6)(Hoskin and Schaltegger, 2003)。在球粒陨石标准化稀土元素配分图上(图 11)，两组锆石配分型式几乎完全一致：均明显富集重稀土，亏损轻稀土，显示为向左陡倾的配分曲线，具有强烈的Ce正异常(Ce/Ce*=4.14~21.99) 及Eu负异常(Eu/Eu*=0.06~0.15)，表明两组锆石具有典型岩浆成因的特点(Belousova et al., 2002; Rubatto, 2002; Hoskin and Schaltegger, 2003)。

表 6 斜长角闪岩锆石稀土和微量元素分析结果(×10-6) Table 6 Trace element contents of zircons selected from amphibolites (×10-6)

图 11 斜长角闪岩锆石稀土元素球粒陨石标准化配分图(标准化值据Taylor and McLennan, 1985) Fig. 11 Chondrite-normalized REE pattern of zircons in amphibolites (normalization values after Taylor and McLennan, 1985)

利用锆石特征微量元素亦可有效判别锆石成因。不同成因锆石有不同的Th、U含量及Th/U比值：岩浆锆石的Th、U含量较高，一般Th/U>0.4；变质锆石的Th、U含量低，一般Th/U < 0.1(Rubatto, 2002)。本文所测锆石Th/U比值介于0.51~1.84(表 5)，10个较大年龄点Th/U平均1.36，7个较小年龄点Th/U平均1.01(图 10b)，反映斜长角闪岩中锆石属岩浆锆石。Wu et al.(2002)研究大别山黄镇榴辉岩锆石时发现，岩浆锆石的Nb、Ta含量及Nb/Ta值均高于变质锆石：岩浆锆石Nb、Ta含量小于100×10-6，Nb/Ta>1；变质锆石Nb、Ta含量约为1×10-6，Nb/Ta < 1.3。本文所测锆石Nb、Ta含量几乎均与岩浆锆石一致，10个较大年龄点Nb/Ta平均4.52，7个较小年龄点Nb/Ta平均3.67(表 6)，表明锆石均为岩浆成因。

5 讨论 5.1 矿床类型与成矿时代

斜长角闪岩为歪头山铁矿主要的赋矿围岩，其野外产状及地球化学分析显示原岩为基性火山岩，表明歪头山矿区成矿与火山活动关系密切，属Algoma型BIF。Huston et al.(2004)认为不同类型的BIF矿石具有不同程度的Eu异常：Algoma型BIF与火山活动密切相关，矿石具有较高的(Eu/Eu*)NASC值(>1.8)；Superior型BIF与火山作用无直接联系，(Eu/Eu*)NASC值较低( < 1.8)。歪头山铁矿石主量元素含量与Algoma型BIF接近(图 4a, b)，且(Eu/Eu*)NASC介于2.25~4.25，平均3.69(NASC标准化值据Gromet et al., 1984)，进一步证明该矿床为Algoma型BIF。

彭澎和翟明国(2002)统计了华北克拉通236个大于16亿年且精度较高的年龄数据，认为在2550~2475Ma期间华北克拉通发生了一起重大地质事件，在该地质事件中成岩作用峰期早于变质作用峰期，而磁铁石英岩主要形成于2600~2500Ma。本文对华北克拉通与BIF相关岩石形成时代与变质时代统计显示，新太古代BIF沉积事件普遍发育于整个华北克拉通，随后又在太古宙-元古宙转换期间发生了一期变质事件(表 7、图 12)。对歪头山铁矿斜长角闪岩的锆石SIMS U-Pb定年结果显示两个年龄峰，且具有两个上交点年龄。所测锆石多呈长柱状，棱角分明，与常具圆化外形的变质锆石不同(Hoskin and Schaltegger, 2003)；锆石均具有高的Th/U与Nb/Ta值；稀土配分型式显示为向左陡倾的曲线，具强烈的Ce正异常及Eu负异常；以上特征均表明斜长角闪岩中的锆石属岩浆锆石。然而，阴极发光图像显示锆石不具有清晰的振荡环带，表明锆石受到后期重结晶作用的影响。本文认为较大年龄组锆石为捕获锆石，较小年龄组锆石为岩浆锆石；二者于2500Ma左右均发生重结晶，但重结晶作用强度不大，对锆石年龄无明显影响，所以这两组年龄仍可代表各自的原岩年龄。Algoma型BIF与火山活动关系密切，条带状铁建造与其顶底板围岩或夹层(变质火山岩) 当属同一火山-沉积事件的产物，故变质火山岩的年龄可以间接代表BIF的形成时代(Wilson et al., 1995; Zhang et al., 2011a; Zhang et al., 2011b; Cabral et al., 2012)。基于上述，本文认为较小年龄代表了斜长角闪岩原岩的形成时代，即歪头山BIF形成于2533±11Ma；在玄武质岩浆喷发过程中，还捕获了一组年龄为2610±5Ma的岩浆锆石。

表 7 华北克拉通与BIF相关岩系形成时代与变质时代统计表 Table 7 The formation and metamorphic ages of rocks associated with BIF in the North China craton

图 12 华北克拉通与BIF相关岩系形成时代与变质时代直方图 Fig. 12 The histogram of formation and metamorphic ages of rocks associated with BIF in the North China craton
5.2 构造背景

目前，对Algoma型BIF形成构造环境的主流认识有两种：岛弧与弧后盆地(Veizer, 1983) 或克拉通内部裂谷带(Gross, 1983)。Henderson (1984)认为大陆裂谷型玄武岩(La/Yb)N和(Ce/Yb)N值均大于12，最高可达20；洋壳型或过渡壳型玄武岩该值小于12。Fitton et al.(1991)认为陆内拉张带或初始裂谷玄武岩Th/Ta>4，一般为4~10。歪头山斜长角闪岩(La/Yb)N=0.80~1.10，(Ce/Yb)N=0.86~1.00，Th/Ta=1.87~2.13，表明其产出环境与大陆裂谷无关，而低的Th/Ta值正是岛弧玄武岩的特征。此外，大陆裂谷型玄武岩在稀土元素球粒陨石标准化的配分图上表现为轻稀土相对富集，重稀土相对亏损，为向右倾的曲线(Wilson, 1989; Gill, 2010)，与歪头山斜长角闪岩稀土配分型式差异很大(图 7a)。

Sm/Nd值是反映物质来源的一个重要参数，如地幔为0.260~0.375，大洋玄武岩为0.234~0.425，源于地壳的花岗岩类及各类沉积岩一般小于0.3(Wilson, 1989; Jahn et al., 1999; Winter, 2001)。歪头山斜长角闪岩Sm/Nd值介于0.324~0.331，平均为0.327，类似于地幔及大洋玄武岩。斜长角闪岩具有低的K2O含量，ΣREE为球粒陨石的10倍左右，与岛弧拉斑玄武岩相似(Wilson, 1989; Gill, 2010)。Condie (1989)认为，岛弧拉斑玄武岩具有较低的Ti/V值( < 30)，而板内玄武岩该值较大(>30)。歪头山斜长角闪岩Ti/V值18.87~21.31，平均20.25，与岛弧拉斑玄武岩一致。在稀土配分型式上，斜长角闪岩轻重稀土分馏不明显，与N-MORB差异较大，而与Mariana岛弧拉斑玄武岩(Elliott et al., 1997) 及Scotia弧后盆地玄武岩(Fretzdorff et al., 2002) 相似(图 7a)。微量元素蛛网图显示斜长角闪岩富集大离子亲石元素，其高场强元素无明显亏损(图 7b)。大离子亲石元素的富集有两种原因：与消减作用有关或是受后期蚀变影响(Ģelik et al., 2011)。微量元素蛛网图显示斜长角闪岩缺乏明显的Nb-Ta槽，与岛弧拉斑玄武岩特征不符(Wilson, 1989; Gill, 2010)，暗示其原岩与消减作用关系并不密切。3个斜长角闪岩样品主量元素测定结果具有较大的烧失量(表 2)，指示斜长角闪岩经历了较强烈的蚀变作用(Ģelik et al., 2011)。然而，斜长角闪岩样品具有非常一致的化学组成(图 6、图 7、图 13)，表明蚀变作用并未影响岩石中的难溶组分，如SiO2、REE及高场强元素。斜长角闪岩中REE及高场强元素含量基本与N-MORB相当，显示洋中脊玄武岩或弧后盆地玄武岩的特征(Fretzdorff et al., 2002)。总体而言，歪头山斜长角闪岩与Scotia弧后盆地玄武岩具有一致的地球化学特征。Falloon et al.(1992)、Gribble et al.(1996, 1998) 及Taylor and Martinez (2003)认为，弧后盆地既能产生具有洋中脊特征(MORB-like)，也能产生具有岛弧特征(Arc-like) 的玄武岩。Fretzdorff et al.(2002)与Gill (2010)认为弧后盆地玄武岩化学组成兼具岛弧拉斑玄武岩及N-MORB的特征。在TiO2-MnO-P2O5、Ti-Sr-Zr、Nb-Zr-Y构造环境判别图解上，斜长角闪岩样品落入岛弧玄武岩及MORB区域(图 13a, b, c)，表明其原岩很可能属于弧后盆地玄武岩，这一点在Y-La-Nb图解上也得到了印证(图 13d)。Zhai and Santosh (2011)研究表明，华北克拉通绿岩带变基性岩几乎均形成于岛弧或弧后盆地背景。Trendall (2002)指出，BIF的形成需要构造稳定的半深水-深水盆地，弧后盆地能够为BIF韵律条带的产生提供稳定的沉积环境。本文认为，歪头山斜长角闪岩原岩形成于弧后盆地，代表了歪头山BIF形成时的构造背景。

图 13 斜长角闪岩构造环境判别图解(底图分别据Mullen, 1983; Pearce and Cann, 1973; Meschede, 1986; Cabanis and Lecolle, 1989) Mariana ARCTH-Mariana岛弧拉斑玄武岩(Elliott et al., 1997); Scotia BABB-Scotia弧后盆地玄武岩(Fretzdorff et al., 2002) Fig. 13 Tectonic discrimination diagrams of amphibolites (the base map after Mullen (1983), Pearce and Cann (1973), Meschede (1986) and Cabanis and Lecolle (1989), respectively) Mariana ARCTH-Mariana arc tholeiite (Elliott et al., 1997); Scotia BABB-Scotia back-arc basin basalt (Fretzdorff et al., 2002)
5.3 沉积环境与物质来源

自然界中Ce一般呈稳定的+3价离子。在氧化条件下Ce被氧化为+4价，Ce+4易发生水解，从而导致REE配分曲线中的Ce负异常。因此，根据Ce异常可以有效判断古海洋的氧化还原状态。按照常规的Ce异常算法，即Ce/Ce*=2CePAAS/(LaPAAS+PrPAAS)，歪头山铁矿石出现明显的Ce负异常(Ce/Ce*=0.74~0.88)。Bau and Dulski (1996)认为常规算法下Ce负异常的出现与La正异常有关，并建立了用Ce/Ce*和Pr/Pr*来判别真正Ce负异常的图解[Pr/Pr*=2PrPAAS/(CePAAS+NdPAAS)]。如图 14所示，歪头山铁矿石均位于正La异常区域，并未落于Ce负异常区域。因此，铁矿石稀土配分图中出现的Ce负异常(图 5a) 以及按照常规算法得出的Ce/Ce* < 1，并非意味着真正的Ce负异常，而是La正异常。本文采用Bolhar et al.(2004)推荐的算法[Ce/Ce*=CePAAS/(2PrPAAS-NdPAAS)](表 1)，亦未见明显的Ce异常(Ce/Ce*=0.92~1.06)，表明铁建造沉积时海水处于缺氧环境，而缺氧环境正是BIF形成的必要条件之一(Cloud, 1973; Klein, 2005; Bekker et al., 2010)。

图 14 Ce异常判别图解(底图据Bau and Dulski, 1996) Fig. 14 Ce/Ce*-Pr/Pr* discrimination diagram for Ce anomaly (the base map after Bau and Dulski, 1996)

歪头山BIF顶底板围岩及夹层主要为斜长角闪岩，其原岩为弧后盆地玄武岩。弧后盆地玄武岩具有高的FeT含量(平均8.30%, Fretzdorff et al., 2002)，表明岛弧火山活动能够为BIF沉积事件提供充分的铁质来源。铁矿石Y/Ho值与海水显示亲缘性，其Sr/Ba值、Ti/V值又暗示成矿与火山作用有关。在PAAS标准化的稀土元素配分图上，铁矿石呈现的重稀土富集、La正异常及Y正异常特征与现代海水稀土配分型式一致(Zhang et al., 1994; Alibo and Nozaki, 1999)，同时铁矿石所具有的Eu正异常又类似于海底高温热液(Danielson et al., 1992; Bau and Dulski, 1996; Douville et al., 1999)。杨凤筠(1980)测定了歪头山矿区磁铁石英岩与围岩中黄铁矿、磁黄铁矿的硫同位素组成，磁铁石英岩中3个黄铁矿的δ34S介于-3.9‰~-3.5‰，围岩的5个硫同位素测试值在-3.7‰~3.1‰之间。二者δ34S值接近于幔源硫(δ34S=0±1‰, Eldridge et al., 1991)，暗示成矿物质具有深源特征。综上所述，本文认为歪头山BIF成矿物质可能来源于由海底火山活动带来的高温热液与海水的混合溶液。Hamade et al.(2003)利用BIF硅层的Ge/Si比值，判别出硅质主要为陆源风化产物。Heck et al.(2011)研究了太古宙与元古宙BIF中石英的硅、氧同位素，认为硅主要来自海底热液，但大陆风化来源的硅也是一个重要组成部分。歪头山铁矿石除Fe2O3T与SiO2外，其他氧化物含量均很低，同时铁矿石贫Th、U、Zr等明显具有陆源性质的元素，表明大陆碎屑物质对BIF贡献极少，与前人的观点不一致。

歪头山铁矿条带状矿石(Fe2O3T平均39.31%) 与块状矿石(Fe2O3T=72.94%) 稀土、微量配分型式颇为相似，对两种类型矿石中的磁铁矿电子探针分析结果也未见明显差异。然而，块状矿石相对条带状矿石具有较高的Al2O3、CaO含量(表 1)，且块状矿石磁铁矿的FeOT含量相对偏低(表 3)，表明有更多其它杂质的加入。此外，块状矿石具有更强的Ce负异常及相对偏小的Eu正异常(图 5a)。Danielson et al.(1992)指出，高温热液作用到低温蚀变作用的转变可以导致BIF正Eu异常的减小。本文认为，两种类型铁矿石的差异可能反映了二者并非形成于同一成矿过程，块状矿石是在条带状矿石的基础上，经后期地质事件的改造富集而成。

6 结论

本文报道了歪头山BIF铁矿石与围岩斜长角闪岩主量元素、微量元素、锆石SIMS U-Pb定年及磁铁矿电子探针分析结果，主要认识如下：

(1) 铁矿石主、微量元素特征反映成矿物质可能来源于由海底火山活动带来的高温热液与海水的混合溶液，而大陆碎屑物质对BIF贡献极少。

(2) 斜长角闪岩变质原岩为弧后盆地玄武岩，形成于2533±11Ma，代表了歪头山BIF的成矿年龄；在玄武质岩浆喷发过程中，还捕获了一组年龄为2610±5Ma的岩浆锆石。

(3) 矿石组成与组构及磁铁矿标型组分分析显示歪头山BIF属沉积变质型铁矿，且条带状矿石与块状矿石可能并非形成于同一成矿过程。

综合分析认为歪头山铁矿属Algoma型BIF，代表了新太古代末华北克拉通普遍发育的一期BIF成矿事件。

致谢 感谢中国科学院地质与地球物理研究所离子探针中心、岩矿分析实验室、微量元素实验室、电子探针与电镜实验室及国家地质实验测试中心帮助完成测试工作。
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辽宁本溪歪头山条带状铁矿的成因类型、形成时代及构造背景
代堰锫, 张连昌, 王长乐, 刘利, 崔敏利, 朱明田, 相鹏