岩石学报  2018, Vol. 34 Issue (2): 398-426   PDF    
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
彭自栋1,2,3 , 张连昌1,2 , 王长乐1,2 , 佟小雪1,2,3 , 南景博3,4     
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
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Al2O3、TiO2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。
关键词: 下甸子     BIF     地质特征     矿石成因     清原绿岩带     新太古代    
Geological features and genesis of the Neoarchean pyrite-bearing Xiadianzi BIF, Qingyuan greenstone belt
PENG ZiDong1,2,3, ZHANG LianChang1,2, WANG ChangLe1,2, TONG XiaoXue1,2,3, NAN JingBo3,4     
1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. Deep Sea Science Division, Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China
Abstract: 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 the silicate type, this might be related to the local hydrothermal activity during the precipitation. The geochemical features of amphibolites indicate they were derived from depleted mantle but obviously contaminatedby the continental crust during its emplacement, meanwhile, it also suggests these rocks formed at continental margin. Combined with previous geochronology and geochemical studies of the supracrustal rocks and TTG of QGB, we suggest the Xiadianzi BIF formed in a volcanic arc or back arc basin setting, which related to the subduction of an oceanic slab in the Neoarchean.
Key words: Xiadianzi     BIF     Geological features     Ore genesis     Qingyuan greenstone belt     Neoarchean    

位于华北克拉通北缘的清原花岗绿岩带是我国最早被识别出的太古宙绿岩带(毛德宝等, 1997),其中发育同时期的火山成因块状硫化物型(Volcanogenic Massive Sulfide, 简称VMS)Cu-Zn矿床和条带状铁建造型(Banded Iron Formation, 简称BIF)铁矿床(万渝生等, 2005a; 张雅静等, 2014; Zhu et al., 2015; 王新, 2015; 彭自栋等, 2015),这在世界上比较少见。目前,国外仅少数地区报道过VMS与BIF的共生现象,如加拿大的Abitibi(Thurston et al., 2008)、澳大利亚的Pilbara(Hollis et al., 2015)和美国的Arizona地区(Slack et al., 2007)。

通过长期的研究,现阶段国际上对BIF的认识已取得显著进展:1)时间上,BIF产出的高峰期为~27亿年、~25亿年和~19亿年,并于18亿年之后骤减并逐渐消失;2)空间上,BIF广泛分布于全球前寒武纪克拉通上,其中西澳的Hamersley盆地和南非的Transvaal盆地及加拿大的Superior地区为其代表性产区(Isley and Abbott, 1999; Huston and Logan, 2004; Bekker et al., 2010; 王长乐等, 2012);矿床类型方面,依据BIF与火山建造的关系和沉积构造环境,其被划分为Algoma型和Superior型(Gross, 1965, 1980),前者与火山岩关系密切,主要发育于太古代-古元古代的绿岩带火山-沉积序列中;而后者的围岩多为沉积岩,不含或含有极少量的火山岩,一般形成于被动大陆边缘的大陆架浅海环境,形成时代集中于古元古代;3)沉积相方面,依据沉积时的水深条件和主要含铁矿物成分差异,早期BIF被划分为四个相,即氧化物相、硅酸盐相、碳酸盐相和硫化物相(James, 1954),但随后一些学者(Fripp, 1976; Phillip et al., 1984; Groves et al., 1987; Simonson, 2003)对硫化物相中黄铁矿是否为原生沉积成因提出了质疑,另一些学者(Beukes and Gutzmer, 2008; Bekker et al., 2010)则认为硫化物相不是BIF的一种类型。

随着研究工作的进展,一些学者注意到在前寒武纪,BIF铁矿与VMS矿床在成矿时间和规模上具一致性,即均大规模产出于~27亿年和~19亿年,且与地幔柱、板块构造等一些重大地质事件密切相关,并据此推测二者之间可能存在成因上的联系(Isley and Abbott, 1999; Peter, 2003; Bekker et al., 2010; Huston et al., 2010; Planavsky et al., 2010)。然而,这两类矿床之间是否存在成因联系、存在何种联系以及如何建立二者的成因联系,是前寒武纪矿产研究的热点和难点(Goodwin, 1973; James, 1983; Zaleski and Peterson, 1995; Isley and Abbott, 1999; Peter, 2003; Bekker et al., 2010)。近期研究表明,在VMS与BIF分布区,往往存在VMS与BIF共生及BIF独立分布、以及含黄铁矿条带BIF等几种情况(Zaleski and Peterson, 1995; Peter, 2003; Lodge et al., 2015; 彭自栋等, 2015)。其中,单一BIF与伴生有VMS的BIF具有一些共同特征,如均由互层的富铁和富硅条带构成,具有相近的主量和微量元素含量。但仔细看来,二者的矿物组合、地球化学特征之间仍存在一定的差异,这些差异性可能反映了其沉积时海水组分和环境的变化以及不同的构造背景(Gross, 1983; Alexander et al., 2008)。因此,开展绿岩带内与VMS共生BIF(纯BIF与含黄铁矿条带BIF)的地质和地球化学特征的研究,一方面可以约束二者共生的海洋环境和构造背景,另一方面也为探讨二者共生机制提供信息。

本文选取下甸子BIF作为研究对象,基于详细的矿床地质特征研究,并结合岩石、矿石地球化学、BIF主要矿物电子探针成分分析以及SIMS锆石U-Pb年代学和原位氧同位素测试工作,对其形成时代、物质来源、沉积环境及构造背景开展了系统分析,期望为进一步研究该区BIF与VMS的成因联系及其与同期火山活动的关系提供依据。

1 区域与矿床地质概况 1.1 区域地质

华北克拉通是世界上著名的古老陆块,大量年代学研究表明,在鞍山地区存在3.8Ga的陆壳岩石(Liu et al., 1992; 刘敦一等, 2007; Song et al., 1996; Wan et al., 2005),冀东、鲁山、信阳、胶东等地区有2.8~3.8Ga的碎屑锆石和残余锆石(Wan et al., 2012),暗示其演化史可追溯到38亿年(Kusky et al., 2007; 翟明国, 2008)。华北克拉通基底由多个微陆块拼合而成的观点已为学者们所接受,但对于微陆块的划分方案及其演化历程仍存在较大争议(伍家善等, 1998; Zhao et al., 1998, 2001; Kusky and Li, 2003; Zhai and Santosh, 2011)。目前,占主导的有两种观点,一是认为华北克拉通基底由胶辽陆块、迁怀陆块、阜平陆块、许昌陆块、集宁陆块和阿拉善陆块这六个微陆块组成(图 1a),晚太古代末期绿岩带呈线性褶皱带围绕古老的微陆块分布,而同时期广泛发育的变质作用和壳熔花岗岩事件表明,华北于~2.5Ga完成克拉通化(翟明国, 2011; Zhai and Santosh, 2011; Zhai, 2014);二是认为华北克拉通可划分为东部陆块、西部陆块以及位于二者之间的中央造山带,这一观点主要基于古元古代构造事件及其相关岩石组合、岩石地球化学特征和区域变质作用P-T轨迹的研究(Zhao et al., 2001; Wilde et al., 2004; Santosh et al., 2010)。

图 1 研究区地质简图 (a)华北克拉通基底构造单元划分(据伍家善等, 1998; 翟明国, 2011修改); (b)清原地区区域地质及VMS、BIF分布简图(据Gu et al., 2007; Zhu et al., 2015修改) Fig. 1 Simplified geological map of study area (a) tectonic subdivision of the North China Craton (after Wu et al., 1998; Zhai, 2011); (b) geological sketch and identified VMS and BIF deposits of the QGB (after Gu et al., 2007; Zhu et al., 2015)

清原花岗绿岩带位于华北克拉通北缘(图 1a),辽宁东北部和吉林南部,面积约25000km2。早期对于清原绿岩带的限定曾存在一定分歧,认为该区存在中太古代高级区和晚太古代绿岩带两种不同地壳类型。以浑河断裂为界,沈保丰等(1993, 1994)将浑河以北称为绿岩带浑河以南称为高级区,毛德宝等(1997)将浑河以南部分称为小莱河花岗-绿岩带。小莱河地区斜长角闪岩2.99Ga的40Ar/39Ar年龄(王松山等, 1987)和3.02Ga的全岩等时线年龄(李俊建和沈保丰, 2000)是清原地区存在中太古代地质体的主要依据。而万渝生等(2005a, b)对浑北和浑南地区的表壳岩以及浑南的TTG花岗岩的年代学研究表明,该区太古代表壳岩系主体形成于2.56~2.51Ga,其后TTG花岗岩侵位,2.52~2.47Ga期间表壳岩系和TTG花岗岩发生强烈变质变形和深熔作用改造,据此他们认为浑河断裂不具备划分太古宙不同时代基底的构造意义,即浑河南北两侧太古宙变质岩在形成时代、岩石组合等方面可以对比,主体均形成于新太古代。

清原地区的太古宙岩系主体由三类岩石组合构成,约70%的TTG片麻岩、同构造花岗闪长岩和表壳岩(图 1b),其中表壳岩由变质的超镁铁-长英质火山岩和沉积岩组成,变质程度为角闪岩相,局部至麻粒岩相(翟明国等, 1984; Zhai et al., 1985; 于凤金, 2006; 张连昌等, 2012; Peng et al., 2015)。前人一般将清原太古代绿岩地层自下而上划分为石棚子组、红透山组和南天门组,各组地层均呈不整合接触(刘宇光, 1982; 翟明国等, 1984; 沈保丰等, 1994; 于凤金, 2006)(图 2)。其中,石棚子组地层广泛分布在浑南汤图至后安河一带,在浑北仅于混合花岗岩中呈大小不等的残留体存在,可细分为石棚子组一段和二段,其岩性包括麻粒岩、片麻岩、变粒岩、斜长角闪岩及少量磁铁石英岩等,原岩主要为拉斑玄武质超基性-基性火山岩,夹中-酸性火山岩,上部具双峰式特征;红透山组主要发育于浑北地区,在浑南通什村地区少量出露,自下而上依次为树基沟段、红透山段和大荒沟段,主要岩性有黑云变粒岩、斜长角闪岩、片麻岩及浅粒岩,局部可见少量的磁铁石英岩,其原岩为一套多旋回的拉斑玄武岩、玄武质安山岩、英安岩及少量流纹岩等钙碱性火山岩、火山沉积岩(夹正常沉积岩)组成;南天门组地层大面积出露于清原东部的斗虎沟-南天门-曾家顶子一带,根据其岩性组合可大致分为上下两段,下段主要以斜长角闪岩夹磁铁石英岩为主,上段为一套包括绢云石英片岩、绢云绿泥片岩、石英岩和大理岩的沉积岩组合,其原岩主要为基性火山岩、火山沉积岩及正常沉积岩;区内VMS矿床主要产于红透山组,代表性矿床为红透山Cu-Zn矿床;BIF矿床主要分布于石棚子组和南天门组,少量位于红透山组,其中规模较大的有小莱河铁矿、下甸子铁矿等(辽宁省地质矿产局, 1989; 沈保丰等, 1994; 于凤金和王恩德, 2005; 于凤金, 2006)。

图 2 清原绿岩带表壳岩系综合地层柱状图(据于凤金, 2006; 张雅静, 2014修改) 本文给出了各组的锆石U-Pb同位素年龄 Fig. 2 Stratigraphic column for supracrustal rocks in the Qingyuan greenstone belt (after Yu, 2006 and Zhang, 2014) Representative U-Pb zircon formation ages from each group are also provided
1.2 矿床地质

下甸子铁矿位于清原县北西枸乃甸乡(图 1b),区内出露地层主要为新太古代南天门组,其整体呈单斜构造,走向为北西向,倾向以北东向为主。南天门组各层位间呈整合接触,由下至上依次为:下甸子层,主要为斜长角闪岩和磁铁石英岩;曾家顶层,以白云质大理岩为主,夹少量石英岩、斜长角闪片麻岩、黑云斜长片麻岩;斗虎沟层和龙王庙层,以角闪斜长片麻岩为主,夹斜长角闪岩、黑云斜长片麻岩及少量的角闪磁铁石英片岩(图 2; 张雅静, 2014)。区内出露的岩浆岩较少,除太古代TTG岩系外,可见到中生代伟晶岩、闪长玢岩、煌斑岩、正长斑岩、辉绿岩等呈岩脉侵入到太古代表壳岩和TTG内。下甸子铁矿赋存于下甸子层上部,受层位控制,其含铁岩系主要为斜长角闪岩、磁铁石英岩,局部可见到少量石榴云母片岩以夹层形式产出(图 3)。

图 3 下甸子BIF地质剖面图及两种类型矿石局部素描图 Fig. 3 Sketch profile of the Xiadianzi BIF, showing relationships between BIF and associated lithologies as well as structures of representative ores

主要矿体有北部的Ⅰ号矿体和南部的Ⅱ号矿体,其中Ⅰ号矿体为隐伏矿体,Ⅱ号矿体地表出露较好,为本次工作的重点。Ⅱ号矿体整体呈似层状-层状、连续性好,总体走向324°,倾向北东,倾角50°~70°,局部近直立(图 3图 4a, b),地表延伸长度达400m,矿体厚度约1.5~10m,平均为3.5m。矿体整体变形较弱,局部可见较强的揉皱(图 4c),近地表矿石有一定的氧化,见赤铁矿和褐铁矿,矿体与围岩斜长角闪岩的接触界线截然,靠近矿体部分的斜长角闪岩发育轻微片理化,矿体上盘可见到斜长角闪岩与BIF互层现象(图 4d)。

图 4 下甸子BIF野外照片 (a)陡倾的BIF与斜长角闪岩整合接触;(b)受后期构造作用影响近直立的BIF;(c) BIF中局部层位富铁条带和富硅条带条带强烈揉皱,其上下部条带依然平直;(d) BIF与斜长角闪岩互层且接触界线截然.图中地质锤柄长30cm Fig. 4 Representative field photographs of the Xiadianzi BIF (a) conformable contact between steeply NE-dipping BIF and plagioclase amphibolite; (b) nearly vertical distributed BIF formed by post-depositional deformation; (c) intraformational bending deformation of iron-and silica-rich bands; (d) comfortable contact between alternating BIF and plagioclase amphibolite thin layers. The length of the hammer is 30cm
2 样品采集与测试方法

基于对下甸子铁矿Ⅱ号矿体及其夹层、围岩详细的野外地质调查,分别采集了不同层位、具代表性的新鲜BIF样品及夹层和上下盘的新鲜围岩样品(采样位置见图 3)。通过对所采取的样品进行清洗、磨片和显微镜下岩相学鉴定,甄选出无蚀变的围岩样品及无氧化、无蚀变的BIF样品,采用钨钢研磨仪将其磨至200目,用于全岩主、微量及稀土元素分析。另外挑选矿体夹层斜长角闪岩样品~5kg进行锆石分选。

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,采用Phillips PW 2404型X荧光光谱仪分析,分析误差优于5%;围岩样品主量测试于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,采用熔片XRF分析法,使用XRF-1500型X荧光光谱仪测试,分析误差优于5%。

围岩及矿石全岩的微量和稀土元素分析工作在中国科学院地质与地球物理研究所微量元素分析实验室完成,采用酸溶法制备样品,然后使用Finnigan Element型ICP-MS进行上机测试,当元素含量大于10×10-6时,分析精度优于5%,当含量小于10×10-6时,精度优于10%。

矿物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与电镜实验室,采用JEOL JXA-8100型电子探针仪完成,工作电压为15kV,加速电流为20nA,束斑直径2~5μm,以天然样品和人工合成氧化物为标准样品,分析精度优于2%。

锆石分选在河北省廊坊市宇能岩石矿物分选技术服务有限公司完成,于双目镜下将透明、无包裹体、少裂的锆石颗粒用双面胶粘于载玻片上,然后使用PVC环及环氧树脂和固化剂进行制靶。透、反射光照相和阴极发光图像分析在中国地质大学(北京)矿物标型实验室完成。锆石U-Pb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),测试束斑为20μm×30μm。分析过程中,采用Qinghu锆石标样作为外标,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)。

锆石微区原位氧同位素分析同样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,首先将做过U-Pb定年的样品靶再次磨去约5μm,以消除前期U-Pb定年时造成的氧同位素污染,详细测试方法参考Li et al. (2010)。仪器质量分馏校正采用Penglai锆石标样,分析过程中每个样品点分析采集20组数据,单组18O/16O数据内精度一般优于0.2‰~0.3‰(1σ)。

3 岩相学特征 3.1 BIF及其矿物化学成分

下甸子BIF一般呈微条带-中等条带状构造,矿石由灰黑色富铁条带和灰白色富硅条带交互形成韵律层,条带宽度2~10mm不等。依据产出的相对空间位置及矿物组合差异,矿石类型可划分为下部和上部的硅酸盐型及中部的含黄铁矿条带型。硅酸盐型为其主要矿石类型,这类矿石通常由~10mm宽的富铁条带和~2mm宽的富硅条带构成,其富铁条带主要矿物有磁铁矿、铁阳起石和石英,部分样品局部可见少量的绿泥石,富硅条带以石英为主,含少量磁铁矿、铁阳起石(图 5a);含黄铁矿条带型与James (1954)定义的硫化物相有显著差异,后者主要指黄铁矿化炭质页岩或板岩,其主要矿物组合为黄铁矿、磁黄铁矿、燧石和菱铁矿,其中黄铁矿含量可达30%~40%,而下甸子矿区含黄铁矿条带型矿石通常由三类宽度、成分差异显著的条带互层构成,其一是宽2~8mm不等的以磁铁矿为主含少量阳起石、石英的富铁条带,其二为宽2~5mm以石英为主含少量磁铁矿、阳起石的富硅条带,其三是宽约1~2mm以黄铁矿为主含微量磁黄铁矿、黄铜矿的硫化物条带,样品整体硫化物含量不超过10%(图 5b)。两类矿石的条带一般平直连续,偶尔可见有揉皱现象,在含黄铁矿条带矿石中未见有黄铁矿条带穿切铁条带、硅条带现象,初步认为其可能为同沉积成因。

图 5 下甸子BIF手标本及显微镜下照片 (a)条带平直的硅酸盐型BIF手标本;(b)含黄铁矿条带型BIF手标本,为了便于观察图中勾勒出部分黄铁矿条带;(c、d)较纯净的硅铁条带,其中部分铁条带的边部可见少量铁阳起石(FA),内部可见少量细粒、半自形黄铁矿(Py) (c为单偏光,d为反射光);(e)硅酸盐型矿石中含大量铁阳起石的富铁条带,铁阳起石聚集成微条带或团簇,其内部可见少量细粒的磁铁矿(Mag) (单偏光);(f)含黄铁矿条带型矿石中富硅条带被共生的方解石(Cal)和阳起石(Act)切穿(正交光);(g)铁条带外围大颗粒的半自形-自形黄铁矿聚集成黄铁矿条带,部分铁条带内可见少量半自形黄铁矿(反射光);(h)半自形-他形的黄铁矿颗粒和磁铁矿颗粒边部接触界线平直,可见到黄铁矿重结晶形成的三结点(反射光) Fig. 5 Field photos and micro-photographs of the Xiadianzi BIF (a, b) field photos of silicate-and pyrite-bearing-facies BIFs; (c, d) pure alternating iron-and silica-rich bands, showing few ferroactinolite and pyrite preserved along the iron-rich layers (c: plane-polarized light; d: reflected light); (e) iron-rich microband of silicate-facies BIF with huge amount of scattered ferroactinolite, locally magnetite inclusions occurring within the ferroactinolite (plane-polarized light); (f) silica-rich microband of pyrite-bearing-facies BIF with calcite and intergrown actinolite cutting across quartz (crossed-polarized light); (g) nearly continuous pyrite mircoband composed of large subhedral-euhedral crystals occurring parallel to the iron-rich bands, locally few subhedral pyrite grains occurring within the iron-rich bands; (h) subhedral-anhedral pyrite grains display a sharp and flat contact with the magnetite crystals, locally triple junctions of pyrite grains were also preserved (reflected light)

硅酸盐型矿石单个富铁条带由更细的(宽约0.05~0.1mm)磁铁矿、石英和铁阳起石条带交替互层构成,其矿石中石英约占40%~60%,磁铁矿约占30%~40%,铁阳起石约占10%~20%,绿泥石 < 1%,局部可见由重结晶作用形成的粒度差异较大的磁铁矿(图 5c-e);含黄铁矿条带矿石中黄铁矿呈半自形-自形颗粒分布于磁铁矿条带的外围,与磁铁矿接触界线近平直,矿石中石英含量约40%~50%,磁铁矿30%~35%,阳起石10%~15%,黄铁矿5%~10%,方解石~5%(图 5f, g)。

3.1.1 磁铁矿

磁铁矿是下甸子BIF的主要铁氧化物矿物,它主要以三类形式分布:一类是呈粒状聚集成连续的条带(磁铁矿占条带矿物总量的60%以上)(图 5c),另一类是呈星点状分布于富硅条带石英颗粒间或包裹于石英颗粒内部(占条带矿物总量的10%左右)(图 5f),此外,部分富硅酸盐条带中可见到少量的磁铁矿以包裹体形式分布于铁阳起石-阳起石内(占条带矿物总量的10%左右)(图 5e)。上述三类磁铁矿均结晶较好,呈半自形-自形、粒度5~100μm不等。

下甸子BIF两类矿石中的磁铁矿单矿物电子探针分析显示,磁铁矿成分基本一致,FeOT平均含量为92.6%,其他组分如SiO2、TiO2、Al2O3、MnO等含量均极低(0.02%~0.03%)。将下甸子BIF磁铁矿单矿物成分与华北克拉通典型Algoma型BIF,如冀东水厂BIF(张晓静, 2011)、鞍本地区BIF(代堰锫, 2014)、五台王家庄BIF(Wang et al., 2014a)进行比较,结果显示下甸子BIF磁铁矿成分与鞍本地区BIF基本一致,均具有极高的FeOT和极低的SiO2、TiO2、Al2O3、MnO含量;不同的是,水厂BIF具有最低的FeOT含量(平均为90.3%),推测可能与其变质程度较高有关(张晓静, 2011);王家庄BIF具较高的TiO2、Al2O3含量,这与其沉积过程中有较多的陆源组分的输入有关(Wang et al., 2014a)(表 1)。

表 1 下甸子BIF磁铁矿电子探针分析结果(wt%) Table 1 Electron probe microanalyses of magnetite from the Xiadianzi BIF (wt%)
3.1.2 石英

石英是BIF各沉积相中普遍发育的一类矿物,通常被认为是BIF沉淀早期形成的燧石在后期变质作用中重结晶的产物(Klein, 2005)。下甸子BIF两类矿石中均含有大量的石英(约40%~60%),在富铁条带中石英含量较低(10%~20%),且粒径较细(约10~50μm),多呈浸染状分布或被闪石矿物所包裹(图 5e);而富硅条带中石英含量明显升高(80%~90%),一般呈粒径不等(50~200μm)的颗粒聚集成连续的条带,其粒间可见到重结晶作用形成的稳定的三边结构,局部可见方解石和阳起石构成的微条带切穿富硅条带的现象(图 5f)。

3.1.3 铁阳起石和阳起石

阳起石为中级变质作用条件下BIF中常见的一类富钙的角闪石(Klein, 2005),下甸子BIF中这类矿物分布广泛,在其富铁和富硅条带中均有出现。阳起石呈浅绿色-深绿色或褐黄色,具正中-高凸起,在富铁条带中多呈针柱状,聚集成团簇状或呈不连续的微条带(图 5e),在富硅条带中阳起石则多呈针柱状、纤维状与碳酸盐矿物(方解石)密切共生(图 5f)。

电子探针分析显示,下甸子BIF中阳起石的成分有着明显差异(表 2),可进一步将其细分为铁阳起石和阳起石。铁阳起石主要分布于硅酸盐相BIF中,其FeOT平均含量为28.3%,MgO平均含量为5.07%,CaO平均含量为11.2%,Mg/(Mg+Fe2+)比值平均为0.29;阳起石则常见于含黄铁矿条带BIF中,其FeO、MgO、CaO平均含量分别为12.0%,15.4%,11.7%,Mg/(Mg+Fe2+)比值介于0.78~0.89之间。相对而言,分布于含黄铁矿条带BIF中的阳起石更为富Mg,可能为其沉淀过程中,受到与火山活动近同期的富Mg热液影响(Duuring et al., 2016)。

表 2 下甸子BIF中的铁阳起石和阳起石电子探针分析结果(wt%) Table 2 Electron probe microanalyses of ferroactinolite and actinolite from the Xiadianzi BIF (wt%)
3.1.4 方解石

方解石主要分布于下甸子BIF含黄铁矿条带型矿石中,其含量相对较低(约5%),在矿石中它主要与阳起石共生。分析表明,方解石中除了CaO含量(54.7%~56.6%)最高外,较高的为FeOT的含量(0.95%~2.05%),其余组分MnO和MgO的含量均较低(分别为0.30%~0.56%和0.17%~0.38%),而SiO2、TiO2、Al2O3等的含量均极低(表 3)。在BIF富硅条带内,共生的方解石和阳起石周围的石英明显减少,局部可见方解石与阳起石构成的微条带切穿富硅条带现象(图 5f),表明阳起石和方解石可能为早期的铁白云石与石英反应的产物(Klein, 2005)。

表 3 下甸子BIF方解石电子探针分析结果(wt%) Table 3 Electron probe microanalyses of calcite from the Xiadianzi BIF (wt%)
3.1.5 黄铁矿

黄铁矿是下甸子BIF含黄铁矿条带型矿石的重要组分,而在硅酸盐相矿石中含量极低,仅部分样品内可见到零星分布的自形黄铁矿颗粒(图 5d)。含黄铁矿条带型矿石中黄铁矿颗粒较粗,粒径可达100~300μm一般大于其他矿物颗粒(如磁铁矿、石英、阳起石等)。黄铁矿主要以两类形式分布,一类是呈半自形-自形粒状(晶形以五角十二面体居多)聚集成条带(条带宽约200~300μm),分布于磁铁矿微条带的两侧;另一类是呈自形粒状(多为立方体)零散分布于磁铁矿条带内(图 5g)。BIF中的黄铁矿与磁铁矿接触界面平直,未见有交代现象,部分黄铁矿内可见到被包裹的黄铜矿(图 5h)。前人关于BIF中的黄铁矿的成因研究工作相对较少,Berner (1970)对显生宙及现代海洋沉积物的研究认为,黄铁矿的原始沉积矿物可能为马基诺矿[Fe(1+x)S]或硫复铁矿(Fe3S4),他们在后期的成岩作用过程中转化为黄铁矿(FeS2);Ayres(1972)对澳大利亚Harmsley盆地中Dales Gorge IF的研究显示,其BIF的黑硬绿泥石条带中有一定含量的黄铁矿,这些矿黄铁矿多为半自形-自形、细-粗粒,或呈浸染状分布或聚集成微条带,其与磁铁矿接触界线平直且局部可见到细粒黄铁矿被自形的磁铁矿所包裹,根据这些现象他认为黄铁矿应为原始沉积成因或至少早于这些重结晶形成的自形磁铁矿;而Garrels and Christ (1965)Klein (2005)通过溶液和矿物达到平衡时的相分析认为,在还原性较强的环境中黄铁矿可与磁铁矿共生。

3.2 斜长角闪岩及石榴云母片岩

斜长角闪岩是下甸子BIF的主要围岩,通常呈灰绿-深灰色,块状构造。主要矿物成分为角闪石(70%~80%)、斜长石(15%~25%)、石英(2%~3%)及少量的绿泥石(< 1%~2%),具纤状变晶结构(图 6a)。角闪石多为绿色或褐色,呈柱状或粒状,粒径0.1~1mm不等,多色性显著,部分角闪石可见其横截面菱形解理,整体呈略定向分布,未见有明显蚀变现象;斜长石呈板状-片状,粒径约0.1mm,分布于角闪石颗粒间;石英含量明显低于角闪石,呈他形粒状(粒径多小于0.1mm)零星分布(图 6b)。由于区内岩石均经历过较为强烈的变质作用,基本已无法见到其原始结构构造,结合前人研究,其原岩应为中-基性火山岩(沈保丰等, 1989; 张雅静, 2014; Peng et al., 2015)。

图 6 下甸子BIF围岩手标本及显微镜下照片 (a)斜长角闪岩手标本;(b)结晶粒度较大的角闪石(Am)略定向分布,颗粒细小的斜长石(Pl)、石英(Q)分布于角闪石之间(单偏光);(c)石榴云母片岩,可见其与BIF呈整合接触,图中磁性笔长约20cm;(d)粒度较大的石榴子石(Grt)外围的细粒的黑云母(Bi)略定向且发生明显的绿泥石化,石英分布于前二者之间(单偏光) Fig. 6 Field photos and micro-photographs of associated rocks from the Xiadianzi BIF (a) Field photo of plagioclase amphibolite; (b) corase-grained euhedral amphibolite shows a preferred crystallographic orientation, while fine-grained plagioclase and quartz filled in the space between amphibolite grains(plane-polarized light); (c) garnet mica schist showing comfortable contact with BIF, the length of the magnetic pen is about 20cm; (d) fine-grained biotite showing moderate to weak shape preferred orientation and distributed around large garnet grain, the biotite grains undergone obviously chloritization, anhedral quartz crystals showing scattered distribution patterns(plane-polarized light)

石榴云母片岩在区内出露极少,仅在局部段可见到其作为BIF夹层(厚约~1m)产出。岩石为灰白色,具片状构造,片理面呈波浪状(图 6c)。主要矿物成分为黑云母(40%~60%)、石英(25%~35%)和石榴子石(15%~25%),具斑状变晶结构,变斑晶为石榴子石,基质为黑云母、石英及极少量的斜长石,基质具鳞片变晶结构。黑云母呈细小的片状(~0.2mm)分布于石榴子石(~1mm)两侧,石英呈半自形-他形粒状(1~2mm)充填于二者之间(图 6d)。

4 分析结果 4.1 主微量地球化学 4.1.1 BIF

下甸子BIF主量、微量元素分析结果见表 4,其中硅酸盐型铁矿石SiO2含量为33.8%~48.5%,平均为43.4%;Fe2O3T含量为44.6%~60.6%,平均为51.7%;CaO含量为0.60%~2.76%,平均为1.80%;MgO含量为0.08%~2.14%,平均为1.17;Al2O3含量为0.11%~1.64%,平均为0.91%,TiO2、MnO、Na2O、K2O、P2O5平均含量均较低,小于1%。含黄铁矿条带型铁矿石SiO2含量为44.5%~60.7%,平均为52.3%;Fe2O3T含量为34.8%~49.1%,平均为42.0%;SO3含量为1.93%~2.56%,平均为2.26%;CaO含量为0.66%~2.50%,平均为1.70%;其他组分如Al2O3、TiO2、MnO、MgO、Na2O、K2O、P2O5平均含量均较低,未超过1%。对比两类矿石的主量成分可以看出,含黄铁矿条带型矿石具有高的S和SiO2含量及较低Fe2O3、Al2O3、MgO含量,两类矿石中TiO2、MnO、Na2O、K2O、P2O5含量均较低(小于1%)。

表 4 下甸子BIF主量元素(wt%)和微量元素(×10-6)分析结果 Table 4 Major elements (wt%) and selected trace elements (×10-6) of the Xiadianzi BIF

大量BIF主量成分的对比性研究显示,全球范围内不同时间、类型、产地,且遭受不同等级变质作用的BIF,除挥发分(尤其是CO2和H2O)会有一定差异外,其余氧化物含量基本不受产出时间、地点和类型以及区域变质作用的影响,且均在一个相对较窄的范围内变化(Klein, 1973, 2005)。将下甸子BIF两类矿石与世界其他典型BIF对比(图 7),发现除含黄铁矿条带型矿石具一定的硫含量外,它们的主要的氧化物含量均无明显差异。

图 7 下甸子BIF与世界典型BIF主量元素含量对比图(据Klein, 2005修改) 图中灰色区域据世界典型BIF共215个样品主量数据绘制,包括Isua、Yilgarn、Montana、Nova Lima Group BIF、Marra Mamba、Dates Gorge、Joffre Member、Kuruman、Griquatown、Biwabik和Labrador;图中所有数据扣除烧失量后以100%计算,其中下甸子硫化物相BIF扣除了烧失量和SO3含量 Fig. 7 Comparison on major elements between the Xiadianzi BIF and other typical BIFs worldwide (after Klein, 2005) The gray area is ploted according to the geochemical results of 215 representative BIFs worldwide, including Isua, Yilgarn, Montana, Nova Lima Group BIF, Marra Mamba, Dates Gorge, Joffre Member, Kuruman, Griquatown, Biwabik and Labrador; all the data are recalculated as 100% after deducted the LOI, for the Xiadianzi BIF the content of SO3 is also deducted

鉴于Y的离子半径和化学性质与Ho相似(Henderson, 1984; Bau and Dulski, 1996, 1999),故将其插入到Dy和Ho之间一起讨论。硅酸盐相矿石稀土总量(ΣREE+Y)为10.2×10-6~22.7×10-6,平均为17.3×10-6,含黄铁矿条带型矿石稀土总量为13.5×10-6~21.0×10-6,平均为16.7×10-6;两类矿石的Y/Ho比值无明显差异,介于28.2~30.4之间,平均为29.2,远小于海水的Y/Ho比值44~74,更为接近陆地岩石与球粒陨石的Y/Ho比值26(Lee and Byrne, 1993; Nozaki et al., 1997; Bau and Dulski, 1999; Bolhar et al., 2004)。PAAS(Post Archean Australian Shale; McLennan, 1989)标准化后,两类矿石显示近似的稀土配分模式(图 8),即轻稀土亏损、重稀土富集[(La/Yb)PAAS=0.10~0.65,平均0.25],且有明显的La正异常[(La/La*)PAAS=1.16~3.11,平均为1.56]、Y正异常[(Y/Y*)PAAS=1.08~1.19,平均为1.14]和显著的Eu正异常[(Eu/Eu*)PAAS=1.53~2.74,平均为2.10],此外所有样品都具有微弱的负Ce异常[(Ce/Ce*)PAAS=0.72~0.83,平均为0.77]。与世界上典型的Isua Algoma型BIF和Dales Gorge Superior型BIF相比较,下甸子BIF具有与前者相似的稀土配分模式,但不同的是其Y/Ho值明显偏低(图 8)。

图 8 下甸子BIF和国外Dales Gorge、Isua铁建造PAAS标准化稀土元素配分图(a)和现代海水、高温热液及低温热液PAAS标准化稀土元素配分图(b) (标准化值据McLennan, 1989) (a)中Dales Gorge IF位于澳大利亚Hamersley盆地为Superior型(Pecoits et al., 2009),Isua IF位于格陵兰地区Isua绿岩带为Algoma型(Bolhar et al., 2004);(b)中SPW (South Pacific Water为南太平洋0.9m和30m深的海水平均值),NPDW (North Pacific Deep Water为北太平洋2500m深的海水),高温和低温流体取自大西洋洋中脊黑烟囱附近(Bau and Dulski, 1999; Bolhar et al., 2004; Bolhar and Van Kranendonk, 2007) Fig. 8 PAAS-normalized REE+Y diagrams for the Xiadianzi, Dales Gorge, and Isua BIFs (a) as well as modern seawater and high-T and low-T marine hydrothermal fluids (b)(normalized values after McLennan, 1989) (a)Dales Gorge(Superior-type) and Isua(Algoma-type)BIFs are located on the Hamersley basin, Australian and Isua greenstone belt, Greenland, respectively(Bolhar et al., 2004); (b)South Pacific Water(SPW, average of 0.9m and 30m depth samples), North Pacific Deep Water(NPDW at depth 2500m), high-T and low-T hydrothermal fluids were sampled from mid-oceanic ridge black smokers of the Atlantic(Bau and Dulski, 1999; Bolhar et al., 2004; Bolhar and Van Kranendonk, 2007)

两类矿石微量元素含量无明显差异,一般小于10×10-6;高场强元素,如Zr、Hf、Th、U等的含量尤其低,通常小于1×10-6;Cr、Ni、Cu、Zn、Sr含量较高,且变化范围大,分别为211×10-6~363×10-6、3.57×10-6~17.9×10-6、0.76×10-6~49.1×10-6、16.4×10-6~58.5×10-6、3.56×10-6~11.9×10-6

4.1.2 斜长角闪岩

下甸子BIF围岩斜长角闪岩主微量分析结果见表 5。斜长角闪岩SiO2含量为50.7%~52.3%,平均为51.5%;Al2O3含量为13.4%~14.8%,平均为14.1%;Fe2O3T含量为10.1%~12.4%,平均为11.7%;MgO含量为7.89%~8.67%,平均为8.37%;CaO含量为6.36%~8.23%,平均为7.45%;Na2O含量为2.87%~4.16%,平均为3.42%;TiO2、MnO、K2O及P2O5含量较低,均小于1%。

表 5 下甸子斜长角闪岩主量元素(wt%)和微量元素(×10-6)分析结果 Table 5 Major elements (wt%) and selected trace elements (×10-6) of plagioclase amphibolites associated with the Xiadianzi BIF

斜长角闪岩稀土元素总量(∑REE)为28.4×10-6~77.6×10-6,平均为53.4×10-6;经球粒陨石标准化后稀土配分模式显示轻稀土(LREE)相对于中稀土(MREE)和重稀土(HREE)略富集[(La/Sm)N=1.59~2.03,平均为1.80;(La/Yb)N=1.92~5.17,平均为3.46] (图 9a),不具备Ce异常[(Ce/Ce*)=0.88~1.01,平均为0.97]和Eu异常[(Eu/Eu*)=0.81~1.11,平均为0.97]。在原始地幔标准化微量元素蛛网图中,Th、Nb明显亏损,但Ti、Zr、Hf等高场强元素均无明显异常,其蛛网图配分模式介于N-MORB与E-MORB之间,与前者更为相似(图 9b)。

图 9 下甸子BIF围岩球粒陨石标准化稀土元素配分图(a)和原始地幔标准化微量元素蛛网图(b) (标准化值及N-MORB、E-MORB、OIB值据Sun and McDonough, 1989) Fig. 9 Chondrite-normalized REE diagram(a) and primitive mantle-normalized multielement diagram(b)for associated rocks of the Xiadianzi BIF(normalizing values and N-MORB, E-MORB as well as OIB values after Sun and McDonough, 1989)
4.2 锆石U-Pb年代学及原位氧同位素分析

U-Pb体系封闭温度高、半衰期长,因此锆石U-Pb年龄能够为前寒武纪地质研究提供最为精确的年代学数据(Valley et al., 1994)。然而,鉴于锆石成因的复杂性,限定锆石的形成环境,正确判断锆石成因类型,对于锆石年龄地质意义的合理解释至关重要(吴元保等, 2004)。本次分析工作的锆石选自下甸子BIF矿体夹层中的斜长角闪岩,显微镜下可见锆石多数透明,呈无色-淡紫色;阴极发光图像显示,锆石颗粒多呈浑圆状、补丁状,粒径约70~120μm(图 10),除个别颗粒外,本次测试的锆石普遍缺乏清晰的震荡环带,推测其可能遭受后期强烈的变质作用影响已发生重结晶,应为典型的变质锆石(Hoskin and Schaltegger, 2003)。

图 10 下甸子斜长角闪岩锆石U-Pb谐和图及代表性锆石阴极发光图像、207Pb/206Pb年龄、δ18O值 括号外数值为锆石年龄,括号内数值为锆石δ18O值 Fig. 10 Concordia diagram of SIMS zircon U-Pb data and representative cathodoluminescence images, 207Pb/206Pb ages as well as δ18O values from plagioclase amphibolites associated with the Xiadianzi BIF

本次测试的锆石U含量为162×10-6~460×10-6,平均为240×10-6;Th含量为2×10-6~17×10-6,平均为9×10-6;Th/U值介于0.01~0.06,平均为0.04,极低的Th/U值进一步印证了锆石的变质成因。13个分析点的207Pb/206Pb年龄介于2487~2505Ma之间(表 6),构成一条不一致线,上交点年龄为2497.8±7.4Ma(MSWD=0.39),其中位于谐和线上的10个点的加权平均年龄为2496.7±3.4Ma(MSWD=0.8)(图 10),二者在误差范围内一致,应为斜长角闪岩遭受变质作用的年龄。

表 6 下甸子斜长角闪岩SIMS锆石U-Pb测年及原位氧同位素分析结果 Table 6 SIMS U-Pb and in situ oxygen analytical results of zircon from plagioclase amphibolites associated with the Xiadianzi BIF

13个锆石测点的原位氧同位素分析见表 6。斜长角闪岩变质锆石的δ18O值为5.3‰~6.2‰,平均为5.9±0.3‰(2SD),与现代地幔δ18O值(5.3±0.6‰; 2SD; Valley et al., 1998)基本一致,且均落入太古代火成岩锆石δ18O值范围内(5.0‰~7.5‰; Valley et al., 2005)(图 11)。

图 11 下甸子斜长角闪岩锆石δ18O值 现代地幔数据来自Valley et al., 1998; 太古代火成岩锆石数据来自Valley et al., 2005 Fig. 11 δ18O values for zircon grains from plagioclase amphibolites of the Xiadianzi BIF Data for modern mantle and Archean igneous rocks sourced from Valley et al., 1998 and Valley et al., 2005, respectively
5 讨论 5.1 成矿与变质时代

Algoma型BIF一般与火山活动关系密切,且常以火山岩(变火山岩)为赋矿围岩,故其围岩的形成、变质年龄能够为BIF的成矿和变质时代提供间接约束(Trendall et al., 1998; Zhang et al., 2012)。下甸子BIF年代学样品均采自矿体夹层斜长角闪岩,锆石U-Pb年龄测试获得了一组2497.8±7.4Ma的谐和年龄。然而,锆石CL图像下的形态和结构特征(整体为补丁状,无核幔结构)及低的Th/U比值(0.01~0.06)均显示其应为变质成因,因此该年龄为斜长角闪岩的变质年龄,也近似代表下甸子BIF遭受变质作用改造的时代。近期大量的高精度锆石U-Pb年代学研究工作表明,清原绿岩带表壳岩系主体形成于2.57~2.53Ga(万渝生等, 2005a, b; Zhu et al., 2015; Peng et al., 2015; Wang et al., 2016);同时,Wu et al. (2016)对下甸子BIF矿区附近南天门组龙王庙层(下甸子层上部层位?)黑云斜长片麻岩的SHRIMP锆石U-Pb年龄测试结果显示其形成于~2.55Ga(图 2)。综上,下甸子BIF的成矿时代可能为~2.55Ga,而其变质作用改造发生于~2.50Ga,该变质年龄与区域变质作用的时间一致,反映了华北克拉通在25亿年前后发生的古陆块拼合与克拉通化事件。

锆石原位氧同位素分析显示,角闪岩变质锆石的δ18O值平均为5.9±0.3‰(2SD),接近于现代地幔的δ18O值(5.3±0.6‰; 2SD),但明显低于太古代沉积岩的δ18O值(9‰~12‰; Shieh and Schwarcz, 1978)。实验岩石学和理论研究认为(Fortier and Giletti, 1989; Cherniak et al., 1997; Watson and Chermiak, 1997; Zheng and Fu, 1998),在以下两种情况下会导致锆石氧同位素体系的再造和破坏:(1)长达数十百万年的高温(700~800℃)热液蚀变;(2)锆石的放射性损伤。前人(沈保丰等, 1994; 于凤金, 2006; 张雅静, 2014)资料显示,在下甸子地区不存在长期的高温热液蚀变作用,而且本次测试的锆石普遍具有低的Th、U含量及较高的U-Pb年龄谐和度,表明其未受到放射性损伤;此外,变质地体的研究也证明,在经历了强烈的角闪岩相-麻粒岩相区域变质作用后,锆石仍能保存其原始的氧同位素特征而不发生明显变化(Wilde et al., 2001; Bindeman and Valley, 2000)。综上,在后期变质作用过程中下甸子斜长角闪岩的锆石氧同位素体系保持稳定,其氧同位素值反映了原始岩浆特征。

5.2 含黄铁矿条带BIF的成因

下甸子BIF中黄铁矿主要以两种形式产出,一是呈半自形-自形粒状(晶形以五角十二面体居多)聚集成条带分布于磁铁矿微条带的两侧,另一类是呈自形粒状(多为立方体)零散分布于磁铁矿条带内(图 5g)。前人对显生宙沉积和热液成因的黄铁矿的研究认为,沉积成因的黄铁矿通常为胶状结构的细粒他形晶体,在后期的区域变质作用过程中,因变质程度差异其晶形会转变为五角十二面体或立方体(徐国风和邵洁涟, 1980);而热液成因黄铁矿的晶形可能反映其形成时的介质条件,中等温度、缓慢冷却和充足的物质来源,有利于五角十二面体和八面体晶形的出现,而太高和太低的温度且物质供应不足,则多发育立方体晶形(Sunagawa, 1957; 陈光远等, 1987)。由于工作区的普遍经历了角闪岩相变质作用,且黄铁矿均已发生明显的重结晶(图 5h),因此其不同晶形是变质作用的结果还是反映了不同的成因,可能已经无法识别。

Klein and Fink (1976)指出,前寒武纪铁建造中含铁矿物成分的变化受控于其沉积时的物理-化学条件,这些条件决定了铁建造中含铁矿物的类型。不同于华北克拉通其他铁建造(如,鞍本、冀东、固阳等地区),下甸子BIF发育一类特殊的含黄铁矿条带型矿石,这类矿石产于矿体中部,与下部的硅酸盐型矿石间有一层石榴云母片岩夹层(图 3),矿石中主要含铁矿物除磁铁矿和阳起石外还有5%~10%的黄铁矿,且黄铁矿与磁铁矿接触界线平直无交代现象,他们或呈孤立的自形-半自形颗粒产于磁铁矿条带内,或呈自形-半自形粒状并聚集成与磁铁矿条带平行的黄铁矿条带(图 5g, h),这些特征综合表明,硅酸盐型矿石与含黄铁矿条带型矿石之间存在明显的沉积间断(出现沉积岩夹层),而且后者中的黄铁矿可能为同沉积成因而非后期热液交代或充填。影响BIF中硫化物形成的物理-化学因素较多,如沉淀时海水的温度、pH值和氧逸度等,但其中最为重要的是同期海水中还原性硫(HS-和H2S)的含量(Garrels and Christ, 1965; Goodwin, 1973),因此,黄铁矿中硫的来源可为其成因提供直接约束。海洋中还原性硫的可能来源主要有两种:(1)生物作用对海水中硫酸盐的还原;(2)海底热液系统的直接供应(Strauss, 1997)。考虑到太古代晚期大气整体处于还原状态,有氧风化作用不发育(Farquhar et al., 2000; Mojzsis et al., 2003),那么来自陆源的硫酸盐贡献基本可以忽略,但在还原的大气中SO2光解作用仍可产生SO42-,所以这一时期的海洋中有一定含量的硫酸盐(Huston and Logan, 2004)。然而,如果下甸子含黄铁矿条带型矿石中的硫来自生物还原作用,那么该类型的矿石在同时期的矿床中应该是普遍存在的,这与实际情况不符,因此,其含黄铁矿条带型矿石中的硫应当主要来源于海底热液系统。

火山活动或地幔柱作用可诱发形成一系列的海底热液系统,他们会向海洋中输送大量的金属物质(如,Fe2+、Cu2+、Zn2+)和还原性气体(如,H2S)(Isley, 1995; Abbott and Isley, 2001; Barley et al., 2005),进而促使形成一系列的VMS和BIF矿床(Slack et al., 2007; Piercey, 2010; Huston et al., 2014),然而由于晚太古代海洋整体富铁(Fe2+>1×10-6: Huston and Logan, 2004),因此还原性硫无法在海洋中积累,所以下甸子含黄铁矿条带型矿石中的硫更可能来自于其沉淀同期的热液喷口,矿石中黄铁矿条带的形成机制可能类似于海底热液喷流沉积。此外,对两类矿石样品的稀土元素分别进行平均并采用PAAS标准化(为避免可能的碎屑物质混染,部分Al2O3含量大于1%的样品,如XD-6、XD-9、XD-11、XD-12未参与对比)(图 12),可以看出它们具有整体类似的稀土配分模式,但明显不同的是相比于硅酸盐型BIF,含黄铁矿条带型BIF具有较低的HREE含量及相对较高的Eu正异常和LREE含量,结合图 8中现代海底热液和海水的稀土配分模式,下甸子含黄铁矿条带型BIF可能更多的继承了海底热液的稀土特征,这与其沉淀过程中存在海底热液活动的推断相符。综上,下甸子BIF的沉淀过程“记录了”同时期的海底的热液活动,其矿石中多层的黄铁矿条带可能与海底热液脉动式喷发对硫的周期性供给有关,而矿石类型的差异则反映了由于海底局部热液活动所导致的海水环境由贫硫到富硫的短暂转变;此外,鉴于下甸子BIF发育含黄铁矿条带型矿石,且与区内VMS矿床成矿时间一致,推测二者的成矿物质来源可能具有同源性。

图 12 下甸子BIF不同类型矿石稀土特征对比(PAAS标准化值据McLennan, 1989) Fig. 12 Comparation of REE+Y features between different types of ores from the Xiadianzi BIF (PAAS values after McLennan, 1989)
5.3 物质来源与沉积环境

BIF的微量元素特征一方面可以用来探究其成矿物质来源,另一方面也能够反应沉淀同期的古海洋和大气的氧化还原状态,因此它在研究工作中广泛应用(Fryer, 1977; Klein and Beukes, 1992; Bau and Dulski, 1996; Frei et al., 2008; Bekker et al., 2010)。但是,BIF的稀土元素特征可能会受到同沉积过程(如各类碎屑物质的混染)及沉积后各类地质作用(如成岩作用、变质作用)的影响(Bau, 1993),所以在利用其判断成矿物质来源和沉积环境前,应对这些干扰因素加以排除。

沉积过程中,影响BIF稀土元素特征的因素主要有三:(1)陆源碎屑物质的混入;(2)火山灰的混入;(3)一些来源未知物质(主要为海水中的磷酸盐矿物)的加入(Thurston et al., 2012)。下甸子BIF部分样品(XD-6、XD-9、XD-11、XD-12)Al2O3含量较高(1.63%~1.71%),可能受到富Al碎屑的混染,在讨论物质来源和沉积环境时将这部分样品排除。其余矿石样品中Al2O3和TiO2的含量(< 1%),高场强元素Th、Zr、Hf等的含量(< 1.5×10-6)均非常低;且Al2O3与∑REY之间,Al2O3与(Sm/Yb)SN之间,以及Zr含量与Y/Ho之间均缺乏相关性(图 13a-c),说明其基本未受到陆源碎屑的混染(Bau, 1993; Manikyamba andnAqvi, 1995; Bolhar et al., 2004),同时也暗示下甸子BIF低的Y/Ho比不是碎屑混染的结果。此外,在Wonder et al. (1988)的Al2O3-SiO2判别图解中(图 13d),未受混染的BIF样品全部落入到热液区,表明成矿物质可能主要来源于海底热液,同时Al2O3与SiO2之间缺乏相关性,也说明其均为纯净的化学沉积物,未受到富Al碎屑的混染。火山灰的混染一方面会导致BIF中Al含量的增加,另一方面也会造成Zr、Sc、Th等元素含量的显著升高,并导致Zr和Th之间呈现正相关(Gourcerol et al., 2016),其中,镁铁质火山物质的混染会导致BIF的Cr/V升高、Y/Ni降低,而长英质火山物质的混染则会导致Cr/V降低、Y/Ni升高(Bhatia and Crook, 1986)。下甸子BIF样品Zr与Th之间缺乏相关性(r=0.3)(图 13e),且不相容元素Zr、Sc、Th等含量均极低,综合表明其未受到火山物质的混染,这也暗示其沉淀可能发生于深海环境,与Superior型BIF有着明显差异,后者多沉积于浅海环境(Gross, 1983; Alexander et al., 2008)。磷酸盐矿物的混染可通过BIF的Th/U比值加以判别,海洋中沉淀的纯净的Fe、Mn氢氧化物普遍具有高的U含量、低的Th含量及极低的Th/U比(< 1)(Thurston et al., 2012),而陆源碎屑、火山灰、磷酸盐等物质的Th/U比较高(3~5)(Condie, 1993),因此若BIF中受到这类物质混染,其Th/U比会明显升高。下甸子BIF的Th/U比为1.16~5.31,平均为3.43,该值高于海洋中纯净的Fe、Mn沉积物。前已述及,这部分样品并未受到陆源碎屑和火山灰的混染,故其高的Th/U比可能为少量海洋磷酸盐矿物混染所致,此外,图 13f中Th/U比值与P2O5含量之间较高的相关性(r=0.8)也印证了这一推论。

图 13 下甸子BIF两种类型矿石主、微量元素二元图解 Fig. 13 Major and trace elements binary diagrams for two types of ore from the Xiadianzi BIF

沉积后成岩作用和变质作用可能会对BIF微量元素特征造成影响。Alexander et al. (2008)认为成岩过程中BIF的稀土特征会发生均一化,使得BIF硅、铁条带具有一致的稀土特征;然而,Bau and Dulski (1992)Morris (1993)Bau (1993)对南非Transvaal盆地和西澳Hamersley盆地中铁建造单独的富铁条带和富硅条带的对比研究显示,二者的稀土特征存在差异,并非完全一致,说明在成岩过程中铁建造的稀土元素是不活动的。BIF在形成之后通常会遭受多期变质作用,但Grauch (1989)Bau (1991)研究显示,变质作用过程中整个系统一般具有较高的封闭性和较低的水岩反应速率(w/r < 100),因此其稀土元素特征应当能够保持稳定。此外,综合对比全球不同时代的BIF(3.80Ga Isua BIF, Bolhar et al., 2004; 2.98Ga Pongola BIF, Alexander et al., 2008; 2.47Ga Dales Gorge BIF, Pecoits et al., 2009; 2.55~2.54Ga石人沟BIF, Zhang et al., 2012; 2.38~2.21Ga袁家村BIF, Wang et al., 2014b),它们均显示一致的稀土配分模式;而且,除元素相对含量有一定差异外,它们均与现代海水的稀土配分模式类似(Derry and Jacobsen, 1990),这也进一步说明后期变质作用不会改变BIF的稀土特征。

综上所述,下甸子BIF的大部分样品未受到碎屑物质的混染,而且在沉积后的各类地质作用过程中,其稀土元素特征基本保持稳定,因此可以利用这部分样品来判断下甸子BIF的物质来源和沉积环境。

5.3.1 物质来源

由于地球演化历程的不可再现性,目前对古海洋及同时期海底热液地球化学特征的认识均建立在现代海洋化学的基础上。一般来讲,现代海洋氧化水体显示一致的La、Gd和Y正异常,Ce负异常,LREE和MREE相对于HREE明显亏损;与海水不同,现代洋底热液无明显的LREE亏损,相比于低温热液(< 250℃),高温热液(>250℃)具有明显的Eu异常(Eu/EuPAAS*>1)以及较高的(Sm/Yb)PAAS值(Bau and Dulski, 1999; Bolhar et al., 2004; Bolhar and Van Kranendonk, 2007)(图 8b)。经PAAS标准化后,下甸子BIF两类矿石均显示显著的La正异常(La/LaPAAS*=1.25~3.11),Eu正异常(Eu/EuPAAS*=1.53~2.74),以及微弱的Y正异常(Y/YPAAS*=1.08~1.19),LREE相对于HREE亏损[(La/Yb)PAAS=0.10~0.65](图 8a),这些特征均表明下甸子BIF是海水与高温热液混合沉淀的产物。此外,图 5a中还给出了Isua和Dales Gorge两个典型BIF的稀土元素配分曲线,对比可知下甸子BIF与二者具有一致的稀土特征,反应它们均继承了海水和高温热液的稀土元素特征,但明显不同的是,下甸子BIF具有更低的Y/Ho比值(28.2~30.4,平均为29.3),考虑到热液中的稀土元素含量远高于海水,因此海水中仅需少量的热液添加即可使BIF具有上述稀土元素特征。鉴于下甸子BIF沉积过程中未受到碎屑物质的混染,且其稀土元素特征在后期地质作用过程中保持稳定,所以其低的Y/Ho比值可能反映沉积时热液组分的参与度略高。

除了高温热液系统,现代海洋中通常也发育一些低温(< 250℃)热液系统(Alexander et al., 2008)。Pichler et al. (1999)Wheat et al. (2002)分别对海岸和洋中脊附近低温热液系统的稀土元素特征进行了研究,结果表明他们具有类似的特征,即PAAS标准化后均无明显的Eu、Ce和Y异常,LREE相对于HREE不亏损且不具有较高的(Sm/Yb)PAAS值。整体而言,下甸子BIF两类矿石具有较大的Eu异常变化范围(Eu/EuPAAS*=1.53~2.74),但其(Sm/Yb)PAAS变化范围较小[(Sm/Yb)PAAS=0.26~0.67],且(Eu/Eu)PAAS*值与(Sm/Yb)PAAS值之间缺乏相关性,因此,无法排除其沉积过程中是否有低温热液的参与(Bau and Dulaki, 1996; Wang et al., 2014a, b)。

为了进一步评估海水和高温热液对下甸子BIF的贡献度,我们引入了Alexander et al. (2008)提出的Y/Ho对Sm/Yb二元判别图(图 14)。从图 14中可以看出热液组分仅需少量(~1%)的添加,就足以获得下甸子BIF的Y/Ho和Sm/Yb特征,这与一些典型的BIF如Isua和Kuruman相类似。然而值得注意的是,在排除碎屑物质混染的前提下,单一高温热液组分的添加会导致BIF的Y/Ho降低、Sm/Yb升高,最终会导致样品投点大致沿图中箭头所示方向分布,但实际情况并非如此,说明BIF沉积过程中除高温热液外,可能有部分低温热液组分的加入。

图 14 下甸子BIF热液组分加入量二元混合模型(底图据Alexander et al., 2008) Isua BIF数据来自Bolhar et al., 2004; Pongola BIF数据来自Alexander et al., 2008; Kuruman BIF数据来自Bau and Dulski, 1996 Fig. 14 Two-component conservative mixing model on hydrothermal components for the Xiadianzi BIF (after Alexander et al., 2008) Data for Isua BIF, Pongola BIF, and Kuruman BIF sourced from Bolhar et al., 2004, Alexander et al., 2008, and Bau and Dulski, 1996, respectively
5.3.2 沉积环境

如前所述,作为发育于古海洋中的化学沉积岩,BIF可以记录其沉淀同期海水的稀土元素特征,这为研究当时的海洋环境(即BIF的沉积环境)提供了极佳的条件(Fryer, 1977; Goldstein and Jacobsen, 1988; Derry and Jacobsen, 1990; Dasgupta et al., 1999)。一般而言,海洋中氧化的水体(经PAAS标准化后)通常具有强烈的负Ce异常,而弱氧化或缺氧的水体则缺乏明显的负Ce异常(German and Elderfield, 1990; Byrne and Sholkovitz, 1996)。这是由于在氧化的水体中,稳定的Ce3+被氧化为Ce4+,导致其溶解度降低并优先被海水中的富铁锰氢氧化物、有机物或者粘土颗粒吸收(Byrne and Sholkovitz, 1996);而在还原或氧化还原分层的水体中,这些氢氧化物会在水体中或氧化还原界面附近发生还原性溶解(Sholkovitz et al., 1992),最终造成水体缺乏负Ce异常,甚至一些层化的水体还会发育正Ce异常(Bau et al., 1997; De Carlo and Green, 2002)。因此,根据BIF的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Ce/Ce*=2CePAAS/(LaPAAS+PrPAAS),下甸子BIF发育明显的负Ce异常(Ce/Ce*=0.72~0.83,平均为0.77);然而,Bau and Dulski (1996)指出常规算法下,La的正异常会导致Ce的负异常,并进一步提出了Ce异常的Ce/Ce*和Pr/Pr*判别图解。如图 15所示,除个别样品外下甸子BIF两类矿石均位于正La异常区域,并未落入负Ce异常区域。因此,矿石稀土元素配分模式图中出现的负Ce异常(图 8a)并非真实的负Ce异常,而是受到正La异常的影响。

图 15 下甸子BIF稀土元素Pr-Ce判别图(底图据Bau and Dulski, 1996) Fig. 15 Plot of Ce and Pr anomalies normalized to PAAS for the Xiadianzi BIF (after Bau and Dulski, 1996)

化学沉积岩的CaO/(CaO+MgO)值可用于判断其沉积时的化学环境,但前提条件是这些化学沉积岩沉积时无碎屑物质的混染且沉积后未遭受碳酸盐化(Murray, 1994; Dasgupta et al., 1999)。统计显示,除新元古代的Rapitan型铁建造具有较高的CaO/(CaO+MgO)值(>0.85)以外(Dasgupta et al., 1999),其他前寒武纪未受到碎屑物质混染的铁建造,如Superior地区的BIF(Gross and McLeod, 1980)、Harmersley盆地的BIF(Ewers and Moris, 1981)、华北鞍本和冀东地区的BIF(Zhang et al., 2012; 李志红等, 2012; 杨秀清, 2013; 王恩德等, 2013; Sun et al., 2014; 代堰锫, 2014)普遍具有较低的CaO/(CaO+MgO)值(0.40~0.50),与现代海水的CaO/(CaO+MgO)值基本一致(< 0.50)(Dasgupta et al., 1999);然而,现代洋底扩张中心和活动热液喷口附近的化学沉积物(如,锰铁结核、燧石等)普遍具有较高的CaO/(CaO+MgO)值(~0.80)(Murray et al., 1991),这与其沉积时受到海水和玄武岩相互反应生成的富Ca贫Mg热液流体叠加有关(Murray et al., 1991; Holland, 1978)。下甸子未受混染的BIF样品中CaO/(CaO+MgO)比值平均为0.72,明显高于现代海水的值,与热液喷口附近的化学沉积物更为接近。

综上,作为晚太古代清原绿岩带中条带状铁建造的典型代表,下甸子BIF应当沉积于靠近热液喷口的还原性水体中,而缺氧环境也正是BIF沉淀的必要条件之一(Cloud, 1973; Klein, 2005)。Barley et al. (1998)认为太古宙BIF矿床的沉淀与同时期的构造运动、岩浆作用以及海平面升降密切相关,Bekker et al. (2010)Thurston et al. (2012)进一步指出,绿岩带中的Algoma型BIF是绿岩带火山-沉积序列演化过程中,局限盆地内火山和热液周期性活动的产物。下甸子BIF的地球化学特征表明其沉积过程中,存在因海底火山热液活动造成的局部高硫逸度环境。

5.4 下甸子BIF构造背景 5.4.1 元素活动性及陆壳混染

清原绿岩带中的岩石普遍经历了角闪岩相变质作用,局部区域可达麻粒岩相(沈保丰, 1993, 1994; 毛德宝等, 1997),因此在使用其地球化学数据反演岩石的原岩类型、成岩过程和产出的构造环境之前,应当考虑变质作用等后期地质作用对岩石化学组成的影响。

大量关于太古宙绿岩带中岩石的地球化学研究证明,在经历了角闪岩相变质作用后,这些岩石中大离子亲石元素普遍是活动的,但其稀土元素、高场强元素和过渡金属元素仍能够保持稳定(Dostal et al., 1980; Kerrich and Fyfe, 1981; Winchester and Floyd, 1997; Polat and Hofmann, 2003)。在此基础上,Polat and Hofmann (2003)还给出了具体的判别标准,对于遭受中-高级变质作用的岩石样品,当其具有下述特征时其元素稳定性是无法保证的:(1)具有较大的烧失量(LOI>6%);(2)初始地幔标准化后,Ce异常大于1.1或小于0.9[Ce异常的计算方法为:Ce/Ce*=CeN/Sqrt(LaN×PrN)];(3)样品中各元素含量与最不易活动的元素Zr含量之间缺乏明显的相关性(r < 0.75)。下甸子斜长角闪岩样品的烧失量较小(1.88~2.54,平均为2.05),Ce/Ce*值介于0.88~1.01(平均为0.97),且在二元相关性图解中(图 16)Zr含量与高场强元素(以Nb、Zr、Th为代表)和稀土元素(以La、Sm、Y、Yb为代表)含量间均呈现良好的相关性(r>0.75),上述特征综合表明这些元素在后期变质过程中是不活动的。此外,各样品经球粒陨石和初始地幔标准化后的元素配分曲线基本一致,也印证了这一结论。

图 16 下甸子斜长角闪岩微量元素相关性二元图解 Fig. 16 Trace elements binary diagrams for plagioclase amphibolites of the Xiadianzi BIF

K、Na、Rb、Ba等元素的活动性较强,与之相关的判别图(如全碱-SiO2、K2O-SiO2)就不再使用,但是一些高场强元素、稀土元素和过渡金属元素具有较高的稳定性,因此它们仍可以用来判别原岩特征。下甸子斜长角闪岩的K2O含量较低(0.23%~0.67%),Zr/Y值变化范围为3.3~4.6,与现代拉斑玄武岩的Zr/Y比值基本一致(2.0~4.5)(Barrett and MacLean, 1994),在Zr/TiO2-Nb/Y图解中,所有样品均落入玄武岩范围(图 17a);在Th/Yb-Zr/Y图解中,样品主要落入拉斑-过渡型系列(图 17b),综合表明斜长角闪岩原岩为拉斑-过渡型玄武岩。

图 17 下甸子斜长角闪岩Zr/TiO2-Nb/Y图解(a, 据Winchester and Floyd, 1997)和Th/Yb-Zr/Y图解(b, 据Ross and Bédard, 2009) Fig. 17 Zr/TiO2 vs. Nb/Y(a, after Winchester and Floyd, 1997) and Th/Yb vs. Zr/Y(b, after Ross and Bédard, 2009)diagrams for plagioclase amphibolites of the Xiadianzi BIF

陆壳的混染会导致幔源岩浆中部分元素含量的明显变化,比如造成Nb、Ta、Ti的负异常和Zr、Hf的正异常以及LREE的富集等(Puchtel et al., 1997),因此在讨论下甸子斜长角闪岩的成岩过程和产出的构造环境前,应对其是否受到陆壳混染进行判断。识别陆壳混染的可靠指标是经原始地幔标准化后出现Nb的负异常,且该异常与La/SmCN比之间具有相关性[Nb异常的计算方法为:Nb/Nb*=NbN/[Sqrt(ThN×LaN)](Taylor and McLennan, 1985),下甸子斜长角闪岩的Nb/Nb*值为0.44~1.09(平均为0.80),明显低于N-MORB(Nb/Nb*=1.44)而更为接近地壳的值(下地壳的Nb/Nb*=0.55,上地壳的Nb/Nb*=0.23),此外Nb异常与La/SmCN比之间具有良好的相关性(图 18),暗示其岩浆演化过程中受到了陆壳的混染。陆壳混染的类型(上地壳或下地壳)及程度可以通过样品的Nb异常大小和REE分异程度进行判断(Nosova et al., 2010; Glendenning et al., 2015),在图 18所示的判别图解中,受混染程度较低的样品特征与N-MORB类似,随着混染程度的增加样品投点更为接近下地壳,此外,斜长角闪岩样品均显示LREE略富集[(La/Yb)N=1.92~5.19],综合表明斜长角闪岩的原始岩浆可能来自亏损地幔,在后期演化过程中受到了下地壳的混染。

图 18 下甸子斜长角闪岩陆壳混染判别图 上地壳和下地壳数据来自Rudnick and Gao, 2003; N-MORB, C1球粒陨石, 原始地幔数据均来自Sun and McDonough, 1989 Fig. 18 Diagrams for plagioclase amphibolites of the Xiadianzi BIF to discriminate crustal contamination Data for upper and lower crust after Rudnick and Gao, 2003; N-MORB, C1 chondrite, and primitive mantle values sourced from Sun and McDonough, 1989
5.4.2 下甸子BIF构造背景

下甸子BIF产于清原绿岩带表壳岩系内,因此绿岩带的形成过程可为其构造背景提供有力约束。目前,关于清原绿岩带成因的认识可归纳为以下两个方面:(1)形成于陆内裂谷环境,与地幔柱活动及其诱发的幔源岩浆底侵作用有关(Zhai et al., 1985; Wu et al., 2013),主要证据为绿岩带浑北地区的含石榴子石角闪片岩(2.56~2.51Ga)记录了逆时针的P-T变质轨迹,同时绿岩带发育超镁铁质-镁铁质火山岩、双峰式火山岩以及大规模与表壳岩近同期的TTG(Bai and Dai, 1998);(2)形成于与现代板块构造体制类似的俯冲过程中(Peng et al., 2015; Wang et al., 2016),可进一步细分为洋-陆俯冲(Peng et al., 2015)和洋-洋俯冲(Wang et al., 2016),该类认识主要基于绿岩表壳岩、花岗岩和TTG的岩相学、U-Pb同位素年代学和地球化学研究。

近年来大量的研究表明,清原绿岩带发育典型与俯冲作用相关的岩石组合,如正常洋中脊玄武岩、岛弧拉斑玄武岩、钙碱性玄武岩-英安岩以及与埃达克岩地球化学特征类似的花岗质片麻岩(万渝生等, 2005a; 张雅静, 2014; 白翔等, 2014; Peng et al., 2015; Wang et al., 2016; Guo et al., 2016),而逆时针的P-T可能与多种地质作用有关,如幔源岩浆的底侵过程(Bohlen, 1987)、花岗岩侵位过程(Sandiford et al., 1991)、初期裂谷和俯冲环境(Hacker, 1991; Aoya et al., 2002; Gerya et al., 2002; Wakabayashi, 2004)以及火山弧岩浆作用过程(Wells, 1980; Pickett and Saleeby, 1993; Lucassen and Franz, 1996; Wakabayashi, 2004),因此,清原绿岩带的形成可能与板块俯冲作用关系更为密切,但具体是洋-陆俯冲还是洋-洋俯冲仍存在争议。

本文分析表明,下甸子斜长角闪岩的原始岩浆在上侵过程中明显受到了陆壳物质的混染,指示在清原绿岩带的形成过程中有大陆岩石圈的参与(Glendenning et al., 2015)。然而,由于样品受到了陆壳的混染,对进一步约束其产出的构造环境造成了一定的影响,但是在Nosova et al. (2010)提出的玄武岩类陆壳混染判别图解中(图 19),样品投点沿图中箭头所示方向均匀分布于N-MORB与LC之间,说明下地壳对原始岩浆的混染是不均匀的,其中部分受混染程度较低的样品仍保留了原始特征,表现为在判别图中更为接近N-MORB。因此,我们尝试将采用Pearce(1982, 2008)提出的Th/Yb-Nb/Yb和Zr-Ti判别图来进一步区分其原始岩浆属性,在图 19中,样品的总体趋势是落于N-MORB和火山弧玄武岩范围内,说明其化学组成与N-MORB和火山弧玄武岩类似。此外,对前人关于清原绿岩带中表壳岩和花岗岩类年代学资料的系统梳理表明,这些岩石中普遍存在年龄为2.7~2.6Ga的继承锆石(部分样品中存在年龄为3.1Ga的继承锆石)(表 7),这一方面暗示清原地区可能存在古老(>2.5Ga)的陆壳,另一方面也说明该绿岩带应当形成于活动大陆边缘环境。结合清原绿岩带的演化历程,我们推测下甸子BIF可能沉淀于晚太古代的洋-陆俯冲作用过程中形成的火山弧或弧后盆地环境。

图 19 下甸子斜长角闪岩Th/Yb-Nb/Yb判别图(a)和Ti-Zr判别图(b)(底图据Pearce, 1982, 2008) N-MORB:正常洋中脊玄武岩;E-MORB:富集洋中脊玄武岩;LC:下地壳;UC:上地壳;TH:拉斑质系列;CA:钙碱性系列;SHO:钾玄质系列;S、C、W、f分别指俯冲带组分、陆壳混染、板内分异和分离结晶(据Pearce, 2008; Angerer et al., 2013);弧后(Backarc)、岛弧(Arc)、弧前(Forearc)区域据现代汇聚型板块边界(数据Metcalf and Shervais, 2008) Fig. 19 Th/Yb vs. Nb/Yb (a) and Ti vs. Zr (b) diagrams for plagioclase amphibolites from the Xiadianzi BIF (after Pearce, 1982, 2008) N-MORB: normal mid-oceanic ridge basalt; E-MORB: enriched mid-oceanic ridge basalt; LC: lower crust; UC: upper crust; fields for convergent margins basalts include the tholeiitic (TH), calc-alkaline (CA), and shoshonitic (SHO) magma series; the vectors S, C, W, and f refer to subduction zone component, crustal contamination, within plate fractionation, and fractional crystallization respectively(after Pearce, 2008; Angerer et al., 2013); back-arc, arc, and forearc fields are of recent convergent margins(after Metcalf and Shervais, 2008)

表 7 清原绿岩带部分表壳岩和花岗岩类年龄统计表 Table 7 Summary of zircon U-Pb ages for supracrustal rocks and granitoids of the Qingyuan greenstone belt
6 结论

(1) 下甸子铁矿属于太古代Algoma型BIF,矿体夹层斜长角闪岩SIMS锆石U-Pb年代学分析表明,下甸子BIF的变质年龄为~2.50Ga,与区域变质作用的时间一致,反映了华北克拉通在25亿年前后发生的古陆块拼合与克拉通化事件。结合前人年代学资料,可认为下甸子BIF的形成时代为~2.55Ga,与区域上红透山等VMS型Cu-Zn矿床的成矿时代基本一致。

(2) 下甸子矿区发育含黄铁矿条带型和硅酸盐型两类矿石,他们的主微量元素特征与国外典型BIF近似,表明其成矿物质主要来自周围海水和高温热液,所有样品均无明显的Ce异常,则指示其沉淀于缺氧水体。

(3) 不同于其他前寒武纪BIF,下甸子两类矿石均具有极低的Y/Ho比(平均为29.3),并且其Y/Ho比与Sm/Y比之间无明显反相关关系,暗示沉积过程中可能有低温热液组分的参与,BIF较高的CaO/(CaO+MgO)比值(平均为0.72),表明其沉积环境距热液喷口较近。此外,相对于硅酸盐型矿石,含黄铁矿条带型矿石与同期海底热液活动关系更为密切,矿石中黄铁矿条带可能与BIF沉淀过程中海底热液脉动喷流沉积有关。

(4) 斜长角闪岩的地球化学分析显示,其原岩玄武质岩石的岩浆可能来自亏损地幔,但岩浆上升过程中受到不同程度的陆壳混染。综合前人关于绿岩带表壳岩和花岗岩类的地球化学、同位素年代学研究,清原绿岩带可能形成于晚太古代洋-陆俯冲过程,即下甸子BIF形成的构造背景可能为火山弧或弧后盆地环境。

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。
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