地球科学  2017, Vol. 42 Issue (12): 2129-2145.   PDF    
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辽东鞍山齐大山韧性剪切带运动学解析及形成机制
刘昕悦1,2, 李婧1,2, 刘永江1,2, 李伟民1,2, 温泉波1,2, 梁琛岳1,2, 常瑞虹1,2     
1. 吉林大学地球科学学院, 吉林长春 130061;
2. 东北亚矿产资源评价国土资源部重点实验室, 吉林长春 130061
摘要:关于太古宙早期地壳演化构造机制的争论已经持续了数十年,其焦点主要集中于水平构造还是垂向构造两大经典构造模式的探讨.对于早期地壳构造演化方面的研究,将会有助于我们更好地理解早前寒武纪的地球动力学机制.本文对华北克拉通东北部鞍山地区花岗-绿岩带内齐大山韧性剪切带的构造变形特征进行了详细的解析,揭示了该区新太古代垂向构造作用样式.研究结果表明,齐大山韧性剪切带内花岗质岩石长英质矿物塑性拉长特征明显,条带状构造发育,面理向NWW方向陡倾,不对称组构特征和矿物拉伸线理产状指示向NWW的陡倾正滑移剪切作用.变形岩石中的长英质矿物均发育中低温显微变形特征,石英C轴电子背散射衍射(EBSD)组构分析揭示石英以菱面<a>和底面<a>滑移系为主,岩石经历了中低温非共轴变形.根据矿物的变形行为以及石英的结晶优选方位推测变形温度约为400~500℃,岩石变形特征以位错蠕变为主.有限应变分析结果表明,靠近铁矿带方向,构造岩类型由L=S构造岩过渡为LS构造岩,岩石应变强度呈明显增强趋势.运动学涡度测量结果显示齐大山韧性剪切带内大多数岩石样品的Wk值大于0.75,岩石形成于以简单剪切作用为主的一般剪切作用.对比花岗-绿岩带西侧的白家坟韧性剪切带,显示二者均具有相向的陡倾正滑移运动学特征,表明新太古代时期鞍山地区地壳构造演化模式以垂向构造作用为主.
关键词韧性剪切带    太古宙片麻岩    组构分析    拗沉作用    垂向构造作用    华北克拉通    构造    
Kinematics Analysis and Formation Mechanism of Qidashan Ductile Shear Zone, Eastern Anshan, Liaoning Province, NE China
Liu Xinyue1,2 , Li Jing1,2 , Liu Yongjiang1,2 , Li Weimin1,2 , Wen Quanbo1,2 , Liang Chenyue1,2 , Chang Ruihong1,2     
1. College of Earth Sciences, Jilin University, Changchun 130061, China;
2. Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources, Changchun 130061, China
Abstract: The controversy over the Archean tectonic regimes has lasted several decades focused on the horizontal and vertical tectonics, the two classical tectonic models for Archean.Thus, more studies of the early crustal tectonic evolution are requisite for better understanding geodynamic regimes in the early Precambrian. In this study, detailed structural analysis of Qidashan down-slip ductile shear zones which developed in the eastern Anshan area was carried out and an example for revealing Neoarchean vertical tectonics is provided. The ribbon structures formed by intensely elongated felsic minerals are widespread in the deformed gneisses. The quartz C-axis fabric patterns obtained by electron backscatter diffraction technique imply low to middle temperature non-coaxial deformation with active rhomb < a > slip and basal < a > slip. Deformation behaviors of minerals and quartz crystallographic preferred orientations demonstrate that the rocks underwent mylonitization at a temperature of 400-500℃ under greenschist facies metamorphic conditions. Dislocation creep is the main rock deformation mechanism within the shear zones. Finite strain measurement results suggest that toward the iron ore belt, the tectonites change from L=S-to LS-type and the strain intensity exhibits an enhanced trend across the shear zones. Kinematic vorticity values (>0.75) indicate that the deformed rocks in ductile shear zones were produced by steady-state simple-shear dominated general shear. Compared to the Baijiafen ductile shear zone to the west, the Qidashan and Baijiafen ductile shear zones both have mutually down-slip kinematic characteristics, indicating that the Neoarchean crust growth and tectonic evolution in Anshan area is dominated by vertical tectonics.
Key Words: ductile shear zone    Archean gneiss    fabric analysis    sag duction    vertical tectonic    North China craton    tectonics    

众所周知,大陆岩石圈的变形主要受控于其流变学行为(Chardon et al., 2009; Gapais et al., 2009; Cagnard et al., 2011),物质组成及其热状态极大程度地影响并决定着早前寒武纪岩石圈演化的动力学机制.太古宙地壳热流值高、极软,呈高塑性的缓慢流动状态,太古宙构造样式也表现出独特的、不同于现代板块构造(李三忠等, 2015a, 2015b, 2015c).垂向构造和水平构造是基于太古宙变形以及地壳生长提出的两大具有显著性差异的构造模型.垂向构造主要以地幔柱、地幔对流方式为主.许多太古宙花岗-绿岩带内发育的穹脊构造(dome and keel structure)也是太古宙垂向构造的典型代表之一(McGregor, 1951; Anhaeusser et al., 1969; Mareschal and West, 1980; Ramberg, 1981; Dixon and Summers, 1983; Collins, 1989; Hippertt and Davis, 2000; Zhao et al., 2001; Van Kranendonk et al., 2004; Lin, 2005; Moyen et al., 2006; Parmenter et al., 2006; Lin et al., 2007; 赵国春, 2009; Van Kranendonk, 2011; Lin and Beakhouse, 2013).水平构造则与现代板块构造作用相似,是一种均变过程,主要表现为俯冲、碰撞、岛弧岩浆作用等地质特征(Kröner, 1981; de Wit, 1998; Kusky and Polat., 1999; Calvert and Ludden, 1999; Zhai et al., 2003; Zhao et al., 2001).

围绕垂向和水平构造两大模式,有关太古宙构造机制的争论已经持续了数十年.加拿大Superior克拉通绿岩带构造演化的最新研究成果显示,太古宙时期水平与垂向构造运动在区域范围内是同时存在的;其中,新太古代可能代表了由早期的垂向构造作用向晚期的水平构造转换的过渡时期(Lin, 2005; Parmenter et al., 2006; Lin et al., 2007; Lin and Beakhouse, 2013).但是总体来说,太古宙构造机制尚且处于探讨之中,与此相关的一些重要的科学问题也有待解决,还需要更多的关于早期地壳生长和构造演化等方面的研究,来进一步帮助我们去查明早前寒武纪的地球动力学机制.

鞍山地区位于华北克拉通东北部(图 1),保存了较为完整的太古宙地质记录,是世界上为数不多的3.8 Ga古老岩石的出露地之一,也是我国太古宙地壳演化及早期构造机制研究的绝佳场所(Zhao et al., 1998, 1999, 2000, 2001, 2005; 赵国春, 2009; 吴福元等, 2008; 翟明国, 2008).通过详细地野外观察我们发现:鞍山东部齐大山-西大背BIF型铁矿带两侧发育两条NNW向的韧性剪切带,按其空间地理位置分别命名为:白家坟韧性剪切带(Li et al., 2017)和齐大山韧性剪切带(图 1).韧性剪切带内太古宙TTG岩石,以及具有绿岩带性质的鞍山群变质层状岩系均具有明显的韧性剪切及流变学特征.两条韧性剪切带表现出相向的陡倾滑运动学特征,推测二者的形成可能与绿岩带内条带状磁铁建造(BIF)的向下拗沉作用相关,反映了太古宙的垂向构造运动.为了进一步查明韧性剪切带形成与绿岩带垂向构造作用之间的相互关系,需对剪切带内岩石的构造变形、运动学特征以及变形机制等问题进行详细的解析与研究.

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图 1 鞍山东部区域地质及韧性剪切带分布图 Fig. 1 Regional geological map showing the ductile shear zone in the Anshan east 据Li et al.(2017)

基于以上问题,本文选择鞍山东部花岗-绿岩带内,与铁矿相关的齐大山韧性剪切带为研究对象,开展韧性剪切带变形岩石的宏观变形、显微组构以及运动学特征分析,及对剪切带内的岩石应变和运动学涡度特征进行了系统的测量、统计与分析.综合以上研究成果,查明韧性剪切带内克拉通早期组成物质的运动学性质、岩石变形温度以及变形机制等问题,初步探讨鞍山东部花岗-绿岩带的构造样式.

1 区域地质概况

鞍山地区位于华北克拉通东北部(图 1),属辽东和吉南太古宙杂岩出露区的一部分,是我国重要的铁矿基地,也是前寒武纪地质演化研究的重要窗口.研究区内80%的面积被太古宙花岗质岩石占据,根据前人获得的精确年代学证据,鞍山地区的太古宙花岗质岩石可划分为4个岩石单元:(1)始太古代花岗质岩石(3.80~3.65 Ga),主要包括了白家坟奥长花岗片麻岩(Liu et al., 1992, 2008)、东山条带状奥长花岗岩(Song et al., 1996)和变质石英闪长岩(Wan et al., 2005)、深沟寺条带状奥长花岗岩(Wan et al., 2012)以及锅底山奥长花岗岩(Wang et al., 2015);(2)古太古代片麻岩杂岩和陈台沟糜棱岩化花岗岩(3.35~3.30 Ga);(3)中太古代立山花岗岩和铁架山花岗岩;(4)新太古代齐大山花岗岩(~2.47 Ga),这些古老基底岩石通常呈卵形穹隆形态产出,周围被新太古界鞍山群以及早元古界辽河群绿片岩相变质沉积岩系覆盖,构造了鞍-本地区典型的花岗-绿岩穹隆构造(Liu et al., 1987).研究区内著名的鞍山式BIF铁矿主要产于新太古界鞍山群樱桃园组地层中,沿齐大山到西大背,空间展布与绿岩带分布一致(张连昌等, 2012, 2014),构成了区内近南北向的铁矿带,本文称之为“齐大山-西大背BIF铁矿带”.鞍山地区BIF铁矿多为Algoma型含铁建造,与大规模的海底火山活动密切相关,前人对齐大山-西大背BIF铁矿带内相关岩系的定年结果显示研究区BIF铁矿应形成于2 551~2 530 Ma(王守伦和张瑞华,1995杨秀清,2013;代堰培等,2013;崔培龙,2014),且经历了2 469 Ma左右的变质事件改造(代堰培等,2013).

徐仲元(1991)指出鞍山地区的铁矿主要处于3条巨大的韧性变形带中,并经历了早期的塑性变形和晚期的脆性变形共同作用.同时,在构造置换、剪切分异和流体参与等因素的影响下,磁铁石英岩中条带的矿物组分重新迁移和分配.范正国等(2013)对鞍山地区进行了重磁剖面反演,反演结果表明,鞍山区域上总体表现为以铁架山花岗岩穹隆为中心,东鞍山以及齐大山向斜构造环绕在穹隆顶部,控制区内BIF铁矿体的分布.其中东鞍山向斜向南倒卧,齐大山向斜向北东倒卧.

近年来,笔者通过详细的野外考查,发现花岗-绿岩带东西两侧,即花岗质岩石与上覆变质-沉积岩系接触部位,岩石遭受了强烈地韧性变形改造,矿物组构定向明显,且均具有向下的陡倾滑运动学特征,两条韧性剪切带在空间分布、岩石变形组构特征等方面具有很强的相似性和可对比性,综合反映了区域上花岗-绿岩带内的垂向构造运动特征.Li et al.(2017)通过宏/微观构造-岩石组构分析、有限应变测量及运动学涡度等研究方法,探讨了铁矿带西侧白家坟韧性剪切带的形成机制和过程.为进一步提供早期地壳垂向生长的构造样式的证据,本文选择了铁矿带东侧的齐大山韧性剪切带进行详细的构造解析.

2 宏观构造特征及运动学分析

齐大山韧性剪切带位于齐大山-西大背BIF铁矿带的东侧,为一走向NNW的狭窄剪切带,沿鞍山群与齐大山花岗岩的接触界面连续分布,沿走向可延伸近5 km.野外观察发现:铁矿带东侧花岗-绿岩接触界面附近的齐大山花岗岩与鞍山群均发生了强烈的韧性变形,矿物定向拉长明显,片麻理、矿物拉伸线理构造发育.韧性剪切变形只局限于花岗-绿岩接触面内,向东方向远离铁矿带,齐大山花岗岩变形程度逐渐减弱,过渡为块状构造,无韧性变形特征.本文对齐大山剪切带内鞍山群和齐大山花岗质片麻岩的构造变形及运动学特征进行了详细的分析,在齐大山铁矿区齐欣选矿厂(剖面A)、胡家庙子铁矿区(剖面B)构造变形强烈部位分别进行了实测构造剖面及定向样品采集等工作(图 2).

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图 2 齐大山韧性剪切带野外实测构造剖面图及采样位置 Fig. 2 Field survey and sample location of the ductile Shear zone in the Qidashan area 剖面A测量于齐大山铁矿区齐欣选矿厂;剖面B测量于胡家庙子铁矿区;具体位置见图 1

在齐大山剖面A上可见矿体发育平直条带状构造,暗色铁质条带与浅色硅质条带相间排列.贫、富铁BIF层以及鞍山群变质沉积岩系为断层接触关系,断层产状与BIF矿体条带产状一致,产状为250°~260°∠73°~88°,线理产状为240°~263°∠71°~85°,倾伏角较陡,反映了近W方向的正滑移特征(图 3a).

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图 3 齐大山韧性剪切带宏观构造变形特征 Fig. 3 Deformation characteristics of the Qidashan ductile shear zone 所有照片均垂直于水平面,朝向S、SSW方向;a.BIFs与云母片岩呈正断层接触关系;b, c.云母片岩、云母石英片岩发育陡倾面理,不对称褶皱指示近W方向的向下剪切作用;d.齐大山花岗质片麻岩发育规则的陡倾片麻理,矿物定向拉长明显,不对称残斑指示近W方向的向下剪切作用

矿体围岩樱桃园组云母片岩、云母石英片岩主要由石英、白云母、绿泥石等矿物组成,受韧性剪切作用,白云母等片状矿物定向排列,石英颗粒塑性拉长明显,片理近于直立,沿花岗-绿岩接触面走向方向产状变化很小,基本保持一致,集中在265°~330°∠69° ~88°,片理面上矿物拉伸线理产状竖直(270°~296°∠64°~81°),部分云母片岩、云母石英片岩中发育不对称褶皱,反映了近W方向向下的剪切滑脱作用(图 3b3c).鞍山群向东逐渐过渡为齐大山花岗质片麻岩,其原岩为中粗粒白云母花岗岩,主要由斜长石、碱性长石、石英、白云母、黑云母等矿物组成.胡家庙子剖面B上可见齐大山花岗质片麻岩发育规则平整的面理,面理由塑性拉长的石英条带和长石集合体构成,局部可见颗粒较大的碱性长石残碎斑晶沿面理展布方向两端尖灭,呈透镜状产出(图 3d).岩石面理与鞍山群以及铁矿接触面的产状较为一致,优势产状为290°∠88°.

沿剖面走向观察,齐大山韧性剪切带内岩石具有细粒化特征逐渐减弱、矿物颗粒粒度逐渐增大的趋势.向东方向逐渐远离接触带观察,约300 m处可见块状构造的齐大山花岗岩,岩石未遭受韧性剪切作用改造,保留完好的中粗粒花岗结构.故推测,齐大山韧性剪切带内岩石变形强弱与距离铁矿带的远近密切相关:越靠近铁矿带,岩石变形程度越强,面理化特征越明显,线理发育越好;反之,越弱.

3 显微构造特征

鞍山群樱桃园组中云母石英片岩的主要组成矿物为石英(60%~75%)和白云母(20%~30%),还有少量的绿泥石(~5%)、绢云母(3%~5%)、黑云母(<2%).云母石英片岩中石英颗粒塑性拉长明显,粒度集中在0.3~0.5 mm,石英具有波状消光特征,呈条带状分布(图 4a).白云母多呈较宽的片状,粒径为0.5~2.0 mm,石英条带边界较平直,多被定向排列的白云母晶体间隔,单个石英颗粒多为不规则状,局部具有膨凸式重结晶和亚颗粒旋转重结晶共存的特征.越靠近铁矿带,岩石中白云母含量明显增加,平直的石英条带逐渐向透镜状过渡,长轴平行于片理方向,矿物颗粒细粒化程度增加(图 4b).局部可见石英压力影,阴影部分由细粒石英及少量白云母组成,核部石英边界或微裂隙内发育膨凸式重结晶特征(图 4c),运动学特征指示左行滑脱剪切.

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图 4 铁矿围岩鞍山群樱桃园组、齐大山花岗质片麻岩显微构造特征 Fig. 4 Microstructure characteristics of Qidashan granitic gneiss from the Yingtaoyuan Formation a.平直的多晶石英条带;b.白云母S-C组构及晚期伸展折劈理;c.扁豆状石英集合体,石英波状消光、亚颗粒,局部可见压力影;d.微斜长石机械双晶;e.矿物塑性拉长、细粒化特征明显,斜长石机械双晶;f.石英条带环绕不对称的微斜长石残斑分布,箭头方向代表其运动学方向

齐大山花岗质片麻岩原岩为中粗粒白云母花岗岩,主要由斜长石(30%~35%)、碱性长石(~25%)、石英(~30%)、白云母(8%~10%)、绿帘石(3%~5%)、绢云母(3%~5%)及黑云母(~2%)组成.显微镜下,胡家庙子剖面A上变形强烈的齐大山花岗质片麻岩主要表现为较平直的多晶石英条带与长石聚集带相间排列(图 4e),构成面理.岩石中斜长石颗粒多发生了强烈绢云母化蚀变,表面浑浊,边界模糊,但仍可见塑性拉长特征,多聚成条带状分布(图 4d).岩石中的碱性长石以微斜长石为主,格子双晶发育,局部可见双晶弯曲、机械双晶等特征(图 4e).较宽的片状白云母多沿面理方向定向分布(图 4e).越靠近铁矿带齐大山花岗质片麻岩的变形程度越强,矿物拉长以及细粒化特征越显著(图 4e).齐大山剖面B上齐大山花岗质片麻岩基质矿物多由细小的绢云母以及石英颗粒组成,白云母定向排列,与鞍山群石英云母片岩的显微组构特征相似,可见大的斜长石、微斜长石斑晶,推测是在构造应力的作用下,靠近接触带的齐大山花岗岩中长石矿物发生了强烈的绢云母化蚀变,导致岩石中长石含量减少,云母含量增多.多晶石英条带多表现出沿面理方向环绕长石残斑弯曲、透镜状产出特征(图 4f).不对称的微斜长石旋转残斑(图 4f)指示岩石经历了左行剪切作用.

定向标本显微尺度观察的左行剪切作用与野外宏观构造变形特征所反映的近W方向的倾滑剪切作用方向上是一致的.

自然界一般韧性剪切带内的岩石和矿物,通常是在一定的温度、压力和应变速率范围内,通过位错蠕变等机制发生变形的.当差异应力、应变速率等物理参数环境一定的前提下,温压条件便成为影响岩石和矿物变形行为的主导因素(Hirth and Tullis, 1992).不同的温压条件下,矿物受不同的变形机制主导,发育不同的变形行为.因此,韧性剪切带内岩石矿物变形行为的详细研究是约束岩石的变形环境的一种有效方法.利用石英重结晶类型Stipp et al.(2002)以及矿物的变形行为(Passchier and Trouw, 2005;向必伟等,2007),齐大山韧性剪切带内变形岩石中均发育中低温(400~500 ℃)变形组构特征(表 1),属于绿片岩相变质环境.

表 1 胡家庙子剖面B内测试样品显微组构特征 Table 1 Microstructure characteristics of analyzed samples in the Hujiamiaozi section B
4 有限应变测量与运动学涡度分析

本次岩石有限应变测量的岩石样品包括齐大山韧性剪切带内的齐大山花岗质片麻岩(6件),云母石英片岩(4件).样品采样位置如(图 2)所示.并采用Fry法(Fry, 1979; 郑亚东和常志忠,1985)对剖面内的岩石样品进行有限应变测量,结果见表 2.

表 2 研究区韧性剪切带内岩石Fry法有限应变测量结果 Table 2 Finite element strain measurement of rocks in the Qidashan ductile shear zone using Fry method

测量结果表明,齐大山韧性剪切带内,剖面B东侧齐大山花岗质片麻岩k值为1.26~1.62,在Flinn图解中以平面-拉伸应变和L=S构造岩特征为主;西侧靠近铁矿带的鞍山群云母石英片岩k值范围为2.94~5.62,均显著大于1,以一般拉伸应变和LS构造岩为典型特征.可见靠近铁矿带方向,东西两侧剪切带内的岩石类型呈现出由L=S构造岩向LS构造岩逐渐过渡的变化趋势.

Hossack图解中,随着应变强度(Es)的增加,罗德参数(v)也逐渐加大(图 5b),根据定量化的应变强度数据做韧性剪切带的应变剖面图,从图中可以看出,齐大山韧性剪切带内齐大山花岗质片麻岩显示出中等-较弱程度的应变强度(0.15~0.48)特征,向西方向,鞍山群云母石英片岩的应变强度(0.53~0.66)明显增强(图 5a).

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图 5 Flinn有限应变判别图解(a)和Hossack图解(b) Fig. 5 Flinn finite strain discrimination diagram (a) and Hossack diagram (b)

运动学涡度(Wk)是检验应力作用类型的重要参数(Xypolias and Koukouvelas, 2001; Roy et al., 2016),该值介于0~1之间,0.75作为界线,Wk<0.75以共轴的纯剪作用为主,而>0.75则简单剪切作用占主导地位(Simpson and De Paor, 1993; Tikoff and Teyssier, 1994).笔者采用极摩尔圆法(张进江和郑亚东,1995)和石英条带斜交面理法(Wk=sin(2θ);Passchier, 1987)对韧性剪切带内变形岩石中石英变形时的运动学涡度进行估算.两种方法获得的运动学涡度结果基本一致(表 3),极摩尔圆法所获得的运动学涡度值介于0.848~0.946,平均值为0.909;石英条带斜交面理法所获得的运动学涡度值介于0.883~0.961,平均值为0.925.获得的Wk值大于0.75,表明剪切带内岩石形成于以简单剪切作用为主的一般剪切作用中.并且显示横穿各韧性剪切带不同岩石样品的运动学涡度值变化幅度很小(图 6a),韧性变形应发生于较稳态的剪切作用过程中.对比白家坟韧性剪切带(Li et al., 2017),铁矿带东侧齐大山韧性剪切带剪切作用中的简单剪切组分明显增多.

表 3 研究区韧性剪切带内岩石的运动学涡度值 Table 3 Kinematic vorticity values of analyzed rocks in the Qidashan ductile shear zone
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图 6 运动学涡度(a)剖面图、同一比例尺下显微组构特征(b) Fig. 6 Kinematic vorticity (a) Sectional view, microstructure characteristics under the same scale (b)

综上所述,韧性剪切带内变形岩石的岩石类型和应变强度均表现出空间对称分布的变化规律,且越靠近铁矿带,岩石最大延伸方向的应变量逐渐增大,宏观上表现为矿物拉伸线理等线状构造的发育,同时岩石整体的应变强度也显示出逐渐增大的趋势,与微观尺度上观察到的靠近铁矿带方向,变形岩石中矿物塑性拉长和细粒化特征逐渐增强的变化规律是一致的(图 6b).

5 石英C轴EBSD组构分析

本次石英C轴组构EBSD测试分析是在中国地质大学(北京)地质过程与矿产资源国家重点实验室完成的.详细的测试分析仪器参数及操作方法参见Liu et al.(2012).选择胡家庙子剖面Ⅱ上具有代表性的定向岩石样品进行测试分析,测试样品的采样位置及岩相学特征见图 2表 1.EBSD测试分析均在运动学方向上的XZ面上进行.测试对象为不同样品中的石英条带或者集合体,石英颗粒多表现出不均匀消光的变形特征.测试结果如(图 7)所示,其中组构图中的XY面代表了糜棱岩的叶理面,组构图中的X轴方向则代表了矿物拉伸线理方向.

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图 7 研究区韧性剪切带内测试样品石英C轴组构图 Fig. 7 Lower-hemisphere projection of C-axis fabric of quartz grains measured by EBSD from the Qidashan ductile shear zone

齐大山韧性剪切带测试样品中,样品15AS13-6和15AS13-9均为齐大山花岗质片麻岩,样品15AS13-10为云母石英片岩.齐大山花岗质片麻岩石英组构图中多表现为位于Y轴附近的点极密,样品15AS13-6在XZ大圆内X轴与Z轴之间发育弱点极密,表明石英以中温柱面<a>滑移系启动为主,含有少量的菱面<a>滑移组分.云母石英片岩15AS13-10石英组构图发育两个点极密,一个点极密位于XZ大圆上靠近Z轴边缘,另一点极密靠近Y轴分布,反映了底面<a>和菱面<a>滑移系共同作用的特征,指示了齐大山韧性剪切带岩石中低温变形特征,岩石变形温度应在400~500 ℃左右,与长石-石英矿物变形温度计分析结果基本一致.韧性剪切带内石英矿物以非共轴变形为主,石英C轴组构的不对称性表明,齐大山韧性剪切带内变形岩石则以左行剪切作用为主,石英组构所反映的岩石变形运动学特征与宏观、显微构造分析结果一致.

6 讨论 6.1 齐大山韧性剪切带构造变形特征

对比分析前人的研究成果可知(表 4),鞍山东部南北向齐大山-西大背BIF铁矿带两侧发育两条狭窄的陡倾滑韧性剪切带:白家坟韧性剪切带和齐大山韧性剪切带.虽然目前没有精确的同位素年龄数据表明白家坟、齐大山韧性剪切带的形成时间是否一致,但根据韧性剪切带内岩石相似的宏观、显微构造变形特征以及对称的运动学和空间应变分布特征推测二者应为同一构造机制作用下的产物.白家坟和齐大山韧性剪切带内变形岩石均发生了以拉伸应变为主的韧性变形,变形的花岗质岩石广泛发育由长英质矿物组成的条带状构造,矿物塑性拉长特征明显.石英波状消光、变形带、膨凸式和亚颗粒旋转重结晶,长石塑性拉长、细粒化等一系列中低温显微变形组构特征以及石英C轴组构特征表明铁矿带两侧韧性剪切带应形成于较为一致的温压环境中,变形温度应为400~500 ℃.

表 4 铁矿带东西两侧韧性剪切带对比 Table 4 Comparison of ductile shear zones on both sides of the Qidashan iron ore belt

最新的重磁交互反演结果显示鞍山地区具有典型的穹脊构造特征,以铁架山穹隆(背斜)为中心,北东、南西侧分别为齐大山、东鞍山向斜构造,发育含铁建造的绿岩体均分布在这些向斜构造内,且控制了铁矿体的分布及产状,研究区内SW方向倒卧的齐大山向斜构造北东翼矿体产状近于直立,南西翼矿体向NE方向倾斜,倾角(约45°~65°)中等(范正国等,2013Fan et al., 2014).根据前人研究资料及空间分布关系可知,出露于地表的齐大山、胡家庙子以及西大背铁矿应位于向斜构造的北东翼一侧,而隐伏于地下的陈台沟铁矿(代堰锫等,2013)应分布于南西翼一侧(图 8a).

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图 8 鞍山东部花岗-绿岩带垂向构造模式及区域韧性剪切带分布(a)、白家坟韧性剪切带XZ有限应变椭圆分布(b)和齐大山韧性剪切带XZ有限应变椭圆分布(c) Fig. 8 Vertical tectonic model for the evolution of granite-greenstone domain and regional ductile shear zones in eastern Anshan area (a), Baijiadu ductile shear zone XZ finite strain elliptic distribution (b) and Qidashan ductile shear zone XZ finite strain elliptic distribution (c) 图b据Li et al.(2017)

平面上观察,齐大山和白家坟韧性剪切带沿齐大山-西大背BIF铁矿带东西两侧线性对称分布,纵向剖面上,二者应位于绿岩带向斜构造的两翼与花岗岩接触部位(图 8a),其中白家坟韧性剪切带内岩石面理构造倾向为NEE方向,倾角中等,与绿岩带向斜构造南西翼产状较为一致,矿物拉伸线理向SEE方向倾伏,倾伏角中等偏大,指示了向斜南西翼一侧SEE方向的陡倾滑剪切作用;齐大山韧性剪切带内变形岩石发育NWW方向陡倾(~90°)的面理构造,与向斜构造北东翼矿体的产状一致,矿物拉伸线理产状反映了向斜北东翼一侧近W方向的陡倾滑剪切作用,东西两侧相向的陡倾滑运动学特征与绿岩带向形的几何学特征相吻合.综合韧性剪切带内岩石的变形、运动学特征以及空间应变分布规律,推测绿岩带两侧岩石强烈剪切变形很可能与花岗-绿岩带垂向穹脊构造的形成作用相关,岩石变形的运动学特征与垂直构造模式所涉及的底辟作用和拗沉作用相一致,反映了向形绿岩带内的垂向构造作用,韧性剪切作用应发生于花岗-绿岩带垂向穹脊构造的形成过程中,主要表现为密度较轻的花岗质岩石与上覆密度较重的富铁建造之间存在一定程度的重力不均衡,尤其是在~2.5 Ga齐大山花岗岩侵位的过程中会促使BIF沉积基底(铁架山花岗岩)的部分熔融,使得基底在物性上变软,最终在区域性地质运动与岩浆作用的驱动下,发生了花岗质岩石向上的底辟作用和含铁建造向下的拗沉作用(Collins, 1989; Van Kranendonk et al., 2004; Lin, 2005; Moyen et al., 2006; Parmenter et al., 2006; Lin et al., 2007; Van Kranendonk, 2011; Lin and Beakhouse, 2013),该垂直运动为花岗-绿岩体接触界面的花岗质岩石和铁矿体围岩提供了向下运动的有利条件(Hippertt and Davis, 2000),在接触界面附近的岩石发生了向下的陡倾滑拉伸变形作用,形成规则的且平行于矿体分布的面理构造(图 8a).由于韧性剪切带主要驱动力来源于重密度铁矿体的向下拗沉作用,故在韧性剪切带内部,越靠近铁矿带,岩石的变形程度越强,矿物塑性拉长特征也越显著,宏观上岩石的线理构造越发育,岩石类型逐渐由L=S构造岩过渡为以线理构造为主的LS构造岩,岩石应变特征也逐渐增强(图 8b, 8c).此外,按几何形态可知,岩石递进变形过程中剪切作用面越陡,平行于剪切作用面的简单剪切组分分量越多,垂直于剪切作用面的纯剪切组分分量越少,绿岩带向斜构造中白家坟韧性剪切带所处的南西翼一侧剪切作用面倾角中等偏大,而齐大山韧性剪切带所处的北东翼一侧倾角较陡,剪切作用面近于直立,故相同的构造应力作用下,北东翼齐大山韧性剪切带的简单剪切分量应多于南西翼白家坟韧性剪切带(Li et al., 2017)的简单剪切分量,进一步验证了本文运动学涡度测量的准确性.

6.2 鞍山花岗-绿岩带的垂向构造

鞍山花岗-绿岩带早前寒武纪构造演化过程复杂,经历了多期次、强度不同的变质变形及构造热事件的改造.根据前人研究成果及野外地质特征来看,研究区花岗-绿岩带基底样式的形成在太古宙期间已基本完成,早元古代(吕梁运动)及其后的构造运动对花岗-绿岩带基底样式的形成贡献并不大,区域上早元古界辽河群与新太古界鞍山群呈角度不整合覆盖关系,太古宙花岗质岩石与鞍山群的接触面,鞍山群与辽河群的接触面应是两个形成时代不同、强度和性质不一的独立构造活动带,前者主要表现为垂向构造运动机制下的强烈韧性剪切作用,后者主要表现为发育程度不同的剪切滑动带,滑动带的产状特征明显受绿岩带基底的形态所控制,且从白家坟、齐大山韧性剪切带各横截剖面上岩石的变形特征来看,早元古代变形作用对鞍山群(包括含铁建造)的影响也是极其微弱的,原因可能有两个方面,一是早元古代时期,鞍山群包括含铁建造已基本固结,二是由于应力到达鞍山群和辽河群的接触面,在剪切滑动过程中得到了释放.此外,显生宙构造运动多表现为脆性构造的发育,局部破坏了区域地质体的连续性,在本文韧性剪切带横截剖面上也可见大量断层和节理构造的发育.

总体来说,鞍山东部花岗-绿岩带基底构造样式的形成和演化与带内垂向构造运动密切相关.在垂向构造机制作用下,花岗岩体底辟隆升,铁矿及其围岩向下拗沉重新就位,逐步形成花岗-绿岩带穹脊构造样式,且在花岗-绿岩体接触面表现出强烈的陡倾滑韧性剪切作用.这一认识与Lin and Beakhouse(2013)关于加拿大Superior克拉通上Hemlo金矿的“钱口袋”模型一致.含铁建造作为绿岩带中的强干岩层,其产出形态及构造现象的辨识对于认识研究区太古宙时期花岗-绿岩带基底样式的形成及构造演化过程具有重要的指示意义.南北向绿岩带内的BIF铁矿体中富铁矿体多为致密的块状构造,富铁矿体两侧的贫铁矿体则广泛发育由暗色的铁质条带和浅色的硅质条带相间排列而成条带状构造,条带状构造的产状较陡或近于直立,与绿岩带向斜构造两翼的产状一致,形态吻合(图 8a).花岗-绿岩体接触面陡倾滑韧性剪切带横截剖面显示靠近铁矿体方向,岩石变形程度和应变强度逐渐增强(图 8b8c).基于鞍山东部花岗-绿岩带的穹脊构造样式,我们推断穹脊构造样式的形成过程中,绿岩带内的条带状铁矿体一定发生过向下的垂向运动,进而引起花岗-绿岩体接触界面这些构造活动带的向下陡倾滑韧性剪切作用,向斜构造内陡倾的条带状铁矿体产状分布并不代表其原始沉积的初始状态,而是在垂向构造机制作用下原始含铁建造发生了方位变化,含铁建造原始水平产状逐渐向直立过渡转变,同时伴随一系列的变质变形作用的发生.

这一期以垂向运动为主且伴随强烈韧性剪切作用的构造热事件对研究区花岗-绿岩带的古构造格架形成具有至关重要的作用,其可能控制现今花岗-绿岩带的基底样式.鞍山花岗-绿岩带早期基底穹脊构造样式的形成表明新太古代时期鞍山地区的地壳构造演化模式以垂向构造运动为主,局部可能伴随小规模的水平剪切作用.花岗-绿岩带整体的基本特征、形态分布及构造变化规律与世界上其他花岗-绿岩带地体是可以类比的,如澳大利亚西部Pilbara克拉通(Collins, 1989; Van Kranendonk et al., 2004)、印度南部Dharwar克拉通(Bouhallier et al., 1995; Chardon et al., 1996)、加拿大Superior克拉通(Lin, 2005; Parmenter et al., 2006; Lin and Beakhouse, 2013)等.此外,垂向构造机制下含铁建造绿岩体向下拗沉过程中伴随的矿体富集特征以及绿岩带向斜构造的形成演化过程可能会为今后沉积变质型铁矿向斜控矿模式的研究以及鞍山地区BIF铁矿带深部找矿等工作提供一定的理论依据.

7 结论

(1) 齐大山韧性剪切带内花岗质岩石中长英质矿物塑性拉长特征明显,条带状构造发育,发育陡倾面理、线理构造,指示了向西的陡倾正滑移运动学特征.

(2) 变形岩石中长石、石英矿物显微变形特征以及石英C轴组构特征显示齐大山韧性剪切带形成于较为一致的中低温变形环境中,变形温度大致为400~500 ℃,位错蠕变是剪切带内岩石变形的主要机制.有限应变测量结果表明剪切带中岩石变形以平面-拉伸应变为主,靠近铁矿带方向,构造石类型由L=S构造岩过渡为LS构造岩,矿物拉伸线理等线状构造越发育,岩石应变强度呈明显增强趋势.

(3) 运动学涡度测量结果显示齐大山韧性剪切带内大多数岩石样品的Wk值大于0.75,岩石形成于以简单剪切作用为主的一般剪切作用.

(4) 鞍山东部花岗-绿岩带穹脊构造样式的形成以及白家坟、齐大山陡倾滑韧性剪切带的发育与含铁建造向下拗沉以及同时期花岗岩底辟隆升作用密切相关,是早期地壳垂向构造运动机制下产物.

致谢 感谢中国地质大学(北京)刘俊来教授在EBSD组构分析测试方面给予的帮助,对两位匿名审稿人提出的宝贵的意见和建议表示忠心的感谢!

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