岩石学报  2013, Vol. 29 Issue (2): 462-484   PDF    
华北克拉通孔兹岩带中段大青山-乌拉山变质杂岩立甲子基性麻粒岩年代学及地球化学研究
刘平华, 刘福来, 蔡佳, 刘建辉, 施建荣, 王舫     
中国地质科学院地质研究所,北京 100037
摘要: 大青山-乌拉山变质杂岩立甲子基性麻粒岩主要由角闪二辉麻粒岩、含榴角闪二辉麻粒岩和黑云角闪二辉麻粒岩所组成,并以变形岩墙和不规则透镜体形式赋存于富铝片麻岩和花岗质片麻岩之中。立甲子基性麻粒岩中变质锆石含有单斜辉石(Cpx)+角闪石(Amp)+斜长石(Pl)+磷灰石(Ap)的包体矿物,与寄主岩石--基性麻粒岩矿物组合及其化学成分十分一致,相应的207Pb/206Pb表面年龄变化于1933±39Ma~1834±40Ma,加权平均年龄为1892±7Ma (MSWD=0.50, n=46),应代表立甲子基性麻粒岩原岩经历中低压麻粒岩相的变质时代。在变质过程中,以大离子亲石元素(K、Na、Sr、Rb)为代表的活动元素发生了显著的改变;而高场强元素(Nb、Zr、Ti)和稀土元素基本无变化,保持稳定。立甲子基性麻粒岩原岩属于拉斑玄武质岩石系列,其SiO2集中变化于45.58%~51.40%,Mg#值集中介于41~54之间;在球粒陨石标准化稀土配分图中,立甲子基性麻粒岩具有平坦型的稀土配分曲线特征((La/Yb)CN=1.30~1.51),Eu异常不明显(Eu/Eu*=0.93~1.04)。与显生宙岛弧拉斑玄武岩类似,立甲子基性麻粒岩所有样品皆具有Nb、Zr、Ti负异常特征。综合分析认为,立甲子基性麻粒岩原岩形成于2450~1930Ma,并于~1900Ma经历中低压麻粒岩相变质作用的改造,其主量元素和微量元素组成具有岛弧拉斑玄武质岩石的地球化学特征,其原岩可能是板块汇聚动力学背景下,岛弧构造环境中形成的辉长岩或辉绿岩。
关键词: 基性麻粒岩     年代学     地球化学     大青山-乌拉山变质杂岩    
Geochronological and geochemical study of the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex, the central Khondalite Belt in the North China Craton
LIU PingHua, LIU FuLai, CAI Jia, LIU JianHui, SHI JianRong, WANG Fang     
Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Abstract: The Lijiazi mafic granulites are predominantly composed of Amp Cpx-Opx granulite, Grt-bearing Amp Cpx-Opx granulite and Bt Amp Cpx-Opx granulite, and have been found as irregular lenses and deformed dykes within the Al-rich gneisses and granitic gneisses in the Daqingshan-Wulashan metamorphic complex. The apparent207Pb/206Pb ages recorded by the metamorphic zircon domains with the middle-to low-pressure granulite-facies mineral inclusions of Cpx+Amp+Pl, which are consistent to the matrix-type metamorphic assemblages of the mafic granulites, range from 1933±39Ma to 1834±40Ma with a weighted mean age of 1892±7Ma, which are interpreted as the timing of the middle-to low-pressure granulite-facies metamorphism of the mafic granulites. Mobile elements such as the large ion lithophile elements (K, Na, Sr, Rb) of the mafic granulites changed in significant levels, high field strength elements (such as Nb, Zr, Ti) and rare earth elements basically unchanged and keep stable during the metamorphic process. The Lijiazi mafic granulites from the Daqingshan complex belong to tholeiitic-basaltic rock series, with the concentrations of SiO2 at 45.58%~51.40%, and Mg# value mainly ranging from 41 to 54. The composition of the rare earth elements of the mafic granulites is characterized by the flat, (La/Yb)CN=1.30~1.51) with no Eu anomalies (Eu/Eu*=0.93~1.04) on the chondrite-normalized patterns. All samples of the mafic granulites share obvious negative anomaly in Nb, Zr and Ti of the Phanerozoic island arc basalts. According to the geochemical and geochronological data of the Lijiazi mafic granulites, it is suggested that the protolith ages of the Lijiazi mafic granulites is 2450~1930Ma, the protoliths of the mafic granulites are undergone middle-to low-pressure granulite-facies metamorphism at~1900Ma, the composition of the major and trace elements of the mafic granulites is characterized by the island arc tholeiitic basaltic rocks, and their protoliths may be gabbro or diabase dykes which formed in the island arc of the plate convergence dynamic setting.
Key words: Mafic granulites     Geochronology     Geochemistry     Daqingshan-Wulashan metamorphic complex    
1 引言

大青山-乌拉山早前寒武纪高级变质杂岩,是华北克拉通北缘孔兹岩带的重要组成部分,主要由太古代兴和岩群麻粒岩系(基底再造岩系)、古元古代孔兹岩系、(紫苏)花岗质片麻岩-闪长质片麻岩、古元古代基性麻粒岩和斜长角闪岩所组成。多年来,许多研究者对大青山-乌拉山变质杂岩进行了深入而系统的成因矿物学、岩石学、变质演化、同位素年代学和构造解析等方面的研究,并在其物质组成、变质演化、原岩性质、地质事件演化序列及其成因机制等方面取得了一系列的重要研究成果和进展(李璞等,1963董启贤和周俊昌,1984崔文元等,1988金巍,1989甘盛飞,1991甘盛飞和钱祥麟, 1991, 1992, 1996金巍等,1991徐学纯, 1991a, b, c19921995刘喜山等,1992Liu et al., 1993a, b; 李树勋等,1994刘喜山, 1994, 1996金巍和李树勋,1996胡凤翔,2000李龙等,2000苗来成等,2000徐仲元等, 2001, 2002, 2003, 2005a, b, 2007, 2011谭应佳等,2002宋海峰和徐仲元,2003杨振升等, 2003, 2006赵庆英,2003贾和义等,2004宋海峰等;2005吴昌华等,2006Xia et al., 2006; Liu et al., 2007; 蒙炳儒,2007陈晓锋等,2008董春艳等, 2009a, b刘正宏等, 2011a, bWan et al., 2009, 2013; 章永梅等,2011Dong et al., 2012; Hu et al., 2012; 董晓杰,2012马铭株等,2012Ma et al., 2012; 万渝生等,2012刘建辉等,2013)。如,有关大青山-乌拉山变质杂岩的锆石U-Pb年代学和Hf同位素研究显示:大青山-乌拉山地区不仅存在~2500Ma、~1950Ma、~1850Ma的岩浆作用以及相应的地壳生长事件,而且,还存在2500~2450Ma的高级变质事件(李璞等,1963苗来成等,2000Wan et al., 2009, 2013; 章永梅等,2011万渝生等,2012Dong et al., 2012; 及其参考文献)。区域变质演化研究表明,与孔兹岩带西段千里山-贺兰山地区以及东段集宁地区出露的孔兹岩系一样,该区不仅存在具有顺时针P-T轨迹和减压反应结构的含堇青石高温中压泥质麻粒岩和含假蓝宝石超高温泥质麻粒岩,而且,还存在具有逆时针P-T轨迹的基底再造麻粒岩系(金巍,1989金巍等,1991Liu et al., 1993b; 刘喜山, 1994, 1996李树勋等,1994徐学纯,1995金巍和李树勋,1996Guo et al., 2012; 蔡佳等,2013;及其参考文献);高精度同位素年代学研究还显示,乌拉山岩群孔兹岩系的原岩沉积时代为2000~1950Ma,此外,无论是基底再造岩系,古元古代变质基性岩,还是广泛分布的古元古代孔兹岩系都记录了1950~1850Ma高级变质事件(吴昌华等,2006Xia et al., 2006; 董春艳等, 2009a, bWan et al., 2009, 2013; Dong et al., 2012Ma et al., 2012; 及其参考文献)。

然而值得注意的是,尽管有关大青山-乌拉山变质杂岩的研究已取得大量研究进展。但是,许多重要的关键问题仍然没有解决。比如,大青山-乌拉山变质杂岩中广泛分布的基性麻粒岩的变质演化以及其原岩的岩石成因与构造环境就是一个没有明确的问题。大青山-乌拉山变质杂岩中是否存在具有顺时针P-T轨迹和减压反应结构的高压基性麻粒岩?其原岩是大陆裂谷构造环境下侵入的辉长质小岩体或辉绿质岩墙,还是形成于汇聚板块边缘的岛弧构造环境?这些问题一直未能得到明确的阐述。

本文选择大青山-乌拉山变质杂岩立甲子基性麻粒岩,在详细的野外地质剖面观察和岩相学研究基础上,重点研究其年代学和地球化学特征。首先,系统地研究其锆石矿物包体及锆石U-Pb年代学特征,而后,充分分析变质过程中元素迁移规律,慎重选择地球化学指标和相关判别图解,并结合年代学资料,探讨大青山-乌拉山变质杂岩立甲子基性麻粒岩可能的原岩性质和构造环境,从而为精细刻画孔兹岩带中段大青山-乌拉山变质杂岩古元古代变质演化历史与造山作用过程,以及明确其地质事件演化序列与岩浆事件性质提供新的科学依据。

2 地质背景

研究区位于黄河以北的内蒙古大青山-乌拉山地区,是华北克拉通北缘沿集宁-大青山-乌拉山-千里山-贺兰山一线展布的孔兹岩带的重要组成部分(图 1),其南北两侧分别为太古代鄂尔多斯陆块和阴山陆块,西侧分别是千里山变质杂岩和贺兰山变质杂岩,东侧与孔兹岩带的集宁变质杂岩相邻(图 1)。大青山-乌拉山地区出露的岩石类型以早前寒武纪变质岩系和显生宙沉积岩系为主,显生宙沉积岩系主要为侏罗纪含煤陆源碎屑岩系。此外,在局部地区有少量中元古代-中生代岩浆岩分布。

图 1 大青山-乌拉山变质杂岩地质简图及采样位置(a, b据Zhao et al., 2005; c, 据刘正宏等, 2007) Fig. 1 Simplified geological map showing the geological setting of the Daqingshan-Wulashan metamorphic complex and the sample locations of the mafic granulites (a, b, after Zhao et al., 2005; c, Liu et al., 2007)

区内早前寒武纪变质岩主要包括新太古代兴和岩群麻粒岩系、古元古代乌拉山岩群和美岱召岩群变质表壳岩系(孔兹岩系)、新太古代深变质岩浆岩、古元古代花岗质岩石以及少量的古元古代(石榴)基性麻粒岩和斜长角闪岩岩墙和小岩体。这些岩石单元曾先后被命名为太古宙桑干群、五台群,太古宙乌拉山群、集宁岩群;孔兹岩系和麻粒岩系,太古宙岩系、古元古代岩系和基底再造岩系。新太古代兴和岩群麻粒岩系,在区内出露广泛,主要出露于哈德门沟-贾浪沟-爬榆树-乌兰此老-五当召-后厂汉大坝一带,此外,在包头南部的水涧沟、杨圪楞煤矿等地和北部阴山陆块内部的下湿壕-后腮忽洞-后哈拉站一带则呈大小不等的残片零星出露(图 1)。其主要由角闪二辉麻粒岩、含榴角闪二辉麻粒岩、黑云紫苏麻粒岩,磁铁紫苏麻粒岩、紫苏斜长麻粒岩、角闪紫苏二长麻粒岩、辉石斜长角闪岩和角闪斜长透辉岩夹条带状铁英岩所组成,在局部地段可以出现基性麻粒岩-中性麻粒岩-酸性麻粒岩的韵律层,总体上具有灰绿色、灰褐色外貌,并以厚层状产出为特征。成分上以中性麻粒岩为主,夹酸性麻粒岩、基性麻粒岩和条带状铁英岩层。大量的原岩恢复和岩石化学资料分析结果表明(李树勋等,1994董晓杰,2012),新太古代兴和岩群麻粒岩系为一套中基性火山沉积建造夹条带状铁矿。最新的高精度同位素定年结果显示,研究区内出露的新太古代兴和岩群麻粒岩系不仅记录了1950~1850Ma高级变质事件,还记录了2500~2450Ma高级变质事件(Ma et al., 2012)。古元古代乌拉山岩群主要分布在包头以北哈德门沟、忽鸡沟、大南沟、五当召和鸡灯湾等地,包括两个岩石单元,其上部岩石单元总体与孔兹岩系相当,主要为石榴黑云二长/斜长片麻岩、夕线石榴堇青黑云二长/斜长片麻岩、(石墨)大理岩、长石石英岩、黑云变粒岩等,而其下部主要是黑云角闪质片麻岩,包括含石英辉石斜长角闪岩、含石英钾长/二长角闪岩、斜长角闪岩、角闪斜长片麻岩、黑云角闪斜长片麻岩、黑云角闪二长片麻岩、黑云钾长/二长片麻岩夹辉石磁铁石英岩。近年来对乌拉山岩群高精度同位素定年结果表明,乌拉山岩群上段变质表壳岩的沉积时代主要介于2000~1950Ma之间,其变质时代为1950~1850Ma (吴昌华等,2006Xia et al., 2006; Wan et al., 2009; Dong et al., 2012Ma et al., 2012; 及其参考文献)。美岱召岩群主要分布于研究区南部的萨拉齐-窑子湾一带,为一套绿片岩相变质的表壳岩系,主要由石英岩、长石石英岩、黑云变粒岩、阳起变粒岩等所组成。美岱召岩群底部石英岩中碎屑锆石的SHRIMP U-Pb年龄在3150~1999Ma之间,说明美岱召岩群最大沉积时代应小于1999Ma (Wan et al., 2009),与乌拉山岩群孔兹岩系的沉积时代大体相当。

太古代深变质岩浆岩主要分布在包头以东萨拉齐一带,在哈德门沟口、沙德盖、明安火车站和包头以北大庙等地均有零星分布,主要岩性为麻粒岩相变质的闪长质片麻岩、TTG质和花岗质片麻岩所组成。其中,花岗质片麻岩主要包括紫苏花岗质片麻岩、石榴花岗质片麻岩、眼球状钾长花岗质片麻岩和斜长花岗质片麻岩。近年来的高精度同位素定年结果表明,大青山-乌拉山地区太古代深变质岩浆岩的原岩形成时代主要为~2500Ma (Wan et al., 2013; 刘建辉等,2013;及其参考文献)。古元古代岩浆岩在区内多以形态各异、规模较小的侵入体和变形岩墙出露,其岩石成分变化也较大,在区域上广泛分布。根据其岩相特征、地球化学特征以及野外地质关系,可以划分为以下三个岩石系列:石英二长岩-石英正长岩系列;黑云角闪花岗岩系列和变质辉长辉绿岩系列。其中,变质辉长辉绿岩,主要分布在包头以东萨拉齐地区,在沙德盖、大庙、明安火车站和五当召等地区有零星分布,包括的主要岩性为石榴基性麻粒岩、含榴角闪二辉麻粒岩、角闪二辉麻粒岩、辉石斜长角闪岩和斜长角闪岩。这些变质基性岩主要以变形岩墙和不规则透镜体形式分布于围岩紫苏花岗质片麻岩、石英闪长片麻岩和富铝片麻岩之中,它们的形成时代具有多期次的特征,主要包括~2450Ma、~1950Ma和~1850Ma (Wan et al., 2013)。

区内中元古代-中生代岩浆岩主要为中元古代辉长岩-石英闪长岩和印支期花岗岩。其中,印支期花岗岩以沙德盖中粗粒黑云母花岗岩为代表,主要分布在沙德盖和查汗沟口牧场一带,出露面积为24km2,侵入于新太古代晚期-古元古代早期紫苏花岗质片麻岩中。其形成时代为~220Ma (侯万荣等,2011)。

3 岩相学特征

立甲子基性麻粒岩剖面出露的岩石主要由富铝片麻岩、花岗质片麻岩和基性麻粒岩所组成(图 2图 3)。其中,富铝片麻岩以含石榴石变斑晶和粗粒夕线石为特征,外观上为青灰色,中粒不等粒粒状变晶结构,片麻状构造发育,其主要组成矿物为石榴石+斜长石+钾长石+黑云母+石英+夕线石,此外还含有~5%的细粒磁铁矿、浑圆状浅绿色尖晶石,常见的副矿物为钛铁矿、磷灰石、独居石和锆石。花岗质片麻岩,外观上具有浅肉红色-砖红色的外貌特征,局部含细粒石榴石,中粒等粒花岗变晶结构,片麻状构造发育,石英和长石因强烈的变形作用,而呈拔丝拉长结构。其主要构成矿物为石英+钾长石(正条纹长石和钾微斜长石)+斜长石,次要矿物为黑云母(已绿泥石化)、石榴石和不透明铁质矿物,常见的副矿物为锆石和磷灰石。

图 2 大青山-乌拉山变质杂岩中立甲子基性麻粒岩地质剖面图 Fig. 2 Geologic section map of the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex

图 3 大青山-乌拉山变质杂岩中立甲子基性麻粒岩典型地质产状 (a)-基性麻粒岩以变形岩墙形式产于富铝片麻岩中;(b)-基性麻粒岩以不规则透镜体形式产于富铝片麻岩中;(c、d)-基性麻粒岩以变形岩墙形式产于花岗质片麻岩中 Fig. 3 Typical field photographs showing the relationships between the Lijiazi mafic granulites and their country rocks from the Daqingshan-Wulashan metamorphic complex (a)-the mafic granulites as deformed dykes preserved within the Al-rich gneisses; (b)-the mafic granulites as irregular lenses preserved within the Al-rich gneisses; (c, d)-the mafic granulites as deformed dykes preserved within the granitic gneisses

基性麻粒岩主要由二辉麻粒岩、角闪二辉麻粒岩、含榴角闪二辉麻粒岩和黑云角闪二辉麻粒岩组成,其中,角闪二辉麻粒岩保存了典型中低压麻粒岩相矿物组合(图 4)。基性麻粒岩外观上为灰黑色,中至细粒等粒粒状变晶结构,块状构造和片麻状构造。其主要组成矿物为单斜辉石、斜方辉石和斜长石,次要矿物角闪石和黑云母(图 4)。其中,单斜辉石和斜方辉石主要以填隙状均匀地分布在斜长石间,当二者相接触时,接触界线平直,显示平衡共生的特点,而角闪石和黑云母,主要沿主期片麻理方向不均匀分布在岩石之中,局部可见黑云母切穿辉石和角闪石,斜长石则因强烈的变形而发育机械双晶。此外在基性麻粒岩中还含有~5%的细粒磁铁矿和细粒浑圆状石英,常见的副矿物为钛铁矿、磷灰石和锆石。

图 4 大青山-乌拉山变质杂岩中立甲子基性麻粒岩典型显微结构照片 (a、b)-角闪二辉麻粒岩主要矿物组合:斜方辉石(Opx)+单斜辉石(Cpx)+角闪石(Amp)+斜长石(Pl);(c)-角闪二辉麻粒岩主要矿物组合:斜方辉石+单斜辉石+斜长石;(d)-含榴二辉麻粒岩主要矿物组合:斜方辉石+单斜辉石+斜长石+石榴石(Grt). 均为单偏光下 Fig. 4 Photomicrographs showing metamorphic assemblages of the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex (a, b)-the matrix-type metamorphic assemblages of Opx+Cpx+Amp+Pl preserved in the Amp Cpx-Opx granulite; (c)-the matrix-type metamorphic assemblages of Opx+Cpx+Pl preserved in the Cpx-Opx granulite; (d)-the matrix-type metamorphic assemblages of Opx+Cpx+Pl+Grt preserved in the Grt-bearing Cpx-Opx granulite. All under plane polarized light
4 样品采集与分析方法

本次分析测试样品均采自大青山-乌拉山变质杂岩中立甲子富铝片麻岩-基性麻粒岩-花岗质片麻岩实测地质剖面,地质剖面的详细描述见蔡佳等(2013)的论述。其中,样品BT35-2、BT35-4、P01-1、P01-3、P01-5、P01-7、P01-8、P01-9与P01-12采自富铝片麻岩系中的变形基性岩墙和构造透镜体,而样品BT35-6、P01-14与P01-16则采自花岗质片麻岩之中的变形岩墙和透镜体(图 2)。

本文共对11件基性麻粒岩样品进行了系统的室内综合研究。首先,在电子显微镜下对立甲子基性麻粒岩及其围岩薄片进行详细的岩相学研究,并在此的基础上,选取基性麻粒岩样品的新鲜部分,在河北省区域地质调查所矿物分选实验室进行全岩粉末样品磨制至200目。而后,进行全岩主量与微量测试分析,全岩主量与微量元素测试在国家地质实验测试中心3080E型荧光光谱仪XRF以及等离子质谱仪(ICP-MS)上完成,具体分析的测试条件和步骤可参阅靳新娣和朱和平(2000)的论述。

此外,本文还对1件基性麻粒岩样品进行了系统的锆石年代学测试。锆石破碎与分选在河北省区域地质调查所矿物分选实验室完成。首先,将样品(约5kg)进行破碎至适当粒级,经清洗、烘干和筛选后,采用磁选和重液分选出不同粒级的锆石晶体,然后在双目镜下挑选出颗粒相对完整的锆石晶体约200粒,制成符合激光拉曼测试、阴极发光图像照相和LA-ICP-MS U-Pb定年的标准锆石靶。

锆石微区矿物包体鉴定在中国地质科学院地质研究所大陆构造与动力学国家重点实验RENISHAW-1000型激光拉曼仪、JSM-561LV型扫描电镜和能谱仪(英国牛津公司INCA软件包版本4.4)上进行,而锆石矿物包体的化学成分测试在北京大学造山带与地壳演化教育部重点实验室JXA-8100型电子探针上进行。锆石U-Pb定年的LA-ICP-MS测试在天津地质矿产研究所同位素实验室Neptune型MC-ICP-MS上进行,上述实验测试条件详见刘平华等(2011)侯可军等(2009)的说明与描述。所有矿物代号据Whitney and Evans (2010)资料。FeOT=0.8998×Fe2O3+FeO,Mg#=100×Mg/(Mg+Fe2+)(摩尔分数),Eu/Eu*=EuCN/(SmCN× NdCN)1/2,CN表示与球粒陨石标准化值(Sun and McDonough, 1989)。

5 分析结果 5.1 锆石U-Pb年代学特征

立甲子基性麻粒岩锆石为暗紫红色,不透明-半透明,以半自形浑圆状晶体为主,少数者为他形不规则粒状晶体,长宽比多为1:1~1:2。锆石颗粒较小,一般长轴为50~150μm,短轴为30~100μm。锆石矿物包体激光拉曼测试-扫描电镜能谱分析-电子探针测试和锆石阴极发光图像研究结果表明,立甲子基性麻粒岩(BT35-2)锆石可分为两种类型。其中,第一类锆石为柱状(以长柱状为主),具有相对弱发光效应特征(灰黑色),从核部至边部具有较模糊的岩浆结晶环带,该类锆石不含矿物包体(图 5)。与此相反,第二类锆石主要为浑圆状-椭圆状晶体,少数者为柱状和不规则粒状晶体,具有相对均匀强发光效应(灰色-灰白色)特征,多数者其内部无分带特征(图 6),少数者发育冷杉叶结构,显示变质锆石成因。锆石矿物包体激光拉曼与扫描电镜能谱综合研究表明,该类变质锆石微区含有的矿物包体主要为单斜辉石、角闪石、斜长石和磷灰石等。该类包体矿物组合与寄主岩石基性麻粒岩的矿物组合完全一致。

图 5 大青山-乌拉山变质杂岩中立甲子基性麻粒岩(BT35-2)中岩浆锆石的阴极发光图像与LA-ICP-MS定年结果 (a)-样品第15粒锆石(BT35-2.15);(b)-样品第9粒锆石(BT35-2.9);(c)-样品第14粒锆石(BT35-2.14) Fig. 5 Cathodoluminescence (CL) images and LA-ICP-MS U-Pb ages of magmatic zircons from the mafic granulite (sample BT35-2) from the Daqingshan-Wulashan metamorphic complex (a)-zircon grain BT35-2.15; (b)-zircon grain BT35-2.9; (c)-zircon grain BT35-2.14

图 6 大青山-乌拉山变质杂岩中立甲子基性麻粒岩(BT35-2)中变质锆石的阴极发光图像与LA-ICP-MS定年结果 (a)-样品第17粒锆石(BT35-2.17)具有相对均匀的弱发光效应(灰黑色);(b)-样品第50粒变质锆石(BT35-2.50)具有相对均匀的弱发光效应(灰黑色);(c)-样品第39粒锆石(BT35-2.39)具有相对均匀的强发光效应(灰白色);(d)-样品第22粒锆石(BT35-2.22)具有相对均匀的弱发光效应(灰黑色);(e)-样品第44粒锆石(BT35-2.44)具有相对均匀的强发光效应(灰白色);(f)-样品第34粒锆石(BT35-2.34)具有相对均匀的强发光效应(灰白色) Fig. 6 Cathodoluminescence (CL) images and LA-ICP-MS U-Pb ages of metamorphic zircons from the mafic granulite (sample BT35-2) from the Daqingshan-Wulashan metamorphic complex (a)-zircon grain BT35-2.17 showing homogeneous relatively low-luminescence (greyish black); (b)-zircon grain BT35-2.50 showing homogeneous relatively low-luminescence (greyish black); (c)-zircon grain BT35-2.39 showing homogeneous relatively high-luminescence (greyish white); (d)-zircon grain BT35-2.22 showing homogeneous relatively low-luminescence (greyish black); (e)-zircon grain BT35-2.44 showing homogeneous relatively low-luminescence (greyish white); (f)-zircon grain BT35-2.34 showing homogeneous relatively low-luminescence (greyish white)

本文对基性麻粒岩(BT35-2)锆石进行了系统的定年研究,共计测试了51个锆石微区,其分析结果列入表 1中,相应的207Pb/235U-206Pb/238U关系图解如图 7所示。5个具有相对弱发光效应(灰黑色)岩浆锆石微区的Th、U含量明显偏高,且变化范围较大,分别介于11×10-6~185×10-6和364×10-6~1468×10-6之间,相应的Th/U比值变化亦较大,变化于0.03~0.64之间(表 1)。5个测试点显示了比较分散的207Pb/206Pb表面年龄,变化于2526±10Ma~2302±18Ma之间。在207Pb/235U-206Pb/238U关系图解中,该类岩浆锆石所有测试点均位于谐和线下方(Amelin et al., 2000),显示出强烈的铅丢失,这与其具有模糊的岩浆结晶环带所指示的铅丢失相一致。因此,该组年龄不具有真实的地质意义。

表 1 大青山-乌拉山变质杂岩中立甲子基性麻粒岩(BT35-2)锆石LA-ICP-MS定年结果 Table 1 LA-ICP-MS ananlyses of zircons from the Lijiazi mafic granulite (sample BT35-2) from the Daqingshan-Wulashan complex

图 7 大青山-乌拉山变质杂岩中立甲子基性麻粒岩(BT35-2)中锆石207Pb/235U-206Pb/238U年龄关系图 Fig. 7 207Pb/235U-206Pb/238U diagrams showing U-Pb analyses for zircons of the Lijiazi mafic granulite (BT35-2) from the Daqingshan-Wulashan metamorphic complex

与此相反,46个具有相对较强发光效应的变质锆石微区的Th、U含量明显偏低,且变化范围较大,分别变化于2×10-6~54×10-6和11×10-6~150×10-6之间,相应的Th/U比值介于0.08~0.48之间(表 1)。所有变质锆石微区测试点记录了十分相近的207Pb/206Pb表面年龄,变化于1933±39Ma~1834±40Ma之间,相应的加权平均年龄为1892±9Ma (MSWD=0.45,n=46;表 1图 7),该类变质锆石含有中低压麻粒岩相特征的变质矿物(Cpx+Amp+Pl)。因此,它们应代表寄主岩石--立甲子基性麻粒岩原岩经历中低压麻粒岩相的变质时代。

5.2 主量元素特征

在主量元素分析表(表 2)中,立甲子基性麻粒岩显示SiO2、TiO2、K2O、Na2O的含量变化较大,其中,SiO2含量介于45.58%~51.44%之间,TiO2含量变化于0.72%~2.21%之间,K2O含量变化于0.35%~1.22%之间,Na2O含量变化于0.77%~2.74%;而CaO、Al2O3和P2O5含量变化相对较小,分别变化于8.55%~11.43%、12.85%~15.75%和0.06%~0.37%之间。所有样品中,FeOT含量变化也较大,且相对较高(10.60%~17.10%),MgO含量变化范围却较小,MgO含量集中变化于6.23%~7.64%之间,相应的Mg#值变化范围也较大(39~54),皆低于原生玄武质岩石(Mg#=70,Dupuy and Dostal, 1984),具有弱演化玄武质岩石的特征。

表 2 大青山-乌拉山变质杂岩中立甲子基性麻粒岩主量(wt%)和微量(×10-6)元素分析结果 Table 2 Major (wt%) and trace (×10-6) elements analyses results for the Lijiazi mafic granulites of the Daqingshan-Wulashan complex
5.3 稀土元素特征

在稀土元素分析表(表 2)中,立甲子基性麻粒岩的稀土元素含量变化范围较大,存在内在差异,根据其稀土元素含量及相关比值,可进一步分为稀土元素含量相对较低的A类(35.81×10-6~64.89×10-6)和稀土元素含量相对较高的B类(120.1×10-6~148.8×10-6)。无论是A类还是B类,其轻、重稀土元素分异程度不明显((La/Yb)CN=1.30~1.52),具有平坦的稀土配分模式(图 8)。立甲子基性麻粒岩Eu异常不明显(Eu/Eu*=0.93~1.04),样品P01-7(Eu/Eu*=0.88)除外,表明立甲子基性麻粒岩的原岩形成过程中无大量的斜长石结晶分离(Woodhead, 1988)。

图 8 大青山-乌拉山变质杂岩中立甲子基性麻粒岩稀土配分图解(球粒陨石值据Sun and McDonough, 1989) Fig. 8 Chondrite-normalized REE diagrams for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex (Chondrite values after Sun and McDonough, 1989)
5.4 微量元素特征

在微量元素分析表(表 2)中,除样品P01-6外,大部分样品Ni含量变化范围很小,分别变化于120×10-6~301×10-6、64.1×10-6~120×10-6,其中,Ni含量总体上与正常洋中脊玄武岩(N-MORB)相当(Ni=149.5×10-6Hofmann, 1988);Co含量变化范围相对较小,介于42.0×10-6~65.9×10-6,总体上与正常洋中脊玄武岩(N-MORB)相当(Co=47.47×10-6; Hofmann, 1988);而大部分样品中Ta (0.14×10-6~0.50×10-6)、Nb (1.88×10-6~7.83×10-6)、Zr (42.9×10-6~169.0×10-6)、Hf (1.30×10-6~4.38×10-6)含量总体上与正常洋中脊玄武岩(N-MORB)含量相当(Hofmann, 1988; Sun and McDonough, 1989)。在原始地幔标准化的微量元素配分图解中(图 9),立甲子基性麻粒岩的配分模式与REE配分曲线一样,存在内在差异,可以进一步划分A类和B类(图 9)。但是,无论A类还是B类样品,所有样品皆与N-MORB或大陆地壳存在明显的差异,即具有平坦的曲线特征和明显的Nb、Zr、Ti的负异常(图 9),与显生宙典型岛弧玄武岩相似(McCulloch and Gamble, 1991)。

图 9 大青山-乌拉山变质杂岩中立甲子基性麻粒岩微量元素地幔标准化配分图解(原始地幔值据Sun and McDonough, 1989) Fig. 9 Primitive mantle-normalized trace element diagrams for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex (Primitive mantle values after Sun and McDonough, 1989)
6 讨论与结论 6.1 原岩形成时代与变质时代

到目前为止,前人已对大青山-乌拉山变质杂岩进行了大量高精度锆石SHRIMP和LA-ICP-MS U-Pb年代学测试,并积累了丰富的同位素年代学数据,为系统建立孔兹岩带中段大青山-乌拉山变质杂岩的地质事件演化序列提供新的科学依据(Guo et al., 2012; Wan et al., 2013)。最新的年代学研究表明,与孔兹岩带东段集宁地区相似(Peng et al., 2010, 2011, 2012),大青山-乌拉山变质杂岩在古元古代构造演化过程中,也存在一系列的基性岩浆活动(~2450Ma,~1950Ma,~1850Ma; Wan et al., 2013)。本文对立甲子基性麻粒岩中5个具有相对弱发光效应(灰黑色)岩浆锆石微区进行了年代学测试,结果显示它们记录了比较分散的207Pb/206Pb表面年龄(2526±10Ma~2302±18Ma),由于这些岩浆锆石微区受到后期强烈的热事件影响,发生了强烈的铅丢失(Amelin et al., 2000),它们不能代表立甲子基性麻粒岩的原岩形成时代的上限。然而,详细的野外地质观察研究表明,立甲子基性麻粒岩侵位于花岗质片麻岩和富铝片麻岩之中,这些野外地质关系说明,立甲子基性麻粒岩的原岩形成时代要晚于花岗质片麻岩和富铝片麻岩的原岩形成时代,而花岗质片麻岩的原岩时代为~2450Ma (另文发表)。因此,立甲子基性麻粒岩原岩形成时代应小于~2450Ma,而立甲子基性麻粒岩中最老的变质锆石记录的谐和207Pb/206Pb表面年龄为1933±39Ma。故本文初步判定立甲子基性麻粒岩原岩形成时代应介于2450~1930Ma之间,这与孔兹岩带中段大青山-乌拉山地区以及东段集宁地区古元古代存在一系列的基性岩浆作用相吻合(Peng et al., 2010, 2011, 2012; Wan et al., 2013)。

如上所述,前人已对大青山-乌拉山变质杂岩进行了大量高精度年代学测试,多数研究者认为在1950~1850Ma期间,孔兹岩带曾经历了一次中高压麻粒岩相变质事件,在此期间,鄂尔多斯陆块和阴山陆块发生陆陆碰撞拼合形成统一的西部陆块(Zhao et al., 2012;及其参考文献)。然而值得注意的是,孔兹岩带中各类变质岩石不仅经历了峰期中高压麻粒岩相变质作用,而且还遭受了峰后中低压麻粒岩相和晚期角闪岩相变质作用的叠加改造,锆石成因十分复杂,单纯依靠锆石SHRIMP或LA-ICP-MS U-Pb定年,很难断定这些变质锆石所记录的年龄所代表的真实地质意义。对于孔兹岩带的各类变质岩石的锆石年代学研究,应将锆石矿物包体激光拉曼测试-扫描电镜测试-电子探针分析、锆石内部结构的阴极发光图像分析与锆石SHRIMP或LA-ICP-MS U-Pb定年技术有机结合,才能获得具有真实地质意义的年龄。系统的锆石矿物包体种类鉴定和成分测试结果显示,立甲子基性麻粒岩变质锆石中含有特征变质矿物包体主要为(图 10),单斜辉石(Cpx),角闪石(Amp)和斜长石(Pl),这些包体矿物不仅在种类上与寄主岩石--基性麻粒岩的矿物组合十分一致,而且,在误差范围内,它们的化学成分也是相同的(表 3)。以上证据充分说明了立甲子基性麻粒岩中变质锆石是在角闪二辉麻粒岩相变质条件下形成的,这些变质锆石记录的207Pb/206Pb表面年龄加权平均年龄为1892±9Ma,它应代表寄主岩石--立甲子基性麻粒岩原岩经历中低压麻粒岩相的变质时代。

图 10 大青山-乌拉山变质杂岩中立甲子基性麻粒岩变质锆石中典型矿物包体的背散射电子图像 (a)-样品中第10粒锆石的背散射电子图像与207Pb/206Pb年龄;(b)-图(a)中含有单斜辉石(Cpx)的变质锆石背散射电子图像;(c)-样品中第52粒锆石的背散射电子图像;(d)-图(c)中含有角闪石(Amp)+磷灰石(Ap)的变质锆石背散射电子图像;(e)-样品中第53粒锆石的背散射电子图像;(f)-图(e)中含有斜长石(Pl)的变质锆石背散射电子图像 Fig. 10 Back scattering electron (BSE) images showing the typical mineral inclusions of metamorphic zircons from the Lijiazi mafic granulite, the Daqingshan-Wulashan metamorphic complex (a)-BSE image of the metamorphic zircon grain BT35-2.10 and a 207Pb/206Pb age; (b)-BSE image of the same zircon grain as (a) showing the zircon rim containing inclusion of Cpx; (c)-BSE image of the metamorphic zircon grain BT35-2.52; (d)-BSE image of the same zircon grain as (c) showing the zircon with inclusions of Amp+Ap; (e)-BSE image of the metamorphic zircon grain BT35-2.53; (f)-BSE image of the same zircon grain as (e) showing the zircon with inclusion of Pl

表 3 立甲子基性麻粒岩锆石包体矿物与寄主岩石特征变质矿物化学成分 Table 3 Chemical compositions of the mineral inclusions from the metamorphic zircons and the metamorphic minerals of the Lijiazi mafic granulites
6.2 变质作用与元素迁移

已有研究表明,在变质过程中,特别是高级变质过程中,REE、高场强元素(Th、Nb、Ta、Zr、Hf、Y)是相对不活动的,而大离子亲石元素常常是活动性元素(Rudnick et al., 1985; Rollison, 1993; Kerrich et al., 1999)。由于立甲子基性麻粒岩曾经历了中低压麻粒岩相高级变质作用,所以,在利用元素讨论岩石成因和构造环境之前应判明元素的活动性。Polat et al.(2002, 2009, 2012;及其参考文献)在研究格陵兰Isua绿岩带与Fiskenæsset高级变质片麻岩地体中变质镁铁-超镁铁质岩时,提出4条判别岩石遭受变质改造的条件:(1)利用元素与最不活动性元素Zr在双变量图中的相关性,相关系数R<0.75被认为是活动性的元素;(2)在原始地幔标准化图解中出现明显的Ce异常,通常Ce/Ce*比值大于1.1或小于0.9的样品被认为不同程度遭受了改造;(3)碳酸盐矿物和石英富集(体积百分数>2.0%)的样品可能遭受改造;(4)样品中出现高的烧失量(LOI>6.0%)。其中Zr与其它元素的相关性是简便有效的判别方法(Polat and Hofmann, 2003)。因此,本文利用Zr与主、微量元素的相关性来判明立甲子基性麻粒岩中元素在变质过程的活动性。

立甲子基性麻粒岩的Ti、P、REE、Nb与Zr的相关图如图 11所示,TiO2与Zr具有非常好的相关性,相关系数R=0.90;稀土元素La、Sm、Nd、Gd、Yb与Zr亦具有非常好的相关性,相关系数R分别为0.98、0.94、0.96、0.88、0.88;高场强元素Nb与Zr也显示了很好的相关性,相关系数为R=0.99。

图 11 大青山-乌拉山变质杂岩中立甲子基性麻粒岩Zr与主、微量元素(REE、Nb、Ti、P)变化图 Fig. 11 Zr vs. selected major and trace elements variation diagrams for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex

与稀土元素、大部分高场强元素相比,在立甲子基性麻粒岩中活动性元素Sr、Rb、Cs、Th、K、Na与Zr没有明显的相关性,具有很强的分散性(图 12),其中,Rb的变化范围最大。其它元素如Mg、Si、Ca、Ni、Cr、U与Zr相关系数R分别为0.46、0.27、0.24、0.36、0.12、0.05(图略)。其它元素如Hf、Ta、Fe、Al与Zr亦具有较好的相关性,其相关系数R分别为0.98、0.93、0.87、0.68(图略)。

图 12 大青山-乌拉山变质杂岩中立甲子基性麻粒岩Zr与主、微量元素(Rb、Th、K、Na)变化图 Fig. 12 Zr vs. selected major and trace elements variation for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex

为进一步明确立甲子基性麻粒岩在变质过程中元素迁移规律,我们结合常用的元素比值作进一步讨论。Shaw (1968)曾建立了一个有关各类火成岩钾和铷变化的“主要趋势”(图 13),其中K/Rb比值的相应变化范围为195~433,一般小于250。在许多早前寒武纪麻粒岩地体中,随钾的减少,K/Rb比值则有急剧的升高(Tarney and Windley, 1977)。说明,在麻粒岩相变质过程中,随着钾的丢失,铷有更大程度的丢失,因而在麻粒岩中往往有低的Rb含量和高的K/Rb比值(Green et al., 1972; Field and Clough, 1976; Tarney and Windley, 1977; Clough and Field, 1980; 沈其韩等,1992Jahn and Zhang, 1984; 王凯怡,1986李树勋等,1994)。立甲子基性麻粒岩大部分样品的K/Rb比值都在300~600之间,显示了麻粒岩相变质作用对立甲子基性麻粒岩的K和Rb的丢失效应比较强烈。然而值得注意的是,尽管样品BT35-4、P01-7和P01-9的K/Rb比值低于200,具有“火成岩变化趋势”的特征,但是,岩相学研究表明,在以上3件基性麻粒岩中晚期角闪岩相退变质作用的叠加十分明显,黑云母和绿色角闪石沿主期片麻理方向切穿早期矿物,说明晚期退变质阶段形成的角闪石和黑云母带入了一定数量的K和Rb,从而降低了这3件样品的K/Rb比值。一般来说,Th比La更为不相容,因此La/Th比值升高的趋势可以代表岩浆的分异结晶趋势,但极高的La/Th比值代表了麻粒岩相变质作用造成的Th的亏损(Rogers and Adams, 1978; Rudnick et al., 1985)。在Th/U和La/Th相关图中(图 13b),立甲子基性麻粒岩大部分样品分布在火成岩方框的右边,显示,Th和U同时丢失,而Th的丢失大于U。仅样品P01-14在火成岩方框的上边,显示,Th和U同时丢失,而U的丢失大于Th。样品P01-7位于火成岩方框内,显示在麻粒岩相变质过程中未发生变化。

图 13 大青山-乌拉山变质杂岩中立甲子基性麻粒岩Rb (×10-6)-K (%) (a)与La/Th-Th/U (b)关系图解 图(a)中MT,OT,DGT分别代表Shaw’s主要趋势(大陆火成岩趋势),大洋玄武岩趋势,亏损麻粒岩趋势;Tarney and Windley, 1977; 图(b)中火成岩数据据Rudnick et al., 1985 Fig. 13 Rb (×10-6) vs. K (%) (a) and La/Th vs. Th/U (b) diagrams for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex MT, OT and DGT represent the Shaw's main trend, ocean tholeiite trend and granulite trend in Fig. 13(a); igneous data in the box Fig. 13(b) after Rudnick et al., 1985

立甲子基性麻粒岩主量、稀土和微量元素与Zr的相关性分析结果和常用元素比值的综合研究表明,Ti、Fe、P、Nb、Ta、Hf与稀土元素在变质过程中元素保持稳定,基本无变化,可以代表原岩的含量和特征;Al、Mg、Si、Ca、Ni在变质过程中可能发生了部分迁移,应可以反映原岩的一些特征;而以大离子亲石元素为代表的其它活动性元素如Sr、Rb、Cs、K与Na,基本已发生强烈的迁移,无法代表原岩的含量和特征,因此这些强活动性元素不能用来讨论岩浆系列和岩石成因。

6.3 岩石成因与构造环境

由于上述变质作用的影响,K、Na等活动性元素多已发生较显著的迁移变化,因此利用TAS图判明岩浆岩系列可能存在一些偏差,而采用没有发生显著变化的惰性组分可判别岩浆岩系列。在Nb/Y-Zr/TiO2(Pearce, 1996)与Nb/Y-SiO2(Winchester and Floyd, 1977)分类图中,立甲子基性麻粒岩几乎所有样品皆位于亚碱性玄武岩区域(图 14)。在进一步区分钙碱性和拉斑玄武岩的Zr-Y与FeOT/MgO-SiO2(Miyashiro, 1974)分类图中,立甲子基性麻粒岩所有样品皆位于拉斑玄武岩区(图 15)。

图 14 大青山-乌拉山变质杂岩中立甲子基性麻粒岩Nb/Y-Zr/(TiO2×104)(据Pearce, 1996)与Nb/Y-SiO2(据Winchester and Floyd, 1977) Fig. 14 Nb/Y vs. Zr/TiO2(after Pearce, 1996) and Nb/Y vs. SiO2(after Winchester and Floyd, 1977) diagrams for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex

图 15 大青山-乌拉山变质杂岩中立甲子基性麻粒岩Zr-Y与FeOT/MgO-SiO2分类图解(据Miyashiro,1974) Fig. 15 Zr vs. Y and FeOT/MgO vs. SiO2diagrams for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex (after Miyashiro, 1974)

如上所述,立甲子基性麻粒岩经历了麻粒岩相变质作用,许多元素已经不能代表岩浆源区本身的特征,选取受变质作用影响较小的(REE、Th、Nb、Y与Zr的相关系数R>0.70)样品进行构造环境判别。在微量元素Ti-Zr判别岩浆形成环境相关图中(图 16aPearce, 1982)立甲子基性麻粒岩位于N-MORB区。在相对不活动的元素MnO-TiO2-P2O5图解(图 16bMullen, 1983)、Nb-Zr-Y图解(图 16cMeschede, 1986)和Th-Hf-Nb图中(图 16d);Wood, 1980),立甲子基性麻粒岩既有位于岛弧玄武岩区,也有位于大洋中脊玄武岩区。在进一步区分岛弧玄武岩与其它构造环境的玄武质岩石的构造环境判别图解,立甲子基性麻粒所有样品皆位于岛弧玄武岩区(图 17)。立甲子基性麻粒岩具有岛弧拉斑玄武质岩石的地球化学特征,与微量元素原始地幔标准化图中所有样品显示出Nb、Zr、Ti的负异常相吻合(Woodhead, 1988; Kelemen et al., 1990; McCulloch and Gamble, 1991)。

图 16 大青山-乌拉山变质杂岩立甲子基性麻粒岩构造环境判别图解 在图(a)中(Pearce, 1982),MORB-洋中脊玄武岩,WPB-板内玄武岩,VAB-火山弧玄武岩;在图(b)中(Mullen, 1983),MORB-洋中脊玄武岩,IAT-岛弧拉斑玄武岩,CAB-岛弧钙碱性玄武岩,OIA-洋岛碱性玄武岩,OIT-洋岛拉斑玄武岩;在图(c)中(Meschede, 1986),AⅠ-板内碱性玄武岩,AⅡ-板内碱性玄武岩和拉斑玄武岩,B-富集型洋中脊玄武岩;C-板内拉斑玄武岩和火山弧玄武岩,D-正常洋中脊玄武岩和火山弧玄武岩;在图(d)中(Wood, 1980),A-正常洋中脊玄武岩,B-富集型洋中脊玄武岩和板内玄武岩,C-板内碱性玄武岩,D-岛弧拉斑玄武岩 Fig. 16 Immobile trace element tectonic discrimination diagrams for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex

图 17 大青山-乌拉山变质杂岩中立甲子基性麻粒岩构造环境判别图解(a,据Pearce, 2008;(b-d,据Agrawal et al., 2008) 在图(a)中,C-陆壳混污趋势;W-板内富集趋势;E-MORB-富集型洋中脊玄武岩;N-MORB-正常型洋中脊玄武岩,OIB-洋岛玄武岩;在图(b-d)中,MORB-洋中脊玄武岩;IAB-岛弧玄武岩;CRB-大陆裂谷玄武岩;OIB-洋岛玄武岩 Fig. 17 Immobile trace element tectonic discrimination diagrams for the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex (a, after Pearce, 2008; b-d, after Agrawal et al., 2008)

对于孔兹岩带中段大青山-乌拉山地区以及东段集宁地区出露的古元古代变质基性岩的地球化学性质和形成构造环境,目前还存在不同的认识。董晓杰(2012)Wan et al. (2013)曾对大青山-乌拉山地区古元古代变基性岩做过初步研究,并认为它们形成与孔兹岩带古元古代裂谷构造环境有关;也有研究者认为孔兹岩带东段集宁地区分布的古元古代哈拉沁火山沉积岩系和徐武家辉长苏长岩以及凉城紫苏花岗岩-S型花岗岩可能是洋中脊俯冲过程中岩浆作用在地壳不同层次的产物(Peng et al., 2010, 2011, 2012)。对立甲子基性麻粒岩具有岛弧拉斑玄武岩的地球化学特征,所指示的构造环境,本文做如下分析。

已有研究表明,地壳物质混染可以造成玄武质岩石的Nb (Ta)、Ti的负异常(Ernst et al., 2005; Polat et al., 2011),然而,立甲子基性麻粒岩所有样品不仅具有明显的Nb (Ta)、Ti负异常(图 9),而且具有明显的Zr (Hf)负异常(图 9),这很难用大陆地壳物质的混染来解释,因为地壳物质通常是无Zr (Hf)负异常,还可能存在正异常(Rudnick et al., 1995Gao et al., 1998; Hawkesworth and Kemp, 2006)。此外,立甲子基性麻粒岩与围岩接触关系非常明显、平整,而且没有围岩和壳源包体,说明基性麻粒岩原岩在上升过程中,没有显著的地壳物质加入。因此,地壳物质混染不是造成立甲子基性麻粒岩具有Nb (Ta)、Zr (Hf)、Ti负异常的根本原因。

综合立甲子基性麻粒岩的微量元素地球化学、构造环境判别图以及野外地质关系等方面特征,本文初步认为立甲子基性麻粒岩在变质过程中Nb、Ti、Zr等元素未发生明显的迁移,其微量元素组成具有岛弧拉斑玄武质岩石的特点,与显生宙岛弧拉斑玄武质岩石的形成环境可以类比(Pearce et al., 1984, 1994; Ewart et al., 1994; Hawkins, 1995),造成其Nb (Ta)、Zr (Hf)、Ti等高场强元素亏损原因应是古俯冲带亏损地幔熔体与俯冲板片部分熔融相互作用的结果(岛弧构造环境;Tatsumi, 1989; Peacock, 1993; You et al., 1996; Tatsumi and Kogiso, 1997; Scambelluri and Philippot, 2001)。而根据立甲子基性麻粒岩具有变形岩墙和透镜体的产状特点以及大青山-乌拉山地区及其邻区古元古代大地构造演化特点来看,其原岩更可能为板块汇聚动力学背景下,岛弧构造环境中形成的辉长岩或辉绿岩。

6.4 结论

综合以上的讨论,本文得出如下几点初步认识:

(1)立甲子基性麻粒岩主要由二辉麻粒岩、角闪二辉麻粒岩、含榴角闪二辉麻粒岩和黑云角闪二辉麻粒岩所组成,且以变形岩墙和不规则透镜体的形式赋存于古元古代富铝片麻岩和钾长花岗质片麻岩之中。

(2)锆石矿物包体与LA-ICP-MS U-Pb定年结果和野外地质关系研究表明,立甲子基性麻粒岩原岩形成时代应介于2450~1930Ma之间,并于~1900Ma经历中低压麻粒岩相作用的改造。

(3)立甲子基性麻粒岩在变质过程中许多元素发生了显著的变化,比如K、Na、Sr与Rb等;与此相反,在大部分样品中,其高场强元素(Th、Zr、Ti、Nb)和稀土元素基本无变化,保持稳定;立甲子基性麻粒岩具有平坦型的稀土配分曲线特征,Eu异常不明显(Eu/Eu*=0.88~1.04);立甲子基性麻粒岩所有样品皆具有Nb (Ta)、Zr (Hf)、Ti亏损特征。

(4)立甲子基性麻粒岩主量元素和微量元素组成具有岛弧拉斑玄武质岩石的地球化学特征,其原岩可能是在板块汇聚动力学背景下,岛弧构造环境中形成的辉长岩或辉绿岩。

致谢 本文所有样品全岩主量元素、微量元素、锆石矿物包体和锆石LA-ICP-MS U-Pb定年测试与分析得到了韩慧明、陈方远、阎玲、舒桂明、李怀坤与耿建珍等老师们的帮助;论文写作过程中得到了沈其韩院士、郭敬辉研究员和杜利林研究员的指导与帮助;在此一并致谢。
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