2020, Vol. 36
2. 中国地质大学(北京) 地球科学与资源学院, 北京 100083
2. School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China
羌塘地体位于青藏高原中部，是青藏高原的重要组成部分，北部以金沙江缝合带为界与松潘-甘孜地体相连，南部以班公湖-怒江缝合带为界与拉萨地体相连(潘桂棠等, 2002; 莫宣学和潘桂棠, 2006; 许志琴等, 2007, 2011; 李才等, 2009a; Zhang et al., 2014)。以其内部的龙木措-双湖缝合带为界，羌塘地体又可划分为南、北羌塘两个地体(李才等, 2006, 2007, 2008; Zhai et al., 2010, 2016, 2017; 胡培远等, 2014; Xu et al., 2015)。羌塘地体中部和南缘分布大量的中生代岩浆岩，这些岩浆活动分别与龙木措-双湖古特提斯洋和班公湖-怒江洋的演化相关(Guynn et al., 2006; Zhai et al., 2011, 2013; Li et al., 2014, 2017a, b; Chen et al., 2016a, b; 吴浩, 2016; Wang et al., 2016; Wu et al., 2016)。但是在羌塘地体东部南缘地区，由于龙木措-双湖缝合带和班公湖-怒江缝合带空间位置的相近(图 1a)，使该地区岩浆活动受龙木措-双湖古特提斯洋和班公湖-怒江洋演化叠加控制，导致该地区三叠纪岩浆岩的构造归属一直存在争议。
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图 1 青藏高原中部构造纲要图(a, 据Chen et al., 2018)及羌塘地体东部类乌齐地区地质简图(b) JSSZ-金沙江缝合带；LSSZ-龙木措-双湖缝合带；BNSZ-班公湖-怒江缝合带；LMFZ-洛巴堆-米拉山断裂带；IYZSZ-印度-雅鲁藏布缝合带；DQZ-丁青蛇绿岩；TKZ-同卡蛇绿岩；BSZ-八宿蛇绿岩 Fig. 1 Tectonic outline of central Tibetan Plateau (a, after Chen et al., 2018) and geological map of Leiwuqi area, eastern Qiangtang terrane (b) |
羌塘地体东部类乌齐地区分布着大量的三叠纪岩浆岩，前人对该地区的花岗质岩石已进行了研究，初步确定了岩石类型和形成时代，并对岩石成因和构造背景进行了讨论(王保弟等, 2011; 沙绍礼等, 2012; 时超等, 2012; Hu et al., 2014; Tao et al., 2014; Chen et al., 2018)。目前对于花岗质岩石的构造归属仍存在不同的认识，一种观点认为形成在澜沧江洋闭合之后的碰撞或后碰撞环境下(王保弟等, 2011; Tao et al., 2014)；另一种观点认为形成在龙木措-双湖古特提斯洋闭合之后的碰撞或后碰撞环境下(Hu et al., 2014; Chen et al., 2018)。但是以上认识都是基于对花岗质岩石的研究成果，对该地区出露的基性岩石却鲜有报道，基性岩石的形成时代、岩石成因和形成的构造背景尚不清楚，对该地区基性岩的研究，有助于揭示该地区不同类型岩浆岩的构造属性。本文选择羌塘地体东部类乌齐地区吉塘岩群内出露的变辉长岩为研究对象，对其开展岩石学、锆石U-Pb年代学、锆石Lu-Hf同位素以及岩石地球化学的研究，以限定变辉长岩形成时代，讨论岩石成因和形成的大地构造背景。
1 地质背景和样品本文的研究区位于青藏高原东部类乌齐地区，大地构造位置上位于羌塘地体东部(图 1a)。该地区出露的地层以中-新生代三叠系-白垩系地层为主，其中以三叠系和侏罗系地层发育最为广泛，三叠系地层为一套紫红色的砂砾岩、碎屑岩夹碳酸盐岩建造，侏罗系地层角度不整合于三叠系地层之上，为互层产出的砂岩、泥岩和灰岩。古生代地层在局部零星出露，主要由石炭系马查拉组的一套沉积于大陆边缘的浅海-海陆交互环境的含煤层沉积岩和二叠系莽措组灰岩组成。该地区构造复杂，北西-南东向的褶皱和脆、韧性断层比较发育(潘桂棠, 2004; 曾庆高等, 2010; 时超, 2011)。
类乌齐地区出露的变质岩被称为吉塘岩群，沿他念他翁山脉南侧呈北西向楔状分布。吉塘岩群普遍经历了绿片岩相-角闪岩相的变质，主要由正片麻岩、片岩、混合岩、斜长角闪岩和大理岩组成。吉塘岩群以往一直被认为是该地区的前寒武纪基底(李才等, 2009b; 何世平等, 2012; 邱军强等, 2015)，近年来的研究发现吉塘岩群还经历了泛非期和印支期的岩浆活动以及印支期的变质作用(李才等, 2009b; 时超, 2011; Hu et al., 2014; 邱军强等, 2015)，时超(2011)认为印支期的变质作用可以与藏东地区发育大规模印支期构造岩浆侵入事件相对应，是发生在古特提斯阶段“威尔逊旋回”中聚敛闭合期的事件。类乌齐地区岩浆岩以花岗质岩石为主，主要沿吉塘岩群两侧呈北西-南东向展布，与吉塘岩群呈断层或侵入接触，西北部发育早二叠世和早侏罗世二长花岗岩(潘桂棠, 2004; 曾庆高等, 2010)，中部和南部出露晚三叠世的花岗岩和花岗闪长岩(Tao et al., 2014; Chen et al., 2018)。此外，在吉塘岩群中还发现原岩年龄为早三叠世和晚三叠世的正片麻岩(王保弟等, 2011; 沙绍礼等, 2012; 时超等, 2012; Hu et al., 2014)以及早二叠世的辉长岩(胡培远等, 2016)。
本次研究的变辉长岩样品采自类乌齐地区的吉塘岩群(图 1b)，呈脉状侵入到花岗质片麻岩中(图 2a)。岩石为黑绿色，粒状变晶结构，块状构造，未见明显变形。主要矿物为斜长石(40%~45%)、角闪石(45%~50%)、黑云母(2%~5%)以及少量的石英、绢云母和绿泥石，未见辉石残留。斜长石呈板状自形-半自形晶，常发育聚片双晶，粒径在0.5~2mm之间，局部被绢云母交代(图 2b)。角闪石呈半自形柱状。黑云母呈片状，局部绿泥石化(图 2c)。
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图 2 变辉长岩野外(a)和显微照片(b、c) 矿物代号：Amp-角闪石；Bt-黑云母；Pl-斜长石；Ser-绢云母；Q-石英；Chl-绿泥石 Fig. 2 Field photo (a) and microphotographs (b, c) of the metagabbro |
样品主量和微量元素(包括稀土)成分分析在中国地质科学院国家地质实验测试中心完成。主量元素采用XRF(X-ray fluorescence)方法测定(仪器型号Phillips 4400)，检测限 < 0.01%，分析精度优于5%；FeO采用重铬酸钾滴定法测定；微量元素和稀土元素采用电感耦合等离子体质谱仪(ICP-MS)来测定(仪器型号PerkinElmer NexION 300D)，检测限为1×10-6~0.05×10-6，分析误差为5%~10%。
锆石分选在河北省区域地质矿产调查研究所实验室完成。锆石阴极发光成像在中国地质科学院地质研究所北京离子探针中心完成。锆石U-Pb同位素LA-ICP-MS分析在武汉上谱分析科技有限责任公司完成，激光剥蚀系统为GeoLas 2005，激光斑束直径为32μm，频率为5Hz。ICP-MS仪器型号为Agilent 7700e。采用锆石91500和GJ-1作为标准样品。使用ICPMSDataCal(V10.7)软件对分析数据进行处理。详细的仪器操作条件和数据处理方法见Liu et al. (2010)。使用Isoplot软件(Ludwig, 2003)计算锆石加权平均年龄和绘制谐和图。
锆石Hf同位素测试在中国地质大学(武汉)地质过程与矿产资源国家重点实验室完成，使用的仪器为激光剥蚀多接收等离子体质谱仪(LA-MC-ICP-MS)，激光剥蚀系统为德国Lamda Physik公司生产的Geolas 200M激光剥蚀仪器，MC-ICP-MS系统为德国Thermo Fisher Scientific公司生产的Neptune Plus仪器。实验过程中采用氦气作为剥蚀物质的载气。使用的激光剥蚀斑束直径为44μm，频率为8Hz。91500和GJ-1作为分析测试的标准锆石。详细的仪器状态和使用流程方法见Hu et al. (2015)。
3 锆石U-Pb年代学和Lu-Hf同位素对3个变辉长岩样品(T15-69-1、T15-69-6、T15-69-7)进行锆石U-Pb同位素测年，测试结果见表 1。锆石为无色透明，自形-半自形柱状，长度为100~300μm。阴极发光(CL)图像显示，锆石颜色为灰-灰黑色，部分颗粒发育弱韵律环带或平行条带，变质边较窄或不发育(图 3d)。锆石Th和U含量分别为127×10-6~6413×10-6和418×10-6~4832×10-6，Th/U比值较大，介于0.30~1.32之间，表明其为岩浆成因锆石。3个样品锆石U-Pb年龄谐和度都较好，206Pb/238U加权平均年龄分别为206±2Ma、210±2Ma和212±2Ma(图 3a-c)，该年龄代表了变辉长岩结晶年龄。
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表 1 变辉长岩锆石U-Pb定年结果 Table 1 Zircon U-Pb dating data for the metagabbro |
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图 3 类乌齐地区变辉长岩锆石U-Pb谐和图(a-c)和代表性锆石阴极发光图像(d, 含分析点位和相应年龄，单位Ma) Fig. 3 U-Pb concordia diagrams (a-c) and cathodoluminescence images (d) of representative zircon grains of the metagabbro in Leiwuqi area, showing the analytical spots and related ages (in Ma) |
对以上3个样品进行锆石Lu-Hf同位素分析，测试结果见表 2。样品(176Hf/177Hf)i=0.282344~0.282552，εHf(t)的范围为-10.6~-3.2，单阶段模式年龄为tDM1=1034~1296Ma。
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表 2 变辉长岩锆石Lu-Hf同位素分析结果 Table 2 Lu-Hf isotopic composition of the metagabbro |
类乌齐地区变辉长岩经历了变质作用，大离子亲石元素(如Rb、Ba、Cs、K等)在以上过程中容易发生改变，而高场强元素、稀土元素以及一些过渡金属元素则比较稳定(Winchester and Floyd, 1977; Humphris and Thompson, 1978)。为此，本文选取相对稳定的元素来进行变辉长岩地球化学特征的分析和讨论。
本文共选取了类乌齐地区6个变辉长岩样品进行了主、微量元素测试分析，分析结果见表 3。主量元素测试结果显示，样品SiO2含量为49.36%~52.82%，TiO2含量为1.28%~1.58%，Al2O3含量为17.10%~17.91%，FeOT为9.35%~9.95%，MgO含量为4.28%~7.17%，Mg#值为56~67。在SiO2-(Na2O+K2O)图中投在了辉长岩区域(图 4a)，在FeOT-(Na2O+K2O)-MgO图解中均落在了钙碱性系列区域(图 4b)。
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表 3 变辉长岩主量元素(wt%)和微量(×10-6)元素分析结果 Table 3 Major (wt%) and trace (×10-6) element data for the metagabbro |
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图 4 类乌齐地区变辉长岩SiO2-(Na2O+K2O)图解(a, 据Middlemost, 1994)和FeOT-(Na2O+K2O)-MgO图解(b, 据Irvine and Baragar, 1971) Fig. 4 SiO2 vs. (Na2O+K2O) (a, after Middlemost, 1994) and FeOT-(Na2O+K2O)-MgO (b, after Irvine and Baragar, 1971) classification diagrams of the metagabbro in Leiwuqi area |
在原始地幔标准化微量元素蛛网图中，样品富集大离子亲石元素(如Rb、Ba、Th、U等)，亏损高场强元素(如Nb、Ta、Zr、Hf、Ti等)(图 5a)。稀土元素含量较高，∑REE为157.7×10-6~193.4×10-6，LREE/HREE=4.45~7.05，(La/Yb)N=3.96~8.61，δEu=0.53~0.98。球粒陨石标准化稀土元素配分曲线为右倾型，轻、重稀土分馏明显，具有弱的负Eu异常(图 5b)。
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图 5 类乌齐地区变辉长岩原始地幔标准化微量元素蛛网图(a)和球粒陨石标准化稀土元素配分曲线(b)(标准化值、OIB、E-MORB和N-MORB值据Sun and McDonough, 1989) Fig. 5 Primitive-mantle normalized spider diagram (a) and chondrite-normalized REE patterns (b) for the metagabbro from Leiwuqi area (normalization, OIB, E-MORB and N-MORB values from Sun and McDonough, 1989) |
在早期的区域地质调查中，类乌齐地区已查明出露有三叠纪花岗质岩石。Tao et al. (2014)通过该地区花岗闪长岩和黑云母花岗岩锆石U-Pb同位素测年，获得~219Ma的岩浆年龄，Chen et al. (2018)也报道了吉塘岩群中片麻状花岗质岩石的锆石U-Pb年龄为213~208Ma。本文所研究的变辉长岩呈脉状侵入到花岗质片麻岩中，其结晶年龄为212~206Ma，稍晚于花岗质岩石。以上研究表明类乌齐地区不仅经历了晚三叠世的酸性岩浆事件，还经历了同时期的基性岩浆事件。
5.2 岩石成因类乌齐地区变辉长岩地球化学特征表明，SiO2、TiO2和Al2O3含量随MgO含量的增加而减少，呈现负相关性特征，而FeOT、CaO、(Na2O+K2O)含量和MgO具有良好的正相关性(图 6)，表明岩石形成过程中岩浆发生了结晶分异作用。样品的Mg#值为56~67(表 3)，低于初始岩浆的Mg#值(~70)(Wilson, 1989)，也说明在成岩过程中岩浆发生了一定程度的结晶分异。Al2O3和MgO的负相关性以及Eu、Sr的负异常说明岩浆在演化过程中发生了斜长石分离结晶(图 5、图 6c)。CaO和MgO的正相关性表明岩浆可能发生了辉石的分离结晶(图 6f)，(Na2O+K2O)含量和MgO的正相关性表明岩浆的碱性逐渐减弱(图 6d)。
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图 6 类乌齐地区变辉长岩MgO与SiO2、TiO2、Al2O3、(Na2O+K2O)、FeOT和CaO图解 Fig. 6 Bivariate plots of MgO versus SiO2, TiO2, Al2O3, (Na2O+K2O), FeOT and CaO for the metagabbro in Leiwuqi area |
稀土和微量元素配分曲线特征显示，样品富集LREEs、Th、U和Ba，亏损Nb、Ta、Zr、Hf和Ti，具有岛弧火山岩的地球化学特征(Pearce, 2014)。样品具有较低的εHf(t)(-10.6~-3.2)，单阶段模式年龄tDM1=1034~1296Ma(图 7)，表明其岩浆来自中元古代岩石圈地幔部分熔融或富集地幔(Stern et al., 2004; Ishizuka et al., 2010)。样品较高的La/Nb比值(1.44~2.42)和低的La/Ba比值(0.03~0.05)也表明其具有受俯冲影响的大陆岩石圈地幔源区特征(Saunders et al., 1992)。微量元素Nb、Ta、Zr、Hf的亏损表明岩浆源区有俯冲带物质的加入，样品具有较高的Th(2.15×10-6~10.5×10-6)和LREE(135.7×10-6~167.1×10-6)含量以及Th/Yb和Th/Nb比值，也显示岩石源区有不同程度俯冲沉积物的加入(图 8a, b; Pearce and Peate, 1995; Woodhead et al., 1998; Aldanmaz et al., 2008)。
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图 7 类乌齐地区变辉长岩锆石εHf(t)与U-Pb年龄图(据Chen et al., 2018) Fig. 7 Zircon εHf(t) values vs. U-Pb ages diagram (after Chen et al., 2018) of the metagabbro in Leiwuqi area |
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图 8 类乌齐地区变辉长岩Nb/Yb-Th/Yb(a, 据Pearce, 2014)、Ta/Yb-Th/Yb(b, 据Pearce and Peate, 1995)、La/Nb-Nb/Th(c, 据Pearce, 1983)和Hf/3-Th-Ta(d, 据Wood, 1980)图解 A-亏损性洋中脊玄武岩；B-富集型洋中脊玄武岩和板内玄武岩；C-板内玄武岩；D-火山弧玄武岩 Fig. 8 Nb/Yb vs. Th/Yb (a, after Pearce, 2014), Ta/Yb vs. Th/Yb (b, after Pearce and Peate, 1995), La/Nb vs. Nb/Th (c, after Pearce, 1983) and Hf/3-Th-Ta (d, after Wood, 1980) diagrams for the metagabbro from Leiwuqi area |
类乌齐地区变辉长岩稀土和微量元素配分曲线特征明显区别于N-MORE、E-MORE和OIB的特征(图 5)，样品富集LREEs、Th、U和Ba，亏损Nb、Ta、Zr、Hf和Ti，是典型的岛弧火山岩的地球化学特征(Pearce, 2014)。在Nb/Yb-Th/Yb构造判别图中，样品均落在大陆弧的范围内(图 8a)；在Ta/Yb-Th/Yb图中，样品均落在了活动大陆边缘的区域内(图 8b)；在La/Nb-Nb/Th图解中，样品也落在了弧火山岩或地壳混染岩浆源区域(图 8c)；而在Hf/3-Th-Ta图解中，样品落在了火山弧区域(图 8d)。综上所述，类乌齐地区变辉长岩形成在大陆弧构造背景下。
类乌齐地区位于羌塘地体东部的南缘，由于龙木措-双湖缝合带和班公湖-怒江缝合带在该地区空间位置的相近(图 1a)，使该地区成为龙木措-双湖古特提斯洋和班公湖-怒江洋叠加控制域，该地区三叠纪岩浆活动也主要与龙木措-双湖古特提斯洋和班公湖-怒江洋的演化相关(王保弟等, 2011; 时超等, 2012; Hu et al., 2014; Tao et al., 2014; Li et al., 2017a; Chen et al., 2018)。
关于龙木措-双湖古特提斯洋的演化历史，目前的研究认为其在中-晚寒武纪打开，形成了505~432Ma洋中脊蛇绿混杂岩(Wang et al., 2008; Zhai et al., 2010, 2016; 胡培远等, 2014; Xu et al., 2015)，在晚泥盆世-中三叠世向北俯冲，在羌塘地体中部形成~243Ma蓝片岩-榴辉岩带以及375~246Ma的岛弧火山岩(Liu et al., 2011; Zhai et al., 2011, 2013, 2017; 江庆源等, 2014; 刘函等, 2015; 吴浩, 2016; Fan et al., 2016, 2017)，至晚三叠世该大洋闭合，南、北羌塘地体碰撞，形成237~203Ma碰撞型花岗岩和高压变质带(李才等, 2006, 2007, 2008; Fan et al., 2017; Zhai et al., 2017; Chen et al., 2018; Wang et al., 2018)。前人对于类乌齐地区晚三叠世花岗质岩石的研究表明，岩石属于过铝质的超钾至高钾钙碱性的S型花岗岩，形成在碰撞环境(Tao et al., 2014; Chen et al., 2018)，但是对于其构造归属仍存在分歧，Tao et al. (2014)认为其形成在澜沧江古特提斯洋闭合之后的后碰撞背景下，而Chen et al. (2018)则认为是形成在龙木措-双湖-丁青古特提斯洋闭合而导致的羌塘地体和同卡微陆块碰撞的构造环境下。本文所研究的变辉长岩形成在晚三叠世，形成的构造背景是俯冲环境下的大陆弧背景，而龙木措-双湖古特提斯洋在晚三叠世已经闭合，与该大洋演化相关的构造环境处于碰撞或后碰撞的背景，所以本文认为类乌齐地区变辉长岩的形成与龙木措-双湖古特提斯洋的演化无关。
关于班公湖-怒江特提斯洋的演化历史，最早的观点认为其打开时间在晚二叠世至早三叠世，在班公湖-怒江缝合带内形成255~248Ma的洋中脊蛇绿岩以及洋岛玄武岩和辉长岩(任纪舜和肖黎薇, 2004; 刘鸿飞和刘焰, 2009; Fan et al., 2015, 2017)，通过对缝合带北侧羌塘地体内242~235Ma岛弧火山岩、高级变质岩和沉积盆地的研究，最早的向北初始俯冲的观点，认为其不晚于晚三叠世(段瑶瑶等, 2016; Chen et al., 2016a, b; Zhu et al., 2013; Zeng et al., 2016; Li et al., 2017b; Fan et al., 2018)。基于以上认识，本文所研究的变辉长岩的形成时代和构造背景与班公湖-怒江特提斯洋的演化更加吻合，所以，本文认为类乌齐地区晚三叠世变辉长岩是形成在班公湖-怒江特提斯洋向北俯冲在羌塘地体下的大陆弧背景下。
6 结论(1) 藏东类乌齐地区变辉长岩属于钙碱性系列，在成岩过程中岩浆发生了一定程度的结晶分异。
(2) 类乌齐地区变辉长岩结晶年龄为212~206Ma，为晚三叠世。岩浆来自中元古代岩石圈地幔部分熔融或富集地幔，源区有不同程度俯冲沉积物的加入。
(3) 类乌齐地区变辉长岩是形成在班公湖-怒江特提斯洋向北俯冲在羌塘地体下的大陆弧背景下。
致谢 感谢付长垒和袁月蕾对本文写作的帮助。感谢两位审稿人审阅全文并提出重要修改意见。
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