博格达与哈尔里克早泥盆世火山岩成因对比及其构造意义

2015-03-14 08:26崔方磊汪晓伟马中平孙吉明朱小辉
地质与勘探 2015年3期
关键词:东天山博格达里克

崔方磊,汪晓伟,马中平,孙吉明,朱小辉

(1. 中国地质调查局西安地质调查中心,陕西西安 710054;2. 长安大学地球科学与资源学院,陕西西安 710064)



博格达与哈尔里克早泥盆世火山岩成因对比及其构造意义

崔方磊1,2,汪晓伟1,2,马中平1,孙吉明1,朱小辉1

(1. 中国地质调查局西安地质调查中心,陕西西安 710054;2. 长安大学地球科学与资源学院,陕西西安 710064)

本次研究在东天山博格达东段下涝坝南部新确认一套以安山岩为主的火山岩,为钙碱性系列,LA-ICP-MS锆石U-Pb年龄为397.5±4.7 Ma,属早泥盆世。该火山岩具有一致的轻稀土与重稀土分馏明显的右倾配分模式,K、Ba等大离子亲石元素显著富集,而Nb、Ta等高场强元素相对亏损,其原始岩浆可能来自于石榴石稳定域地幔橄榄岩2%~5%的部分熔融,同化混染程度较小。对应的哈尔里克南麓下泥盆统大南湖组火山岩为一套基性到酸性连续演化的钙碱性系列岩石,稀土丰度较下涝坝火山岩高,轻、重稀土之间分馏较弱,整体富集大离子亲石元素而亏损高场强元素。其岩浆可能来源于尖晶石稳定域地幔橄榄岩<3%的部分熔融,受地壳物质混染明显,岩石后期蚀变较强。Hf/3-Th-Ta图解和Yb标准化的Th-Nb判别图解显示这两套泥盆纪火山岩均形成于与俯冲有关的活动陆缘环境,暗示博格达和哈尔里克地区在早泥盆世时期可能具有相同的大地构造环境。

博格达与哈尔里克 早泥盆世 火山岩 岩石成因 大地构造环境

Cui Fang-lei,Wang Xiao-wei,Ma Zhong-ping,Sun Ji-ming,Zhu Xiao-hui. Comparison of genesis of Devonian volcanic rocks in the Bogda and Harlik areas and its tectonic implications[J]. Geology and Exploration, 2015, 51(3):0519-0533.

0 前言

天山造山带构造上处于西伯利亚、准噶尔-哈萨克斯坦和塔里木板块的交汇处,是中亚增生造山带的关键组成部分,以晚古生代新生陆壳和小陆块的拼合为特征。东天山在古生代分别属于塔里木板块及其南缘活动带(南天山弧后盆地、中天山岛弧带、阿齐山-雅满苏岛弧带)、哈萨克斯坦-准噶尔洋壳板块(哈尔里克岛弧带)、西伯利亚板块及其南缘活动带(麦钦乌拉岛弧海沟系)。自北向南分为五个构造单元,主要包括:哈尔里克造山带、博格达造山带、觉罗塔格造山带、吐哈盆地和中天山地块。长期以来,东天山因其复杂的构造环境,多变的构造样式,广泛发育的构造-岩浆活动以及丰富的矿产资源而倍受国内外学者关注(Yakubchuk,2004;Maoetal.,2005;Wangetal.,2007;Zhangetal.,2008;Charvetetal.,2011;王春艳,2013)。

博格达山和哈尔里克山位于准噶尔盆地与吐哈盆地之间,主要由晚古生代地质体组成。对于哈尔里克构造带是与东准噶尔南部卡拉麦里洋盆向南俯冲有关(马瑞士等,1997;楼法生等,1995;王赐银等,1996;顾连兴,2001;赵明等,2002),还是与康古尔塔格一带洋盆向北俯冲的岩浆相应(周济元等,1994;李文明等,2002)一直存在争议,但对其主体为泥盆纪-石炭纪岛弧的构造环境已达成共识。而对于博格达造山带晚古生代的构造属性,仍存在有岛弧(成守德,1986;Coleman,1989;马瑞士等,1993;Allenetal.,1993)和裂陷槽(肖序常等,1992)或裂谷(吴庆福,1986;何国琦等,1994;顾连兴等,2000,2001b;夏林圻,2007b)等不同观点。同时,研究者对于博格达与哈尔里克两条造山带的关系依然没有定论,方国庆(1994)曾根据博格达-哈尔里克地区的岩石组合及火山岩和沉积岩的地球化学系列推断博格达和哈尔里克均为泥盆纪-石炭纪岛弧。孙桂华等(2005)通过对哈尔里克地区石炭纪闪长岩的研究,并结合近年来大量高精度同位素年代学资料认为,博格达与哈尔里克具有大体相同的岩浆活动历史。而王赐银等(1996)在研究东天山晚古生代的岩浆作用时,发现博格达以基性、中性火山岩和侵入的辉长岩、辉绿岩为特征,罕见花岗岩类,地壳成熟度较低,哈尔里克火山弧则以钙碱性火山岩、火山碎屑岩和花岗岩组合为标志。卢苗安(2007)在总结前人的基础上,综合研究了二者在沉积构造和重磁差异后也认为它们是构造属性不同的独立单元。笔者最近在博格达东段下涝坝地区野外工作中发现一套火山岩,其LA-ICP-MS锆石U-Pb年龄为397.5±4.7Ma,属早泥盆世。因此作者从岩石地球化学方面讨论该套火山岩的岩石成因、岩浆演化以及构造属性,并将其与哈尔里克地区早泥盆世火山岩进行对比,来确认博格达与哈尔里克地区在晚古生代初期是否具有相同的大地构造背景,为进一步理解东天山晚古生代构造演化提供新的依据。

图1 东天山区域构造简图(a:据李亚萍等,2006);博格达东段下涝坝区域地质简图 (b:据1∶25万三道岭幅,长安大学地质调查研究院,2013①,有修改)Fig. 1 Geological sketch of the Eastern Tianshan (a:after Li et al., 2006); Geological sketch of the Xialaoba area in eastern part of Bogda orogenic belt (b:modified from 1∶250000 areal geological map, Chang’an university geology surveying institute,2013①) 1-下泥盆统大南湖组;2-上泥盆统康古尔塔格组;3-下石炭统七角井组;4-下石炭统黑山头组;5-下石炭统姜巴斯套组;6-上石炭统阿什喀腊组;7-上石炭统奥尔吐组;8-上石炭统柳树沟组;9-下二叠统石人子沟组;10-下二叠统三塘胡组;11-第四系;12- 二辉橄榄岩;13-辉长岩;14-二长花岗岩;15-正长花岗岩;16-地名;17-采样点 1-Lower Devonian Dananhu Formation;2-Upper Devonian Kangguertage Formation;3-Lower Carboniferous Qijiaojing Formation;4-Lower Carboniferous Heishantou Formation;5-Lower Carboniferous Jiangbasitao Formation;6-Upper Carboniferous Ashikala Formation;7-Upper Carboniferous Aoertu Formation;8-Upper Carboniferous Liushugou Formation;9-Lower Permian Shirenzigou Formation;10-Lower Permian Santanghu Formation;11-Quarternary;12-lherzolite;13-gabbro;14-monzonitic granite;15-syenoqranite;16-place name;17-sampling location

1 区域地质背景

研究区位于东天山东段,吐哈盆地北缘。根据中国地层区划的基本准则及参照《新疆维吾尔自治区岩石地层》(1999)、《新疆区域地质志》(1993)等重要论著对新疆各断带地层所做的地层区划方案,并结合研究区近年来众多新的地质科研成果,将研究区综合地层区划为南准噶尔-北天山地层分区,博格达-哈尔里克地层小区。研究区中部以晚古生代地层为主,北部为红井子大断裂与白敦子大断裂岩浆带,南部为柳树沟岩浆带,七角井石炭纪沉陷盆地夹于两条岩浆带中间。红井子大断裂与白敦子大断以北发育大套石炭系,柳树沟岩浆带以南为大面积的第四系冲洪积物覆盖。下泥盆统大南湖组(D1d)以浅海相火山碎屑岩、熔岩以及凝灰质细砂岩为主,夹少量大理岩和燧石条带;上泥盆统康古尔塔格组(D3k)为一套陆相火山-火山碎屑岩+河流相或三角洲相的陆源碎屑岩建造,二者呈角度不整合或平行不整合接触。下石炭统底部为一套以砾岩、砂砾岩为主、夹少量薄层状砂岩和粉砂岩的陆源碎屑岩,向上为一套灰绿色细砂岩-粉砂岩、钙质粉砂岩、硅质粉砂岩等陆源细碎屑岩和火山岩及火山碎屑岩;上石炭统为一套含大量植物化石的浅水相细砂岩、粉砂岩、泥质粉砂岩和泥岩夹中基性-中酸性的爆发相、溢流相和喷溢相的火山岩和火山碎屑岩。二叠系仅发育早二叠世石人子沟组和三塘胡组。其中石人子沟组(P1s)为一套正常沉积碎屑岩,偶夹火山碎屑岩,以底部不整合面上发育一套底砾岩为特征;三塘胡组(P1st)下部为一套火山碎屑岩,上部为一套中性-酸性火山熔岩,加少量细碎屑岩。

图2 博格达东段下涝坝泥盆纪火山岩照片 Fag.2 Photos of the Devonian volcanic rocks from Xialaoba area in eastern part of Bogda orogenic belt a-野外露头;b-镜下显微特征,基质玻晶交织结构,长石斑晶环带; Cpx-斜方辉石; Py-斜长石 a-field outcrop;b-microcharacteristics,hyalopilitic texture for groundmass,girdle band for plagioclase porphyritic crystal; Cpx-clinopy roxene; Py-plagioclase

对比的哈尔里克山区内晚古生代泥盆纪地层出露面积有限,零星分布,缺失上泥盆统,集中分布在哈尔里克山南坡靠近吐哈盆地边缘部位,伊吾县以南和大石头泉以北,为一套海相火山碎屑岩、火山岩、碎屑岩地层,火山岩岩石组合为玄武岩-安山岩-英安岩-流纹岩,以中性安山岩为主体,区域上归属于下泥盆统大南湖组,与上覆和下伏地层均为不整合接触(靳刘圆等,2013)。

2 采样位置及岩相学

博格达东段下涝坝早盆纪火山岩分布于巴里坤-木垒公路下涝坝段,以灰绿色玄武安山岩、安山岩等中性火山岩为主体(野外发现流纹岩转石)。采样坐标位置自E:91°46′51.8″;N:43°45′34.1″至E:91°46′55.7″;N:43°45′40.2″。沿途共采集玄武安山岩、安山岩等5个化学样、5个薄片及1套同位素测年样品,后经薄片鉴定均为安山岩。岩石野外呈红褐色或深褐色,风化色为灰黑色-灰褐色。露头风化较强烈,出露不连续,节理发育使得岩石破碎呈小块状,其与上覆地层和下伏地层的接触关系难以识别。镜下观察,岩石为斑状结构,斑晶基本全为斜长石,偶见辉石,斜长石斑晶呈自形板状,粒径0.2mm×0.05mm~0.8mm×0.5mm,含量约15%,以中长石-更长石为主,少量基性斜长石,沿环带纹发育绢云母化和绿帘石化,个别斜长石斑晶中可见继承颗粒。基质主要为斜长石微晶构成,粒径为0.01~0.02mm,分板状和针状两种,自形程度较低,呈杂乱分布,局部充填辉石微晶,显示一定的玻晶交织结构。岩石整体蚀变较弱,局部裂隙中显示淋滤后Fe3+浸染形成红褐色。

3 分析测试方法

本次研究在野外采集样品共计6件,其中全岩及微量元素测试样品5件,LA-ICP-MS锆石U-Pb定年样品1件,引用全岩及微量元素数据5件。

博格达东段下涝坝泥盆纪火山岩全岩及微量元素测试分析在西安地质矿产研究所国土资源部岩浆作用成矿与找矿重点实验室完成。将样品去皮及杂质后,先用颚式破碎机粗碎至约50目,再细碎磨制200目以上后上机分析测试,其中除FeO和LOI采用标准湿化学法分析外,其他主量元素均采用PW4400型X萤光光谱仪XRF测定,分析误差低于5%。微量元素和稀土元素采用X-SeriesII型电感耦合等离子质谱仪ICP-MS测定,检测误差优于5×10-9,相对标准偏差优于5%。锆石阴极发光(CL)显微照相在长安大学地质工程与矿产资源教育部重点实验室完成。锆石原位U-Pb同位素年龄分析在中国地质调查局天津地质调查中心(天津地质矿产研究所)完成,锆石定年分析所用仪器为Finnigan Neptune型MC-ICP-MS及与之配套的NewwaveUP193激光剥蚀系统,激光剥蚀斑束直径为35m,激光剥蚀样品的深度为20~40m锆石年龄计算采用国际标准锆石91500作为外标,元素含量采用美国国家标准物质局人工合成硅酸盐玻璃NISTSRM610作为外标,29Si作为内标元素进行校正,数据处理采用ICP MS Data Cal 4.3程序(Liuetal.,2008),并对测试数据进行普通铅校正(Andersen,2002),年龄计算及谐和图绘制采用Isoplot(3.0版)(Ludwig,2003)软件完成。

4 分析测试结果及地球化学特征

4.1 锆石U-Th-Pb同位素

本次博格达地区泥盆纪火山岩锆石U-Pb定年样品共挑选出锆石50余颗,多为半自形状,粒径50~130μm。阴极发光图像显示其整体呈亮白色,可见明显的环带结构且没有后期蚀变痕迹(图3),206Pb/238U表面年龄一致表明其为同一期岩浆作用的产物。激光剥蚀等离子体质谱(LA-ICP-MS)U-Pb对样品进行微区同位素测试,测试的30个分析点结果(表1)显示,锆石的232Th/238U比值集中于0.4152~0.8520,变化区间较小,反映各环带之间以及各锆石颗粒之间具有比较密切的成因。其中4个分析点谐和度较差,而另外26个分析点的206Pb/238U表面年龄在389Ma~410Ma之间,加权平均年龄为397.5±4.7Ma(MSWD=0.23)(图4a),206Pb/238U-207Pb/235U谐和年龄为388.6±3.9Ma(MSWD=4.0)(图略)。从谐和图上可以看出,所有的年龄点都位于不一致曲线以下,表示该年龄具有明显的Pb丢失特征,而真正的结晶年龄应该大于388.6 Ma。因此笔者利用207Pb/207Pb-206Pb/238U进行谐和年龄模拟显示该火山岩结晶年龄为398.3±9.2 Ma(MSWD=0.14)(图4b),与加权平均年龄397.5±4.7Ma在误差范围内一致。因此本文选取的博格达东段下涝坝地区火山岩的形成年龄应该为397.5±4.7 Ma~398.3±9.2 Ma,属早泥盆世。

4.2 主量元素

博格达东段下涝坝早泥盆世火山岩SiO2含量集中于64.10%~67.07%,均属安山岩范畴且偏酸性端元(图5),与岩石镜下观察基本一致(表2)。样品Al2O3含量比较稳定,为14.51%~15.63%;CaO含量较低为0.83%~2.46%,平均1.49%,暗示岩浆可能发生过辉石的结晶分离作用;MgO含量偏低为2.12%~2.98%,平均2.59%;TiO2普遍偏低,变化于0.40%~0.69%,平均仅0.51%;Na2O含量为2.21%~3.10%,K2O含量相对较高为4.60%~5.19%,且所有样品Na2O

图3 博格达东段下涝坝火山岩锆石阴极发光图像Fig.3 CL images of zircon from the volcanic rocks in the Xialaob area,the eastern part of Bogda表1 博格达东段下涝坝地区火山岩LA-ICP-MS锆石U-Th-Pb测试结果Table 1 LA-ICP-MS zircon U-Th-Pb isotopic analyses of the volcanic rocks from Xiaolaoba area in the east of Bogda

点号同位素比值同位素年龄(Ma)232Th/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ10.63820.00090.09090.00460.79700.05640.06360.000814459759542397520.41520.00070.06640.00470.58810.04230.06430.000581814947034402330.61570.00060.10530.00580.93930.06870.06470.0008172010167349404540.58830.00080.07810.00410.69940.03740.06490.0006115010453829406450.69680.00370.08490.00420.75760.03840.06470.000513149657329404360.44890.00110.09260.00510.81680.05120.06400.0007147910360638400470.77310.00070.05910.00170.51070.01560.06270.00045716441913392280.70430.00080.07770.00270.68910.02430.06430.000411396853219402390.54550.00190.08460.00490.73980.05450.06340.00071308112562413964100.59940.00070.06920.00240.61240.02150.06420.000490372485174013120.79400.00110.07680.00230.66720.02300.06300.0004111761519183943130.53120.00170.06460.00270.55480.02420.06230.000476287448203893150.46650.00080.08710.00270.76810.02780.06400.0005136260579214003160.51220.00190.07160.00230.61680.02140.06240.000497665488173903170.49120.00110.08410.00540.75070.04860.06480.00051294124569374053180.47980.00120.10750.00540.95920.05040.06470.0006175892683364044190.50870.00380.06950.00410.61010.03540.06370.0005913123484283983200.63980.00250.06140.00190.52720.01760.06230.000465365430143893210.56730.00110.06520.00580.56730.05110.06310.0005782186456413943220.47590.00190.08790.00720.78380.06390.06470.00051381157588484043230.57890.00260.08190.00210.74170.02170.06570.0005124250563164103240.53820.00090.07880.00320.67760.03000.06240.0005116779525233903

续表1

Continued Table 1

点号同位素比值同位素年龄(Ma)232Th/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ260.51100.00190.08690.00840.76480.07420.06380.00071359186577563994270.43560.00090.06790.00430.59770.03800.06380.0005865130476303993290.46690.00120.09480.00850.83670.07700.06400.00061525170617574004300.85200.00080.06210.00250.54090.02220.06320.000467887439183952

测试单位:天津地质矿产研究所实验测试中心;测试时间:2014-04。

图4 博格达东段下涝坝火山岩LA-ICP-MS 年龄测试结果Fig.4 LA-ICP-MS zircon U-Pb age result of the volcanic rocks from Xialaob area in the east part of Bogda a-加权平均年龄图;b-U-Pb谐和年龄图 a-weighted average diagram;b-U-Pb concordia diagram

哈尔里克山南麓早泥盆世火山岩为玄武岩-安山岩-英安岩-流纹岩的钙碱性组合,以中性安山岩为主体(图5;靳刘圆,2013)。岩石SiO2含量49.16%~71.55%,Al2O3含量12.55%~15.72%,;MgO含量在0.80%至7.26%之间变化;而CaO含量明显较博格达泥盆纪火山岩高,为1.41%~8.69%;全碱含量Alk较博格达泥盆纪火山岩稍低,集中于4.05%~7.35%,而相反的是所有样品Na2O含量均大于K2O;计算得样品的里特曼指数δ=1.04~2.66<3.8,同样为钙碱性系列,Mg#介于47.31~75.55,说明岩浆经过一系列不同程度的演化,长英指数为31.79~83.90和镁铁指数=58.80~81.48也反映出岩浆的分异演化程度相差较大。

4.3 稀土与微量元素

在稀土元素丰度及配分曲线方面,博格达地区早泥盆世火山岩显示出一致的轻稀土富集,而重稀土显著亏损的下凹式右倾模式(图6a)。稀土元素丰度总量为48.75×10-6~65.44×10-6,(La/Yb)N值为6.87~12.92,(La/Sm)N和(Gd/Yb)N分别为2.57~3.71和1.77~2.05,说明轻、重稀土之间分馏作用较强且轻稀土较重稀土分馏程度高,基本无负Eu异常。在微量元素蛛网图中,博格达地区早泥盆世火山岩表现出一致的K、Rb、Ba等大离子亲石元素富集,而Nb、Ta及HREE等高场强元素明显亏损,这些特征类似形成于俯冲带火山岩的地球化学特征(McCullochetal.,1991;Innocentietal.,2005),因为Nb和Ta负异常暗示了安山岩的物质源区中具有一定含量的Nb和Ta难熔矿物(钛铁矿、金红石或韭闪石),这通常存在于俯冲消减带中流体作用交代的亏损地幔中(Hawkins,2003),Zr、Hf轻度富集却与之不同(图6b)。哈尔里克地区早泥盆世火山岩稀土配分曲线形状相似(图6a),暗示其应该为同源岩浆演化的产物;稀土元素丰度总量为86.81×10-6~117.86×10-6,(La/Yb)N值为2.83~6.01,整体较博格达地区早泥盆世火山岩稀土丰度总量高(重稀土更显著)而分馏程度弱,显示微弱的负Eu异常,暗示后者的岩浆形成深度较前者浅。其蛛网图中也表现出大离子亲石元素不同程度的富集,明显的Nb、Ta负异常以及P和Ti的适度亏损。但各样品间的变化趋势不一致,相同的强不相容元素均显示出不同程度的富集和亏损(图6b)。

图5 博格达-哈尔里克泥盆纪火山岩类型及岩石系列判别图解Fig. 5 Rocks type and series discrimination diagrams for Bogda-Harlik area Devonian volcanic rocks a-Nb/Y-Zr/TiO2图解(据Winchester和Floyd,1977);b-TAS图解(据Le Bas等,1986);c-AFM图解,(据Irvine et al,1971) a- Nb/Y-Zr/TiO2diagram(after Winchester and Floyd,1977);b-TAS diagram(after Le Bas et al.,1986);c-AFM diagram(after Irvine et al.,1971)表2 博格达下涝坝和哈尔里克地区早泥盆世火山岩主量(%)和微量(10-6)元素含量对比表Table 2 Major element (%) and trance element (10-6) abundance of the volcanic rocks from Bogda and Harlik regions

样品编号博格达下涝坝地区哈尔里克地区13BD-124H13BD-125H13BD-126H13BD-127H13BD-128H*HK-1*HK-2*HK-3*HK-4*HK-5SiO266.2267.0764.1066.5366.6249.1653.7256.4465.2571.55TiO20.430.400.690.530.510.941.381.000.840.52Al2O315.3614.9115.6314.5114.9015.6215.4515.7212.5513.10Fe2O31.551.221.951.701.306.133.363.613.262.17FeO2.221.942.652.362.444.235.244.453.951.35MnO0.070.070.110.080.080.170.140.190.190.11MgO2.982.122.542.582.747.263.882.751.970.80CaO0.831.562.461.461.158.696.536.453.141.41Na2O2.953.102.212.882.622.604.453.434.055.65K2O4.604.664.934.825.191.450.701.930.761.70P2O50.130.130.190.160.150.240.290.560.110.10灼失2.672.842.532.402.303.854.192.881.262.44总量100.01100.0299.99100.01100.00100.3499.3399.4197.33100.90K2O+Na2O7.557.767.147.77.814.055.155.364.817.35δ2.452.502.422.522.582.662.472.141.041.89Mg#70.7366.3063.3166.3066.9075.5557.1352.6647.3151.61FL90.1083.2674.3884.0687.1731.7944.0945.3960.5083.90MF55.8559.8564.4361.1457.7258.8068.9174.5678.5481.48La11.413.810.711.28.7715.8011.7115.5021.0315.12Ce24.127.822.222.319.534.9226.5934.3041.6428.20Pr2.983.412.872.702.454.794.205.976.084.81Nd11.112.011.010.09.4120.1618.8028.7024.2120.73Sm2.412.342.572.342.154.384.726.525.285.06Eu0.610.640.750.620.591.221.601.991.351.40Gd1.871.832.361.911.814.295.256.865.195.11

续表1

Continued Table 1

样品编号博格达下涝坝地区哈尔里克地区13BD-124H13BD-125H13BD-126H13BD-127H13BD-128H*HK-1*HK-2*HK-3*HK-4*HK-5Tb0.260.250.350.280.270.670.860.940.900.88Dy1.501.422.101.681.583.975.355.715.385.54Ho0.300.270.400.330.320.791.081.101.011.15Er0.790.741.080.880.842.053.003.072.703.40Tm0.120.110.160.130.130.320.450.440.390.52Yb0.780.721.050.870.811.872.792.992.363.33Lu0.130.110.160.130.120.290.410.440.340.50∑REE58.3565.4457.7555.3748.7595.5286.81114.53117.8695.75δEu0.850.910.920.870.890.850.980.900.780.83δCe0.970.950.950.951.000.960.910.860.880.79(La/Yb)N9.8512.926.878.687.305.702.833.496.013.06(La/Sm)N2.983.712.623.012.572.271.561.502.511.88(Gd/Yb)N1.932.051.811.771.801.851.521.851.771.24Rb39.956.38.4813.312.110.2543.3013.9013.0014.25Sr334309424384351211.50472.00668.00156.00426.70Ba122011609281100948113.50455.50286.00233.00336.00Nb2.952.813.893.533.393.406.553.255.906.69Ta0.240.230.290.280.270.320.420.190.490.53Zr10791.710610198.877.00129.2591.20190.00202.00Hf2.482.402.682.582.491.802.952.825.804.67U1.451.461.071.221.190.580.531.150.581.10Th4.074.033.003.223.080.901.832.091.401.89Y7.877.5610.58.998.6418.1428.4332.1024.9230.82La/Nb4.015.102.853.292.684.651.794.773.562.26Nb/U2.031.923.642.892.855.8612.362.8310.176.08Zr/Sm44.4039.1941.2543.1645.9517.5827.3813.9935.9839.92Th/Ta16.9617.5210.3411.5011.412.814.3611.002.863.57Ce/Th5.926.907.406.936.33-----Ba/Th9.257.6216.3913.2012.96-----

注:*为引用数据,数据来源于靳刘圆(2013);其余数据为本文测试数据,测试单位:西安地质矿产研究所分析测试中心,2013年12月;Mg#=100×Mg2+/(Mg2++Fe2+)。

图6 球粒陨石标准化的稀土配分曲线图(a) 和原始地幔标准化的蛛网图(b)Fig. 6 Chondrite-normalized REE distribution patterns (a) and primitive mantle-normalized multi-elements spider diagram(b) (球粒陨石标准化值据Boytton,1984;原始地幔标准值Sun and McDonough,1989) (chondrite data after Boynton,1984 and primitive mantle data after Sun and McDonough,1989)

在热液蚀变过程中,一些大离子亲石元素如Cs、K、Rb、Ba、Sr等是容易活动的;而稀土元素和一些高场强元素(如Nb、Ta、Zr、Hf、Ti、Th等)活动性弱;其中,Zr在热液蚀变过程中是不活动的(Sun and McDonough,1989;Saundersetal.,1988;Zindleretal.,1986)。所以,我们可以通过研究其它元素与Zr的关系来判断这些元素的活动性(Gibsanetal.,1982; Pearceetal.,1992)。在Zr-Rb和Zr-La图解中,两套火山岩并没有表现出相关关系(图7),但Zr-Ta相关性明显,证明哈尔里克地区早泥盆世存在后期热液蚀变作用,导致活泼元素发生不同程度的带入和带出现象,而高场强元素则基本不受影响。

5 讨论

5.1 岩石成因

研究表明,镁铁质岩浆同化陆壳物质,会增加SiO2、K2O、Rb、Ba、Zr、Th等氧化物或元素的丰度,同时会升高La/Nb、K/P、Zr/Nb和87Sr/86Sr比值,降低Ti/Yb、Ce/Pb比值和Sr丰度(Barkeretal.,1997;McDonaldetal.,2001)。博格达和哈尔里克地区早泥盆世火山岩的SiO2-K2O/P2O5、Ti/Yb-Zr/Nb之间没有明显的相关关系(图8),La/Nb值分别为2.68~5.10和1.79~4.65,高于亏损地幔(地壳萃取过后的亏损地幔La/Nb=0.68,Sun and McDonough,1989),且随Sr含量的降低而降低,证明博格达和哈尔里克地区泥盆纪火山岩均同化了一定的陆壳物质,但是博格达地区下涝坝早泥盆世火山岩Nb/U=1.92~3.64,远低于大陆地壳相应值(平均地壳Nb/U=9.7,McDonough,1990),且SiO2含量比较均一,说明博格达泥盆纪火山岩同化混染程度十分微弱;而哈尔里克地区泥盆纪火山岩Nb/U=2.83~12.36,且元素的起伏变化较博格达地区大,则意味着哈尔里克地区早泥盆世火山岩岩浆在不同演化阶段混染程度可能不同。

博格达地区下涝坝早泥盆世火山样品Sr含量介于309×10-6~424×10-6,非常低的Y和Yb含量表明其源区的深度较大,可能为石榴石稳定域,这是因为该深度下斜长石不稳定,大量分解进入熔体,导致熔体中高的Sr含量,Y和Yb因与石榴子石相容而被大量残留在源区。同时,所有样品的Zr/Sm比值均大于10,反映源区可能有角闪石的残留,因为Sm在角闪石中的分配系数远大于Zr。哈尔里克地区早泥盆世火山岩不论是重稀土含量,还是Zr/Sm比值都较下涝坝早泥盆世火山岩明显偏高,标志其源区深度相对较浅。用稀土元素相关图解来模拟地幔岩石的部分熔融程度及发生的熔融深度,LaN-(La/Yb)N图解显示,博格达下涝坝早泥盆世火山岩可能由石榴石二辉橄榄岩在2%~5%的部分熔融程度下产生,而哈尔里克地区早泥盆世火山岩则可在尖晶石二辉橄榄岩<3%的部分熔融程度下来模拟(图9a)。Denieletal.(1998)研究认为,来源于不同矿物相源区不同部分熔融程度形成的岩浆具有不同的Zr/Nb和Ce/Y比值,因而用Zr/Nb-Ce/Y图解对两套火山岩进行判别,同样显示博格达和哈尔里克地区早泥盆世火山岩分别是发生在石榴石稳定域和尖晶石稳定域地幔橄榄岩较低程度部分熔融的产物(图9b),这些都与稀土元素配分图解所得结论一致。

图7 Zr-K/Rb/La协变图解Fig.7 Zr-K/Rb/La diagrams

图8 SiO2-K2O/P2O5、Zr/Nb-Ti/Yb和Sr-La/Nb协变图解Fig.8 SiO2-K2O/P2O5、Zr/Nb-Ti/Yb and Sr-La/Nb diagrams

图9 石榴石二辉橄榄岩与尖晶石二辉橄榄岩部分熔融模拟LaN-(La/Yb)N图解(a)(据Kerr et al., 1997)和Zr/Nb-Ce/Y图解(b)(据Deniel,1998)Fig.9 The LaN-(La/Yb)N diagram for fraction melting of garnet lherzolite(a;after Kerr et al., 1997) and the Zr/Nb-Ce/Y diagram (b;after De niel, 1998)

前述博格达和哈尔里克地区早泥盆世火山岩微量元素(包括稀土元素)有显著的造山带弧火山岩的性质,包括下凹式右倾稀土配分模式、富集大离子亲石元素而亏损高场强元素以及Nb、Ta、Ti、P不同程度的亏损等(Rollinson,1993)。一般来说岛弧环境中的岩浆会有:(1) 地幔楔中橄榄岩的部分熔融;(2) 俯冲带流(熔)体;(3) 俯冲板片部分熔融;(4) 陆壳同化混染等多种来源(Macdonaldetal.,2000)。不同地幔岩在相应的温压条件下部分熔融均可产生原生岩浆(Gast,1968;Greenetal.,1967,1968,1970,1976;鄂莫岚等,1987;池际尚,1987;Yaxley,2000),而不同环境或源区产生的岩浆会表现出不同的地球化学特征(Wilson,1989;Forster,1997),因此可以用火山岩的微量元素特征对其源区性质进行约束。

博格达地区下涝坝早泥盆世火山岩表现出Nb、Ta和Ti的负异常以及Ba、Th、Sr的富集,这可能是因为地幔楔受到了洋壳流(熔)体的交代作用(Pearceetal.,1995)。元素Th和Ta在地幔演化过程中表现较为稳定,但在受俯冲带流(熔)体影响的地幔楔发生部分熔融时,二者会发生分异,表现为Th的富集(Wilson,1989),从而形成较高的Th/Ta比值。博格达地区泥盆纪火山岩Th/Ta比值高达10.34~17.52,尽管地壳物质混染也可以表现出Nb、Ta负异常等岛弧火山岩的特征(Wilson,1989;夏林圻等,2007a,2007b),但是前文已经证明岩浆同化地壳物质十分有限,因此这种特征显然是由受俯冲流(熔)体交代的地幔部分熔融形成的。哈尔里克早泥盆世火山岩尽管其岩浆存在地壳混染作用,且岩石后期也发生过显著的热液蚀变作用,导致K、Rb、Ba等活泼元素的岩浆特征被不同程度的掩盖,但所有样品具有一致右倾下凹式的稀土配分模式,整体富集大离子亲石元素而亏损高场强元素,显著亏损Nb、Ta,依然可以判断其形成于与俯冲作用有关的环境(Gill,1981;Pearce,1982),其对应岩浆的形成主要是俯冲洋壳及其沉积物在脱水过程中形成的流体或熔体交代上覆地慢楔的结果(Eileretal.,2000;Elliottetal.,1997)。

图10 博格达-哈尔里克地区早泥盆世火山环境判别图 (底图分别据Meschede,1986和Pearce et al.,1995)Fig.10 Tectonic discrimination diagrams for Bogda-Harlik area Devonian volcanic rocks (the base diagram after Meschede,1986 and Pearce et al.,1995)

5.2 构造环境与地质意义

博格达地区下涝坝和哈尔里克地区早泥盆世火山岩均表现出富集LILE和相对亏损HFSE的地球化学特征(图6b),与形成于俯冲带的火山岩的特征相当一致(McCulloch and Gamble,1991)。其中Nb、Ta的显著亏损表明其岩浆来源于俯冲带熔融的上地幔或俯冲板片,而Ba、Rb和Th的相对富集则可能是地幔楔被俯冲流体交代所致。

在Hf/3-Th-Ta三角图解(图10a)中,两套火山岩均投影于钙碱性玄武岩区域,并且每套火山岩均有从岛弧型拉斑玄武岩向钙碱性玄武岩演化的线性趋势,表明这些火山岩可能形成于会聚板块边缘,即俯冲型火山岛弧环境。用Yb标准化的元素比值可以削除或减少部分熔融和分离结晶作用对元素产生的影响,从而得到岩浆源区的地球化学性质(Maedonaldetal.,2000)。因此,笔者用Yb标准化的Th-Nb图解对两个地区的火山岩进行环境判别(图10b),结果显示哈尔里克火山岩样品落在大陆岛弧火山岩和大洋岛弧火山岩的重叠区域,而博格达火山岩明显偏向大陆岛弧区域,表明这两套火山岩均具有大陆火山岛弧亲缘性。

卡拉麦里-莫钦乌拉-哈尔里克构造带在晚海西期均大量发育有复理石和火山岩建造,现今博格达山山体向东经巴里坤山自然弯曲并交入哈尔里克山,构成地貌上紧密相连的东西走向山脉。方国庆(1990)和何国琦(1994)认为尽管在海西期博格达造山带与哈尔里克造山带在地层和火山作用方面存在一定差异,但其地层呈过渡接触,野外并未发现截然的构造拼合界线。马瑞士(1993;1997)和Lamb(1997)则进一步研究认为它们是连续的构造带,二者构成了准噶尔-吐哈地块北缘连续的晚古生代岛弧。近年来,大量的研究包括火山岩、花岗岩以及沉积相方面均肯定了哈尔里克造山带为泥盆纪-石炭纪晚古生代岛弧这一认识(李锦轶,2002,2004,2006;贺军慧,2006;侯广顺,2006;左国朝,2006;张洪瑞,2010;夏芳,2012;赵同阳,2014),如李锦轶等(2004)在哈尔里克一带下泥盆统大南湖组和中泥盆统头苏泉组选取的38件火山岩样品均为从基性到酸性连续演化的钙碱性系列,具有相似的地球化学特征,属活动陆缘火山岩。同时,孙桂华等(2005)认为博格达与哈尔里克具有大体相同的岩浆活动历史,后期构造变动改造使二者的剥蚀深度不同,从而表现出博格达山主要由石炭纪的火山沉积岩系构成,而只出露很少的深成侵入岩体,与之相连的哈尔里克山主要由泥盆纪火山岩构成并出露大量的深成侵入岩体。相对而言,前人对博格达泥盆纪构造属性的研究工作甚少。此次在博格达地区下涝坝南部确定的早泥盆世火山岩LA-ICP-MS锆石U-Pb平均年龄为397.5±4.7Ma,属早泥盆世。岩石地球化学特征证明该套火山岩具有活动陆缘的性质,因此我们认为博格达造山带(至少在东段)和哈尔里克地区在早泥盆世时期应该具有相同的大地构造背景,这为理解博格达-哈尔里克造山带晚古生代的构造演化提供了新的证据。

6 结论

(1) 博格达地区下涝坝南部火山岩LA-ICP-MS锆石U-Pb年龄为397.5±4.7Ma,属早泥盆世。

(2) 博格达地区下涝坝早泥盆世火山岩和哈尔里克早泥盆世火山岩均为钙碱性系列,岩性组合前者以中性为主,后者则为基性到酸性的连续演化。

(3) 博格达地区下涝坝早泥盆世火山岩岩浆可能是石榴石稳定域地幔橄榄岩发生2%~5%的部分熔融产生,上升过程中同化混染程度较小,而哈尔里克早泥盆世火山岩可能是尖晶石稳定域中地幔橄榄岩<3%的部分熔融形成,地壳混染较明显。

(4) 博格达地区下涝坝早泥盆世火山岩都形成于与俯冲有关的活动陆缘环境,暗示博格达地区和哈尔里克地区在早泥盆世时期可能具有相同的大地构造环境。

致谢 本文在野外工作中得到长安大学翁凯博士、张涛硕士和中国地质大学(北京)贠杰硕士的关心,数据测试过程中受到天津地调测试中心耿建珍工程师的热情帮助,审稿老师的意见使作者受益良多,在此一并致谢。

[注释]

① 长安大学地质调查研究院.2013.1∶25万三道岭幅地质图[R].

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Comparison of Genesis of Devonian Volcanic Rocks in the Bogda and Harlik Areas and its Tectonic Implications

CUI Fang-lei1,2,WANG Xiao-wei1,2,MA Zhong-ping1,SUN Ji-ming1,ZHU Xiao-hui1

(1.Xi’anCenterofGeologicalSurvey,ChinaGeologicalSurvey,Xi’an,Shaanxi710054;2.CollegeofEarthScienceandResourcesofChang’anUniversity,Xi’an,Shaanxi710064)

This work has identified a set of calc-alkali volcanic rocks from the southern Xialaoba area, east section of Bogda, East Tian Shan, which is dominated by andesite with zircon U-Pb ages 397.5±4.7 Ma. The samples' REE abundance shows a rightly-oblique pattern type characterized by fractionation. Their trace elements are rich in large-ion-lithophile elements (LILE) such as Ba and K, uniformly and observably depleted in high-field-strength elements (HFSE) such as Nb,Ta, Ti and Y. Their primitive magma might emanate from mantle peridotite 2%~5% partial melting in the garnet stability domain with a slight assimilation and hybridization. The corresponding Early Devonian Dananhu formation volcanic rocks from southern Harlik are calc-alkali series that evolved from basalt to rhyolite. Their REE abundance is higher than the Xiaolaoba volcanic rocks, but have a low fractionation between LREE and HREE and are rich in LIEL and depleted HFSE as a whole. Their primitive magma could originate from mantle peridotite <3% partial melting in the Spinel stability domain, assimilating some crust material, and rocks were strongly altered in later time. Both the two kinds of volcanic rocks formed in an active continental margin environment related to subduction,which implies that the Bogda and Harlik areas shared a same tectonic setting in Early Devonian.

Bogda and Harlik, Early Devonian, volcanic rocks, genesis, tectonic environment

2014-12-26;

2015-03-09;[责任编辑]郝情情。

国家自然科学基金项目(编号:41272089)、天山关键地区区域地质调查项目(编号:12120113042200)和西北基础地质调查成果集成与综合研究项目(编号:1212011220649)联合资助。

崔方磊(1990年-),男,2013年毕业于长安大学,获学士学位,长安大学在读硕士研究生,主要从事火成岩成因研究工作。E-mail:cuifanglei@126.com。

P618

A

0495-5331(2015)03-0519-15

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