内蒙古西拉木伦河两岸志留-泥盆系碎屑锆石年龄及其构造意义*

程胜东　方俊钦　赵盼　徐备　鲍庆中　周永恒　邓荣敬

1.北京大学造山带与地壳演化教育部重点实验室，北京大学地球与空间科学学院，北京　1008712.沈阳地质矿产研究所，沈阳　1100341.

1　引言

作为中亚造山带东段的重要组成部分，兴蒙造山带记录了华北板块与其北部各块体的碰撞拼合过程，是研究中亚造山带构造演化历史的重要区域(黄汲清等,1977,1980; 任纪舜等,1980; 王鸿祯,1981; 李春昱等,1982; 何国琦和邵济安,1983; Tang,1990; 邵济安,1991; Sengör and Natal’in,1996)。关于兴蒙造山带的构造发展历史仍存在重大争论，焦点集中于古亚洲洋最终闭合的时代及缝合带的位置。其闭合时间归结起来主要有晚志留世(Tang,1990; 赵春荆等,1996)、早-中泥盆世(任纪舜等,1980; 李春昱等,1982; 王鸿祯,1981; 邵济安,1986; 唐克东,1992)、晚泥盆-早石炭世(郭胜哲,1986; 曹从周等,1987; 邵济安,1991; Dobretsovetal.,1995; Xu and Chen,1993; 徐备和陈斌,1997; Khainetal.,2002; Xuetal.,2013)、二叠纪早期(樊志勇,1996; 王玉净和樊志勇,1997; 王友等,1999)、二叠纪末期(Wang and Liu,1986; 王荃等,1991; 和政军等,1997; Badarchetal.,2002; Xiaoetal.,2003,2009; 孙德有等,2004; 尚庆华,2004; Li,2006; Zhangetal.,2007; 李朋武,2007; Wuetal.,2007; 邓胜徽等,2009; 李益龙等,2009)等截然不同的观点。关于最终闭合的位置，早期的观点根据地球物理、植物古地理分区和放射虫化石的存在等提出索伦科尔-西拉木伦为缝合线(任纪舜等,1980; 李春昱等,1982; Tang,1990; 李锦轶等,1986,1987; 王玉净和樊志勇,1997; 和政军等,1997; 尚庆华,2004; Xiaoetal.,2003; Windleyetal.,2007)，而唐克东(1992)则认为锡林浩特带是晚志留世到中泥盆世期间形成的南、北两大陆块的缝合带。李锦轶等(2007)从造山带构造学分析的角度认为西拉木伦河地区存在晚古生代碰撞杂岩，从而进一步认定西拉木伦河断裂带为西伯利亚板块与华北板块的缝合线。而另一些学者提出“双冲造山带”观点，即兴蒙造山带沿南、北两条缝合线闭合，北缝合线位于艾力格庙-白音宝力道-锡林浩特一线，南缝合线则位于图古日格-温都尔庙地区(徐备和陈斌,1997; Jianetal.,2008,2010; Xuetal.,2013)。

综上所述，西拉木伦河地区处于上述争议的关键地区，通过该地区古生代早期碎屑岩的物源研究，将为古亚洲洋闭合提供重要的证据。笔者选择了西拉木伦河北侧林西双井子地区和南侧奈曼旗下石碑地区的志留纪地层中的碎屑岩，进行了锆石U-Pb年代学研究，并试图通过物源对比讨论该区古亚洲洋的闭合时限。

2　地质概述与样品采集

2.1　区域地质概况

研究区位于松辽盆地以西、大兴安岭南段的林西至敖汉旗一带，构造位置属于兴蒙造山带东南段(图1b)。以西拉木伦河为界将研究区分为南、北两部分(图1c)，其两侧地层分述如下。

根据1:20万刘家营子幅(内蒙古自治区地质局第二区域地质测量队,1968*内蒙古自治区地质局第二区域地质测量队.1968.1:20万刘家营子幅区域地质调查报告)、林西县幅(辽宁省地质局第二区域地质测量队,1971a*辽宁省地质局第二区域地质测量队.1971a.1:20万林西县幅区域地质调查报告)、巴林左旗幅(辽宁省地质局第二区域地质测量队,1971b*辽宁省地质局第二区域地质测量队.1971b.1:20万巴林左旗幅区域地质调查报告)和1:25万林西县幅(中国地质大学(武汉)地质调查研究院,2007*中国地质大学(武汉)地质调查研究院.2007.1:25万林西县幅区域地质调查报告)区域地质矿产报告可知，北侧地层包括元古界、志留系、二叠系、中生界和新生界。西拉木伦河北岸出露一套灰黑色黑云石英片岩、二云石英片岩夹黑云斜长片麻岩及灰黑色黑云变砾岩，前人将其划归为元古代宝音图群。上志留统杏树洼组为一套碎屑-碳酸盐岩沉积组合，包括深灰色-浅绿色粉砂质板岩、绢云母片岩以及细晶灰岩，与周围地层呈断层接触关系(王友等,1999)。粉砂质板岩经历了低绿片岩相变质作用，岩石变形强烈，劈理发育；灰岩呈巨厚层状，未发生变质作用。在小杏树洼南灰岩中采集到丰富的珊瑚化石，经沈阳地质矿产研究所鉴定，有珊瑚：Pachyfavositessp.,Zelophyllumsp.indet.,Neomphymasp.,Favositessp.,Brachyelasmasp.,Polyorophesp.,Tryplasmasp.,Thedstromi(Wedekind)；层孔虫：Stromatoporasp.。据此将杏树洼组的时代确定为晚志留世 (王友等,1999)。二叠系下二叠统青凤山组为中性熔岩、熔岩角砾岩、局部夹有较多的凝灰粉砂岩、凝灰砂岩和砂砾岩；黄岗梁组为板岩、泥质板岩、灰黑色凝灰岩和少量结晶灰岩；上二叠统林西组为轻度变质的巨厚状细砂岩、粉砂岩和粉砂质板岩，局部夹灰岩透镜体。中生界地层主要为灰黄色酸性凝灰岩夹薄层凝灰砂岩、凝灰粗砂岩、砂岩、紫红色砾岩、砂砾岩。

图1　研究区地质简图(据内蒙古自治区地质矿产局,1991; 李锦轶等,2007; Han et al.,2010; Xu et al.,2013修改)Fig.1　Sketch geological map of study area (modified after BGMRI,1991; Li et al.,2007; Han et al.,2010; Xu et al.,2013)

图2　野外及镜下照片(a)-林西浅绿色粉砂质板岩与灰岩断层接触关系；(b)-林西浅绿色粉砂质板岩显微结构(+)；(c)-奈曼旗下石碑组砂岩露头；(d)-奈曼旗下石碑组砂岩显微结构(+).Q-石英;Lm-变质岩岩屑;Cc-碳酸盐岩屑;Pl-长石;Mu-白云母Fig.2　The field and microscopic photographs(a)-faulted contact relationship between shallow green silty slate and limestone,Linxi area; (b)-microscopic structure of green silty slate,Linxi area (under cross-polarized light); (c)-sandstone of Xiashibei Formation,Naiman Banner; (d)-microscopic structure of sandstone of Xiashibei Formation,Naiman Banner (under cross-polarized light).Q-quartez; Lm-metamorphic rock’s debris; Cc-carbonate rock’s debris; Pl-feldspar; Mu-muscovite

根据1:20万下洼幅(内蒙古自治区地质局第二区域地质测量队，1967a*内蒙古自治区地质局第二区域地质测量队.1967a.1:20万下洼幅区域地质调查报告)、赤峰幅(内蒙古自治区地质局第二区域地质测量队,1967b*内蒙古自治区地质局第二区域地质测量队.1967b.1:20万赤峰幅区域地质调查报告)、敖汉旗幅(辽宁省地质局第二区域地质测量队二分队,1970a*辽宁省地质局第二区域地质测量队二分队.1970a.1:20万敖汉旗幅区域地质调查报告)、翁牛特旗幅(辽宁省地质局第二区域地质测量队二分队,1970b*辽宁省地质局第二区域地质测量队二分队.1970b.1:20万翁牛特旗幅区域地质调查报告)、五分地幅(辽宁省地质局第二区域地质测量队,1971c*辽宁省地质局第二区域地质测量队.1971c.1:20万五分地幅区域地质调查报告)、克什克腾旗幅(辽宁省地质局第二区域地质测量队,1971d*辽宁省地质局第二区域地质测量队.1971d.1:20万克什克腾旗幅区域地质调查报告)区域地质矿产报告可知，南侧地层包括太古界、奥陶系、志留系、泥盆系、石炭系、二叠系、中生界和新生界。太古界称鞍山群，由一套斜长角闪片麻岩、角闪斜长片麻岩、混合岩、混合花岗岩、大理岩、石英岩夹角闪云母片麻岩、石英云母片岩组成。奥陶系由大理岩、灰岩、结晶灰岩夹泥质页岩及砂岩、黑云斜长片岩、黑云二长片岩、黑云斜长片麻岩组成。志留系包括5个组，自下而上依次为：中志留统晒勿苏组结晶灰岩、泥质灰岩夹砂岩；八当山组片理化流纹斑岩、凝灰岩夹板岩及薄层泥质灰岩；上志留统四道杖棚组黑云斜长片岩、角闪片岩、变质砂砾岩、板岩；大朝阳沟组片理化流纹斑岩、绢英片岩；下石碑组灰岩及砂岩。其中，下石碑组地层仅出露于下洼公社下石碑一带，由海相石灰岩及砂岩、板岩组成，层序清晰，与周围地层呈断层接触关系。下部为灰白、黑灰色石灰岩；中部主要为黑灰色泥质灰岩及石灰岩，泥质灰岩中局部含有白云质灰岩块体，石灰岩中富含珊瑚、层孔虫及苔藓虫化石，根据小泡沫珊瑚(Microplasmasp.)吉林中巢珊瑚(Mesofavositesex gr.jilinensisTchi sp.)和厚巢珊瑚(Pachyfavositessp.)等的存在，将该组划入志留系上统(1:20万下洼幅地质图，内蒙古自治区地质局第二区域地质测量队，1967a)。上部主要为褐灰色、灰绿色板岩与粉砂岩互层，板岩呈薄板状。下泥盆统前坤头沟组为硬砂岩、板岩夹灰岩。石炭系下部为朝吐沟组绢英片岩和变质火山岩、后房身沟组砂岩夹灰岩透镜体；中部为夹道沟组砂岩、板岩及灰岩；上部为酒局子组砂岩夹石灰岩透镜体。二叠系青凤山组为一套碎屑-火山岩岩系，主要有砂岩、板岩、含砾石英砂岩夹灰岩及火山岩。中生界地层主体为一套中酸性火山岩及凝灰岩，局部夹碎屑岩和煤层。

2.2　样品描述

样品LX0831-11为粉砂质板岩，采于西拉木伦河北岸上志留统杏树洼组中(GPS坐标：43°22′54″N，118°19′24″E)(图2a)，地层产状不清，与上下地层多呈断层接触关系。镜下观察主要成分为岩屑(50%)、单晶石英(30%)、长石(15%)和少量白云母(5%)。样品分选差-较差，次棱角状，杂基支撑，长石蚀变较强。岩屑的成分为多晶石英、变质岩等(图2b)。

样品130417-06为石英岩屑砂岩，采于奈曼旗下石碑组顶部砂岩层位中(GPS坐标：42°29′56″N，121°00′03″E)(图2c)。地层大致呈近东西向延伸，倾角近直立。镜下观察显示样品主要成分为岩屑(65%)、单晶石英(30%)、和长石(5%)。样品为镶嵌粒状结构，分选中等、次棱角状、成熟度低，方解石胶结。石英均匀消光，颗粒粒径为0.05～0.2mm，部分石英见有港湾状熔蚀边(图2d)。岩屑粒径为0.05～0.3mm，成分较为复杂，主要为各类变质岩和碳酸盐岩屑。

3　样品测试与年龄分析

3.1　测试方法

样品采用常规方法进行破碎，经浮选和磁选后，在双目镜下挑选出晶形和透明度较好的锆石颗粒制成样品靶，锆石样品靶的制备与SHRIMP定年锆石样品制备方法相同(宋彪等，2002)。锆石的阴极发光(CL)显微照相在北京大学造山带与地壳演化教育部重点实验室完成。

锆石LA-ICP-MS原位U-Pb同位素年龄分析在北京大学造山带与地壳演化教育部重点实验室完成。测试仪器为电感耦合等离子体质谱仪(Agilent 7500c)和准分子激光剥蚀系统(COMPExPro102)联机，激光器为ArF准分子激光器。激光剥蚀束斑直径为32μm，激光能量密度为10J/cm2，剥蚀频率为5Hz。实验中采用He作为剥蚀物质的载气，Ar为辅助气。锆石年龄计算采用标准锆石Plesovice(337Ma)作为外标(Slámaetal.,2008)，标准锆石91500为监控盲样(Wiedenbecketal.,1995)。元素含量采用国际标样NIST610作为外标，29Si为内标元素进行校正。剥蚀样品前先进行15次激光脉冲的预剥蚀，采集20s的空白，随后进行62s的样品剥蚀，剥蚀完成后进行2min的样品池冲洗。采样方式为单点剥蚀，每完成5个测点的样品测定，加测标样一次。在15个锆石样品点前、后各测2次NIST610。样品的同位素比值和元素含量数据处理采用GLITTER4.4.2程序计算，普通铅校正使用Anderson(2002)给出的程序计算，加权平均年龄及谐和图的绘制使用Isoplot/Ex(3.0)完成(Ludwig,2003)。分析数据及锆石U-Pb谐和图给出误差为1σ，95%的置信度。

3.2　分析结果

对西拉木伦河北岸志留纪粉砂质板岩样品(LX0831-11)随机选取75颗锆石进行测定，其测试结果见表1。锆石年龄均处于谐和线上，结合锆石CL图像(图3)及U-Pb谐和图(图4)特征，可将其分为三组，由新自老依次为：385～531Ma(N=52)、872～1097Ma(N=11)、1344～1901Ma(N=11)。第一组年龄(385～531Ma)的峰值为412Ma，锆石CL图像多呈自形柱状，长宽比为1:1～2:1，晶体形态总体较好，岩浆振荡环带清晰，Th/U值在0.12～2.02之间，显示了岩浆锆石的特征。第二组年龄(872～1097Ma)和第三组年龄(1344～1901Ma)的锆石CL图像显示结构较为复杂，晶体形态较差，呈半自形-他形，环带发育不均，Th/U值在0.02～1.62之间，除其中45号点的Th/U值为0.02，具有变质锆石的特征之外，多为岩浆成因。

对奈曼旗下石碑地区砂岩样品(130417-06)随机选取75颗锆石进行年龄测试，其结果见表1。锆石CL图像见图3。所测试的锆石点中，除了6个点的207Pb/235U年龄与206Pb/238U年龄相对误差大于10%而未参加计算之外，其余点的锆石年龄均处于谐和线上(图5)。根据年龄值，将其由新到老依次分为四组：370～523Ma(N=34)、884～1481Ma(N=21)、1573～1900Ma(N=6)、2369～2588Ma(N=8)。第一组年龄(370～523Ma)的峰值为412Ma，锆石CL图像显示具有一定的磨圆度，锆石振荡环带清晰，Th/U值为0.3～1.23。第二组年龄(884～1481Ma)、第三组年龄(1573～1900Ma)和第四组年龄(2369～2588Ma)的CL图像显示多数锆石环带发育，但晶形较差，呈半自形-他形；Th/U值为0.14～1.79，显示岩浆锆石特征。

表1林西杏树洼组粉砂质板岩和奈曼旗下石碑组砂岩锆石U-Pb测年结果
Table 1Results of LA-ICP-MS zircon U-Pb age dating for the Xingshuwa Formation silty slate in Linxi area and Xiashibei Formation sandstone in Naiman Banner area

序号232Th(×10-6)238U(×10-6)Th/U同位素比值年龄(Ma)207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U比值1s比值1s比值1s年龄1s年龄1s年龄1sLX0831⁃11175 98187 510 410 054870 002620 493780 023110 065270 001407774071640862111 95189 640 590 057160 002670 627550 028860 079630 001249875495184947354117 670 460 055330 004280 514580 039210 067460 001324261364222642184122 38147 120 830 054890 005810 478410 050170 063210 001254082003973439585200 74553 040 360 055370 001470 510680 013330 06690 0008342736419941756109 26201 720 540 05460 00260 486320 022740 06460 0009839678402164046764 7102 350 630 072340 003161 598430 068990 160260 002459966397027958148537 09868 460 620 161710 0023710 429140 153890 467790 00519247412247414247423952 6754 30 970 074980 004831 854130 11840 179370 00317106810010654210641710191 35652 20 290 06910 001391 430510 028440 150160 001779022290212902101170 92234 880 300 069420 002061 469910 042820 153570 0021891137918189211212267 97341 770 780 055290 002010 518310 018510 067990 000924245542412424613178 3247 140 720 054330 002280 491390 020240 06560 00097385664061441061479512 770 150 1090 001874 598470 078820 306010 003491783161749141721171565 4458 411 120 074010 007231 792240 174330 175640 00332104216810436310431816151 76246 80 610 055590 002290 527980 021340 068890 000984366543014429617201 13249 930 800 05490 002250 496750 019950 065630 00093408644101441061875 23125 430 600 054480 005390 478590 046980 063720 001163911893973239871964 3103 580 620 054980 004540 498310 04060 065740 001274111494112841082035 7271 840 500 05470 007470 481330 065210 063820 001484002633994539992157 7152 670 380 054590 002810 481510 024290 063980 001053958439917400622140 12219 270 640 05460 002640 482050 022930 064040 00096396803991640062394 91186 770 510 11110 002714 953780 12070 323410 004211817261811211806212490 84207 250 440 055180 002450 500860 021820 065840 0014207041215411625417 64605 810 690 055770 001480 548510 014290 071340 0008744336444944452686 59114 870 750 098620 003443 585110 123980 263680 0037215984315462715091927153 82159 990 960 055410 007530 525570 071080 068810 001274292714294742982848 54337 420 140 055450 00270 529360 025530 069240 000934308443117432629110 73148 30 750 054720 004390 487090 038610 064570 001164011474032640373088 08193 830 450 075580 002361 913540 059080 183640 002610843910862110871431105 44228 160 460 055650 002380 550010 022990 071690 00108438664451544663293 68165 630 570 054890 006310 486580 055710 06430 001084082284033840273383199 420 420 101780 002414 107120 097020 29270 0037916572516561916551934127 55197 810 650 054470 002540 470180 021460 062610 000963917539115391635374 57428 740 870 054140 001630 488790 014390 065480 000853774340410409536129 85354 690 370 070010 001941 490930 04050 154480 0021492933927179261237107 74182 40 590 055190 005140 497020 046080 065320 001024201814103140863882 72220 780 370 090160 002323 094420 079010 248940 003131429301431201433163981 66124 840 650 055290 004610 517260 042560 067860 0012942415142328423840118 7235 690 500 055520 002770 528970 025960 069110 001034338343117431641146 39234 030 630 094610 002553 46860 092860 265930 0034815203115202115201842454 89303 951 500 109160 002014 348220 079790 288940 003481785171703151636174346 4975 930 610 054230 004860 46030 040670 061570 001283811623842838584454 01446 090 120 056350 001580 529040 014520 068110 0008646639431104255459 66528 50 020 112950 002695 152260 122750 330870 0049318472318452018432446116 94238 560 490 055080 00310 515880 028680 067940 00101415984221942464745 6790 160 510 055650 005580 53910 053580 070270 0013643818943835438848196 65238 930 820 069940 001861 480960 038850 15360 00195927339231692111

续表1
Continued Table 1

序号232Th(×10-6)238U(×10-6)Th/U同位素比值年龄(Ma)207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U比值1s比值1s比值1s年龄1s年龄1s年龄1s4932 0943 860 730 086240 004352 760620 138080 232210 004111344691345371346225048 2223 872 020 056960 00970 674030 113990 085840 00246490324523695311551148 5191 821 620 116330 003185 497310 149680 342790 0047119012919002319002352133 89209 290 640 054940 005980 495710 053560 065440 0011941021040936409753274 97436 940 630 055590 001670 538510 015890 070270 000924364243710438654196 43170 231 150 055240 00590 51330 054470 067410 001174222084213742175577 05159 810 480 054890 002930 494860 025970 06540 00102408904081840865681 17143 230 570 055580 003390 532830 032040 069540 0011643610443421433757122 08230 930 530 055230 002670 509840 024270 066960 000994228041816418658111 14128 80 860 054990 003950 501830 035660 06620 00114121304132441375982 89337 580 250 055030 001770 50830 015990 0670 000894134741711418560223 62288 880 770 05520 002190 506470 019780 066550 000914206341613415661117 58179 460 660 076060 001831 943630 046010 185370 0024110972710961610961362259 15397 610 650 055980 001820 567630 018070 073550 00102452464561245866354 37179 670 300 056160 002570 573420 025770 074070 00114459724601746176452 06109 390 480 054890 003190 489920 027980 064750 0011408974051940476536 17399 760 090 069450 001531 387480 030040 144920 0017891225884138721066170 96190 220 900 055370 004190 531140 039880 069590 00114271414332643476736 02107 60 330 070560 002241 535050 047860 157820 002289454094519945136865 74207 730 320 055830 002170 561390 021330 072940 00106446584521445466940 53123 970 330 055460 004080 515280 037470 06740 0011743113242225420770114 46177 630 640 054880 00270 496310 024010 06560 001024078040916410671156 23275 860 570 055580 002050 531710 019150 06940 000994365543313433672125 32203 540 620 09290 002373 314310 083840 25880 0032814862914842014841773151 99269 810 560 057140 001890 63050 020420 080040 00114974749613496774127 61182 250 700 055490 003220 535450 030690 070 001074321014352043667576．53258．260．300．055450．00240．519490．02210．067970．0009943069425154246130417⁃06177 55174 530 440 054680 000910 487360 00770 064660 0006539918403540442113 17206 170 550 302160 0044924 11220 269310 578750 00567348124327311294423393 0384 261 100 076990 001622 016380 040880 190020 00228112122112114112112467 32146 220 460 068670 0011 391220 019090 1470 001468891488588848588 54166 080 530 054680 001740 477050 014580 06330 0008339945396103965666 71174 510 380 162160 0018310 468710 110440 468390 0045224788247710247620721 1854 190 390 083380 001742 507390 050560 218180 002671278211274151272148512 57421 311 220 055060 000820 504820 00710 066520 0006541515415541549180 02249 320 720 0550 000960 498630 008270 065780 0006741220411641141039 2870 480 560 055630 001520 539540 014230 070370 00084438384389438511226 4274 850 820 062280 00110 55390 00930 064530 0006668419448640341258 59178 080 330 110720 001254 956550 052050 324810 00307181191812918131513113 49189 460 600 054670 001190 478470 009970 06350 0006939928397739741477 71136 270 570 055640 001780 482540 014860 062920 00083438454001039351572 35152 060 480 054180 001680 453750 013520 060760 00078379443809380516356 54760 480 470 054620 000940 471560 007680 062640 0006339719392539241792 59200 40 460 054540 001110 470140 009150 062540 0006739325391639141894 7195 770 990 167070 0042810 015450 22670 434770 0051925294424362123272319159 6357 610 450 054420 001060 460820 008510 061440 0006438823385638442099 1165 60 600 054560 000970 471750 007950 062730 00063394213925392421247 8280 50 880 072450 000971 553160 019520 155540 00151999129528932822631 37511 341 230 053840 000920 43830 00710 059060 0005936419369537042347 68171 640 280 074990 001131 863670 026590 180310 0018410681410689106910

续表1
Continued Table 1

序号232Th(×10-6)238U(×10-6)Th/U同位素比值年龄(Ma)207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U比值1s比值1s比值1s年龄1s年龄1s年龄1s2466 2133 160 500 054460 001480 467970 012210 062340 00073390383908390425131 61203 970 650 054580 001980 478170 016720 063560 0009395533971139752655 89106 030 530 05490 001370 490150 011760 064780 00073408344058405427110 95348 570 320 165770 0018110 949010 110220 479210 0044825158251992524202821 551 490 420 08770 001752 605950 050070 215580 0025813762013021412581429158 46162 240 980 086710 001182 697750 03460 225740 0022513541113281013121230235 6321 320 730 054430 001330 466120 01090 062130 0007138932389838943160 59113 460 530 078560 002322 010830 054920 185630 0020911616011191910981132168 44252 070 670 054950 000930 499690 007980 065980 00066410194115412433163 6270 830 600 070850 000861 568830 01760 160650 00151953109587960834331 37529 870 630 165760 0017810 861730 106650 475430 00438251572511925071935243 59305 080 800 090870 002452 369730 058890 189130 0019714445312341811171136149 47208 990 720 101080 001324 059420 049960 291390 0029116441016461016481537167 24218 730 760 055630 001840 530040 016850 069120 00093438474321143163821 2761 060 350 07870 00122 159570 031270 199090 0020411651411681011701139163 65510 110 320 055970 00070 562360 006530 07290 00068451124534454440172 09189 80 910 055740 001060 54590 009880 071060 00074442224426443441149 45182 760 820 173120 0020711 767730 132910 493170 00495258882586112584214277 32418 710 180 071490 001071 599530 022520 162340 0016397114970997094346 6441 261 130 152040 004789 500920 268890 453220 006192369552388262410274475 4101 420 740 09730 001233 72160 044230 27750 0027115731015761015791445275 34336 340 820 076180 0011 848030 022620 1760 0017110011106381045946144 22236 210 610 073550 004671 499690 093330 147890 001821029132930388891047122 41294 130 420 076480 00091 964730 02140 186370 00175110810110471102104859 19105 170 560 103330 001494 250010 058550 298420 0031416851216841116831649264 66574 790 460 056040 000710 559240 006560 072410 00068454124514451450211 31455 030 460 116310 001335 51840 058230 344250 00324190081903919071651166 22111 381 490 090550 001783 116850 059130 249750 003041437191437151437165260 28146 480 410 103390 001234 24770 047150 298090 0028516869168391682145359 16117 710 500 079050 001062 171690 027330 199320 00195117312117291172105416 8843 820 390 072680 001421 690970 03160 168810 001881005211005121006105583 28109 650 760 164440 0020110 737810 124360 473770 004792502925011125002156133 88232 120 580 05490 001360 485710 011550 064190 0007440833402840145734 7368 010 510 222790 0025618 172650 196550 59180 005823001829991029972458193 38502 40 380 056020 000760 558410 007060 072330 00069453134505450459251 13455 990 550 05580 000940 554910 008790 072160 00072444184486449460101 36268 540 380 056080 000910 56720 008670 073380 0007345617456645646156 7102 780 550 157460 00229 89340 134350 455870 0050924291024251324212362104 74186 370 560 055350 001270 525750 011540 068910 00077426294298430563128 89670 370 190 155150 001799 138670 098170 427350 00409240382352102294186446 1325 080 140 072640 000891 680350 019050 167830 0015810041010017100096555 13247 780 220 072530 000971 665220 020710 166580 001611001129958993966114 11169 60 670 055680 001020 533930 009330 069570 00071440214346434467141 66383 710 370 055840 001280 55250 012130 071790 00081446294478447568151 52330 060 460 056020 001580 535520 014470 069360 000864533843510432569709 541337 350 530 07510 001670 588380 011810 056820 000551071464708356370112 9562 871 800 070290 002211 500880 045650 154930 002299373893119929137188 21170 940 520 054850 000990 498050 008490 065880 00067406214106411472209 53349 20 600 055320 001070 520810 009540 06830 0007142523426642647336 4385 070 430 058240 002560 678070 028770 084470 00141539635261752387462 79204 870 310 056250 001430 571730 013890 073750 00086462334599459575171 23211 870 810 092680 001453 109380 046180 243410 00259148114143511140413

图3　林西地区杏树洼组粉砂质板岩和奈曼旗下石碑组砂岩部分锆石CL图像Fig.3　CL images of selected zircons from Xingshuwa Formation silty slate in Linxi area and Xiashibei Formation sandstone in Naiman Banner area

图4　林西地区杏树洼组粉砂质板岩锆石U-Pb谐和图与频率曲线图Fig.4　Zircon U-Pb concordia diagram and relative probability plot for detrital zircons from Xingshuwa Formation silty slate in Linxi area

图5　奈曼旗下石碑组砂岩锆石U-Pb谐和图与频率曲线图Fig.5　Zircon U-Pb concordia diagram and relative probability plot for detrital zircons from Xiashibei Formation sandstone in Naiman Banner area

4　讨论

4.1　北侧粉砂质板岩锆石年龄意义

林西地区西拉木伦河北岸浅绿色粉砂质板岩第一组锆石年龄为385～531Ma，其峰值为412Ma，可能与早古生代艾力格庙-苏左旗-锡林浩特岩浆岩带的剥蚀有关 (Xuetal.,2013)。沿该带前人报道了大量岛弧岩浆岩的锆石U-Pb年龄，年龄分布在421～490Ma之间(Chenetal.,2000; 石玉若等,2005; Jianetal.,2008; 郭锋等,2009; 葛梦春等,2011;孙立新等,2013)。第二组锆石年龄为872～1097Ma。根据前人的年代学研究，华北板块内部没有该期岩浆作用记录，但它在兴蒙造山带却普遍存在。例如Xuetal.(2013)通过对蛇绿混杂带基质的碎屑锆石研究，认为内蒙古中部存在一个未出露的浑善达克地块，其基底年龄主要集中于576～1044Ma；王炎阳等(未发表资料)对克什克腾旗五道石门基性火山岩锆石U-Pb年龄分析发现存在760～950Ma的继承性锆石。第三组年龄(1344～1901Ma)的锆石也只见于兴蒙造山带内，例如前人在本研究区以北的二叠纪沉积岩中发现中-新元古代碎屑锆石，其时代介于564～1557Ma(韩国卿等,2011)。显然，第二、三组元古代锆石应来源于兴蒙造山带的元古代基底，第一组锆石与早古生代岩浆弧的剥蚀有关，而最年轻的锆石U-Pb谐和年龄值为385±8Ma，限定样品所在地层的沉积下限为中-晚泥盆世。

4.2　南侧砂岩锆石年龄意义

位于西拉木伦河南岸的奈曼旗下石碑组砂岩第一组锆石年龄为370～523Ma，其峰值为412Ma，其中的较老锆石(500～400Ma)可能与早古生代华北板块北缘发育的图古日格-达茂旗-温都尔庙-镶黄旗-解放营子岛弧岩浆岩带有关。这些岛弧岩浆岩的年龄为490～404Ma(图1b; 刘敦一等,2003; 张维和简平,2008; 秦亚等,2013; 刘建峰等,2013; Jianetal.,2008; Xuetal.,2013; Zhangetal.,2012)。另外，年龄谱中还包含较多380～400Ma的碎屑锆石年龄，可能来源于该岛弧岩浆岩带南侧的早-中泥盆世碱性岩(曾俊杰,2009; 张晓辉和翟明国,2010; Miaoetal.,2002)及基性侵入岩(Zhangetal.,2009)。样品中最年轻的锆石U-Pb谐和年龄值为370±4Ma，限定地层沉积下限为晚泥盆世。值得注意的是，样品中出现多颗0.9～1.5Ga的锆石年龄(共21颗)。大量研究表明，该年龄段是兴蒙造山带范围的典型特征(Rojas-Agramonte,2011; Xuetal.,2013)。第四组2369～2588Ma的锆石反映出华北板块的物源信息。综上，奈曼旗下石碑组的锆石既有兴蒙造山带、也有华北板块的物源，这表明志留-泥盆纪时期华北板块与兴蒙造山带之间已经没有沉积阻隔，兴蒙造山带的剥蚀物已经到达西拉木伦河南侧的华北板块北缘。

4.3　南北两侧锆石年龄与松辽地块的对比

本文测试的林西地区西拉木伦河浅绿色粉砂质板岩(LX0831-11)和奈曼旗下石碑组砂岩(130417-06)分属西拉木伦河两岸，但却显示出一致的志留-泥盆纪及晚元古代碎屑锆石年龄谱，表明两者从泥盆纪即具有相同的沉积物源(图6a,b)。松辽地块处于华北板块以北的兴蒙造山带内，Wangetal.(2013)通过对松辽-张广才岭塔东群碎屑锆石的研究，提出了松辽地块的锆石年龄分布模式(图6c)。对比发现，西拉木伦河南、北两侧样品的碎屑锆石年龄与松辽地块有着明显的相似性，不仅均具有早古生代末期的年龄信息，而且其继承性锆石也具有相同的年龄分布(图6)，暗示西拉木伦河南、北两侧的沉积物源与松辽地块有关。

图6　锆石年龄频率分布与松辽地块年龄谱对比(松辽地块数据来源于Wang et al.,2013)Fig.6　Comparison of probability plots for our two samples and Songliao Block (the data of Songliao Block comes from Wang et al.,2013)

5　构造意义

本文所测试的两个样品分别位于西拉木伦河北岸的双井子地区和南岸的奈曼旗地区，分属前人所述的晚古生代“西拉木伦河缝合线”两侧，二者从泥盆纪到元古代碎屑锆石年龄的相似性说明它们从中-晚泥盆世以来已经具有共同的物源；而代表兴蒙造山带的元古代碎屑锆石在奈曼旗地区的出现，说明泥盆纪以来兴蒙造山带的剥蚀物已到达华北板块北缘。因此，本次碎屑锆石年代学研究暗示华北板块与其北部松辽地块在中-晚泥盆世之前已经完成拼合过程，即此时两者间已不存在古亚洲洋。

另外，前人根据古生物化石依据在林西地区西拉木伦河北岸划分出志留纪杏树洼组，并与南岸奈曼旗地区志留系下石碑组对比(内蒙古自治区地质矿产局,1991)。从本次研究获得的碎屑锆石年龄数据看，这两个地区存在中、晚泥盆世地层。因此，对西拉木伦河两岸志留-泥盆系做更详细的研究将会为古亚洲洋的构造演化过程提供更多的约束。

6　结论

(1)西拉木伦河北岸的双井子地区杏树洼组碎屑锆石年龄分为三组，由新自老依次为：385～531Ma(N=52)、872～1097Ma(N=11)和1344～1901Ma(N=11)，碎屑锆石的最小年龄限定地层沉积下限为中-晚泥盆世；南岸的奈曼旗地区下石碑组碎屑锆石年龄分为四组，由新自老依次为：370～523Ma(N=34)、884～1481Ma(N=21)、1573～1900Ma(N=6)、2369～2588Ma(N=8)。碎屑锆石的最小年龄限定地层沉积下限为晚泥盆世。

(2)西拉木伦河南、北两侧原志留纪样品显示出一致的志留-泥盆纪及晚元古代碎屑锆石年龄谱，表明两者从志留-泥盆纪即开始接受来自兴蒙造山带的沉积物源。物源相似性表明华北板块与其北部松辽地块在中-晚泥盆世已经完成拼合过程。

致谢感谢王炎阳、廖闻、张辰昊、罗志文等在野外工作中给予的大力帮助;感谢北京大学造山带与地壳演化教育部重点实验室马芳高级工程师在年龄测试过程中进行的指导。

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