据文献[18]修改。1.早二叠世火山‒沉积岩系; 2.早二叠世裂谷岩浆岩带; 3.中二叠世碱性岩浆岩带; 4.造山带; 5.国界; 6.研究区位置
图1 兴蒙造山带晚古生代岩石‒构造单元示意图
Fig. 1 Late Paleozoic litho-tectonic units of Xing-Meng orogenic belt
摘要 为了查明兴蒙造山带西段二连‒满都拉裂陷盆地的充填过程以及其中火山‒沉积岩系的时空分布, 对西里庙组开展地层层序和同位素年代学研究, 探讨盆地形成的构造背景。实测剖面查明, 二叠纪西里庙组地层可分为 3 段, 上段和下段为晶屑凝灰岩及流纹质岩屑晶屑凝灰岩, 中段为暗色沉积岩, 构成独特的火山‒沉积岩系。上段流纹质凝灰岩的锆石 206Pb/238U 年龄为 282±2Ma, 代表形成年龄; 中段碎屑岩的两组锆石年龄峰值分别为 304Ma 和 450Ma, 另有 1 颗锆石的年龄为 870Ma, 分别代表最大沉积年龄、早古生代造山带的形成时代和晚元古代基底的年龄。中段碎屑岩的年代学测试结果表明, 西里庙组火山‒沉积岩系的形成时代为304~282Ma, 其层位应置于哲斯组之下。西里庙组中段沉积地层碎屑锆石的物源分析结果为从中古生代构造挤压过程向晚古生代构造伸展过程转化的沉积响应提供了证据。
关键词 兴蒙造山带; 艾力格庙地区; 西里庙组; 二叠纪; 火山‒沉积岩系; 锆石年代学
中亚造山带西起乌拉尔山, 向东延伸至太平洋沿岸, 北部以前寒武纪西伯利亚克拉通为界, 南部以塔里木板块和华北克拉通为界。中亚造山带是世界上最大的增生型造山带, 也是显生宙以来增生最显著的地区之一[1‒11]。兴蒙造山带位于中亚造山带东南段的中国内蒙古自治区和东北地区, 早‒中古生代的构造演化过程表现为由额尔古纳地块、兴安地块、松辽‒浑善达克地块和华北克拉通等拼贴而形成南造山带和北造山带[12‒14]。兴蒙造山带形成后, 在晚古生代转为伸展构造背景, 发育多个裂陷盆地, 形成分布广泛的火山‒沉积岩系[15‒16](图 1)。该岩系是晚古生代伸展构造的沉积响应, 研究其时空分布和沉积特征, 有助于恢复兴蒙造山带晚古生代裂陷盆地的构造格局及形成过程。
艾力格庙地区处于二连‒满都拉裂陷盆地, 发育连续出露的二叠纪西里庙组火山‒沉积岩系[17], 是开展二叠纪沉积演化和年代学分析的理想地区。本文选择二叠纪火山‒沉积岩系, 进行详细的沉积剖面研究和锆石 U-Pb 年龄测定, 并初步探讨其沉积环境, 以便为进一步恢复晚古生代的构造格局提供新的依据。
艾力格庙地区的地理坐标为 42°10′—43°10′N, 东经 111°05′—111°25′E, 属于兴蒙造山带西段。该区出露的地层单元主要包括新元古代艾力格庙群、早古生代温都尔庙群、石炭纪本巴图组、二叠纪西里庙组和哲斯组(图 2)。
艾力格庙群零星出露于研究区北部, 主要由石英岩和绢云石英片岩组成, 属于南蒙微大陆的一部分, 向西与南蒙托托尚山块体相连, 已获得片麻状花岗岩的锆石 U-Pb 年龄为952±2 Ma[16,19‒20]。
温都尔庙群集中出露于研究区南部, 出露范围东西长约 13km, 南北宽约 7km, 主要岩性为绢云石英片岩、二云母石英片岩和含铁石英岩等, 遭受褶皱变形和绿片岩相变质。在本区以东的查干诺尔和苏尼特左旗南, 已获得温都尔庙群碎屑锆石年龄加权平均值为 463±3Ma, 绿片岩中锆石 U-Pb 年龄为 460±4Ma, 因此该群属于早古生代地层[21‒22]。
石炭纪本巴图组出露于研究区南部, 由巨厚的灰岩、砂岩和粉砂岩组成。
二叠纪西里庙组见于研究区中北部, 由流纹岩、凝灰岩、砂岩和灰黑色板岩互层组成, 形成巨厚的火山‒沉积岩系, 是本文研究重点。二叠纪哲斯组零星出露于研究区南部, 由砾岩、砂岩和泥岩组成沉积旋回, 不整合地覆盖于温都尔庙群之上。
晚古生代侵入岩大部分位于研究区北部, 包括泥盆纪闪长岩、二叠纪闪长岩和花岗岩。泥盆纪闪长岩分布面积较小, 侵入于艾力格庙群; 二叠纪闪长岩和花岗岩侵入于艾力格庙群和西里庙组等。
据文献[18]修改。1.早二叠世火山‒沉积岩系; 2.早二叠世裂谷岩浆岩带; 3.中二叠世碱性岩浆岩带; 4.造山带; 5.国界; 6.研究区位置
图1 兴蒙造山带晚古生代岩石‒构造单元示意图
Fig. 1 Late Paleozoic litho-tectonic units of Xing-Meng orogenic belt
据地质图脑木根幅(K-49-X)①修改。1.艾力格庙群; 2.温度尔庙群; 3.本巴图组; 4.二叠纪哲斯组; 5.西里庙组下段; 6.西里庙组中段; 7.西里庙组上段; 8.侏罗系; 9.花岗岩; 10.年龄采样点; 11.剖面位置; 12.古生物化石
图2 研究区构造背景
Fig. 2 Tectonic setting sketch map of the research area
西里庙组见于研究区北部, 连续出露, 倾向北西, 分为 3 段 19 层, 实测剖面如图 3 所示, 地层描述如下。
-----------未见顶-----------
19 浅灰褐色晶屑凝灰岩, 厚度>30 m
18 灰褐色流纹质晶屑凝灰岩, 厚 14 m
17 灰褐色晶屑凝灰岩, 厚 23 m
16 黄白色流纹质晶屑凝灰岩, 厚 9 m
15 黄白色流纹质晶屑凝灰岩与灰褐色岩屑晶屑凝灰岩互层, 厚 54 m
14 黄褐色凝灰岩, 厚 15 m
13 灰褐色流纹质晶屑凝灰岩与灰黑色板岩互层, 厚45 m
12 灰褐色凝灰质砂岩, 厚 10 m
11 黄褐色变质砂岩、灰褐色凝灰岩、灰黑色板岩互层, 厚 5 m
10 黄褐色变质粉砂岩, 厚 5 m
9 中薄层灰黑色板岩与变质砂岩互层, 厚 8 m
8 中薄层灰黑色板岩, 有逆冲断层和形态各异的小褶皱构造, 厚 300 m
7 红褐色变质粉砂岩夹层, 厚 2 m
6 灰黑色板岩, 厚 15 m
-----------断层-----------
5 灰褐色岩屑晶屑凝灰岩, 厚 25 m
4 黄褐色晶屑凝灰岩, 厚 46 m
3 片理化浅黄褐色流纹质岩屑晶屑凝灰岩, 厚 12 m
2 黄褐色英安质岩屑晶屑凝灰岩, 厚 49 m
1 片理化浅黄褐色流纹质晶屑凝灰岩, 厚 90 m
-----------未见底-----------
下段(图 2 中剖面 A-A′位置)总厚度约为 240m, 分为 5 层, 是一套以酸性‒中酸性火山岩‒火山碎屑岩为主的岩层, 其层序及特征见图 3 中剖面 A-A′。流纹质晶屑凝灰岩呈凝灰结构, 块状构造。晶屑以石英、斜长石和条纹长石为主, 棱角分明(图 4(a)~ (b))。英安质岩屑晶屑凝灰岩呈凝灰结构, 假流动构造。晶屑包括斜长石和石英, 其中斜长石含量为15%, 聚片双晶特征明显; 石英含量为 10%, 呈港湾状。岩屑以凝灰岩为主, 含量约为 8%。
A-A′:西里庙组下段剖面; B-B′:西里庙组中段剖面; C-C′:西里庙组上段剖面。1.英安质岩屑晶屑凝灰岩; 2.流纹质岩屑晶屑凝灰岩; 3.岩屑晶屑凝灰岩; 4.流纹质晶屑凝灰岩; 5.晶屑凝灰岩; 6.凝灰岩; 7.凝灰质砂岩; 8.变质砂岩; 9.变质粉砂岩; 10.板岩; 11.灰岩透镜体; 12.断层; 13.地层编号; 14.采样点
图3 二叠纪西里庙组地质剖面
Fig. 3 Geological sections of Permian Xilimiao Formation
中段(图 2 中剖面 B-B′位置)总厚度为 450m, 分为 9 层, 主体部分是暗色板岩建造, 夹少量薄层及中厚层砂岩, 顶部为碎屑岩与中酸性火山岩密集互层, 层序特征见图 3 中剖面 B-B′。灰褐色及黑色板岩为巨厚层, 内部粒度变化均匀。由于发育密集劈理而呈薄板状, 矿物以绢云母和石英为主, 含量各占约 50%。绢云母由泥质或火山灰变质而成, 凝灰质结构, 变质程度较浅(图 4(c)~(d))。中厚层变质砂岩为浅褐色, 中粒砂状结构, 块状构造。碎屑成分以岩屑和石英为主, 具有不明显递变层理。底部与下伏粉砂岩为突变关系, 连续延伸(图 5(b))。下部含透镜状黄褐色灰岩透镜体, 顺层不连续分布, 单个透镜体规模可达 1×8 m2 (图 5(c))。
上段(图 2 中剖面 C-C′位置)与中段呈整合渐变关系, 总厚度约为 145m, 分为 5 层, 为流纹质晶屑凝灰岩夹岩屑晶屑凝灰岩, 其层序及特征见图 3 中剖面 C-C′。灰褐色岩屑晶屑凝灰岩中的晶屑以石英和斜长石为主, 呈圆‒次圆状, 晶屑粒径为 0.1~0.2mm, 其中石英含量约占 15%, 大部分被溶蚀; 绢云母化的斜长石含量约为 15%, 并含少量岩屑。黄白色流纹质晶屑凝灰岩的晶屑以石英和长石为主, 粒径为 0.5~1mm, 其中石英含量为 20%, 长石含量约25%。
上述实测剖面揭示, 西里庙组地层上段和下段为浅色火山岩段, 以晶屑凝灰岩及流纹质岩屑晶屑凝灰岩为主, 代表火山喷发环境。
中段为暗色沉积岩段, 以巨厚层板岩为特征(图 5(a), 内部均一, 无层理, 表明处于快速沉降和快速沉积的低能水动力环境; 局部可见中厚层砂岩(图 5(b)), 说明短期处于滨海沉积环境, 有较强的水动力条件。
中段下部的暗色板岩夹褐黄色灰岩透镜体(图5(c)), 可能代表滨海‒泻湖的低能沉积环境。结合徐备等[22]发现的底栖生物海百合茎, 本区应处于较低能的滨海‒泻湖环境。
总而言之, 西里庙组地层层序应代表火山喷发与沉积交替的陆相‒滨海‒泻湖环境。
(a)和(b)流纹质晶屑凝灰岩野外和显微镜下照片; (c)和(d)暗色板岩的野外和显微镜下照片。Pl: 斜长石; Qz: 石英
图4 研究区样品野外及镜下照片
Fig. 4 Field and microscopic photos of the samples
(a)西里庙组中段实测剖面宏观照片; (b)中厚层砂岩(白色箭头指示正韵律层理); (c)灰岩透镜体
图5 西里庙组中段沉积现象野外照片
Fig. 5 Field photos of sendimentary phenomenon in Xilimiao Formation
LA-ICP-MS 锆石 U-Pb 年代学测试在北京大学造山带与地壳演化教育部重点实验室完成。激光剥蚀使用德国相干公司(Coherent) COMPExPro102 型ArF 准分子激光器, 质谱仪为美国安捷伦公司 7500c型四极杆等离子质谱。激光条件: 激光束斑的直径为 32μm, 激光能量密度为 10J/cm2, 剥蚀频率为 5Hz。样品剥蚀前, 先进行 20 次激光脉冲的预剥蚀, 采集 20s 的空白, 随后进行 60s 的样品剥蚀。剥蚀完成后, 进行 2min 的样品池冲洗。载气采用高纯度 He 气, 气流量为 0.70L/min。不同同位素的采集时间, 202Hg, 204Hg 和 208Hg为20ms, 206Hg为 30ms, 207Hg 为 50ms, 其余元素为 10ms。使用标准锆石Plesovice (337Ma)[23]作为外标, 进行同位素比值校正, 以标准锆石 91500 为监控盲样。元素含量以国际标样 NIST610 为外标、Si 为内标元素进行计算, NIST612 和 NIS641 为监控盲样。用 GLITTER4.4.2 软件进行同位素比值及元素含量计算。使用 Isoplot/ Ex(3.0)程序计算谐和年龄和绘图。普通铅校正使用 Andersen 等[24]的程序计算。分析数据及锆石 U-Pb 谐和图给出误差为 1σ,表示 95%的置信度。
共测试 4 个样品, 分别是二叠纪西里庙组中段的变质粉砂岩(100830-13)、上段的两块凝灰岩(160708-02 和 160710-07)以及一块侵入于上段的花岗岩(120828-19), 样品采集位置见图 2 中标记的 4个采样点a, b, c和d。
3.2.1 花岗岩
样品 120828-19 为花岗岩, 侵入于西里庙组上段地层(图 2 中采样点 a)。锆石晶体形态好, 绝大部分呈长柱状, 个别呈椭圆状和短柱状, 粒径在 0.1~0.2mm 之间, 长宽比变化较大, 多数为 2:1~ 3:1, 晶形基本上为自形, 晶面整洁光滑, 大多数锆石发育较清晰的韵律环带, 显示岩浆锆石的特点(图 6)。30 颗锆石的离子探针结果见表 1, Th/U 比值均大于 0.42, 表明锆石属于岩浆锆石[25]。在锆石U-Pb 谐和图上, 集中形成两个点群, 较年轻的一期为 268±1Ma (图 7(a)), 代表岩浆侵位年龄; 较老的一期为 280±1Ma, 代表继承性锆石年龄(图 7(b))。由于花岗岩与二叠纪西里庙组上段地层之间为侵入关系, 因此可以限定西里庙组上段地层形成时代不晚于268 Ma。
图6 样品120828-19 (花岗岩)锆石阴极发光(CL)图像
Fig. 6 Zircon CL image of sample 120828-19 (granite)
图7 样品120828-19 (花岗岩)锆石U-Pb谐和图
Fig. 7 Zircon concordia age of sample 120828-19 (granite)
3.2.2 流纹质(晶屑)凝灰岩
凝灰岩样品共两块, 采集于西里庙组上段地层(图 2 中采样点 b 和 c)。
样品 160708-02 为流纹质凝灰岩, 锆石晶体形态好, 绝大部分呈长柱状, 个别呈椭圆状和短柱状, 粒径约为 0.1mm, 长宽比变化较大, 多数为 2:1~ 3:1, 少部分是 1:1 或 4:1, 晶形基本上为半自形和自形, 晶面整洁光滑, 大多数锆石发育较清晰的韵律环带, 显示岩浆锆石的特点(图 8)。共测试 25 颗锆石，其中 22 颗获得有效数据(表 1)。Th/U 比值均大于 0.4, 表明是岩浆成因锆石。在 U-Pb 谐和图(图 9)上, 数据投影点基本上落在谐和线上及其附近, 锆石年龄值集中分布在 283Ma 附近, 谐和年龄值为283±2Ma, 加权平均年龄为 282±3Ma, 据此可以限定西里庙上段地层的形成年龄约为 282 Ma。
表1 艾力格庙地区样品LA-ICP-MS U-Pb测年数据
Table 1 LA-ICP-MS U-Pb age results in Arigin Sum Area
点位含量/(μg·g−1)Th/U207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U调谐度/% ThU比值误差(1σ)比值误差(1σ)比值误差(1σ)年龄/Ma误差(1σ)年龄/Ma误差(1σ)年龄/Ma误差(1σ) 120828-19 2819-1237.52180.421.320.051700.004020.311090.023840.043640.000752721432751827550.00 2819-2280.37564.060.500.046050.004310.246940.022860.038890.0005620018224192463−8.94 2819-3198.69204.500.970.063700.002701.056270.043810.120260.001777326373222732100.00 2819-4714.62533.681.340.051620.002190.301270.012490.042330.00059269692671026740.00 2819-5879.58751.491.170.051440.001990.294700.011100.041550.0005726161262926240.00 2819-6113.45203.620.560.051950.004520.317280.027190.044300.000882831582802127950.36 2819-7172.41344.730.500.051740.002950.305170.017100.042780.000652741012701327040.00 2819-8278.10379.510.730.052020.002310.328170.014220.045750.00068286722881128840.00 2819-9368.25286.961.280.051690.005390.312850.032380.043900.00073272203276252775−0.36 2819-10531.191170.330.450.052060.002140.330140.013290.045990.00061288682901029040.00 2819-11354.80504.230.700.051900.002250.318140.013370.044460.00069281682801028040.00 2819-12218.72313.170.700.051650.002610.304990.015050.042830.00066270852701227040.00 2819-13488.22630.160.770.051690.001990.308150.011500.043240.0006027260273927340.00 2819-14681.80793.380.860.051670.001970.304140.011270.042690.0005827160270926940.37 2819-15224.45278.330.810.051800.002780.314060.016490.043970.00070277912771327740.00 2819-16385.54416.150.930.051520.004050.299380.022970.042140.000892641362661826660.00 2819-17211.13246.870.860.051220.006350.282830.034890.040050.000672512482532825340.00 2819-1886.08244.620.350.052800.005630.377410.039830.051830.00100320203325293266−0.31 2819-1993.28181.650.510.052180.007020.337880.045050.046960.001042932612963429660.00 2819-20396.17506.100.780.051930.003950.318940.023930.044540.000712821422811828140.00 2819-21165.50320.480.520.051600.002550.302190.014550.042470.00066268822681126840.00 2819-22387.30626.070.620.051700.002020.303830.011500.042620.0006027261269926940.00
续表
2819-23225.40280.430.800.051520.002830.296840.015910.041790.00068264932641226440.00 2819-24661.81685.520.970.052050.002010.326780.012190.045530.0006428859287928740.00 2819-25375.62494.290.760.051540.002930.301640.016840.042440.000632651012681326840.00 2819-2687.92179.520.490.051900.004960.316300.029690.044200.000982811722792327960.00 2819-27189.78339.460.560.050980.003930.310740.023550.044200.00076240141275182795−1.43 2819-281345.16951.101.410.051780.001930.306690.011040.042950.0005927657272927140.37 2819-29260.21518.650.500.051930.002540.321440.015250.044890.00070282802831228340.00 2819-30150.26257.780.580.050590.013240.251510.065240.036050.001392224082285322890.00 160708-02 0802-02154.19256.380.600.051130.001910.319670.011760.045330.000522476328292863−1.40 0802-03276.77268.901.030.054630.001910.342860.011780.045510.0005139757299928734.18 0802-04132.95439.820.300.057290.002620.402720.017820.050980.000605031033441332147.17 0802-05317.53416.810.760.051950.003120.307850.018120.042980.000522831392731427130.74 0802-07597.34609.780.980.055540.001430.337890.008480.044120.0004743437296627836.47 0802-08143.52232.520.620.053740.002030.348700.012910.047050.00054360633041029632.70 0802-09114.37190.580.600.054190.002270.331320.013620.044330.00053379712911028033.93 0802-10328.12270.791.200.056770.002720.319470.014960.040810.00056483792811225838.91 0802-11228.50381.100.600.053260.001620.330190.009820.044960.0005034047290728432.11 0802-12186.71229.050.810.051120.002010.322470.012490.045740.0005324668284102883−1.39 0802-13861.24815.681.050.055590.001400.329000.008070.042920.0004643636289627136.64 0802-14159.56236.960.670.052440.002300.335540.014460.046400.00058305752941129240.68 0802-15135.25247.550.550.050860.001970.309160.011760.044070.000512346627492783−1.44 0802-16542.90643.080.850.053240.001390.323330.008240.044030.0004733938284627832.16 0802-171725.521136.691.520.049020.001850.309220.011360.045740.000561496327492883−4.86 0802-18127.98160.340.800.047750.002470.294090.015050.044660.000548791262122823−7.09 0802-19485.20545.530.890.053580.001480.329990.008870.044660.0004935341290728232.84 0802-2056.53142.880.400.052100.002720.326470.016800.045440.00056290952871328630.35
续表
0802-22483.93538.670.900.052370.002960.318680.017650.044130.000513021322811427831.08 0802-23181.26179.021.010.053820.002330.328390.013980.044240.00053364742881127933.23 0802-24176.24250.220.700.052950.002010.323890.012060.044350.0005132764285928031.79 0802-25126.46289.310.440.048710.005350.314320.033820.046800.00101134243278262956−5.76 160710-07 1007-01558.71436.621.280.054830.001940.353530.012240.046750.0005540556307929530.00 1007-02114.83226.100.510.053580.002080.329800.012590.044620.00053353652891028132.85 1007-0388.01143.040.620.052930.002780.326550.016950.044730.00056326952871328231.77 1007-04313.73320.880.980.054320.002280.334220.013750.044610.00056384702931028134.27 1007-0581.46173.340.470.056710.002450.364110.015490.046550.00056480733151229337.51 1007-06277.30261.251.060.051270.002190.314210.013150.044430.0005625373277102803−1.07 1007-071101.55608.831.810.054720.001500.330190.008800.043750.0004840140290727635.07 1007-08334.50219.411.520.054380.002150.343860.013350.045840.00054387663001028933.81 1007-09142.75248.000.580.052450.001990.321530.011990.044450.0005230564283928031.07 1007-10174.36173.631.000.050550.002730.316950.016860.045460.0006022099280132874−2.44 1007-11120.38143.200.840.057260.003560.326660.019960.041360.000635021082871526149.96 1007-12165.90145.921.140.053880.003360.327090.020060.044010.000663661122871527843.24 1007-1384.91131.310.650.059780.003620.364900.021700.044260.000672931962782027740.36 1007-14325.51233.271.400.051740.002020.323830.012410.045380.0005327466285102863−0.35 1007-15329.66222.661.480.054590.002120.331380.012650.044010.00052395642911027834.68 1007-16306.41231.571.320.051290.002070.314180.012460.044410.0005325469277102803−1.07 1007-17159.78184.220.870.053610.002430.320940.014340.043400.00053355792831127433.28 1007-18197.80264.310.750.051430.001950.318760.011880.044940.000522606428192833−0.71 1007-1946.6880.950.580.053370.004480.345630.028760.046950.000693451633012229641.69 1007-20149.54259.340.580.051780.001970.323510.012050.045300.000532766428592863−0.35 1007-21202.62190.911.060.052480.002280.332380.014190.045920.00056306752911128930.69
续表
1007-22181.62161.201.130.054650.002620.333850.015760.044290.00056398832931227935.02 1007-23222.61300.110.740.051890.002120.323760.012990.045230.00056281692851028530.00 1007-24438.87355.211.240.052700.001750.324550.010550.044650.0005131653285828231.06 1007-25556.03375.711.480.050330.001670.317100.010270.045680.000522105428082883−2.78 100830-13 3013-152.9498.420.540.053140.002150.386730.015690.052800.00124335513321133280.00 3013-297.3946.432.100.051930.004840.316620.029140.044230.001282821552792227980.00 3013-364.1780.430.800.052500.002240.347630.014790.048040.00115307553031130270.33 3013-470.65224.520.310.055490.001520.527170.014950.068920.00154432294301043090.00 3013-5214.04323.510.660.052510.001940.348690.013090.048180.00108308463041030370.33 3013-6124.42176.070.710.053170.001600.392070.012070.053490.0012133633336933670.00 3013-743.08108.410.400.052680.003430.370630.024180.051040.001193151063201832170.31 3013-824.5948.030.510.055070.005840.508170.053820.066950.0016741519341736418100.24 3013-9212.78382.090.560.052650.001370.362800.009830.049990.0011131428314731470.00 3013-1071.21143.640.500.052690.002430.359800.016660.049540.00115315643121231270.00 3013-1135.1185.690.410.052910.002530.381390.018250.052290.00123325663281332980.30 3013-12117.84286.090.410.052330.001460.347100.009970.048120.0010730030303830370.00 3013-1373.98144.690.510.056210.001840.572450.019160.073880.001674613746012459100.22 3013-1481.67230.130.350.052330.001820.343020.012130.047560.0010730042299930070.33 3013-15104.76185.380.570.052590.001890.364730.013200.050320.00116311433161031670.00 3013-1646.1968.290.680.052230.002510.334150.015040.046410.00135295532931129280.34 3013-17251.88348.270.720.054650.001790.478570.016070.063530.00141398383971139790.00 3013-18263.89364.750.720.052510.001430.348700.009820.048180.0010730830304730370.33 3013-19243.40326.030.750.052360.003270.342980.021480.047520.001103011012991629970.00 3013-20144.70168.140.860.056140.002030.563190.020350.072780.001734584145413453100.22 3013-21165.52228.960.720.052850.001560.373430.011310.051260.0011532233322832270.00 3013-22148.71233.460.640.055650.002250.536050.021950.069880.00157438524361543590.23
续表
3013-2368.44122.400.560.052360.003030.348520.020190.048290.00113301893041530470.00 3013-24197.10247.700.800.052570.002920.348750.019430.048130.00111310853041530370.33 3013-2537.17117.680.320.055720.001760.543700.017110.070790.001674413344111441100.00 3013-2682.9679.111.050.052300.004120.339000.026580.047030.001162991342962029670.00 3013-27355.62393.100.900.057270.006110.371400.038460.047030.001205022423212829678.45 3013-2842.7266.640.640.054760.002910.482540.025630.063930.00151402774001839990.25 3013-29324.33312.901.040.055760.001620.544450.016340.070840.00157443324411144190.00 3013-30628.87474.071.330.052420.001380.348420.009530.048220.0010630428304730470.00 3013-31233.49195.861.190.052820.001590.368580.011320.050620.0011432133319831870.31 3013-32312.01302.361.030.053320.002030.404230.015580.055000.00123342483451134580.00 3013-3354.74236.210.230.052890.001580.375230.011480.051470.0011532433324832470.00 3013-34132.55202.570.650.052880.001510.376160.010990.051610.0011632431324832470.00 3013-35140.94199.710.710.051870.001690.320270.010600.044790.0010128038282828260.00 3013-36245.30369.490.660.053510.001600.407670.012480.055270.0012235033347934770.00 3013-3757.5263.050.910.047130.032110.327730.222720.050440.0025356103628817031716−9.15 3013-38156.20237.110.660.051730.001760.303910.010420.042620.0009727340269826960.00 3013-39170.23241.410.710.052930.001730.372900.012380.051110.0011532638322932170.31 3013-40614.13496.131.240.052640.001500.358200.010540.049370.0010931332311831170.00 3013-41338.81519.390.650.056180.001380.571050.014620.073740.00162459254599459100.00 3013-4266.0165.471.010.052470.003610.352640.024150.048760.001193061123071830770.00 3013-4358.0570.330.830.052460.003710.351480.024720.048610.001203061153061930670.00 3013-4495.95122.800.780.052900.002080.381400.015030.052310.0012132549328113297−0.30 3013-4572.42180.050.400.052100.001810.331840.011630.046200.0010629041291929170.00 3013-4631.9345.320.700.068240.002481.373730.050550.146040.00339876408782287919−0.11 3013-4779.16105.620.750.052770.002130.365400.014810.050240.00116319523161131670.00 3013-48979.22579.101.690.052400.001370.347290.009340.048080.0010730328303730370.00
续表
3013-49412.98421.580.980.054260.001480.454280.012750.060740.0013538229380938080.00 3013-501220.26737.121.660.053020.001580.382380.011650.052320.0011733033329932970.00 3013-51192.62303.920.630.052640.001470.363830.010380.050140.0011231330315831570.00 3013-52181.08253.440.710.052460.001550.348810.010530.048230.0010830633304830470.00 3013-53166.73298.040.560.053270.001550.395750.011770.053890.0012034032339933870.30 3013-54119.41183.170.650.051730.002310.312880.013930.043880.0010327360276112776−0.36 3013-5599.52132.690.750.052590.001760.349700.011690.048240.0011231138304930470.00 3013-568.4321.660.390.051780.008110.309320.047680.043340.0016927627227437274100.00 3013-57179.03233.050.770.052300.001720.340500.011360.047230.0010629938298929770.34 3013-58140.44365.920.380.056440.001990.584610.020980.075150.001664704246713467100.00 3013-59241.01333.400.720.052250.001600.340390.010590.047260.001062963429782987−0.34 3013-60285.64297.900.960.052660.001500.363180.010550.050040.0011131431315831570.00 3013-61274.49320.670.860.052030.001480.322230.009350.044930.0010028731284728360.35 3013-6273.8855.991.320.052660.004710.355330.031610.048950.001243141553092430880.32 3013-63267.02331.330.810.052460.001460.350230.009950.048430.0010830630305730570.00 3013-64130.25244.720.530.052550.003310.353130.022260.048750.001133091013071730770.00 3013-6584.88154.450.550.052470.002450.355200.016630.049110.00113306653091230970.00 3013-66274.55373.910.730.052500.002140.345800.014140.047790.00110307533021130170.33 3013-6784.94162.680.520.052380.001720.345940.011440.047910.0010930237302930270.00 3013-68130.39237.870.550.052560.001750.350280.011790.048340.0010931039305930470.33 3013-6933.2554.030.620.052120.003950.331250.024320.046100.001452911112911929190.00 3013-70349.71350.531.000.053590.001480.411800.011650.055750.0012335430350835080.00 3013-71244.96223.181.100.055850.001770.553800.017840.071930.00160446354471244810−0.22 3013-7269.01145.340.470.052420.002380.347990.015680.048160.00116304603031230370.00 3013-7336.7569.040.530.053520.004560.409450.034420.055490.0015635113934825348100.00 3013-74220.53201.891.090.052120.001790.328470.011320.045720.0010429140288928860.00 3013-7548.95161.190.300.053320.001700.403350.013000.054870.0012434236344934480.00
另一块样品 160710-07 为流纹质晶屑凝灰岩, 锆石绝大部分呈长柱状, 个别呈椭圆状和短柱状, 粒径在 0.1~0.2mm 之间, 长宽比变化较大, 多数为2:1~3:1, 少部分是 1:1。晶形大多为半自形和自形, 晶面整洁光滑, 大多数锆石发育清晰的韵律环带, 显示岩浆锆石的特点(图 10)。25 颗锆石的测试结果见表 1, Th/U 比值均大于 0.47, 表明是岩浆锆石。在锆石 U-Pb 谐和图(图 11)上, 数据投影点基本落在谐和线上及其附近, 锆石年龄值较集中地分布在282 Ma 附近, 谐和年龄值为 282±2 Ma, 加权平均年龄 282±3 Ma, 代表西里庙组上段的形成时代。
图8 样品160708-02 (流纹质凝灰岩)锆石CL图像
Fig. 8 Zircon CL image of sample 160708-02 (rhyolitic tuff)
图9 样品160708-02 (流纹质凝灰岩)锆石U-Pb谐和图
Fig. 9 Zircon concordia age of sample 160708-02 (rhyolitic tuff)
图10 样品160710-07 (流纹质晶屑凝灰岩)锆石CL图像
Fig. 10 Zircon CL image of sample 160710-07 (rhyolitic crystal tuff)
图11 样品160710-07 (流纹质晶屑凝灰岩)锆石U-Pb谐和图
Fig. 11 Zircon concordia age of sample 160710-07 (rhyolitic crystal tuff)
3.2.3 变质粉砂岩
样品 100830-13 为变质粉砂岩, 采于二叠纪西里庙组中段地层(图 2 中采样点 d)。碎屑锆石呈次棱角至次圆状, 粒径在 0.2~0.4mm 之间, 长宽比变化较大, 多数为 2:1, 少部分是 3:1, 多数锆石发育较清晰的韵律环带, 显示岩浆锆石特点(图 12)。75颗锆石的测试结果见表 1, Th/U 比值绝大部分大于0.4, 表明为岩浆锆石。样品锆石的 206Pb/238U 年龄相对频率分布(图 13)显示两个年龄组, 第一组由约50 多颗锆石的年龄值组成, 峰值为 304Ma; 第二组由 13 颗锆石的年龄值组成, 峰值为 450Ma。此外, 还有1颗锆石的年龄值为870 Ma。
西里庙组地层分为 3 段 19 层, 上段和下段为晶屑凝灰岩及流纹质岩屑晶屑凝灰岩, 中段为暗色沉积岩, 构成独特的火山‒沉积岩系。根据本次碎屑岩(样品 100830-13)以及凝灰岩(样品 160708-02 和160710-07)锆石年代学测定结果, 西里庙组应形成于 304~282 Ma, 并被268±1 Ma (晚石炭世晚期到早二叠世早期)的花岗岩(样品 120828-19)侵入。这些结果精确地限定了西里庙组的地层时代, 也揭示了本区晚古生代岩浆活动的时代。
图12 样品100830-13 (变质粉砂岩)锆石CL图像
Fig. 12 Zircon CL image of sample 100830-13 (metamorphic siltstone)
图13 样品100830-13 (变质粉砂岩)锆石206Pb/238U年龄相对频率分布
Fig. 13 Zircon 206Pb/238U age histogram of sample 100830-13 (metamorphic siltstone)
在 1:20 万脑木根幅地质图中, 由于缺乏化石资料, 西里庙组被置于哲斯组之上, 代表研究区早二叠世最年轻的一个组。根据本次测年结果, 西里庙组应置于哲斯组之下。这一新的认识重新厘定了研究区石炭‒二叠系地层的层序关系。
已有研究表明, 本研究区存在以艾力格庙群为代表的晚元古代基底, 该区以西的中蒙边界英巴地区也被证实有晚元古宙杂岩[26], 早古生代为古亚洲洋西南的沟‒弧‒盆体系, 中古生代形成兴蒙造山带的北造山带, 晚古生代发育以裂陷盆地为特征的构造格局[21‒22,26]。碎屑锆石的物源分析结果印证了上述构造演化历史: 碎屑岩(样品 100830-13) 304Ma的年龄峰值揭示出与伸展构造环境有关的晚古生代岩浆活动记录; 具有 450Ma 年龄峰值的锆石应代表早古生代沟‒弧‒盆体系的岩浆活动; 最老的一颗870Ma 的锆石可能来自兴蒙造山带的晚元古代基底。因此, 碎屑锆石的物源分析结果提供了从中古生代造山带挤压过程向晚古生代伸展过程构造转化的沉积响应证据。
致谢 河北地质大学王志伟副教授和北京大学颜林杰同学协助野外工作, 在此表示衷心感谢。
参考文献
[1]Eizenhöfer P R, Zhao G C, Zhang J, et al. Final closure of the Paleo-Asian Ocean along the Solonker Suture Zone: constraints from geochronological and geochemical data of Permian volcanic and sedimen-tary rocks. Tectonics, 2014, 33(4): 441‒463
[2]Jahn, Wu B M, Chen F, et al. Granitoids of the Central Asian Orogenic Belt and continental growth in the Phanerozoic. Transactions of the Royal Society of Edinburgh Earth Sciences, 2000, 91(1/2): 181‒193
[3]Jahn B M. The Central Asian Orogenic Belt and growth of the continental crust in the Phanerozoic. Geological Society London Special Publications, 2004, 226(1): 73‒100
[4]Khain E V, Bibikova E V, Kröner A, et al. The most ancient ophiolite of the Central Asian fold belt: U-Pb and Pb-Pb zircon ages for the Dunzhugur Complex, Eastern Sayan, Siberia, and geodynamic implications. Earth & Planetary Science Letters, 2002, 199(3): 311‒325
[5]Kröner A, Windley B F, Badarch G. Accretionary growth and crust formation in the CentralAsian Orogenic Belt and comparison with the Arabian-Nubian shield. Geological Society ofAmerica Me-moir, 2007, 200: 181–209
[6]Li J Y. Permian geodynamic setting of Northeast China and adjacent regions: closure of the Paleo-Asian Ocean and subduction of the Paleo-Pacific Plate. Journal of Asian Earth Sciences, 2006, 26(3): 207‒224
[7]Rytsk E Y, Kovach V P, Kovalenko V I, et al. Struc-ture and evolution of the continentalcrust in the Bai-kal fold region. Geotectonics, 2007, 41(6): 440–464
[8]Sengör A M C, Natal’in B A, Burtman V S. Evolution of the Altaid tectonic collage and Paleozoic crustal growth in Eurasia. Nature, 1993, 364: 299‒307
[9]Windley B F, Alexeiev D, Xiao W J, et al. Tectonic models for accretion of the Central Asian Orogenic Belt. Journal of the Geological Society of London, 2007, 164: 31–47
[10]Xiao W J, Windley B F, Hao J, et al. Accretion leading to collision and the Permian Solonker suture, Inner Mongolia, China: termination of the Central Asian Orogenic Belt. Tectonics, 2003, 22(6): doi: 10.1029/2002TC001484
[11]Jian P, Liu D, Kröner A, et al. Evolution of a Permian intraoceanic arc–trench system in the Solonker suture zone, Central Asian Orogenic Belt, China and Mon-golia. Lithos, 2010, 118(1/2): 169‒190
[12]Xu B, Charvet J, Chen Y, et al. Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China):framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt. Gondwana Research, 2013, 23 (4): 1342–1364
[13]徐备, 王志伟, 张立杨, 等. 兴蒙陆内造山带. 岩石学报, 2018, 34(10): 2819–2844
[14]Xu B,Zhao P,Wang YY,et al. The pre-Devonian tectonic framework of Xing’an-Mongolia orogenic belt (XMOB) in north China. Journal of Asian Earch Sciences, 2015, 97:183–196
[15]Chen Y, Zhang ZC, Li K, et al. Detrital zircon U-Pb ages and Hf isotopes of Permo-Carboniferous sand-stones in central Inner Mongolia, China: implications for provenance and tectonic evolution of the south-eastern Central Asian Orogenic Belt. Tectonophysics,2016,671:183–201
[16]Yarmolyuk VV, Kovalenko VI, Sal’nikova EB, et al. U-Pb age of syn- and postmetamorphic granitoids of South Mongolia: evidence for the presence of Grenvi-llides in the Central Asian Foldbelt. Doklady Earth Sciences, 2005, 404: 986–990
[17]张焱杰, 徐备, 田英杰, 等. 兴蒙造山带晚古生代伸展过程: 来自二连浩特东北部石炭‒二叠系沉积地层的证据. 岩石学报, 2018, 34(10): 3083–3100
[18]徐备, 赵盼, 鲍庆中, 等. 兴蒙造山带前中生代构造单元划分初探. 岩石学报, 2014, 30(7): 1841–1857
[19]Demoux A, Kroner A, Hegner E, et al. Devonian arc-related mag-matism in the Tseel terrane of SW Mongolia: chronological and geoche mical evidence. Journal of the Geological Society, 2009, 166: 459–471
[20]周印章, 韩宝福, 徐钊, 等. 中蒙边界英巴地区元古宙杂岩的时代及其对南戈壁微陆块分布范围的限定. 地质通报, 2013, 32(2/3): 319–326
[21]徐备, 徐严, 栗进, 等. 内蒙古西部温都尔庙群的时代及其在中亚造山带中的位置. 地学前缘, 2016, 23(6): 120–127
[22]李承东, 冉皞, 赵利刚, 等. 温都尔庙群锆石的LA-ICP-MS U-Pb年龄以及构造意义. 岩石学报, 2012, 28(11): 3705–3714
[23]Sláma J, Kostler J, Condon D J, et al. Pleŝovice-a new natural reference material for U-Pb and Hf isotopic analysis. Chemical Geology, 2008, 197: 111‒142
[24]Andersen T, Griffin W L. Lu-Hf and U-Pb isotope systematics of zircons from the Storgangen intrusion, Rogaland Intrusive Complex, SW Norway: implica-tions for the composition and evolution of Precam-brian lower crust in the Baltic Shield. Lithos, 2004, 73(3): 271‒288
[25]Rubatto D, Hermann J. Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones. Geochimica et Cosmochimica Acta, 2003, 67 (12): 2173‒2187
[26]邵济安, 唐克东, 何国琦. 内蒙古早二叠世构造古地理的再造. 岩石学报, 2014, 30(7): 1858–1866
Stratigraphy, Geochronology and Tectonic Signification of the Permian Volcanic-Sedimentary Rock Series in Airgin Sum Area, Inner Mongolia
Abstract Several rift basins developed in the western part of Xing’an-Mongol orogenic belt during late Paleozoic, in which volcanic-sedimentary series is important to understand the late Paleozoic tectonic evolution, however, there is not a clear space-time distribution of the series. Stratigraphy and geochronology of Xilimiao Formation have been researched to understand the basin filling process and tectonic setting. According to the field sections, Xilimiao Formation can be divided into three segments. Upper and lower segments are composed of crystal tuffs and rhyolitic lithic crystal tuffs, and middle segment are dark sedimentary rocks. The zircon 206Pb/238U age of 282±2 Ma achieved from the rhyolitic tuff of upper segment represents the forming age. The two peak ages of 304 Ma and 450 Ma represent the upper limit of the sedimentary event, the forming age of early Paleozoic orogenic belt, respectively, and the 870 Ma represents the age of Neoproterozoic basement. The data of middle detrital rocks indicate that the forming age of Xilimiao Formation was between 304‒282 Ma, older than Zhesi Formation. Provenance of the detrital zircon from the middle Xilimiao Formation provides sedimentary evidence for transform process from compress of the middle Paleozoic orogenic belt to extension of the late Paleozoic rift basin.
Key words Xing’an-Mongol orogenic; Arigin Sum Area; Xilimiao Formation; Permian; volcanic-sedimentary rock series; zircon geochronology
doi: 10.13209/j.0479-8023.2018.026
收稿日期: 2018‒04‒04;
修回日期: 2018‒06‒08;
网络出版日期: 2019‒01‒24
国家自然科学基金(41672214)和国家重点基础研究发展计划(2013CB429806)资助
①内蒙古自治区地质局. 区域地质调查报告: 脑木根幅, 1980