大兴安岭扎兰屯地区格根敖包组碎屑锆石U-Pb年代学、地球化学特征及其地质意义

张渝金，吴新伟，江 斌，郭 威，杨雅军，刘世伟，崔天日，李 伟，李林川，司秋亮，张 超

1.吉林大学地球科学学院，长春 130061 2.沈阳地质矿产研究所/中国地质调查局沈阳地质调查中心，沈阳 110034



大兴安岭扎兰屯地区格根敖包组碎屑锆石U-Pb年代学、地球化学特征及其地质意义

张渝金1,2，吴新伟1,2，江 斌1,2，郭 威1,2，杨雅军2，刘世伟2，崔天日2，李 伟2，李林川2，司秋亮2，张 超2

1.吉林大学地球科学学院，长春 130061 2.沈阳地质矿产研究所/中国地质调查局沈阳地质调查中心，沈阳 110034

格根敖包组广泛分布在大兴安岭中段扎兰屯地区，为一套偏中性的火山岩及碎屑岩组合，主要为安山岩、安山质火山碎屑岩、英安岩及细-粉砂岩，夹薄层泥岩。笔者对格根敖包组中细碎屑岩进行了锆石LA-ICP-MS U-Pb年代学及古生物地层学研究，结果表明：格根敖包组细碎屑岩形成于晚石炭世至早二叠世，锆石U-Pb年龄集中于348.9，385.8，428.0和507.3 Ma 4个峰值；粉砂岩中存在Artisiasp.,Eusigillarissp.等化石。格根敖包组碎屑岩主要以细粒杂砂岩和细粒长石(岩屑)砂岩为主，主量元素平均质量分数w(SiO2)=69.04%，w(Al2O3)=14.76%，w(MgO)=1.05%，w(CaO)=0.66%，w(Na2O)=2.34%，w(K2O)=2.65%；镁铝比值M为3.05～9.98，平均值7.18；稀土元素总量∑REE平均184.46×10-6((124.06～261.96)×10-6)，δEu平均值0.71，δCe平均值0.99，LREE富集，HREE亏损。上述结果表明，格根敖包组地层形成于大陆岛弧-活动陆缘附近，古地理显示为温暖潮湿气候下的海陆交互相沉积环境。

格根敖包组; 碎屑锆石; 地球化学; 物源分析; 扎兰屯地区; 大兴安岭中段

0 引言

大兴安岭地区位于中亚造山带东段，古生代主要表现为多个微陆块之间的拼合和古亚洲洋的闭合，这些微陆块自北向南依次为额尔古纳地块、兴安地块和松嫩地块(图1a)。特殊的大地构造位置、复杂的地质演化历史使得该区成为基础地质研究的热点之一。上述3个地块碰撞拼贴的时间学术界一直存在争议[1-8]，特别是兴安和松嫩地块碰撞拼贴的时间：一种观点认为兴安和松嫩地块于晚泥盆世--早石炭世末期闭合[9-11]；另一种观点则认为兴安和松嫩地块于泥盆纪之前已经拼贴[12-13]；还有一种观点则认为兴安和松嫩地块于早中生代[14]拼贴。区内晚石炭世--早二叠世的海陆交互相格根敖包组发育，其成岩环境、物质来源和成岩时代的研究成为研究该地区构造环境的重要环节。

碎屑沉积岩源区与构造环境和地壳演化关系密切[15]，碎屑锆石能较好地保存源区岩石组成的信息，碎屑锆石U-Pb同位素示踪是研究沉积物源区的有效方法。笔者对广泛出露于扎兰屯地区的格根敖包组砂岩样品进行了地球化学和U-Pb锆石年代学研究，旨在示踪物质源区，探讨晚石炭世--早二叠世沉积盆地性质。

1 地质概况及岩石学特征

研究区位于大兴安岭北部扎兰屯根多河地区，兴安地块和松嫩地块的结合部位附近(图1)。区内晚古生代地层发育，包括大民山组(D2-3d)、红水泉组(C1h)和格根敖包组(C2-P1g)。泥盆系大民山组主要为一套海相火山-沉积建造。石炭系红水泉组为一套浅海相陆源碎屑岩沉积建造。晚石炭世--早二叠世格根敖包组，为一套海陆交互相火山-沉积建造。

本次研究对象为扎兰屯根多河地区的格根敖包组，格根敖包组在平面上呈断续的北东向条带状展布，出露面积54 km2，总厚度大于1 270 m。该套地层顶底多被中生代火山-侵入岩覆盖，地层内部多处被中生代侵入岩脉破坏，从而使得接触带外侧发育明显的蚀变作用。 地层层序具明显的两分性：下部为深灰色安山岩，火山碎屑岩；上部主要为一套黄绿色、灰绿色正常细碎屑岩夹少量火山碎屑岩，含海百合茎化石和Calamitissp.,Artisiasp.,Eusigillarissp. 等化石[16]，其与下部层位呈整合接触。

图1 扎兰屯根多河地区地质简图Fig.1 Simplified geological map of Genduohe area in Zalantun

本次测试样品采自格根敖包组上部沉积层位，采样点2011RZ09(121°48′20″N；47°47′30″E)和2011RZ1088(122°04′50″N；47°44′20″E)见图1b。其中：前者岩性为细砂岩，黄褐色，中细粒砂状结构，块状构造；由长石、石英、岩屑组成；粒径为0.2～0.5 mm，除石英碎屑(体积分数20%)外，长石碎屑(体积分数30%±)绢云母化较强，岩屑(体积分数50%)黑云母化、绢云母化较强。岩石为颗粒支撑，泥质胶结，碎屑分选和磨圆较差，显示近源快速堆积的特征。后者岩性为粉砂岩，灰黑色，粉砂结构，块状构造；矿物成分为石英(体积分数70%)和长石(体积分数25%)，长石多蚀变为绢云母，呈鳞片状。此外，含有5%±的鳞片状绿泥石；岩石为颗粒支撑，泥质胶结，石英磨圆度和分选度较好。

2 样品处理与测试方法

2.1 全岩分析

笔者对5件砂岩样品进行了主量、微量、稀土元素测试，测试结果见表1。样品的测试在国土资源部东北矿产资源监督检测中心完成，整个过程均在无污染设备中进行。主量元素采用X 射线荧光光谱法(XRF)，微量、稀土元素的分析则采用电感耦合等离子质谱法(ICP-MS)完成。主量元素分析精度和准确度优于5%，微量元素的分析精度和准确度优于10%。

2.2 锆石U-Pb定年分析

样品的破碎和锆石的分选工作由河北省廊坊市科大矿物分选技术股份有限公司完成。锆石阴极发光图像在中国地质科学院北京离子探针中心完成。样品制靶和LA-ICP-MS锆石U-Pb测年在中国地质科学院国家地质实验测试中心完成。本次实验所采用的仪器为Thermo Element II型MC-ICP-MS 及与之配套的New wave UP 213激光剥蚀系统。激光剥蚀所用斑束直径为30 μm，频率为10 Hz，能量密度为16～17 J/cm2，以He 为载气。普通铅校正采用Anderson(2002)的方法，并采样哈佛大学国际标准锆石91500作为外部校正，Plesovice作质量监控，年龄计算采用国际标准程序Isoplot(ver3.0)，测试数据、加权平均年龄的误差均为1σ。对于所测锆石年龄> 1 000 Ma 的数据，采用207Pb/206Pb 年龄，而对于<1 000 Ma的数据，用206Pb/238U年龄[17-18]，以206Pb/238U年龄和207Pb/206Pb年龄的比值为标准筛选U-Pb年龄数据[17-21]，谐和度在95%以上(包含95%)的数据为有效数据。

3 岩石地球化学特征

3.1 主量元素地球化学

从表1可以看出，岩石具有如下特征：SiO2质量分数为63.99%～78.20%，平均69.04%；Al2O3质量分数为11.11%～16.72%，平均14.76%；MgO质量分数较低，值为0.51%～1.66%，平均1.05%；CaO质量分数为0.06%～2.20%，平均为0.66%；Na2O质量分数为1.68%～2.97%，平均2.34%；K2O质量分数为1.97%～4.06%，平均2.65%。在SiO2/Al2O3-Na2O/K2O 图解(图2)中，样品多落入杂砂岩和长石(岩屑)砂岩区，表明源区岩石没有经历充分的搬运与分选，成熟度较低，形成于活动性较大的构造地带。

表1 研究区格根敖包组岩石地球化学分析结果

注：样品由国土资源部东北矿产资源监督检测中心分析；常量元素质量分数单位为%，稀土及微量元素质量分数单位为10-6；文中提到的2011RZ09与RZ09为同一个样品，2011RZ1088和RZ1088为同一个样品；RZ09、D011、D004、P121为细砂岩；RZ1088为粉砂岩。

底图据Pettijohn等，1972。图2 SiO2/Al2O3-Na2O/K2O图解Fig. 2 SiO2/Al2O3-Na2O/K2O diagram

岩石化学镁铝含量比值可表示为M=100×MgO/Al2O3，是根据沉积岩石中MgO 的亲海性和Al2O3的亲陆性建立起来的[22]，在由淡水向海水过渡的沉积环境中，M值随沉积水体中盐度增加而增大，淡水沉积环境M<1，海陆过渡的沉积环境1

3.2 微量元素岩石化学

沉积岩的稀土元素特征主要受控于物源区类型和沉积作用过程，REE 受风化作用和成岩及变质作用的影响很小[23-25]，其携带的物源区信息一般不会丢失，因此，稀土元素可以作为良好的地球化学演化指示剂，通过对比岩石中的轻、重稀土比值以及整体的分配特征，可以准确判断沉积物源及演化特征。

扎兰屯地区的格根敖包组砂岩稀土总量较高，w(∑REE)=(124.06～261.96)×10-6，平均为184.46×10-6，球粒陨石标准化配分曲线[26]表现出总体右倾的特点(图3)，LREE 富集，HREE 亏损，轻重稀土分馏明显，具有明显的负Eu异常，δEu=0.43～0.92，平均0.71，与上地壳平均负Eu异常值0.71一致[27]；δCe=0.88～1.10，平均0.99，Ce 基本无异常。据现代海洋沉积物及深海钻探表明，在边缘海、浅海及被大陆封闭的海水环境中，Ce浓度基本正常，而在开阔海水环境中Ce亏损严重[28]。因此，本区当时沉积水盆处于边缘海，形成环境应属较温暖、潮湿的气候条件，这样利于Ce的迁入[29]。

底图据Boynton,1984。图3 格根敖包组砂岩稀土元素配分曲线图Fig. 3 REE chondritic standardization plot for sandstones of Gegen’aobao Formation

图4 格根敖包组砂岩样品2011RZ09和2011RZ1088的碎屑锆石CL图像Fig. 4 Representative CL images for zircons from sample 2011RZ09 and 2011RZ1088 of Gegen’aobao Formation

4 锆石U-Pb测年及沉积时限

4.1 锆石U-Pb测试结果

样品2011RZ09和2011RZ1088的锆石分析结果见表2，代表锆石CL图像见图4，锆石谐和图和相对频率图见图5、6。

对样品2011RZ09进行了65个测试点分析，CL图像显示锆石均呈自形--半自形，多呈短柱状，少量呈长柱状，粒径90～160 μm，长宽比值多为2∶1，具有清晰的振荡环带，具有较高的Th/U值(0.24～1.42)，反映了岩浆成因锆石的特点。测试结果显示，6个测试点的数据偏离谐和线，其余均在谐和线上及其附近。按照年龄及频率分布特征大致可以划分出4组：第1组206Pb/238U年龄集中在323.6～368.5 Ma，峰值年龄为(348.5±4.5)Ma，(n=31，MSWD=3.70)；第2组206Pb/238U年龄集中在384.4～398.9 Ma，峰值年龄为(388.7±5.0)Ma，(n=9，MSWD=0.56)；第3组206Pb/238U年龄集中在406.7～448.3 Ma，峰值年龄为(422.2±8.2) Ma，(n=12，MSWD=2.70)；第4组206Pb/238U年龄集中在491.8～518.8 Ma，峰值年龄为(504.0±12.0) Ma，(n=6，MSWD=1.60)。此外，还存在1个测试点757.9Ma(图4a-58)。

对样品2011RZ1088进行了65个测试点分析，CL图像显示锆石晶形完好，多呈短柱状，少量呈长柱状，粒径80～100 μm，长宽比值多为2∶1，内部结构清晰，发育典型的振荡环带，Th/U值为0.17～4.46，反映了岩浆成因锆石的特点。测试结果显示，5个测试点的数据偏离谐和线，其余均在谐和线上及其附近。按照年龄及频率分布特征大致可以划分出5组：第1组206Pb/238U年龄集中在336.5～369.8 Ma，峰值年龄为(350.5±4.5) Ma，(n=18，MSWD=1.90)；第2组206Pb/238U年龄集中在375.5～411.9 Ma，峰值年龄为(384.8±4.5)Ma，(n=22，MSWD=2.00)；第3组206Pb/238U年龄集中在417.9～422.4 Ma，峰值年龄为(420.3±7.6) Ma，(n=4，MSWD=0.05)；第4组206Pb/238U年龄集中在437.1～463.0 Ma，峰值年龄为(450.0±15.0) Ma，(n=5，MSWD=2.10)；第5组206Pb/238U年龄集中在500.9～519.8 Ma，峰值年龄为(511.8±8.1) Ma，(n=5，MSWD=0.95)。还存在(750±40) Ma(n=3，MSWD = 1.40)，1个测试点814.0 Ma(图4b-13)。此外，2个测试点207Pb/206Pb年龄分别为2 500.7 Ma(图4b-23)和1 951.5 Ma(图4b-1)，锆石具有环带结构，可能为古老的捕获锆石。

4.2 格根敖包组沉积岩沉积时限

本区格根敖包组砂岩中最年轻的碎屑年龄为323.6 Ma，表明该组的最大沉积年龄不早于早石炭世谢尔普霍夫阶。

图5 砂岩样品2011RZ09碎屑锆石U-Pb谐和年龄和年龄样品频率图Fig.5 U-Pb Concordia diagram and relative probability plot of detrital zircons from sample 2011RZ09

图6 砂岩样品2011RZ1088碎屑锆石U-Pb谐和年龄和年龄样品频率图Fig. 6 U-Pb Concordia diagram and relative probability plot of detrital zircons from sample 2011RZ1088

点号Th/U同位素比值207Pb/206Pb±1σ207Pb/235U±1σ206Pb/238U±1σt/Ma207Pb/206Pb±1σ207Pb/235U±1σ206Pb/238U±1σ2011RZ09_11.090.055170.001720.420550.015340.055510.00103419.067.49356.410.96348.26.292011RZ09_20.540.054770.001660.517570.019090.068280.00126402.765.92423.512.77425.87.622011RZ09_30.770.055860.001740.378140.013500.051510.00095446.467.67325.79.95323.85.852011RZ09_40.340.051590.001730.404980.016120.058760.00110267.275.23345.311.65368.16.692011RZ09_50.440.052960.002490.379760.022020.053200.00107327.1103.01326.916.2334.26.552011RZ09_60.540.057480.002270.430170.021010.057420.00112509.684.70363.314.92359.96.822011RZ09_70.680.054510.001810.394400.015450.054310.00102392.372.14337.611.25340.96.232011RZ09_80.730.055240.001940.392320.016440.052730.00100421.976.05336.111.99331.36.122011RZ09_90.470.057400.001550.644790.020970.080810.00148506.658.65505.312.95501.08.812011RZ09_100.810.053870.001690.417600.015420.056900.00106365.469.11354.311.05356.76.452011RZ09_110.910.055650.001870.404010.016230.053970.00102438.273.18344.611.74338.96.222011RZ09_120.240.054480.001670.453410.016480.061980.00115391.067.15379.711.51387.66.972011RZ09_130.520.056660.001620.400080.012920.054140.00099477.861.91341.79.37339.96.082011RZ09_140.910.054020.001690.430330.015960.056960.00106371.968.95363.411.33357.16.462011RZ09_150.480.054290.002780.465520.031070.061380.00127382.9110.59388.121.53384.07.702011RZ09_160.620.058700.002120.436660.019430.056870.00109555.976.87367.913.73356.66.652011RZ09_170.590.067880.002570.450820.021190.053150.00104864.976.59377.914.83333.86.352011RZ09_180.780.060310.002240.435140.020010.055980.00108614.778.35366.814.15351.16.592011RZ09_190.750.051790.002680.367160.023240.054760.00110276.2114.26317.517.26343.76.722011RZ09_201.100.064920.002380.581090.027900.067620.00131771.775.45465.217.92421.87.902011RZ09_210.530.056950.001790.537200.021060.070800.00132489.168.68436.613.91441.07.952011RZ09_220.470.055240.001950.440330.019050.058840.00112421.676.59370.513.43368.56.812011RZ09_230.590.053440.002100.465660.023230.061570.00118347.686.34388.216.09385.27.162011RZ09_240.860.054700.001540.393710.012600.054770.00101382.262.25337.19.18343.86.152011RZ09_250.670.058000.001350.649870.016440.082780.00149529.450.56508.410.12512.78.862011RZ09_260.460.057840.001330.603280.014780.079360.00143523.649.99479.39.36492.38.522011RZ09_270.530.055740.001660.604580.022380.079280.00147441.764.98480.114.16491.88.752011RZ09_280.530.057730.001590.637350.021160.083800.00153519.359.55500.713.13518.89.132011RZ09_290.540.057560.002690.531230.032950.065770.00134512.8100.01432.621.85410.68.102011RZ09_300.620.055630.001810.520350.020940.069040.00129437.470.57425.413.98430.47.762011RZ09_310.680.059830.002600.661830.039870.082280.00165597.591.56515.724.36509.79.822011RZ09_320.730.056010.001680.424250.014800.056300.00104452.565.33359.110.55353.16.362011RZ09_330.700.053410.001560.439110.014830.061450.00113346.264.71369.610.47384.46.872011RZ09_340.620.054740.002350.400910.021140.056050.00110401.692.75342.315.32351.66.722011RZ09_350.690.058580.002410.534950.028570.072020.00141551.787.20435.118.90448.38.502011RZ09_360.880.053410.001440.424120.012750.058050.00106346.359.81359.09.09363.76.452011RZ09_370.680.052700.002440.422260.024890.061640.00124315.8101.78357.717.77385.67.542011RZ09_380.700.058980.001700.495500.016840.063250.00117566.461.54408.711.44395.47.082011RZ09_390.950.065530.002180.499030.020250.056690.00107791.368.44411.013.72355.56.532011RZ09_400.540.053910.002740.478970.031510.065750.00131367.0109.91397.421.63410.57.912011RZ09_410.600.055040.002320.397450.020650.053260.00105413.791.11339.815.00334.56.422011RZ09_420.460.053480.001370.450670.012750.067490.00123349.356.93377.78.92421.07.412011RZ09_430.400.055290.001680.495800.018200.065310.00121424.066.01408.912.35407.87.352011RZ09_440.710.057450.001780.430200.015660.056370.00105508.367.14363.311.12353.56.412011RZ09_451.420.054890.001830.434710.017230.062180.00117407.972.09366.512.19388.97.092011RZ09_460.660.053430.001800.382330.015090.055860.00105347.074.33328.811.09350.46.402011RZ09_470.440.054130.001570.356330.011590.049900.00092376.263.86309.58.68313.95.652011RZ09_480.490.055110.002000.364050.015670.049890.00095416.678.58315.211.66313.85.832011RZ09_490.480.052880.001620.346390.012070.050800.00094323.568.25302.09.10319.45.78

表2(续)

表2(续)

在工作区根多河林场东6 km 6686高地采到Artisiasp.,Eusigillarissp.等化石，其中Artisia见于苏联顿巴斯的下石炭统和库兹巴斯上石炭统、德国和英国的上石炭统，Eusigillaris见于苏联顿巴斯的上石炭统中下部*黑龙江省区测二队.1:20万华安公社幅区调报告.哈尔滨:黑龙江省地质调查院,1976.。此外，1：20万华安公社幅内与该套地质体相当的蘑菇气、尕拉城、哈多河一带原宝力高庙组剖面下部为火山熔岩，上部为凝灰岩、细砂岩及粉砂质板岩等，含Angaropteridiumcordiopteroides(Schm.)Zal.，Neuropterissp.，Angaridiumcf.mongolicum，Dicranophyllumsp.，Noeggerathiopsisderzavinii等，时代置于晚石炭世至早二叠世。综上所述，格根敖包组细碎屑岩应形成于晚石炭世至早二叠世。

5 讨论

5.1 岩石地球化学特征反映的物源信息和构造背景

与在各类构造背景下的砂岩岩石地球化学成分进行对比(表3)，扎兰屯地区格根敖包组砂岩的样品除个别比值与大洋岛弧和被动大陆边缘类似外，总体相当于大陆岛弧-活动陆缘的化学成分，显示了活动大陆边缘构造环境下砂岩特点，并且砂岩M值为3.05～9.98，显示为海陆交互相沉积环境。

在SiO2/Al2O3-Na2O/K2O 图解(图2)中，样品多落入杂砂岩和长石(岩屑)砂岩区；稀土配分曲线显示，LREE 富集，HREE 亏损，轻重稀土分馏明显，具有明显的负Eu异常，可以判断该区格根敖包组沉积碎屑岩物源主要为长英质岩石。

综合分析岩石主、微量元素地球化学特征，扎兰屯地区格根敖包组细碎屑岩物源构造背景主要为大陆岛弧-活动陆缘环境，极少数样品的个别参数显示了大洋岛弧和被动陆缘特点。

5.2 碎屑锆石年龄谱及其物源区信息

格根敖包组砂岩的碎屑锆石LA-ICP-MS U-Pb年代学研究显示，样品2011RZ09和2011RZ1088的碎屑锆石年龄具有相似的年龄区间，但锆石所占比例不同，暗示物源区既有相似性又有差异性。同时时代跨度较大，反映了沉积物源区岩石组成的复杂性和多样性。

对2个样品共130颗锆石的年龄研究显示，主要集中在4个年龄群(图7)，反映了古生代的年龄信息:

1)491.8～519.8 Ma，峰值年龄为(507.3±7.1)Ma。这组年龄在额尔古纳地块和兴安地块内普遍存在，它们在早古生代早期同时经历了强烈的区域变质作用和花岗质岩浆侵入作用，为一次重要的造山(造陆)事件[30]，与额尔古纳地块和兴安地块的拼贴作用有关[8,31]。

2)406.7～463 Ma，峰值年龄为(428.0±7.4) Ma。这组年龄与阿尔山地区出露445～450 Ma花岗岩[32-33]，及新林地区出露的438 Ma的碰撞花岗岩[34]一致，区域上岩浆活动与多宝山--伊尔斯洋盆板块碰撞和隆升阶段变质作用有关[35]。

3)375.5～411.9 Ma，峰值年龄为(385.8±3.5)Ma，该组年龄与小兴安岭西北部洪湖吐河组粉砂岩的碎屑锆石峰值年龄385.0 Ma[36]一致，与大兴安岭北部--东乌旗一带发育的弧火山岩年龄(365.0～373.0 Ma)[37]相近。该时期额尔古纳与兴安地块地块已经拼贴为一起，在其南部发生了新的弧火山作用[35]。

4)323.6～369.0 Ma，峰值年龄为(348.9±3.2) Ma，这组年龄跟额尔古纳--兴安地块与松嫩地块拼贴事件一致[35]，在小兴安岭嫩江--黑河地区普遍发育352～355 Ma的火山岩[36]，博克图--扎兰屯一带出露(347.4±1.1) Ma的碰撞花岗岩[35]以及与大兴安岭乌奴尔地区碰撞花岗岩(354 Ma)[35,38]相吻合。

表3 研究区含砂粉砂泥岩样品与不同构造背景砂岩元素特征值

Table 3 Comparison of characteristics between arenaceous and silty mudstone samples in the study area and those in different tectonic backgrounds

构造背景SiO2Al2O3Na2OK2OFe2O3+MgOAl2O3/(Na2O+CaO)Al2O3/SiO2LaCe大洋岛弧58.8317.114.101.6011.731.720.298.00±1.719.00±3.7大陆岛弧70.6914.043.121.896.792.420.227.00±4.559.00±8.2活动大陆边缘73.8612.892.772.904.602.560.1837.0078.00被动大陆边缘81.958.411.071.712.894.150.1039.0085.00本区69.0414.772.342.644.044.920.2133.7673.02构造背景∑REELREE/HREEδEu(Gd/Yb)NLa/Yb(La/Yb)NLa/YRb/Sr大洋岛弧58.003.80±0.91.04±0.111.314.20±1.34.20±1.30.48±0.120.05±0.05大陆岛弧146.007.70±1.70.79±0.131.4911.00±3.67.50±2.51.02±0.070.65±0.33活动大陆边缘186.009.100.601.2612.508.501.33±0.090.89±0.24被动大陆边缘210.008.500.562.7515.9010.801.31±0.261.19±0.40本区184.467.580.711.2212.028.101.110.59

注：常量元素质量分数单位为%，稀土及微量元素质量分数单位为10-6。不同构造背景砂岩元素特征值引自文献[17] 。

a. 全部锆石年龄；b. 显生宙锆石年龄。图7 格根敖包组砂岩样品碎屑锆石年龄分布图Fig.7 Relative probability plot of detrital zircons from all samples of Gegen’aobao Formation

因此，格根敖包组沉积物物源具有复杂性及多样性，物源区既有古老地块物质，也有年轻的火山喷发物，与大陆岛弧-活动陆源环境相符。这也与额尔古纳--兴安地块与松嫩地块于早石炭世末期完成碰撞拼合[35]吻合。

6 结论

综合岩石学、地球化学以及年代学系统研究，可得出以下结论：

1)扎兰屯地区格根敖包组砂岩中最年轻的碎屑年龄为323.6 Ma，表明该组的最大沉积年龄不早于早石炭世谢尔普霍夫阶，结合古生物地层学研究，格根敖包组细碎屑岩应形成于晚石炭至早二叠世。

2)格根敖包组源岩物质以长英质岩石为主，形成于大陆岛弧-活动陆缘环境，古地理显示为较温暖潮湿气候下的海陆交互相沉积环境。

3)格根敖包组锆石U-Pb年龄集中于348.9，385.8，428.0和507.3 Ma 等4个峰值，显示出其沉积物物源具有复杂性及多样性，物源区既有古老地块物质，也有年轻的火山喷发物。

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U-Pb Geochronology of Detrital Zircon and the Constraint of Geochemistry from the Gegen’aobao Formation in Middle of Zalantun Area of Da Hinggan Mountains and Its Tectonic Significance

Zhang Yujin1,2，Wu Xinwei1,2，Jiang Bin1,2，Guo Wei1,2，Yang Yajun2，Liu Shiwei2，Cui Tianri2，Li Wei2，Li Linchuan2，Si Qiuliang2， Zhang Chao2

1.CollegeofEarthSciences,JilinUniversity,Changchun130061,China2.ShenyangInstituteofGeologyandMineralResources/ShenyangCenterofGeologicalSurvey,ChinaGeologicalSurvey,Shenyang110034,China

The Gegen’aobao Formation is widely exposed in the middle of Da Hinggan area. It is a combination of neutral partial volcanic and clastic rocks with main composition of andesite, andesitic pyroclastic rocks, dacite, fine-siltstone, and thin mudstone. We obtained the age of fine grained clastic rock in Gegen’aobao Formation by means of LA-ICP MS zircon U-Pb dating; and also discussed the paleontology stratigraphy of Gegen’aobao Formation. The results show that the fine clastic rock was formed in Late Carboniferous to Early Permian. Four main ages were observed, 348.9, 385.8, 428.0 and 507.3 Ma. Several fossils likeArtisiasp.,Eusigillarissp. were found in siltstone. Fine-grained greywacke and feldspathic greywacke (debris) sandstones are the major clastic rocks in the Gegen’aobao Formation. The average bulk chemical composition of these rocks is SiO269.04%, Al2O314.76%, MgO 1.05%, CaO 0.66% , Na2O 2.34%, and K2O 2.65% in weight. Mg/Al ratio (M) is 3.05 to 9.98, with an average value of 7.18. The total weight of REE (∑REE) ranges from 124.06 × 10-6to 261.96× 10-6with the average value of 184.46 × 10-6. The value forδEu andδCe are 0.71 and 0.99 respectively. The clastic rocks are characterized by LREE enrichment and HREE depletion. These results indicate that Gegen’aobao Formation was formed between the continental island arc and the active continental margin, under a warm and humid paleoclimate condition.

Gegen’aobao Formation；detrital zircons；geochemistry；provenance analysis；Zalantun area；Middle Part of the Da Hinggan Mountains

10.13278/j.cnki.jjuese.201502106.

2014-06-06

中国地质调查局项目(1212011120664 ，1212011120665，1212011120666)

张渝金(1984--)，男，博士研究生，工程师，主要从事区域地质调查工作，E-mail:yujin--123@163.com。

10.13278/j.cnki.jjuese.201502106

P59

A

张渝金，吴新伟，江斌，等. 大兴安岭扎兰屯地区格根敖包组碎屑锆石U-Pb年代学、地球化学特征及其地质意义.吉林大学学报:地球科学版,2015,45(2):404-416.

Zhang Yujin，Wu Xinwei，Jiang Bin，et al. U-Pb Geochronology of Detrital Zircon and the Constraint of Geochemistry from the Gegen’aobao Formation in Middle of Zalantun Area of Da Hinggan Mountains and Its Tectonic Significance.Journal of Jilin University:Earth Science Edition,2015,45(2):404-416.doi:10.13278/j.cnki.jjuese.201502106.