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内蒙古乌兰盖盆地中生代火山岩年代学、地球化学特征及其地质意义
孙守亮, 李永飞, 郜晓勇    
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
摘要:笔者对大兴安岭西部乌兰盖盆地南、北缘出露的中生代火山岩进行了详细的岩石学、激光全熔40Ar/39Ar测年及地球化学研究,探讨了中生代火山岩成因与地质意义。乌兰盖盆地南、北缘的中生代火山岩主要为中性岩(安山岩)与酸性岩(流纹岩)类,各类岩石总体激光全熔40Ar/39Ar定年结果为(151.8±1.5)~(120.2±1.8) Ma,说明其形成时代总体为晚侏罗世晚期至早白垩世;岩石总体为一套钙碱性系列到高钾钙碱性系列的中性至酸性岩石组合,各类岩石地球化学特征与壳源岩石的地球化学特征基本一致,表明它们应来自于地壳物质局部熔融形成的壳源岩浆系列。该套火山岩形成于蒙古鄂霍茨克洋(古太平洋)闭合碰撞造山构造背景,在早白垩世151.8 Ma左右区内曾发生地壳加厚的造山过程,其岩浆深部动力学背景与岩浆源区的性质主要归因于增厚的造山带下地壳发生的部分熔融作用。
关键词乌兰盖盆地     中生代     火山岩     激光全熔40Ar/39Ar测年     地球化学    
Mesozoic Volcanic Rocks Geochronology and Geochemical Characteristics in Wulangai Basin of Inner Mongolia and Their Geological Significance
Sun Shouliang, Li Yongfei, Gao Xiaoyong    
Shenyang Institute of Geology and Mineral Resources/Shenyang Center of Geological Survey, China Geological Survey, Shenyang 110034, China
Abstract:The detailed petrology, Laser 40Ar/39Ar dating and the geochemical characteristics of the Mesozoic volcanic rocks from Wulangai basin of the Western Great Xing'an Mountain have been studied on the ground of their genesis and geological implication. We suggest that the Mesozoic volcanic rocks of Wulangai basin are mainly medium (andesite) and acid rocks (rhyolite) in the age of (151.8±1.5)-(120.2±1.8) Ma by the methods of Laser 40Ar/39Ar dating, identical to the Late Jurassic to Early Cretaceous. The volcanic rock types are medium to acid rocks of the Calc-Alkaline to high-K Calc-Alkaline. The geochemical characteristics is the same as crustal rocks'. This indicates that it derived from the partial melting of the crustal material. The volcanics from Wulangai basin should be result from the collision of Mongol-Okhotsk(Paleo-Pacific) orogenic belt and the thickened lower crust in Early Cretaceous around 151.8 Ma. Combined with the background of the magma dynamics and the nature of the magma, the volcanics were from the partial melting of the thickened orogenic belt.
Key words: Wulangai basin     Mesozoic     volcanic rocks     Laser 40Ar/39Ar dating     geochemical    

0 引言

乌兰盖盆地位于内蒙古西乌珠穆沁旗与东乌珠穆沁旗之间,地理坐标为东经116°6′ ——119°30′和北纬44°45′ ——45°56′,面积约15 000 km2,为呈北东向展布的小型断陷盆地[1]。乌兰盖盆地隶属于二连盆地群东北段,是在兴蒙海西期软对接、软碰撞褶皱基底上发育起来的中、新生代盆地。该盆地的基础地质勘查研究程度相对薄弱,油气勘查程度较低,缺少中生代火山岩覆盖之下盆地的规模、类型和盆地边界资料,因而限制了该地区油气资源潜力评价工作[2]

乌兰盖盆地作为以火山岩充填为主的中生代盆地,其火山岩地层属性与年代学格架对比分析,是厘定盆地构造样式、沉积特征、盆地构造演化规律及其对盆地上古生界含烃层系的控制与改造分析研究的基础工作。近年来,随着火山岩定年方法的不断改进与应用,大兴安岭中北段火山岩的精细同位素年代学研究已经取得了长足的进展[3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13],但是,关于乌兰盖盆地中的中生代火山岩精细年代学的研究与报道目前仍相对薄弱。

由于乌兰盖盆地第四系覆盖严重,笔者选择出露并发育于乌兰盖盆地南北缘的中生代火山岩为研究对象,利用精细的激光全熔40Ar/39Ar法对其进行详细的年代学与地球化学特征研究,以期为乌兰盖盆地发育的火山岩形成构造背景提供基础资料与依据。

1 区域地质背景

乌兰盖盆地是二连盆地群东北向乌尼特坳陷中的一个小型断陷盆地,它是在大兴安岭海西褶皱基底上发育起来的中、新生代断陷沉积盆地,其大地构造位置处于中朝板块与西伯利亚板块的缝合线上[14, 15, 16]。北侧为前苏联和蒙古境内的贝加尔、加里东和海西褶皱系,南侧为东西向的中朝板块,东侧为中生代北东向构造系,西侧为南北向构造系。

出露于乌兰盖盆地南、北缘的中生代火山岩地层自下而上主要为满克头鄂博组(J3mk)、玛尼吐组(J3mn)、白音高老组(J3b)(图 1)及梅勒图组(K1m)、大磨拐河组(K1d)。满克头鄂博组为灰——灰绿色流纹岩、流纹质熔结凝灰岩夹角砾岩;玛尼吐组为紫灰色、红色气孔状玄武岩及安山岩;白音高老组为灰白色、灰紫色凝灰岩、流纹岩夹凝灰质角砾岩; 梅勒图组为一套灰黑色安山岩、玄武安山岩、安山质集块角砾岩,安山质岩屑晶屑凝灰熔岩、深灰色英安岩及英安质晶屑凝灰岩;大磨拐河组是乌兰盖盆地中的下白垩统含煤岩系,《内蒙古自治区区域地质志》称之为巴彦花组,《全国地层多重划分对比研究内蒙古自治区岩石地层》改称大磨拐河组,其下部与梅勒图组呈平行不整合接触[1]

图 1 乌兰盖盆地地质简图及样品取样位置

Fig. 1 The simplified geological map of the Wulangai basin and sampling location

2 样品采集与特征

乌兰盖盆地中生代火山岩样品主要采自于盆地北缘及南缘出露的满克头鄂博组、白音高老组及玛尼吐组,各样品的采样位置见图 1。样品10-120采于满克头鄂博组流纹斑岩,坐标为117°03′28.29″E,44°24′58.37″N;样品10-122采于白音高老组流纹岩,坐标为117°03′30.17″E,44°24′46.93″N;样品22-2-1采于满克头鄂博组流纹岩,坐标为117°52′56.56″E,45°47′18.28″N;样品22-8-1采于玛尼吐组安山岩,坐标为117°56′41.15″E,45°42′28.12″N;样品07-070采于玛尼吐组安山岩,坐标为118°16′40.14″E,45°46′07.45″N。

根据该区区测资料,采集的满克头鄂博组岩石样品多为流纹岩、流纹斑岩;上侏罗统的白音高老组岩石样品为流纹岩;玛尼吐组岩石主要为安山岩、玄武安山岩。

岩石镜下特征如下:

安山岩:灰紫色,斑状结构,气孔状构造。斑晶主要为斜长石、角闪石、黑云母,体积分数为20%~30%,大小为0.5~1.5 mm;斜长石多呈自形——半自形,角闪石多呈半自形;基质主要由微晶斜长石与玻璃质等组成。

流纹岩(白音高老组):浅灰、灰白色,少斑状结构,基质隐晶结构,流动构造。斑晶为斜长石、石英及少量黑云母,体积分数为8%~10%,大小为0.3~2.0 mm。

流纹岩(满克头鄂博组):灰色、灰绿色,少斑状结构,流纹构造。斑晶为斜长石和少量黑云母,体积分数为10%~12%,大小为0.5~3.0 mm。

3 测年样品的制备及结果

样品的激光全熔40Ar/39Ar年代学研究在北京大学造山带与地壳演化教育部重点实验室进行,利用全时标全自动高精度高灵敏度激光全熔40Ar/39Ar定年系统测试完成。采用聚焦激光对单颗粒或多颗粒的矿物岩石样品进行一次性熔融测定。激光能量1.0~3.5 W,激光束斑直径为0.5 mm。激光在5 s内逐渐升温到1.0~3.5 W,升温后熔样释气时间持续40 s。系统分两个阶段使用两个锆铝泵对释出气体进行纯化,第一阶段纯化时间180 s,第二阶段60 s。系统通过测量已知摩尔数的空气对5个氩同位素(40Ar、39Ar、38Ar、37Ar、36Ar)质量歧视进行日常监测与校正,质量歧视因子D=1.001 8±0.000 3。基准线和5个氩同位素均使用电子倍增器进行13个循环测量。信号强度的测量采用电流强度测量法,信号强度以纳安(nA)为单位记录。测量已知摩尔数的空气的氩同位素信号强度,获得系统在电子倍增器单位增益下的绝对灵敏度为2.394×10-18moles/nA。通过绝对灵敏度可以将氩同位素信号强度由nA换算为mol。电子倍增器增益(与法拉第杯测量信号强度的比值)为3 000~4 000倍。测试设备的平均本底信号强度(nA)为:40Ar=(2.77±0.15)×10-339Ar=(2.85±0.15)×10-538Ar=(1.57±0.60)×10-637Ar=(4.59±0.59)×10-636Ar=(9.28±0.96)×10-6

系统测试过程、原始数据处理、模式年龄和等时线年龄的计算均采用美国加州大学伯克利地质年代学中心Alan L. Denio博士[17]编写的“MASS SPEC”软件自动控制,并进行数据处理。

采自乌兰盖盆地中生代火山岩5件样品的激光全熔40Ar/39Ar定年结果见图 2,原始数据见表 1。5件样品激光全熔40Ar/39Ar等时线年龄的概率统计结果误差范围一致(图 2,3),等时线年龄与表观年龄均可靠。由于5件测年样品的40Ar值均大于尼尔值((295.5±5.0) Ma),说明这些样品中40Ar略有过量或者丢失,从而导致表观年龄稍有偏老,因而其等时线年龄更可靠。

表 1 乌兰盖盆地中酸性火山岩激光全熔40Ar/39Ar测年数据 Table 1 Laser 40Ar/39Ar dating results of the volcanic rocks from the Wulangai basin
照射通量序号样品号40Ar
*/%
年龄/
Ma
±σn(39Ar)/
10-15mol
信号强度/nA
40Ar39Ar38Ar37Ar36Ar
J=0.005 3881766-0110-12059.1140.320.8816.195.3960.212 270.005 4260.012 50.007 477
1766-0210-12054.6140.960.9621.356.7300.243 62-0.002 6330.015 20.010 339
1766-0310-12055.2141.190.9219.175.9840.218 750.006 0290.012 50.009 069
1766-0410-12053.7143.710.9714.064.6000.160 480.004 6080.016 60.007 214
1766-0510-12052.3141.060.9614.974.9260.170 830.004 7330.011 20.007 947
1766-0610-12049.3142.391.0213.774.8550.157 110.005 7780.010 80.008 330
1766-0710-12053.6140.530.9425.738.2380.293 730.007 7350.018 50.012 936
1766-0810-12052.2140.380.9620.916.8610.238 670.006 2480.013 50.011 091
1766-0910-12058.8140.130.8816.434.7820.187 430.005 2740.015 80.006 676
1766-1010-12054.7143.800.9515.745.0600.179 640.004 9060.013 20.007 765
1766-1110-12053.7145.030.9817.725.8510.202 240.005 5960.012 10.009 168
1766-1210-12052.8143.510.9825.368.4210.289 270.007 8250.014 70.013 455
1766-1310-12057.7141.810.9120.126.0380.229 610.005 5230.015 00.008 641
1766-1410-12052.0143.901.0015.975.4070.182 330.005 2760.015 20.008 794
1766-1510-12052.4142.380.9820.926.9450.238 580.006 8470.016 10.011 201
1766-1610-12057.0143.190.9118.885.7950.215 390.005 7170.014 30.008 437
1766-1710-12061.5140.520.8621.285.9370.242 800.005 6750.013 40.007 740
1766-1810-12054.4141.470.9518.305.8060.208 740.005 6060.013 10.008 955
1766-1910-12055.4142.490.9417.315.4360.197 370.005 1480.013 10.008 211
1766-2010-12056.3141.640.9212.973.9860.147 920.003 9330.008 30.005 904
1766-2110-12054.7143.600.9513.374.2840.152 490.004 2350.009 20.006 563
J=0.005 4041767-0110-12270.6123.940.954.941.0520.056 400.004 0360.029 70.001 057
1767-0210-12276.0120.000.8412.932.4690.147 390.009 8800.082 20.002 035
1767-0310-12272.2121.360.8713.212.6890.150 700.010 0800.081 60.002 560
1767-0410-12253.6124.021.029.682.7120.110 390.007 5290.065 10.004 280
1767-0510-12262.5122.000.9211.072.6130.126 200.008 7270.072 00.003 336
1767-0610-12261.0119.700.9210.662.5290.121 620.008 6770.069 10.003 358
1767-0710-12274.7121.190.869.261.8170.105 620.007 0440.059 70.001 573
1767-0810-12271.7124.600.938.151.7160.093 010.006 4230.053 30.001 661
1767-0910-12253.3123.161.084.851.3550.055 290.004 1490.029 70.002 150
1767-1010-12260.0122.681.103.540.8750.040 320.002 8580.021 80.001 192
1767-1110-12274.5127.240.966.441.3320.073 490.004 9230.074 60.001 170
1767-1210-12262.2120.700.965.941.3950.067 770.005 1420.034 20.001 797
1767-1310-12258.1123.011.064.431.1340.050 480.003 5670.031 70.001 616
1767-1410-12258.2122.801.103.070.7830.034 960.002 5690.017 60.001 114
1767-1510-12268.9124.070.946.721.4680.076 750.004 9090.045 70.001 560
1767-1610-12269.9119.860.891.082.2340.122 880.008 3930.063 90.002 296
1767-1710-12272.4120.760.899.131.8420.104 100.006 8370.059 00.001 741
1767-1810-12276.0121.080.889.191.7710.104 810.006 9370.059 00.001 455
1767-1910-12278.7121.690.899.931.8560.113 190.007 6450.059 80.001 355
1767-2010-12264.0121.920.948.091.8650.092 270.006 3920.048 00.002 286
1767-2110-12258.9124.611.114.961.2710.056 620.004 2270.035 00.001 777
J=0.005 4201765-0122-2-144.4133.110.9719.126.9390.218 13-0.023 1770.010 30.013 042
1765-0222-2-143.0132.561.009.174.0060.122 560.015 1900.006 80.007 730
1765-0322-2-160.2132.180.7217.584.6740.200 610.081 1610.008 70.006 330
1765-0422-2-156.6133.700.7715.294.3790.174 490.225 3720.004 80.006 538
1765-0522-2-143.4133.320.9916.246.0340.185 220.019 7950.007 20.011 558
1765-0622-2-150.4135.420.8816.835.4780.192 100.220 2100.010 40.009 288
1765-0722-2-144.3132.720.9619.076.9240.217 650.003 4390.010 70.013 057
1765-0822-2-139.3134.641.1118.427.6470.210 230.208 4430.020 10.015 802
1765-0922-2-155.5134.090.7917.485.1140.199 500.138 3390.015 40.007 757
1765-1022-2-143.9133.840.9817.166.3430.195 860.021 1860.010 50.012 056
1765-1122-2-141.3134.631.0414.915.8890.170 170.017 5960.005 30.011 701
1765-1222-2-133.1134.511.3218.599.1440.212 020.021 5380.006 30.020 701
1765-1322-2-151.2135.640.8918.505.9420.211 220.153 7220.007 30.009 879
1765-1422-2-140.5137.491.1017.367.1570.198 160.253 0000.010 00.014 536
1765-1522-2-142.3134.711.0116.896.5120.192 680.019 5580.004 30.012 716
1765-1622-2-147.1137.230.9416.265.7480.185 580.281 3360.005 60.010 419
1765-1722-2-142.8136.211.0125.539.8610.291 480.034 3100.010 40.019 110
1765-1822-2-142.7134.771.0121.258.1360.242 560.030 9450.008 80.015 798
1765-1922-2-143.6135.061.0121.648.1340.246 980.028 1930.007 50.015 547
1765-2022-2-137.5135.221.1624.5610.7270.280 360.027 5950.008 20.022 686
1765-2122-2-153.0133.820.8120.026.1240.228 650.026 4160.006 30.009 740
J=0.005 5301764-0122-8-192.9132.360.488.291.4020.094 650.005 5310.136 20.000 378
1764-0222-8-191.9132.050.654.620.7880.052 780.002 9110.143 00.000 257
1764-0322-8-190.4132.690.517.991.3920.091 180.004 7250.137 30.000 493
1764-0422-8-191.5132.400.614.470.7660.050 960.002 8720.075 30.000 242
1764-0522-8-186.8133.100.545.711.1670.073 230.004 1970.108 90.000 551
1764-0622-8-191.6130.670.536.761.1440.077 210.004 2410.118 50.000 359
1764-0722-8-189.6132.050.613.940.6890.044 970.002 5860.065 20.000 262
1764-0822-8-193.1132.830.564.510.7630.051 470.003 0180.072 50.000 200
1764-0922-8-192.5132.860.516.421.2820.085 840.004 6960.116 30.000 361
1764-1022-8-192.1131.780.525.520.9360.062 990.003 8950.094 40.000 278
1764-1122-8-191.7133.200.4710.21.7670.117 010.006 7590.167 00.000 544
1764-1222-8-193.3132.670.596.651.1210.075 830.003 9590.111 60.000 286
1764-1322-8-190.7133.620.516.291.0980.071 720.003 9690.110 50.000 377
1764-1422-8-191.7133.100.477.861.3540.089 720.004 9690.147 30.000 425
1764-1522-8-187.8134.610.527.141.3010.081 550.004 5410.112 60.000 572
1764-1622-8-181.9133.360.536.811.3150.077 690.004 1730.114 80.000 837
1764-1722-8-184.1134.200.555.281.0000.060 230.003 5090.091 40.000 566
1764-1822-8-182.5134.630.538.021.5540.091 560.005 2630.120 30.000 954
1764-1922-8-187.2134.150.508.101.4780.092 430.004 5420.157 70.000 683
1764-2022-8-190.3134.010.526.471.1380.073 790.004 0560.110 10.000 404
1764-2122-8-186.6133.210.557.251.3240.082 790.003 8300.112 70.000 633
J=0.005 3421755-0107-07095.3157.480.5210.912.2340.124 660.002 7360.041 60.000 369
1755-0207-07095.3156.660.599.121.8580.104 290.002 3280.041 00.000 305
1755-0307-07093.8157.150.5212.032.4960.137 440.003 0810.051 10.000 539
1755-0407-07095.2156.400.539.011.8330.102 910.002 4170.034 30.000 308
1755-0507-07091.9156.020.5310.112.1250.115 510.002 5870.037 40.000 591
1755-0607-07093.6156.140.629.151.8890.104 540.002 2800.033 30.000 417
1755-0707-07088.7155.210.549.882.1390.112 800.002 6490.040 00.000 829
1755-0807-07091.9157.970.5510.252.1880.117 350.002 6280.037 40.000 611
1755-0907-07095.9158.030.5410.392.1220.118 760.002 5910.036 00.000 303
1755-1007-07095.9157.610.5014.222.8950.162 510.003 6570.055 10.000 415
1755-1107-07094.9156.500.5018.253.7290.208 560.004 6120.072 20.000 668
1755-1207-07095.6154.720.5211.892.3930.136 470.002 9590.049 20.000 371
1755-1307-07096.5155.160.539.211.8360.105 300.002 3080.033 00.000 230
1755-1407-07094.3151.610.587.511.4940.085 810.002 1660.031 40.000 299
1755-1507-07095.3155.950.5311.892.4080.135 810.003 0770.043 80.000 392
1755-1607-07097.1157.070.5210.962.1980.125 330.002 8440.041 40.000 227
1755-1707-07094.5154.520.589.061.8340.103 530.002 3490.034 00.000 350
1755-1807-07094.5155.350.5112.102.4650.138 460.003 2490.047 20.000 469
1755-1907-07094.3154.310.618.651.7530.098 920.002 0830.032 70.000 345
1755-2007-07095.8155.420.5015.573.1250.177 690.003 9330.064 50.000 465
1755-2107-07095.5155.830.549.581.9360.109 470.002 5090.036 40.000 307
1755-2207-07095.7154.900.539.161.8360.104 660.002 4060.039 30.000 281
图 2 激光全熔40Ar/39Ar等时线图(左)和表观年龄的概率统计图(右)

Fig. 2 Laser 40Ar/39Ar isochron age(left) and age-probability diagrams(right)

因此从本次乌兰盖盆地中生代火山岩的激光全熔40Ar/39Ar定年结果分析:白音高老组样品(10-122)年龄为(120.2±1.8)Ma;另外,编号为10-120的样品在区测图幅中为白音高老组,定年结果为(136.0±3.0)Ma,依据其年龄数据,该样品所处填图单元不排除为满克头鄂博组;盆地北缘玛尼吐组样品(22-8-1)年龄为(131.7±0.8)Ma;满克头鄂博组样品(22-2-1)年龄为(132.6±1.5) Ma;根据K/J界线(145.0±0.8) Ma,上述样品相当于早白垩世[17]

通过年龄数据分析,笔者所采用区测图幅资料中乌兰盖盆地南缘玛尼吐组样品(07-070)年龄为(151.8±1.5) Ma,其年龄大于满克头鄂博组等样品,说明本区火山岩可能存在相应的穿时性,所属地层时代应为晚侏罗世。总体来说,乌兰盖盆地中生代火山岩时代总体位于晚侏罗世晚期——早白垩世。

4 地球化学特征

4.1 主量元素

乌兰盖盆地中生代火山岩主量、微量、稀土元素分析结果见表 2。主量元素数据经过去水归一化后[18],火山岩在TAS岩石分类图解(图 3a)上显示为中性(安山岩)与酸性岩类(流纹岩)(表 2),均属于亚碱性系列[18, 19, 20, 21, 22];在w(SiO2)-w(K2O)图解(图 3b)上,火山岩大部分为高钾钙碱性系列;在A/CNK-A/NK图解(图 3c)上,样品为准铝质——过铝质。各样品Mg#较低,平均为29.8。

表 2 乌兰盖盆地中生代火山岩常量元素和微量元素、稀土元素分析结果 Table 2 Analytical results of major and trace element for the volcanic rocks of the Manitu Formation in the Wulangai basin
样品岩性地层坐标SiO2Al2O3TFe2O3MgOCaONa2OK2OMnOP2O5TiO2烧失量
22-2-1流纹岩J3mk117°52′56.56″E45°47′18.28″N77.6911.631.660.080.142.894.390.060.10.110.96
22-8-1安山岩J3mn117°56′41.15″E45°42′28.12″N58.8316.957.423.046.163.291.990.090.411.390.56
22-8-3流纹岩J3mk117°56′31.05″E45°42′34.23″N69.9914.452.260.471.242.905.570.090.080.422.56
24-15-1安山岩J3mn117°51′40.23″E45°48′43.49″N57.9416.777.923.096.122.932.110.120.381.380.96
24-28-1安山岩J3mn117°53′34.34″E45°47′36.02″N56.8516.018.713.175.732.602.190.120.801.882.10
24-27-1安山岩J3mn117°53′90.08″E45°47'00.24″N59.8815.648.731.184.902.962.600.090.791.850.79
10-122流纹岩J3b117°03′30.17″E44°24′46.93″N79.7011.121.170.080.372.774.220.010.030.110.76
10-120流纹斑岩J3mk117°03′28.29″E44°24′58.37″N76.4711.182.810.150.682.454.320.030.050.201.02
24-3-2安山岩J3mn117°49′30.68″E45°50′10.99″N61.2115.507.051.863.553.941.240.190.651.383.28
24-8-1安山岩J3mn117°51′15.87″E45°48′55.53″N58.9917.367.931.985.803.122.100.100.391.380.98
10-107安山岩J3mn117°03′17.38″E44°25′29.87″N61.7916.866.180.664.453.113.220.110.531.501.28
样品总量Mg#AsBBaBeBiBrCdCoClCrCsCuGeHfHg
22-2-199.6410.043.411.91083.40.112.274.31.23126.75.04.81.069.79.5
22-8-199.7748.800.93.25761.30.070.775.120.010989.51.026.71.264.55.3
22-8-399.9132.631.415.19753.30.181.381.81.915017.77.55.81.1210.45.3
24-15-199.3647.612.83.67161.40.060.5114.823.35667.04.821.31.324.410.0
24-28-199.9445.862.83.28002.40.081.2112.523.315145.91.033.91.329.012.1
24-27-199.3723.951.93.28062.10.070.469.416.510338.21.023.20.978.96.9
10-122100.3113.085.613.63322.50.041.324.11.1637.82.74.20.877.38.4
10-12099.3211.302.828.82414.60.050.439.31.8826.69.910.21.269.25.3
24-3-299.4138.115.64.65731.40.040.4132.29.72917.11.510.32.085.48.4
24-8-199.8136.794.23.46831.30.060.482.017.64865.79.020.21.184.46.9
10-10799.5620.043.67.78622.50.160.646.512.613154.95.017.40.926.45.8
样品MoNbNiPbRbSbSeSrTaThTlUZnZrCeDyEr
22-2-11.9215.23.818.2139.91.680.040210.9312.70.482.9261.426182.495.552.94
22-8-11.258.225.48.727.20.170.0326770.502.60.180.67107.718442.333.841.74
22-8-34.9414.93.025.9168.71.280.0381951.2916.01.035.0460.139693.755.583.08
24-15-11.098.216.910.332.70.130.0496360.513.30.200.95113.019546.023.961.90
24-28-12.3520.920.717.053.10.130.0535991.235.40.341.23147.243999.435.312.31
24-27-11.6320.414.815.254.10.130.0425611.205.20.131.17101.343097.125.462.40
10-1220.8913.25.018.3120.10.680.034561.1212.50.242.2239.6163113.687.733.78
10-1202.1419.63.426.5211.60.690.036731.9026.40.773.5359.922896.269.455.22
24-3-21.097.75.110.422.90.440.0433440.504.40.171.0688.819556.886.343.56
24-8-11.167.814.09.431.50.170.0666770.483.00.170.79100.219347.834.382.10
10-1071.7714.512.718.498.40.260.0365821.0010.90.552.37105.528587.064.762.14
样品EuGdHoLaLuNdPrSmTbTmYYbNb/TaTi/Y(La/Yb)N∑REE
22-2-10.246.381.0248.30.4944.012.08.031.010.4931.003.1316.3813.2811.06215.97
22-8-11.825.020.6618.20.2425.96.05.810.770.2618.641.5816.3227.888.26114.16
22-8-31.456.611.0640.60.5342.211.17.951.020.5131.503.2911.556.428.86218.70
24-15-11.704.840.7020.30.2726.36.25.530.740.2920.431.7816.0524.188.19120.62
24-28-12.587.980.8944.80.3253.813.110.161.120.3325.172.0717.0623.4615.53244.18
24-27-12.668.170.9245.20.3154.613.310.411.150.3525.542.0914.4662.8515.52244.12
10-1220.749.021.4059.20.5951.614.610.401.410.6140.573.7311.7437.5711.38278.45
10-1200.729.551.7961.90.9254.814.611.041.610.8853.575.7910.2812.407.66274.50
24-3-21.967.041.2524.40.6133.77.67.551.120.5834.803.7015.4441.4017.09218.43
24-8-11.885.540.8021.40.3128.76.76.170.830.3122.211.9416.3232.588.19120.62
10-1072.107.050.8143.10.2847.411.98.730.990.3022.791.8114.4712.8215.53244.18
注:主量、微量元素数据由国土资源部沈阳地质矿产研究所实验室测定; Mg#由Geoplot程序计算,Eu/Eu*=Eucn/(Smcn·Gdcn)0.5。主量元素质量分数单位为%;
微量元素和稀土元素质量分数单位为10-6
图 3 火山岩TAS分类命名图解(a)、w(SiO2)-w(K2O)图解(b) 与A/CNK-A/NK图解(c)

Fig. 3 w(SiO2)-w(K2O+Na2O) of classification diagram (a) and w(SiO2)-w(K2O) diagrams (b) for the study rocks and A/CNK-A/NK diagrams(c)
4.2 微量元素地球化学特征

在球粒陨石标准化稀土元素配分图解(图 4a)上,火山岩表现出富集轻稀土(LREE),轻重稀土分异明显的特征。其中安山岩(La/Yb)N值平均为12.92,而流纹岩(La/Yb)N值平均为9.74,均位于上、下地壳(La/Yb)N值之间(5.3~15.5)[23]。安山岩w(∑REE)为:(114.16~244.18)×10-6,平均为188.30×10-6;流纹岩w(∑REE)平均为246.90×10-6,整体较安山岩要高,为(215.97~278.45)×10-6;岩石有明显Eu异常,其中安山岩样品Eu值为(1.70~2.66)×10-6,平均为2.17×10-6;而流纹岩则出现了明显的低异常,为(0.24~1.45)×10-6,平均0.79×10-6。岩石的稀土元素标准化配分曲线与上、下地壳的配分曲线相似(图 4 a)[23]。在微量元素配分图(图 4 b)上,火山岩明显富集大离子亲石元素Rb、Ba、Th、U、K和LREE,而亏损高场强元素Nb、Ta、P、Ti的特征,并且从中性岩到酸性岩,Rb、Ba、Th、U、K富集程度微弱增强,而Nb、Ta、P、Ti亏损程度逐渐增强,表现出典型的壳源岩浆的特征。岩石的Nb/Ta值(10.28~17.06)也和典型的壳源岩浆比较接近[23]

图 4 稀土元素球粒陨石标准化配分图(a)与不相容元素原始地幔标准化配分图(b)

Fig. 4 Rare earth element chondrite-normalized distribution patterns(a)and primitive mantle-normalized trace element spiderdiagram(b)for the study rocks

5 岩石成因与构造背景

乌兰盖盆地盆缘发育的中生代火山岩,总体为一套钙碱性系列到高钾钙碱性系列的中酸性岩石组合;安山岩Rb/Sr平均值为0.185,接近于上地壳该比值(0.22)[23],而明显高于原始地幔(0.03)、E-MORB(0.033)和OIB(0.047)的Rb/Sr值[24];而满克头鄂博组和白音高老组的酸性火山岩Rb/Sr的平均值分别为2.51、6.53。不同岩类Sr含量的变化,主要归因于加厚下地壳不同压力范围内不同残留相物质部分熔融的结果[25](图 5)。安山岩、流纹岩Ti/Y平均值分别为391、35;安山岩接近于下地壳的Ti/Y值(307)[23];而流纹岩较低的Ti/Y值可能归因于钛铁矿的分离结晶作用。各类岩石地球化学特征与壳源岩石的地球化学特征基本一致,表明它们应来自地壳物质局部熔融形成的壳源岩浆系列。

图 5 乌兰盖盆地中生代火山岩w(Sr)-w(Yb)图解

Fig. 5 w(Sr)-w(Yb) diagram for the rocks from the Manitu formation in Wulangai basin

对于壳源的中、酸性岩石而言,Sr、Yb丰度被认为是识别与判别岩浆源区成因与性质的有效判别指标[23]。在w(Sr)-w(Yb)图解(图 5)中,乌兰盖盆地中生代火山岩从中性到酸性,大致对应于埃达克型、闽浙型、南岭型的岩石地球化学特征,说明本区早白垩世中酸性火山岩是由加厚下地壳不同残留矿物相部分熔融产生。本区131.7 Ma左右发育的具有埃达克型火山岩可能是在榴辉岩相下地壳(以石榴石为主,压力>1.0 GPa,对应地壳厚度大约为50 km)的部分熔融的产物;闽浙型酸性岩可能是在角闪岩相下地壳(以角闪岩+斜长石为主,压力0.8~1.0 GPa,对应地壳厚度大约为30 km)的部分熔融的产物;具有南岭型的流纹岩则是正常厚度下地壳(厚度<30 km,压力<0.8 GPa)的部分熔融的产物[26]

从时间与岩石类型空间分析,本区存在着一个由埃达克型(151.8 Ma±)——南岭型(136.0 Ma±)——埃达克型(131.7 Ma±)——南岭型(120.2 Ma)发育过程,对应于造山带演化的不同阶段,即: 由一个正常地壳发育为加厚地壳(高原)再转化为正常地壳的过程。己有的资料表明,蒙古——鄂霍茨克洋在中——晚侏罗世闭合[27],但该大洋呈剪刀式自西向东逐渐闭合,西部最终闭合时间为晚侏罗世,东部最终闭合碰撞造山为早白垩世[28]。笔者研究的乌兰盖盆地南、北缘中生代火山岩形成于晚侏罗世晚期至早白垩世,恰为该洋的闭合碰撞造山期阶段,其必然引起区域性的挤压并产生地壳不均匀缩短加厚至麻粒岩相,促使造山带岩石圈增厚,最后导致下地壳底部熔融形成中酸性火山岩[29],进而导致造山带重力坍塌,形成具有伸展型的南岭型中酸性岩浆。近年来,在研究蒙古——鄂霍茨克洋(古太平洋)构造演化对于大兴安岭中生代岩浆与成矿作用时发现,大兴安岭中侏罗世——白垩纪,存在着多期近乎东西向的南向逆冲推覆构造事件,而各期逆冲构造必然造成地壳的不均匀加厚以及随之而来的伸展。研究区发育火山岩特征,正是多期逆冲推覆构造事件的反映:埃达克型(151.8 Ma±)前期推覆加厚地壳;南岭型(136.0 Ma±)推覆加厚地壳的垮塌伸展;埃达克型(131.7 Ma±)新的推覆加厚地壳;南岭型(120.2 Ma)该期推覆加厚地壳的垮塌伸展。根据本次年代学数据分析,每次推覆构造事件的间隔持续5~15 Ma。因此,大兴安岭乌兰盖盆地中生代火山岩是蒙古——鄂霍茨克洋(古太平洋)闭合碰撞造山作用过程深部岩浆作用的反应。

6 结论

1)乌兰盖盆地南、北缘中生代火山岩主要为中性(安山岩)与酸性岩类(流纹岩),火山岩激光全熔40Ar/39Ar定年结果为(151.8±1.5)~(120.2±1.8) Ma,说明其时代总体位于晚侏罗世晚期至早白垩世。

2)各类岩石总体为一套钙碱性系列到高钾钙碱性系列,岩石地球化学特征与壳源岩石的地球化学特征基本一致,表明它们应来自于地壳局部熔融形成的壳源岩浆系列。

3)乌兰盖盆地中生代火山岩形成于蒙古——鄂霍茨克洋(古太平洋)闭合碰撞造山构造背景,在早白垩世区内曾发生地壳加厚的造山过程,其岩浆深部动力学背景与岩浆源区的性质主要归因于增厚的造山带下地壳发生的部分熔融作用。

岩石元素测试得到了沈阳地质矿产研究所何炼教授、岳明新高级工程师等人员的热心帮助;激光氩-氩全熔年代学实验测试得到了北京大学造山带与地壳演化教育部重点实验室周晶博士的热心帮助;在此,对以上人员表示最诚挚的谢意。

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http://dx.doi.org/10.13278/j.cnki.jjuese.201502109
吉林大学主办、教育部主管的以地学为特色的综合性学术期刊
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文章信息

孙守亮, 李永飞, 郜晓勇
Sun Shouliang, Li Yongfei, Gao Xiaoyong
内蒙古乌兰盖盆地中生代火山岩年代学、地球化学特征及其地质意义
Mesozoic Volcanic Rocks Geochronology and Geochemical Characteristics in Wulangai Basin of Inner Mongolia and Their Geological Significance
吉林大学学报(地球科学版), 2015, 45(2): 441-452
Journal of Jilin University(Earth Science Edition), 2015, 45(2): 441-452.
http://dx.doi.org/10.13278/j.cnki.jjuese.201502109

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收稿: 2014-06-18

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