北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究-袁洪林

中国科学D辑地球科学2005,35(9:821~836821北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究*袁洪林①**柳小明①刘勇胜②高山①②凌文黎②(①西北大学地质学系教育部大陆动力学重点实验室,西安710069;②中国地质大学地球科学学院,武汉430074摘要对北京西山髫髻山组和东岭台组火山岩锆石U-PbSHRIMP和LA-ICP-MS定年及地球化学的研究结果表明,髫髻山组上部火山岩时代为137.1±4.5Ma(2σ,东岭台组中部火山岩时代为130~134Ma.前者略老于后者,但两者时代相差<5Ma,证明两组火山岩的喷发是在很短的时间内完成的.北京西山地区髫髻山组火山岩的年龄与承德盆地髫髻山组火山岩的年龄明显不同,表明髫髻山组火山岩年龄可能存在明显区域变化.北京西山髫髻山组中存在典型的埃达克岩(SiO2含量约为56%,Na2O=3.99~6.17,Na2O/K2O=2.2~3.1,Sr=(680~1074×10−6,Y=(13.2~16.3×10−6,Yb=(1.13~1.52×10−6,Sr/Y=43~66,高镁埃达克岩和高镁安山岩(Mg#=54~55.髫髻山组埃达克岩及与其共生的其它中酸性火山岩均具有典型的Nb-Ta负异常和Pb正异常等大陆地壳特征,表明它们是陆壳岩石部分熔融的产物.上述结果表明,北京西山地区髫髻山组埃达克岩代表了中生代曾存在于华北克拉通的加厚榴辉岩下地壳与岩石圈地幔一同拆沉至软流圈中,榴辉岩随后发生部分熔融,熔体上升过程中与地幔橄榄岩发生相互作用的产物.髫髻山组火山岩的年龄制约了拆沉作用至少延续至137Ma.东岭台组火山岩为Mg#正常的流纹岩和英安岩,表明它们没有与地幔发生反应,它们可能代表了拆沉作用导致的软流圈地幔物质上涌造成的深部地壳物质重熔的产物.关键词埃达克岩榴辉岩地球化学华北克拉通下地壳拆沉作用2004-07-13收稿,2005-03-28收修改稿*国家自然科学基金资助项目(批准号:40302015,40472099,40373013**E-mail:yhonglin@华北克拉通中生代燕山期以强烈的构造-岩浆活动、成矿作用和盆地形成过程,而明显不同于世界上其他太古宙克拉通,其深部地球动力学背景可能与岩石圈地幔的减薄、去根或置换作用或下地壳和岩石圈地幔的拆沉作用或其他地球动力学过程有关[1~16].然而,这些动力学过程发生的确切时间和机制仍然822中国科学D辑地球科学第35卷是有争议的问题[1~16].火山岩的精确定年和地球化学研究是制约地球动力学过程时间和机制的重要途径.本文对北京西山地区中生代髫髻山组和东岭台组火山岩进行了SHRIMP和LA-ICP-MS单颗粒锆石U-Pb原位定年和地球化学研究.在此基础上重点讨论了髫髻山组火山岩的成因及其动力学意义.1地质背景和样品来源北京西山地区是燕山运动研究的经典地区(图1.该区中生代地层从老至新依次为晚三叠世杏石口组,主要由页岩、砂岩和砾岩组成;该组被南大岭组橄榄玄武岩不整合覆盖,之上为窑坡组、龙门组和九龙山组砂砾岩、粉砂岩、泥岩和煤系.它们被髫髻山组火山岩不整合覆盖.髫髻山组主要由安山岩、英安岩、玄武岩、流纹岩和火山碎屑岩组成.髫髻山组又被东岭台组不整合覆盖.东岭台组主要由英安岩、流纹岩和火山碎屑岩组成.2002年出版的《中国地质图集》将南大岭组时代归属于早侏罗世,髫髻山组归属于晚侏罗世,东岭台组归属于早白垩世[17].李伍平等[18]曾获得北京西山髫髻山组下部三段底部粗安岩斜长石Ar-Ar坪年龄为148±3Ma,其等时线年龄为147±3Ma.汪洋等[19]报道了北京西山髫髻山组火山岩的Sm-Nd等时线年龄为188±19Ma.但该等时线仅有6个样品,且这些图1北京西山地区地质简图(据文献[17]简化剖面A和B分别为髫髻山组和东岭台组火山岩采样剖面分布位置第9期袁洪林等:北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究823样品的147Sm/144Nd与143Nd/144Nd之间的相关性不高(r=0.98.由于作者没有给出各样品的岩性,也没有讨论该“等时线”是否可能是混合线,因此该年龄的意义并不明确.Davies[13]获得河北兴隆县黄土梁髫髻山组安山岩的Ar-Ar年龄为148±2Ma.牛宝贵等[20]获得河北承德盆地髫髻山组顶部火山岩的锆石U-PbSHRIMP年龄为163±6Ma.赵越等[21]获得北京西山马兰村髫髻山组火山岩底部安山岩锆石U-PbSHRIMP年龄为157±3Ma,但这一样品并非取自西山地区髫髻山组标准剖面.对东岭台组火山岩的年龄研究较少.汪洋和姬广义[22]获得东岭台组火山岩的Rb-Sr等时线年龄为131±3Ma,但该等时线仅包括4个样品.由于现有髫髻山组火山岩年龄存在很大的变化范围,加之Ar-Ar定年方法受多种因素的影响,如矿物的封闭温度,过剩40Ar的存在或部分Ar丢失等,可见髫髻山组和东岭台组火山岩的年龄仍需进一步研究.由于华北克拉通中生代火山岩均产在陆相盆地中,横向对比不易,深入研究的重点应是对标准剖面火山岩的锆石U-Pb定年.为此,本研究在北京西山地区沿髫髻山组和东岭台组标准剖面分别采取了典型火山岩样品(图1和2.其中用于SHRIMP和LA-ICP-MS锆石定年的髫髻山组上部(旋回3英安岩样品DT-37采自大台煤矿,GPS坐标为:北纬39°58.707′,东经115°54.850′.东岭台组中部(三段紫图2北京西山地区髫髻山组和东岭台组火山岩采样剖面(据李伍平1[18]和样品分布图剖面分布位置见图11李伍平.燕山-辽西中生代火山岩特征及其构造北京研究.中国地质大学(武汉博士学位论文,1999824中国科学D辑地球科学第35卷红色流纹岩样品YHC-13采自沿河城医院大门旁,GPS坐标为:北纬40°3.956′,东经115°42.886′.2分析方法本研究分析的样品均为未风化蚀变的新鲜火山岩样品.锆石分选采用人工重砂分选,然后在双目镜下挑纯.用于主量和微量元素分析的样品粗碎在刚玉颚板破碎机中进行,细碎在日本CMT公司生产的T1-100型WC细碎机中最终碎至200目.因此样品加工均在无污染设备中进行.主量和微量元素以及LA-ICP-MS锆石U-Pb原位定年分析均在西北大学教育部大陆动力学重点实验室进行.SHRIMP锆石U-Pb原位定年在北京离子探针中心完成.主量元素分析采用XRF(RigakuRIX2100玻璃熔饼法完成.微量元素采用ICP-MS(PerkinElmer公司具动态反应池的Elan6100DRC法完成,样品溶解在Teflon高压溶样弹中进行.对国际标准参考物质BHVO-1(玄武岩、BCR-2(玄武岩和AGV-1(安山岩的分析结果表明(表1,主量元素分析精度和准确度优于4%,微量元素分析的精度和准确度一般优于10%.XRF粉末压饼法微量元素分析结果表明,同一样品的Rb,Sr,Ba,Zr和Nb采用XRF和ICP-MS的两套分析结果的相对偏差<10%.SHRIMP锆石U-Pb年龄分析在北京离子探针中心SHRIMPII上完成,分析方法见文献[23,24].所使用的离子束直径为30µm.LA-ICP-MS锆石U-Pb原位定年分析所使用的ICP-MS为Elan6100DRC,激光剥蚀系统为德国LamdaPhysik公司的GeoLas200M深紫外(DUV193nmArF准分子(excimer激光剥蚀系统.该剥蚀体系相对常规的266nm或213nmNd:YAG剥蚀系统具有明显较小的元素分馏效应[25,26].这对于U-Pb锆石定年至关重要.分析中采用的激光斑束直径亦为30µm,以29Si作为外标,哈佛大学标准锆石91500作为外标校正.91500锆石的同位素比值和年龄已被前人广泛研究,是国际上常用的3个标准锆石之一[27~29].同位素比值数据处理采用GLITTER(4.0版软件和本文作者的数据平滑方法进行,年龄计算采用ISOPLOT(3.00版软件[30]进行.详细分析方法见文献[29,31].对澳大利亚国立大学标准锆石TEMORA和已知SHRIMP年龄的锆石分析结果表明,本方法对2.5Ga的太古宙至31Ma的渐新世锆石获得的年龄与SHRIMP年龄相当[29,31].3锆石U-Pb年龄东岭台组流纹岩样品YHC-13中锆石为典型的浅褐色自形岩浆锆石,未见有残留核部发育(图3.对该样品11个锆石颗粒的SHRIMP分析结果见表2和图4.由表2和图4可见,除分析点YHC-13-11.1不谐和外,其余10个分析点均落在谐和线上或附近.所有11分析点的206Pb/238U年龄范围为121~145Ma,206Pb/238U加权平均年龄为133.8±4.7Ma(2σ;MSWD=2.5,它们的Th/U比值为0.63~1.33(表2,属典型的岩浆锆石范围.对该样品12个锆石颗粒的LA-ICP-MS分析结果见表3和图5.其中大部分点落在谐和线上或附近.这些分析点的206Pb/238U加权平均年龄为130.1±4.6Ma(2σ;MSWD=0.38,它们的Th/U比值为0.84~1.27(表3.在图4和5上,分析点的207Pb/235U比值变化较大是由于样品中U的含量低且该样品较年轻,207Pb含量相对较低,使得ICP-MS测定的离子计数很低,导致分析不确定性较大所致.SHRIMP和LA-ICP-MS两种不同方法获得的YHC-13样品的年龄在误差范围内一致,东岭台组火山岩的形成年龄为130~134Ma,时代为早白垩世.髫髻山组英安岩样品DT-37的锆石亦为褐色自形岩浆锆石,亦未见有残留核部发育(图3.对该样品16个锆石颗粒的SHRIMP分析结果见表2和图6.由表2可见,除分析点DT37-1.1和DT-37-8.1明显较老,207Pb/206Pb年龄分别为2331和1247Ma外,其余14个分析点的206Pb/238U年龄范围为121~147Ma,这14个分析点的206Pb/238U加权平均年龄为137.1±4.5Ma(2σ;MSWD=2.4.除DT37-1.1Th/U比值为0.28外,其余所有15个分析点的Th/U比值为0.60~1.10,为典型的岩浆锆石(表2.因此,137.1±4.5Ma应代表髫髻山组英安岩的形成年龄.如果采用135Ma作为第9期袁洪林等:北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究825表1国际标准参考物质分析结果aBCR-2BHVO-1AGV-1测定值S/%推荐值N测定值S/%推荐值N测定值S/%推荐值NSiO254.10.0654.16749.830.2249.94158.97−0.2258.841TiO22.250.382.26672.73−0.742.7111.013.811.051Al2O313.40.8913.56713.591.5213.8116.871.6317.151TFe2O313.8−0.2713.86712.35−0.9812.2316.81-0.596.771MnO0.19−4.010.18670.170.000.1710.090.000.091MgO3.70−2.993.59677.210.287.2311.54-0.651.531CaO7.17−0.647.126711.271.1411.414.851.824.941Na2O3.26−3.153.166661K2O1.81−1.101.79670.53−1.920.5212.94-0.682.921P2O50.35−1.220.35670.270.000.2710.490.000.491总量100.0099.85100.14100.5397.7898.04Li9.465.219.9844.93−7.284.62811.27.071226Be2.2−37.11.641.09.481.1282.22−5.822.126Sc34−2.4733431.60.6031.82812.2−0.0312.230V437−4.974164318−0.1731728123−1.9712130Cr17.43.131842841.602892810.9−7.8010.126Co46−24.737445.3−0.69452815.6−2.0415.330Ni13.5−4.111341210.081212816.6−3.581628Cu35−85.5194141−3.351362857.54.196027Zn153−20.41274113−7.271052884.34.198830Ga222.7223421.3−1.61212820.2−1.082030Rb53−10.948410.177.56112869.3−3.0267.330Sr3333.7434644000.69403286541.2066230Y363.6237427.460.5127.62820.2−1.122030Zr1870.3218841752.4517928228−0.4622730Nb12.311.814419.2−0.85192814.43.901530Cs1.24−116.970.13251.28−0.131.2830Ba723−5.8168341381.021392812081.46122630La27−9.4625415.80.0015.82838.6−1.543830Ce56−5.8553438.51.17392868.3−1.916730Pr7.0−3.136.845.434.715.7288.18−7.577.623Nd31.1−11.028425.9−2.6125.22832.51.553330Sm7.0−5.196.746.34−2.326.2285.831.205.930Eu2.02−1.11242.031.502.06281.66−1.091.6430Gd6.9−1.256.846.114.556.4285.20−4.03530Tb1.09−2.011.0740.97−0.570.96280.692.110.730Dy6.54−3.206.3445.29−1.685.2283.570.733.630Ho1.320.911.3341.00−0.840.99280.670.660.6730Er3.532.833.6342.42−0.862.4281.74−2.341.730Tm0.54−0.840.5440.34−1.600.33280.25−5.790.2430Yb3.50.263.542.09−3.222.02281.682.401.7230Lu0.54−5.100.5140.30−4.050.29280.264.080.2730Hf5.1−6.534.844.46−1.754.38285.011.695.130Ta0.83−2.830.8141.24−0.921.23280.900.450.930Pb10.72.981142.369.222.62637.2−3.463630Th6.28−7−17.61.08286.332.546.530U1.72−1.711.6940.42−1.130.42281.863.141.9230aBCR-2和BHVO-1为玄武岩;AGV-1为安山岩.S为测定值与推荐值(http:///geochem/之间的偏差.N为分析次数826中国科学D辑地球科学第35卷图3东岭台组流纹岩样品YHC-13和髫髻山组英安岩样品DT-37的阴极发光图图4东岭台组流纹岩样品YHC-13SHRIMP锆石U-Pb同位素年龄谐和图图5东岭台组流纹岩样品YHC-13LA-ICP-MS锆石U-Pb同位素年龄谐和图侏罗纪与白垩纪之间的年龄分界,则该年龄属晚侏罗世.而明显较老的锆石颗粒DT37-1和DT-37-8为捕获锆石.4地球化学髫髻山组火山岩主要由Mg#正常的安山岩、英安岩、玄武岩、流纹岩和高镁(Mg#>45安山岩和埃达克岩组成(表4.其中,高镁安山岩DT-24和DT-26的Cr含量为104~111×10−6,是Mg#正常安山岩的2~50倍,甚至高于玄武岩样品DT-19和DT-32.埃达克岩样品DT-34、DT-36和DT-40的SiO2含量约为56%,Na2O=3.99~6.17,Na2O/K2O=2.2~3.1,Sr=680~1074×10−6,Y=13.2~16.3×10−6,Yb=1.13~1.52×10−6,Sr/Y=43~66,为典型的埃达克岩[32~37](图7.其中,DT-36的Mg#=55,属高镁埃达克岩.东岭台组火山岩由流纹岩和英安岩组成.与髫髻山组中酸性火山岩相比,东岭台组英安岩和流纹岩具有明显低的Sr/Y和LaN/YbN比值(图7;表5.髫髻山组和东岭台组无论是玄武岩、安山岩(包括正常Mg#及高Mg#者,下同、英安岩、埃达克岩,还是流纹岩均表现出明显的Nb和Ta负异常以及明显的Pb正异常等典型的大陆地壳特征,同时还表现出明显的轻稀土元素(LREE富集(图8和9.髫髻山组玄武岩、安山岩和埃达克岩还表现出明显正的Sr异常,除高镁安山岩具正Eu异常(Eu/Eu*=1.14外,无明显Eu异常;而东岭台组火山岩则表现出明显的负Sr异常,除英安岩样品YHC-15和YHC-17外,其余样品第9期袁洪林等:北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究827828中国科学D辑地球科学第35卷第9期袁洪林等:北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究829图6髫髻山组英安岩样品DT-37SHRIMP锆石U-Pb谐和图图7髫髻山组和东岭台组中酸性火山岩的YbN对LaN/YbN(据文献[35]与Y对Sr/Y图(据文献[35]具明显Eu负异常(图9.髫髻山和东岭台组火山岩的Sr-Nd同位素分析(表6表明,130Ma时的初始εNd为−11至−20之间,初始87Sr/86Sr比值为0.705~0.708,具大陆地壳特征.5讨论高山等曾根据秦岭-大别造山带和华北克拉通地质、地球物理和地球化学特征与地幔包体Re-Os同位素研究提出[7,8,38~40],受三叠纪扬子克拉通向华北克拉通的大陆深俯冲作用影响,上述地区早中生代形成了加厚的榴辉岩下地壳,该榴辉岩下地壳与岩石圈地幔一同发生了拆沉作用.张旗等[41]根据中国东部广泛存在的燕山期埃达克质岩石提出当时曾存在中国东部高原.近年许文良等在徐淮地区年龄为132Ma[42]的早白垩世侵入杂岩中发现了榴辉岩类包体,该包体的石榴石-全岩Sm-Nd等时线年龄为219±5Ma[43].该年龄代表了包体榴辉岩相变质作用的年龄.该年龄与大别-苏鲁榴辉岩的峰期变质年龄吻和(220~240Ma[44~46].郑建平等[47,48]在河南信阳地区中生代火山碎屑岩中也发现了榴辉岩包体.这些榴辉岩包体的发现以及前人和本研究确证的埃达克岩的广泛存在,证明华北克拉通中生代确实曾存在加厚的榴辉岩下地壳.对华北克拉通地壳成分的动力学模拟结果表明,这种加厚榴辉岩下地壳的厚度可能>30km[7].吴福元等和张宏福等也曾提出华北克拉通岩石圈地幔的减薄与拆沉作用有关[4,16].关于地壳加厚和岩石圈减薄作用的机制前者认为与古太平洋板块的俯冲有关,而后者则认为与扬子克拉通陆壳和Monolo-Okhotsk洋壳的双向俯冲有关.高山等[9]最近对辽西晚侏罗世兴隆沟组高镁埃达克岩、高镁安山岩和高镁英安岩的矿物学和地球化学研究,发现这些火山岩不仅具有高镁-铬-镍-锶和低钇含量,还含有铬铁矿,斜方辉石斑晶具有核部低镁与幔部高镁的反环带,含有大量25亿年华北克拉通前寒武纪岩石特征的继承锆石,锶-钕同位素组成与徐淮下地壳榴辉岩包体一致.上述特征排除了兴隆沟组火山岩是下地壳部分熔融、含水上地幔部分熔融或俯冲洋壳部分熔融产物的可能性,而揭示了它们是三叠纪加厚的华北克拉通太古宙榴辉岩下地壳与岩石圈地幔一同拆沉再循环进入软流圈,随后榴辉岩部分熔融产生的熔体在上升喷发至地表过程中与地幔橄榄岩相互作用的结果.这一研究不仅为拆沉作用导致的下地壳再循环过程提供了重要证据,830中国科学D辑地球科学第35卷表4北京西山地区髫髻山组火山岩地球化学成分DT-05DT-06DT-09DT-11DT-12DT-15DT-16DT-17DT-19DT-24DT-25岩性高镁埃达克岩高镁安山岩高镁安山岩安山岩安山岩安山岩埃达克岩安山岩玄武岩高镁安山岩流纹岩SiO260.4560.2358.6156.6758.7160.8363.7255.9350.7553.9670.24TiO20.710.830.660.841.140.590.680.861.101.050.65Al2O316.6815.9317.8417.8717.1317.4514.9418.8117.2517.1613.29TFe2O35.936.876.246.876.995.655.907.299.218.324.07MnO00.140.080.100.000.06MgO2.833.232.732.602.422.151.542.805.275.401.27CaO4.224.013.684.934.684.004.895.665.017.012.59Na2O4.814.104.544.233.374.723.914.044.393.692.97K2O2.342.403.282.764.112.681.883.032.411.462.91P2O50.260.300.340.510.550.340.340.440.350.350.18LOI2.012.452.142.551.491.592.071.612.942.612.84总量100.35100.45100.1699.97100.67100.1099.95100.5998.81101.11101.07Na2O/K2O2.061.711.381.530.821.762.081.331.822.531.02Mg#48.6048.2346.4342.8540.6842.9834.0943.2153.1356.2538.20Li13.2117.1715.7617.8014.979.7012.1412.3029.2925.9511.61Be1.721.581.261.301.731.371.291.790.840.901.53Sc6.7011.002.404.928.453.815.926.3915.5414.607.94V111132931101367811711118716660Cr2633194124452211137Co2853512734274934313326Ni12158212445174711Cu2224202435192025253114Zn819489981158670116827741Ga21.0919.7418.1823.9624.2521.6821.2924.5618.7118.2813.21Rb3842596565414951662698Sr6248625489699699879421211531930361Y1214151619131214171521Zr159173178181263192179211109122293Nb10.2710.288.029.2213.679.287.6310.095.126.1910.94Cs0.430.830.551.130.680.751.451.034.020.775.84Ba12851483136811362042113797817278419361039La33.1936.4133.1435.9753.5437.6334.5441.9222.9027.4636.50Ce68.8477.3667.3379.47111.7778.9569.6290.6351.9259.4173.16Pr7.758.698.269.3112.558.698.2610.136.296.837.80Nd28.0532.2830.7335.8147.2731.4329.2037.8630.0531.3332.11Sm4.835.4015.134.616.295.615.385.40Eu1.481.651.671.892.281.581.451.991.751.681.24Gd3.744.224.064.625.653.883.684.664.954.875.17Tb0.540.610.590.660.800.550.540.660.670.610.69Dy2.713.053.033.403.982.832.453.263.302.883.56Ho0.470.520.530.580.660.480.410.540.610.520.70Er1.201.381.361.491.761.331.211.411.501.301.83Tm20.180.29Yb01.341.491.221.021.281.451.161.95Lu010.170.30Hf3.443.733.973.875.774.053.154.432.772.957.49Ta0.580.560.430.450.680.520.510.260.310.74Pb12.8613.706.5012.9613.3712.299.4211.317.137.3922.16Th4.974.922.742.714.643.491.993.391.592.029.17U40.631.010.930.480.740.440.481.78Sr/Y5262376151767987316217LaN/YbN19.5719.6618.6118.0924.2920.8222.9922.1310.6915.9412.63Eu/Eu*1.071.061.111.091.061.081.081.131.021.000.72aTFe2O3为全铁;Mg#=100Mg/(Mg+Fe(原子数;LOI为烧失量.主量和微量元素含量单位分别为10-2和10−6第9期袁洪林等:北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究831续表DT-26DT-28DT-31DT-32DT-33DT-34DT-35DT-36DT-37DT-38DT-39DT-40岩性高镁安山岩高镁安山岩玄武岩玄武岩安山岩埃达克岩安山岩高镁安山岩英安岩安山岩安山岩安山岩SiO253.2661.3151.5751.1558.1456.3457.3555.3066.1558.1858.0355.24TiO21.010.631.101.281.110.870.821.010.480.770.791.21Al2O316.4816.8917.5417.4816.6318.5117.9116.4516.7717.9118.1817.82TFe2O38.095.308.829.037.297.166.987.813.776.566.728.05MnO0.130.0020.080.110.080.09MgO4.812.555.614.962.192.642.504.811.322.292.293.14CaO6.753.407.896.544.814.835.166.052.405.255.614.30Na2O3.424.563.273.313.325.474.413.995.133.564.176.17K2O1.863.761.943.024.522.462.741.863.753.362.661.99P2O50.340.240.330.360.550.530.510.340.200.410.420.42LOI4.191.712.082.941.292.041.882.531.071.481.152.22总量100.34100.44100.27100.2299.95100.98100.38100.27101.1299.88100.10100.65Na2O/K2O1.841.211.691.100.732.221.612.151.371.061.573.10Mg#54.0848.8055.7552.1137.3142.2141.5154.9640.9640.8840.3043.59Li34.948.7615.5216.818.425.014.8310.745.909.058.959.19Be0.941.440.860.791.591.131.071.071.39Sc13.498.6617.7418.789.917.747.4614.164.557.378.0511.77V154821802161211111011524895100100Cr10417120491.01.62.736Co292436283227263329363632Ni44949263.92.02.68.6Cu201332382917172410151518Zn716180818481788551737284Ga17.9716.5520.4620.9419.8221.6520.2418.5617.1120.0619.3718.78Rb446625847237442763454326Sr85465989085510481152133389269015371269731Y131515202116171412151516Zr115176130121247172163143210181180176Nb5.789.726.145.8513.118.868.487.169.758.878.818.91Cs4.190.830.411.430.700.330.390.800.850.780.800.41Ba205414368799721368136012928221327149314381210La25.6937.5627.4524.4955.7038.7137.3031.0341.4839.1439.3738.92Ce54.1072.6259.1555.19101.1782.8078.4365.6178.9480.3881.7079.20Pr6.237.886.936.6612.489.338.897.347.948.859.069.16Nd28.5632.6331.7231.8653.0241.3339.4732.9031.0637.6938.5340.92Sm4.855.135.595.838.106.616.415.544.385.835.896.62Eu1.731.601.731.822.291.981.921.651.301.821.832.00Gd4.464.825.035.197.305.835.574.895.96Tb0.540.590.650.700.860.720.690.600.460.600.630.72Dy2.552.803.123.363.993.413.282.812.132.812.923.27Ho0.450.510.570.610.680.600.580.490.370.490.510.57Er1.121.341.431.521.751.561.511.251.011.291.331.42Tm0Yb1.021.331.341.451.621.521.451.131.04Lu8Hf2.765.563.963.803.384.844.064.094.12Ta0.290.610.320.310.660.440.420.360.600.450.460.44Pb7.9714.028.857.2714.079.3310.059.7615.5911.139.709.07Th1.935.862.312.054.872.912.752.375.393.153.083.16U0.451.360.560.461.010.660.630.511.150.630.650.66Sr/Y6644594350727864581028546LaN/YbN17.0219.1113.8311.4523.1817.2317.4118.4826.4121.4821.2021.13Eu/Eu*1.140.981.001.010.910.980.980.970.941.011.000.97832中国科学D辑地球科学第35卷表5北京西山地区东岭台组火山岩地球化学成分YHC-02YHC-06YHC-08YHC-10YHC-11YHC-12YHC-13YHC-14YHC-15YHC-17岩性流纹岩英安岩英安岩流纹岩流纹岩流纹岩流纹岩英安岩英安岩英安岩SiO276.3365.5365.8278.8378.2476.9170.7464.6565.2765.48TiO20.120.700.530.480.830.550.56Al2O312.7814.7516.9012.4512.4613.1415.3715.6317.0817.07TFe2O30.995.173.860.961.051.522.875.163.984.01MnO0.060.040.120.060.060.060.040.060.080.10MgO0.152.660.270.561.451.161.09CaO0.252.081.720.342.291.492.64Na2O2.881.825.593.623.394.065.095.036.275.35K2O5.113.523.424.904.664.644.863.883.313.25P2O50.010.010.040.04LOI1.053.721.240.690.640.810.791.731.261.12总量99.74100.14100.64101.98100.94101.80101.23100.95100.69100.91Na2O/K2O0.560.521.630.740.730.881.051.301.891.65Mg#23.0950.2938.1121.1523.1926.0327.8835.7636.6135.00Li15.2314.5126.0317.3420.3614.4826.18Be2.532.741.911.971.722.122.451.841.721.80Sc10.650.921.401.812.736.039.805.025.73V0.98111901316Cr481.0380.61.0Co511726Ni1.6221.30.1270.0730.6602.33412.0580.6480.688Cu5.4254.8134.55.7Zn37768633293969778579Ga20.4520.8619.7016.3415.4817.2820.9419.5619.3219.60Rb15914863156144117122726056Sr74301818607468203557750758Y18212523201928192122Zr178202265144164193502375261254Nb15.6315.5411.2121.9423.7523.8025.4218.9612.1311.14Cs2.341.270.820.670.362.221.46Ba1286231596891164791679126213841493La27.2848.9638.8243.1344.1040.7375.0959.3045.4045.90Ce60.1899.1279.7483.8888.3482.21141.10113.7491.1095.34Pr6.5210.759.9616.1512.5710.2110.86Nd21.9235.4236.5635.7035.8430.1666.3151.8845.1646.24Sm3.895.734.7610.338.047.387.32Eu20.280.270.602.291.682.102.11Gd3.344.874.825.475.154.268.676.936.266.43Tb0.510.740.700.740.670.571.100.810.780.83Dy2.924.143.873.783.323.035.313.843.844.04Ho0.570.800.740.840.740.731.110.770.820.75Er1.532.181.902.091.851.892.711.852.032.00Tm0.230.300.240.340.300.320.430.270.310.32Yb1.552.011.672.362.162.312.961.862.212.11Lu0.220.290.240.340.320.350.430.270.330.32Hf6.415.045.885.135.555.7711.408.876.195.75Ta0.231.010.601.491.481.311.261.080.590.57Pb17.2319.8013.4119.9915.3725.1023.3417.8813.3914.45Th11.499.403.3316.6116.9913.0711.6513.504.354.20U2.952.411.032.872.451.451.682.540.900.87Sr/Y414333447293634LaN/YbN11.8816.4815.7512.3713.8111.9117.1321.4913.9114.72Eu/Eu*0.170.691.010.740.690.940.94第9期袁洪林等:北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究833图8髫髻山组和东岭台组火山岩的原始地幔标准化微量元素分布图图9髫髻山组和东岭台组火山岩的稀土元素分布图834中国科学D辑地球科学第35卷表6北京西山地区髫髻山组和东岭台组火山岩Sr-Nd-Pb同位素组成髫髻山组东岭台组岩性DT-05DT-06DT-11DT-15DT-16DT-17YH-08埃达克岩安山岩安山岩安山岩埃达克岩安山岩英安岩147Sm/144Nd0.10040.10780.10210.09620.09750.09740.0986143Nd/144Nd0.5119040.5115050.5119680.5118150.5118330.5117600.5117742σ64277876TDM(Ga1.672.361.611.731.721.821816763354147Sm/144Nd(130Ma0.100460.107930.102180.096270.097540.097490.09866143Nd/144Nd(130Ma0.5118190.5114130.5118810.5117330.5117500.5116770.511690εNd−14.28−22.06−13.03−16.02−15.66−17.09−16.82εNd(130Ma−12.69−20.59−11.46−14.35−14.02−15.45−15.1987Rb/86Sr0.16980.14820.17550.13190.21610.11950.350587Sr/86Sr0.706020.705400.705900.705520.708140.708140.706252σ6114486387Rb/86Sr(130Ma0.17020.14850.17580.13210.21650.11970.351287Sr/86Sr(130Ma0.705710.705130.705570.705280.707740.707920.70560206Pb/204Pb16.61416.46616.13516.30816.2912σ0.0010.0040.0040.0010.002207Pb/204Pb15.21915.17915.08115.17815.1472σ0.0010.0040.0040.0020.002208Pb/204Pb36.70136.48936.08536.436.432σ0.0030.0090.0090.0040.005而且为中国东部中生代强烈岩浆活动和壳幔交换与大规模成矿作用提供了动力学背景,为华北克拉通岩石圈地幔减薄作用提供了时间制约.该研究还表明,虽然组成华北克拉通下地壳和岩石圈地幔拆沉作用的机制是否与古太平洋或Monolo-Okhotsk洋壳的俯冲作用有关,现有资料尚难以确定,但Sr-Nd同位素和锆石年龄研究证明下地壳加厚的机制与上述洋壳俯冲无关,而更可能与扬子克拉通向华北克拉通的俯冲有关[9].如前所述,李伍平等[18]曾获得北京西山髫髻山组下部三段底部粗安岩斜长石Ar-Ar坪年龄为148±3Ma,等时线年龄为147±3Ma.本研究获得髫髻山组上部(旋回3样品DT-37(图2的U-Pb锆石年龄为137.1±4.5Ma.这两个年龄应分别代表了北京西山髫髻山组标准剖面下部和上部的年龄,时代均为晚侏罗世.而上覆东岭台组火山岩年龄(130~134Ma为早白垩世.髫髻山组上部火山岩略老于东岭台组火山岩,但两者时代相差<5Ma,证明北京西山地区两组火山岩的喷发时间间隔很短.此外,本研究获得的北京西山地区髫髻山组上部火山岩的年龄比前述牛宝贵等[20]获得的河北承德盆地髫髻山组顶部火山岩的锆石U-PbSHRIMP年龄(163±6Ma年轻26Ma,表明髫髻山组火山岩的年龄可能存在明显的区域变化.埃达克岩一般被认为是俯冲洋壳部分熔融的产物[32~37].髫髻山组埃达克岩、安山岩和高镁埃达克岩与高镁安山岩表现出与兴隆沟组火山岩类似的地球化学特征.它们均具有典型的Nb-Ta负异常和Pb正异常和演化的Sr-Nd同位素组成等大陆地壳特征.本研究及李伍平等[18]获得的髫髻山组火山岩共8个样品(SiO2>56%的初始Sr-Nd同位素组成与SiO2之间没有明显相关性(相关系数为0.08~0.19,表明上述微量元素和同位素特征不是同化混染的结果,而代表了岩浆的原始特征.因此所研究火山岩是陆壳岩石部分熔融的产物,而不是俯冲洋壳(143Nd/144Nd=0.5125~0.5130,εNd=+5~+10;86Sr/87Sr=0.702~0.704[49]部分熔融的产物.高Sr,Yb和Y特征说明源第9期袁洪林等:北京西山晚中生代火山岩U-Pb锆石年代学及地球化学研究835区岩石含石榴石,应为榴辉岩或石榴角闪岩[50,51].髫髻山组安山岩、埃达克岩、英安岩及其高镁成员的Nb/Ta比值变化范围为16~20,平均为19.Foley等[51]的研究表明,角闪岩和石榴角闪岩的部分熔融产生的熔体以低的Nb/Ta比值(<17.5为特征,而榴辉岩的部分熔融则以较高的Nb/Ta比值(>17.5为特征.因此,髫髻山组火山岩的Nb/Ta比值表明,它们更可能是榴辉岩部分熔融的产物.高镁埃达克岩和高镁安山岩的存在表明,它们曾与地幔橄榄岩发生作用[9,52~54].扬子克拉通宁镇地区发现的早白垩纪埃达克质侵入岩也具有高镁特征,此外还具有富K的特征[55].与宁镇埃达克质侵入岩不同,髫髻山组埃达克质火山岩具有典型的富Na特征(Na2O/K2O>2.0.我们认为北京西山地区髫髻山组火山岩与辽西兴隆沟组火山岩上述相似的地球化学特征,说明它们具有相似的成因,即代表了早中生代曾存在于华北克拉通的加厚榴辉岩下地壳与岩石圈地幔一同拆沉至软流圈中,榴辉岩随后发生部分熔融,熔体上升过程中与地幔橄榄岩发生相互作用,然后喷发至地表的产物[9].兴隆沟组火山岩的SHRIMP锆石U-Pb年龄为159±3Ma,该年龄证明拆沉作用至少自159Ma已开始[9].而本研究获得的髫髻山组火山岩的年龄表明,拆沉作用至少延续至137Ma.东岭台组火山岩为Mg#正常的流纹岩和英安岩,表明它们没有与地幔发生反应,它们可能代表了拆沉作用导致的软流圈地幔物质上涌造成的深部地壳物质重融的产物.致谢野外采样和SHRIMP分析过程中分别得到李伍平和简平博士的帮助,张斌辉绘制了图1,在此表示感谢.参考文献1MenziesA,FanWM,ZhangM.PaleozoicandCenozoiclitho-probesandthelossof>120kmofArcheanlithosphere,Sino-Ko-reancraton,China.In:PrichardHM,AlabasterHM,HarrisT,etal,eds.MagmaticProcessesandPlateTectonics.London:GeolSocLondon,1993,71~812邓晋福,等.中国东部岩石圈根/去根作用与大陆“活化”-东亚型大陆动力学模式研究计划.现代地质,1994,8:349~3553吴福元,孙德有,张广良,等.论燕山运动的深部地球动力学本质.高校地质学报,2000,6:379~3884吴福元,葛文春,孙德有,等.中国东部岩石圈减薄研究中的几个问题.地学前缘,2003,10:51~595郑建平.中国东部地幔置换作用与中新生代岩石圈减薄.北京:中国地质大学出版社,19996翟明国,樊祺诚.华北克拉通中生代下地壳置换:非造山过程的壳幔交换.岩石学报,2002,18:1~87GaoS,ZhangBR,JinZM,etal.Howmaficisthelowercontinentalcrust?.EarthPlanetSciLett,1998,106:101~1178GaoS,RudnickRL,CarlsonRW,etal.Re-Osevidenceforre-placementofancientmantlelithospherebeneaththeNorthChinaCraton.EarthPlanetSciLett,2002,198:307~3229GaoS,RudnickRL,YuanHL,etal.RecyclinglowercontinentalcrustintheNorthChinacraton.Nature,2004,432:892~89710GriffinWL,ZhangAD,O’ReillySY,etal.Phanerozoicevolu-tionofthelithospherebeneaththeSino-KoreanCraton.In:FlowerMFJ,ChungSL,LoCH,etal.,eds.MantleDynamicsandPlateInteractionsinEastAsia.WashingtonDC:AmericanGeophysicalUnion,1998,107~12611MenziesA,XuY.GeodynamicsoftheNorthChinacraton.In:MantleDynamicsandPlateInteractionsinEastAsia.In:FlowerMFJ,ChungSL,LoCH,etal.,eds.MantleDynamicsandPlateInteractionsinEastAsia.WashingtonDC:AmericanGeophysicalUnion,1998,155~16512XuYG.Thermo-TectonicDestructionoftheArchaeanLitho-sphericKeelBeneaththeSino-KoreanCratoninChina:Evidence,TimingandMechanism.PhysChemEarth(A,2001,26:747~75713DavisGA,ZhengYD,WangC,etal.Mesozoictectonicevolu-tionoftheYanshanfoldandthrustbelt,withemphasisonHebeiandLiaoningprovinces,northernChina.GeologicalSocietyofAmericaMemoir,2001,194:171~19714ZhouXH,SunM,ZhangGH,etal.Continentalcrustandlitho-sphericmantleinteractionbeneathNorthChina:isotopicevidencefromgranulitexenolithsinHannuoba,Sino-Koreancraton.Lithos,2002,62:111~12415WildeSA,ZhouXH,NemchinAA,eta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