【作者】 柳小明；

【导师】 高山；

【作者基本信息】 西北大学 ， 地球化学， 2004， 博士

【摘要】 华北克拉通是世界上最古老的太古宙克拉通之一。在中生代时期，该古老克拉通的下地壳和岩石圈地幔拆沉进入了软流圈地幔，产生了壳-幔之间的物质交换。本文试图从辽西中生代火山岩和龙岗地幔包体的地球化学特征揭示这一过程发生的机制和作用时限。由于准确的分析数据是地球化学问题研究的基础，因此本文第一章对地质样品的处理和分析方法进行了研究。 1．地质样品的处理和分析方法研究 选用纯净的石英作为研究对象，分别采用玛瑙和碳化钨钵体进行样品的细碎，通过ICP-MS对所碎样品和未经细碎样品的对比分析显示，除WC钵体会对样品产生W、C和Co的严重污染外，对所测定的其它元素均未造成明显的污染。 采用38个标准参考物质建立的XRF主量元素分析方法对BCR-2、GSR-1和GSR-3的测定显示对氧化物含量小于0.5％的元素测定平均值与推荐值的相对误差小于5％，MgO和Na₂O的误差小于3％；其它氧化物的误差一般小于1％。用相同标准参考物质建立的部分微量元素分析方法对GSR-9的测定结果显示Ni、Cu、Pb、Zn和Nb的相对误差大于10％，V和Cr在7％左右，所测其它元素均小于5％。 建立的ICP-MS溶液方法在近两年的时间内对USGS的AGV-1和BHVO-1标准参考样品的近40次分析结果表明对所测定的元素平均值与推荐值之间的相对误差一般小于10％，大部分元素小于5％。过渡族元素、轻质量元素和含量相对较低的元素分析误差相对较大。采用NlsT610作外标建立的LA一ICP一MS微量元素原位分析方法对BCR一ZG、BIR一IG、BHVO一ZG和AGV一ZG的测定结果显示，所测的4个主量元素和38个微量元素的相对标准偏差和平均值与参考值之间的相对误差一般小于10%。分析的精密度和准确度可以与溶液方法的分析媲美。 采用标准错石91500作外标建立的LA一ICP一MS错石U一Pb定年方法对标准错石几MoRAI的测定所获得的206Pb/z3 su加权平均年龄为417.肚l.3Ma。该错石的In一TD吐s206Pb/2，su年龄为416.75士o.4sMa。二者年龄数值非常一致。 XRf对辽西火山岩样品中微量元素Rb、sr、Y、Zr、Nb和Ba含量的测定值与ICP一MS的测定值之间的相对偏差一般小于10%。在参加三次GcoPT测试的48种元素中，只有在GcoPTll中的Cu和sc两个元素的Z值超出了+2，其它所测元素的Z值均在+2和一2之间，且绝大多数元素的Z值在+l和一1之间。2.辽西地区中生代中性一中酸性火山岩地球化学特征及其对下地壳拆沉作用的示踪 辽西中生代义县组和兴隆沟组中的中性一中酸性火山岩分布于华北克拉通东部辽西义县一北票地区相互孤立的盆地中，这些火山岩中的高镁伽酬>54)岩石具有较高的sr（477一1 618林9/9）及较低的Y（9一17林g/g）和Yb（0.98一1.65林g/g）含量，SrfY值为32一119，亏损HREE伍aN/YbN为12一31)，在蜘蛛图上相对于La出现Nb一Ta的负异常。特别是一些样品中的OPX斑晶在核部具有低Mg、Cr和Ni，高Zr、Sr和Ga的特征，而在慢部恰恰相反。火山岩的Nd同位素模式年龄TnM达1 .SGa，火山岩中的继承错石具有2500、1800、780、420、310、230、160和130Ma的年龄值。初始吕6Sr/s7S:与Rb含量之间不存在相关性。由以上事实推断辽西中生代义县组和兴隆沟组中的中性一中酸性火山岩是由增厚的华北克拉通下地壳榴辉岩拆沉进入地慢后发生部分熔融，并在上升过程中与地慢橄榄岩发生作用形成的。加厚榴辉岩下地壳的形成与23OMa左右扬子克拉通向华北克拉通俯冲形成世界上最大的秦岭一大别一苏鲁造山带超高压变质带有关。火山岩中最年轻的错石年龄限定了下地壳的拆沉作用可能发生157一125Ma之间。3.龙岗地慢包体的地球化学特征及其对地慢交代作用的示踪龙岗地慢包体的主量元素含量变化范围较大，5102%为41 .13一45.24、Mgo%为37.99一47，77、Cao%为0.55一2.78和Feo%为7.90一9.16，反映这些包体既有接近于原始地慢的二辉橄榄，又有高度亏损的方辉橄榄岩。球粒陨石标准化模式有轻稀土富集型、轻稀土亏损型、U型和平坦型四种分布形式，显示这些地慢包体经历了不同程度的交代作用改造。依据Pb的异常，可以推断交代物质可能为硅酸盐熔体，且熔体具有较高的Pb和较低的Y含量以及轻稀土富集的特征。与汉诺坝二辉橄榄岩包体的交代物质相似，交代熔体可能来自中生代拆沉榴辉岩质下地壳的部分熔融。更多还原

【Abstract】 The North China craton is one of the world’s oldest Archaean cratons, its lower crust and lithospheric mantle delaminated into asthenospheric mantle during the Mesozoic, which caused the material exchange between mantle and crust. The geochemicl characteristics of western Liaoning Cenozoic lavas and Longgan mantle xenoliths reveal the occurrence and the period of this process in this paper. Because the accurate analytical data are the basis of geological research, the proper sample preparations and analysing methods for geological samples are discussed in the first chapter.1. The research on the analytical method and the preparation ofgeological samplesThe pure quartz grains were grinded in both agate and tungsten carbide mortars and were then analysed by ICP-MS. The analytical results of grinded pure quartz grain compared with those of ungrinded show that tungsten carbide and agate mortars have no apparent contamination to geological samples for measured trace elements except for W, C and Co in tungsten carbide mortars.Major element concentrations of BCR-2, GSR-1 and GSR-3 were determined as unknown samples by XRF method that was established by using 38 national reference samples. The obtained values usually agree with recommended values within 1% relative, 5% relative for the oxide concentrations less than 0.5% and 3% relative for MgO and Na2O. Some trace element concentrations of GSR-9 were determined as unknown sample by XRF method that was also set up by using 38 national reference samples. The obtained values agree with recommended values within 10% relativeexcept for Ni, Cu, Zn, Pb and Nb.Trace element concentrations of USGS reference rokcs AGV-1 and BHVO-1 have been determined nearly 40 times within two years by ICP-MS solution method as unknown samples. The relative errors of obtained values from recommended values were less than 10%, most of which less than 5% except for transition elements, lighter mass elements and lower concentration elements.4 major and 38 trace elements of USGS glass reference materials BCR-2G, BIR-1G, BHVO-2G and AGV-2G were determined as unknown samples by LA-ICP-MS. The calibration was performed using NIST 610. The relative standard deviations and relative errors between obtained and recommended values were less than 10% for most elements, which were comparable with ICP-MS solution method.The weighted mean 206Pb/238U age of standard zircon TEMORA 1 determined by LA-ICP-MS method using standard zircon 91500 as external caliberate is 417.6+1.3Ma, which agrees with the age 416.75+0.48Ma determined by ID-TIMS method.The concentrations of trace elements Rb, Sr, Y, Z, Nb and Ba in Liaoning lavas determined by XRF and ICP-MS solution methods. The results obtained by XRF agree with those measured by ICP-MS within 10% relative for most elements. The Z-score for all 48 elements measued in GeoPTIO, GeoPTll and GeoPTIS respectively are between -2 and +2 except for Cu and Sc in GeoPT11. Most of them are in fact between +1 and -1.2. Characteristics of Mesozoic Liaoning intermediate-acidicvolcanic rocks and their tracing for delamination of lower crustMesozoic Liaoning intermediate-acidic volcanic rocks in Yixian and Xinglonggou Formations occur in isolated continental basins in the Yixian-Beipiao area of western Liaoning Province in the Eastern block of the North China craton. The features of the high-Mg volcanic rocks from these two formations include higher Sr (477 to 1618ug/g), lower Y (9-17ug/g) and Yb(0.98-1.65ug/g), depletion in HREE (LaN/YbN= 12-31), higher Sr/Y (32-119) ratio and Nb-Ta depletion relative to La inprimitive mantle normalized spider diagram. A striking feature of some lavas is the presence of zoned orthopyroxene phenocrysts which have lower Mg, Cr and Ni and higher Zr, Sr and Ga cores and just reversed mantles. The Nd isotopic model ages (TDM) of these lavas are as old as 1.8 Ga. The inherited zircon age populations include 2500, 1800, 780, 420, 310, 230, 160 and 130Ma and there is no correlation between initial 86Sr/87Sr ratio and Rb conte更多还原