【作者】 刘盛遨；

【导师】 李曙光； 滕方振；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2011， 博士

【摘要】 本博士论文对中国中东部四个不同地区(华北东南缘蚌埠地区、徐-淮地区,郯庐断裂带南段,长江中下游)中生代埃达克质岩石进行了系统的元素地球化学和放射成因(Sr-Nd-Pb-Hf)以及稳定同位素(氧)地球化学研究,以探讨其岩石成因和地质意义。主要研究目标是：(1)通过对蚌埠地区中生代低镁埃达克质岩石进行系统的年代学和地球化学研究,并结合普通花岗岩和辉绿岩的年代学和地球化学研究,探讨岩石圈减薄的机制和过程以及EM-1型基性岩石的成因；(2)通过对郯庐断裂带南段中生代高镁埃达克质岩石的年代学和地球化学研究,探讨早白垩纪郯庐断裂带的活动在诱发断裂带两侧加厚下地壳拆沉方面的作用；(3)通过对郯庐断裂带不成矿高镁埃达克质岩和长江中下游成矿高镁埃达克质岩全面的地球化学对比研究,探讨长江中下游埃达克岩成因和埃达克岩成矿机理；(4)通过对徐-淮地区中生代高镁埃达克质岩石及其所含深源包体和五河群变质岩进行全面的地球化学研究,探讨其岩石成因和华北克拉通东南缘古老下地壳高放射成因Pb的成因。作为《联合培养博士研究生》出国访问研究的一部分,开展了系统的岩浆分异过程中Mg同位素分馏研究,主要包括：(5)对花岗质岩石及其所含镁铁质单矿物进行了系统的Mg同位素地球化学研究,探讨高温下Mg同位素在花岗岩结晶分异过程中的分馏行为以及探索Mg同位素在示踪地壳再循环等方面的应用前景；(6)对一套具有较大温度变化的橄榄岩包体中镁铁质单矿物进行了系统的Mg同位素地球化学研究,探讨高温下Mg同位素分馏机制及其潜在的地球化学应用；(7)分析了上述中国中东部四个地区中生代埃达克质岩石的镁同位素组成,探讨Mg同位素能否应用于埃达克岩的判别指标以及能否应用于区分高、低镁埃达克质岩石的成因。主要研究内容和成果如下：1.华北东南缘蚌埠地区低镁埃达克质岩成因及其对岩石圈减薄的意义蚌埠地区广泛出露中酸性侵入岩(花岗闪长岩和花岗岩)和基性岩,本文获得其中花岗闪长岩的锆石U-Pb年龄为118-115Ma,早于该区普通的花岗岩的锆石U-Pb年龄112Ma。与普通花岗岩相比,蚌埠花岗闪长岩具有高的Sr、低的Y含量,而且具有彼此呈正相关关系的高Sr/Y,(La/Yb)N,(Dy/Yb)N,具有低镁(Mg#<48)埃达克岩的地区化学特征。这些低镁埃达克质岩具有与古老下地壳相似的Sr和Nd同位素组成,87Sr/86Sr(i)=0.70752 to 0.70926,εNd(t)=-21.8 to-18.5,和低的放射成因Pb同位素组成(e.g.,206Pb/204Pb(t)<17.0),以及与地幔相似的锆石氧同位素组成。此外,它们含有大量～1.8和～2.5Ga的继承锆石。这些特征表明蚌埠低镁埃达克质岩来自加厚华北下地壳的部分熔融,而普通花岗岩起源于较浅部地壳的部分熔融。因此,华北克拉通东南缘在中白垩世(～115Ma)以前仍存在加厚地壳,但加厚地壳在115-112Ma的较短时间内发生了显著的减薄。本文同时对蚌埠地区辉绿岩脉进行了研究,其形成时代为112Ma(全岩Ar-Ar年龄),晚于低镁埃达克质岩,而与普通花岗岩一致。其具有典型EM-1似的Sr-Nd-Pb同位素特征、低的Nb/U和Ce/Pb比值、富集轻稀土、亏损重稀土、亏损高场强元素,为拆沉的古老下地壳和上涌软流圈地幔混合部分熔融的产物。但蚌埠地区辉绿岩Pb同位素组成较大别山早白垩世辉绿岩和角闪辉长岩有显著区别,在给定206pb/204pb时,前者较后者有较低的208pb/204pb值,这一特点与华北中生代镁铁质侵入体类似。这表明混染蚌埠地区中生代辉绿岩源区的拆沉下地壳是华北加厚下地壳,与该区低Mg埃达克质岩给出的结论一致。根据低镁埃达克质岩、普通花岗岩和辉绿岩的时空分布特征,综合文献报道的年龄,本文提出华北克拉通东南缘加厚下地壳在130-115Ma发生部分熔融,熔融残留相(榴辉岩)由于长英质熔体的提取从而密度进一步增加发生拆沉进入地幔,并与上涌软流圈地幔发生混合熔融产生具EM-1同位素特征和富集大离子亲石元素的基性岩浆,同时诱发已减薄的地壳发生部分熔融产生普通花岗岩。本文同时提出加厚下地壳的熔融是导致加厚下地壳进一步重力不稳定和拆沉的重要机制,拆沉下地壳进入地幔与地幔混合是产生具有典型下地壳特征的内陆基性岩浆的重要方式。2.郯庐断裂带高镁埃达克质岩的成因及其对下地壳拆沉机制的制约本文在郯庐断裂带南段识别四个高镁埃达克质岩体,它们分别是小李庄、大毛村、方蒋庄和桥头集,其形成时代分别为125、128、129、130Ma,并具有远离断裂带年龄逐渐变年轻的趋势。这些岩石具有高的Sr含量(743-1040ppm)和Sr/Y比值(43-75),低Y(10.1-18.1ppm)和Yb含量(0.90-1.48ppm),高Mg#(50-67)和MgO(2.0-6.1wt.%),高K20/Na2O(O.8-1.1),和与古老下地壳相似的同位素特征：87Sr/86Sr(i)=0.70569-0.70696,εNd(t)=-17.4 to-11.4,206Pb/204Pb(i)=16.258-16.384, 207Pb/204Pb(i)=15.343-15.398,208Pb/204Pb(i)=36.564-36.900.这些特征表明郯庐南段高镁埃达克质岩可能起源于拆沉下地壳的部分熔融,并经历了与地幔相互作用。本文识别的四个高镁埃达克质岩体与前人在大别山造山带东缘识别的三个高镁埃达克质岩体一起,构成了一个沿郯庐断裂带分布的高镁埃达克质岩带。这表明郯庐断裂带在早白垩纪的大规模活动可能在诱发加厚下地壳拆沉方面起到关键作用。本文发现的远离断裂带高镁埃达克质岩石年龄变年轻的趋势进一步支持该动力学机制。3.长江中下游埃达克质岩石及其金属成矿意义长江中下游是中国东部最重要的成矿带之一,其成矿斑岩一般为埃达克质岩石。前人多指出这些成矿埃达克质岩为拆沉下地壳成因,并强调拆沉下地壳来源的埃达克岩也能成矿。本文对长江中下游2个代表性成矿地区(安庆和铜陵)埃达克质岩石进行了详细的地球化学研究。此外,本文选取郯庐断裂带南段不成矿高镁埃达克质岩石作为对比对象,并收集所有长江中下游成矿埃达克质岩地球化学数据,对长江中下游和郯庐断裂带高镁埃达克岩进行了全面的地球化学对比研究,发现两者存在如下重要地球化学差异：(1)长江中下游埃达克岩富Na和Al,而郯庐断裂带埃达克质岩相对富K贫Al；(2)在一定的SiO2含量下长江中下游埃达克岩具有与俯冲洋壳成因埃达克岩相似Mg#和MgO含量,而郯庐断裂带埃达克岩具有更高的Mg#和MgO含量；(3)长江中下游埃达克岩具有相对低的(La/Yb)N但高的Sr/Y,而郯庐断裂带埃达克质岩具有高的且相互呈正相关的(La/Yb)N和Sr/Y;(4)长江中下游埃达克岩具有比郯庐断裂带埃达克质岩显著更高的Ce/Pb和Sr/La,后者与下地壳一致,前者与MORB接近；(5)长江中下游埃达克岩具有EM-2型Sr-Nd同位素组成,暗示岩浆源区可能有沉积物的加入,而郯庐断裂带埃达克岩具EM-1型同位素特征；(6)长江中下游埃达克岩具高度放射成因Pb,如206Pb/204Pb(t)达到18.8,而郯庐断裂带埃达克质岩具低放射成因Pb,如206Pb/204Pb<16.4;(7)长江中下游埃达克岩具有高度变化的且相对于地幔更重的氧同位素组成,支持岩浆源区有沉积物的加入,而郯庐断裂带埃达克岩具均一的且与地幔接近的氧同位素组成。根据这些地球化学差异,本文提出长江中下游埃达克岩起源于俯冲洋壳的部分熔融,不支持前人(如Wang et al.,2004a,2004b,2006,2007a)提出的拆沉下地壳成因。一般认为,俯冲带由于携带有大量的沉积物,因此具有高的氧逸度。而大陆下地壳是相对干的,因此一般具有低氧逸度特点。因此,尽管拆沉大陆下地壳熔融的熔体和俯冲洋壳部分熔融的熔体均与地幔发生了相互作用,但前者不能氧化地幔的硫化物,而后者因具有高的氧逸度能够氧化并分解地幔硫化物,从而使Cu、Au等亲硫元素能够进入熔体,随后富集成矿。这很好地解释了为什么郯庐断裂带高镁埃达克质岩均不成矿,而长江中下游埃达克岩成矿。我们对这两个埃达克质岩系列进行锆石原位微量元素分析发现长江中下游埃达克质岩具有显著更高Ce4+/Ce3+和Eu3+/Eu2+,证实前者确实具有更高的氧逸度。4。华北东南缘徐-淮地区高镁埃达克质岩成因及该区下地壳演化前人认为徐淮地区高镁埃达克质岩是华北大陆下地壳拆沉的产物,其携带的石榴辉石岩包体可代表其源区。本文研究发现徐淮地区高镁埃达克质岩虽然具有高的Sr/Y和(Dy/Yb)N,但它们具有相对较低的(La/Yb)N比值,而且富Na贫K,这些特征与拆沉下地壳来源的高镁埃达克质岩(例如郯庐断裂带南段)不一致,而与洋壳俯冲成因埃达克岩和长江中下游的埃达克质岩石相似。俯冲洋壳LREE亏损特征可导致熔体具有相对低的(La/Yb)N,但低温洋壳蚀变可提高其Sr/Y。此外,徐淮地区高镁埃达克质岩与镁铁质包体的Sr-Nd-Pb同位素组成并不完全相同：高镁埃达克质岩的Sr-Nd同位素具正相关关系,而镁铁质包体表现为从地幔排列向下地壳延伸的趋势。高镁埃达克质岩的高(87Sr/86Sr)i和高εNd(t)值端元落入大陆边缘岛弧区,无法单纯用下地壳源区解释。尽管徐淮地区高镁埃达克质岩与镁铁质包体均具有高放射成因Pb,但包体的Pb同位素比值系统偏高,高镁埃达克质岩落在由包体和五河群变质岩确定的Pb-Pb等时线之下并落入MORB区。这显然与前人(Xu et al.,2006)用包体代表寄主岩石的下地壳源区的解释相矛盾。此外,徐淮地区高镁埃达克质岩含有大量镁铁质包体,下地壳拆沉也不能携带源区的岩石上升至地表。而且,徐淮镁铁质包体均经历了强烈的角闪岩相退变质叠加,且不像是寄主岩浆快速抬升过程中降压分解的结果,因此包体不能代表寄主岩浆的源区岩石(榴辉岩)。根据上述已得到的观测数据,本文提出徐淮地区高镁埃达克质岩可能是俯冲洋壳的部分熔融成因,但岩浆在侵入华北克拉通古老地壳时受到中下地壳的显著污染。徐-淮埃达克质岩携带的石榴辉石岩/石榴角闪岩包体和蚌埠地区五河群石榴-辉石-角闪岩透镜体以及大理岩围岩构成了一条年龄为1.95Ga的Pb-Pb等时线,与它们锆石U-Pb年代学给出的变质时代吻合,暗示早元古代麻粒岩相变质作用导致华北东南缘下地壳大规模Pb同位素均一化。五河群中表壳物质(如大理岩,杂砂岩)的加入可能是导致华北东南缘古老下地壳普遍具有高放射成因Pb的主要原因。此外,本文的发现还指出使用古老下地壳平衡硅酸盐地球的Pb同位素组成需要考虑这些具有高放射成因Pb的古老的下地壳岩石。5.花岗岩及镁铁质矿物Mg同位素地球化学研究本文首次系统地测定了花岗质岩石(花岗岩和花岗闪长岩)及其所含共生的镁铁质矿物(黑云母和角闪石)的Mg同位素组成,发现这些具有较大组成(化学组成和矿物丰度)变化的花岗质岩石在现有分析精度(±0.07‰;2SD)下具有相似的Mg同位素组成(δ26Mg=-0.26 to-0.12‰;n=11)。这一发现表明花岗质岩浆结晶分异过程中没有显著的Mg同位素分馏。共生的黑云母和角闪石在现有分析精度下具有相似的Mg同位素组成(△26MgHbl-Bt=-0.06±0.08‰；n=7),进一步表明花岗岩结晶分异过程中主要含Mg矿物黑云母或角闪石发生分异没有显著改变岩浆的Mg同位素分馏,与全岩分析得到的结论一致,因此花岗质岩石的全岩Mg同位素组成(如果排除了侵入过程中围岩混染和岩浆侵入后地壳风化的影响)可以示踪源区组成而无需考虑分离结晶的影响。这为我们尝试用Mg同位素示踪埃达克质岩石源区奠定了基础。共生黑云母和角闪石之间有限的Mg同位素分馏也与根据Mg-0键长理论预测的结果相符：Mg原子在黑云母和角闪石中均为6配位,因此两者在高温下不会产生显著的平衡同位素分馏。6.橄榄岩及其镁铁质矿物Mg同位素研究前人对地幔橄榄岩包体及其所含镁铁质矿物进行过大量的Mg同位素研究。然而,针对辉石-橄榄石之间是否有可检测到的Mg同位素分馏、这种分馏是否达到平衡、以及尖晶石与橄榄石之间是否有显著的分异存在较大争议。为了解决这些争议,本文系统分析了华北克拉通新生代玄武岩中一套形成温度变化大于300℃的橄榄岩/辉石岩包体中橄榄岩、斜方辉石、单斜辉石、尖晶石的Mg同位素组成,并首次分析了金云母的Mg同位素组成。分析结果发现所有矿物之间的Mg同位素分馏均与温度有非常好的相关性,表明这些同位素分馏均达到了平衡,其Mg同位素组成有如下顺序：尖晶石>>金云母>单斜辉石>斜方辉石=橄榄石。尤其以尖晶石和橄榄石之间的分馏最为显著(△26Mgspl-Ol=+0.25 to+0.55‰;n=10),这与理论预测的Mg在尖晶石中是4配位而在橄榄石中是6配位一致。因此,显著的且平衡的尖晶石-橄榄石Mg同位素分馏暗示其能够被广泛应用于地质温度计。7.中国中东部中生代埃达克质岩石Mg同位素研究在证明花岗岩结晶分异过程中没有Mg同位素分馏基础上,本文对中国中东部中生代埃达克质岩开展了系统的Mg同位素研究,包括长江中下游高镁埃达克质岩、大别山低镁埃达克质岩、郯庐断裂带南段高镁埃达克质岩、以及徐-淮地区高镁埃达克质岩。结果发现除了一个徐-淮埃达克岩样品具有相对轻的Mg同位素组成外,所有分析的样品均具有与地幔相似的Mg同位素组成。该发现表明大别山的加厚山根下地壳主要是源于地幔的底垫镁铁质下地壳且较少受陆壳表壳岩混染,同时还表明拆沉下地壳/俯冲洋壳部分熔融的低镁埃达克质熔体与地幔反应过程中尽管Mg#和MgO显著提高,但Mg同位素变化没有发生改变,这与镁铁质下地壳、俯冲洋壳(MORB)以及OIB自身具有与地幔相似的Mg同位素组成一致。因此,Mg同位素不能有效地应用于区分高、低镁埃达克质岩和判断各种埃达克质岩的成因。长江中下游埃达克岩均一的且与地幔相似的Mg同位素组成进一步表明尽管Sr-Nd-Pb同位素揭示其岩浆源区有大量沉积物(具有比地幔重的Mg同位素组成)的加入,但在与地幔橄榄岩发生反应过程中熔体的Mg同位素组成将与地幔达到充分的交换平衡。具有轻的Mg同位素组成的徐-淮样品也具有高CO2含量,可能是源区有碳酸盐加入的结果。更多还原

【Abstract】 One objective of this dissertation is to constrain the petrogenesis and explore geological implications of adakitic rocks through a combined chronoglocial and geochemical study of Mesozoic adakitic rocks from four different regions in eastern-central China. The major research subjects include (1) chronological and geochemical investigations of low-Mg adakitic rocks and associated normal granites and mafic dyke in the Bengbu area, to study the processes and mechanisms of lithospheric thinning of the North China Craton (NCC) and generation of intraplate EMI-type basaltic magmas; (2) chronological and geochemical studies of high-Mg adakitic rocks along the south Tan-Lu fault zone (STLF), to explore the potential role of movement of Tan-Lu fault in triggering delamination of the dense LCC; (3) comprehensive geochemical comparisons between high-Mg adakites from the STLF and the Lower Yangtze River Belt (LYRB), to elucidate the petrogenesis of the LYRB adakites and explore their implications for Cu-Au mineralization; (4) geochemical studies of Xu-Huai high-Mg adakitic rocks and entrained mafic xenoliths as well as the Wuhe group metamorphic rocks in the Bengbu area, to constrain generation of the adakitic rocks and origins of the highly radiogenic Pb isotopic compositions of the ancient lower continental crust (LCC) in the Southeastern NCC.The other objective is to study high-temperature inter-mineral Mg isotope fractionation during igneous differentiation. The major subjects include (5) studies of granitoids and their associated mafic minerals, to investigate the behavior of Mg isotopes during granite differentiation at high temperatures and explore the potential application of Mg isotopes to trace granite source; (6) systematic studies of peridotite xenoiths formed over a wide temperature range of 300℃and their associated mafic minerals, to investigate inter-mineral Mg isotope fractionations among peridotite xenoliths; and (7) analysis of Mg isotopic compositions of adakitic rocks from central-eastern China, to explore if Mg isotopes can be used to discriminate adakites with different origins and discriminate high-Mg adakites from low-Mg adakites.The main results related to this thesis are given in details in the following.1. Low-Mg adakitic rocks from the Bengbu area in the North China Craton: Implication for lithospheric thinningFelsic (granodiorite and granite) and mafic rocks widely occur in the Bengbu area. The granodiorites have zircon U-Pb ages of 118-115Ma, significantly older than zircon U-Pb age of the granites (112Ma). The granodiorites also contain～2.5 and～1.8 Ga inherited zircons. Compared to the granites, the granodiorites have higher Sr contents, lower Y contents, coupledly higher Sr/Y, (Dy/Yb)N and (La/Yb)N, which are similar to low-Mg adakitic rocks (Mg#<48). In addition, the granodiorites have high initial 87Sr/86Sr (0.70752 to 0.70926), lowεNd(t) (21.8 to-18.5), low radiogenic Pb (e.g.,206Pb/204Pb(t)< 17.0), as well as the mantle-likeδ18Mg value. These features are typical of the ancient LCC. The granodiorites were likely derived from partial melting of the thickened LCC of the NCC, while the normal granites were derived from a shallower crustal source. Therefore, there was still a thick LCC in the southeastern NCC before-115Ma, but the thick LCC was suddenly removed between 115 and 112Ma.Besides, the thesis studied diabase dykes in the Bengbu area. The diabases have Ar-Ar age of 112Ma that is younger than the granodiorites but similar to the granites. The Bengbu diabases are characterized by enriched light Rare Earth Elements (e.g., LaN/YbN>10), depleted Nb, Ta and Ti, and a typical EMI-like Sr-Nd-Pb isotopic signature (initial 87Sr/86Sr=0.70641 to 0.70816,εNd(t)=-6.1 to-5.8,206Pb/204Pb= 16.78 to 17.26), indicating an important contribution of the ancient LCC in their mantle source. These features, together with low Ce/Pb and Nb/U and high Ba/Nb and La/Nb, suggest derivation of the Bengbu diabases from partial melting of upwelling asthenosphere mantle, which was hybridized by the delaminated ancient LCC. However, Pb isotopes of the Bengbu diabases are different from the early Cretaceous mafic rocks in the Dabie orogen due to their lower 208Pb/204Pb at for a given 206Pb/204Pb, and instead, they are similar to Mesozoic mafic rocks from the NCC. In particular, the Pb isotopic features are similar to those of the Bengbu low-Mg adakites. Based on spatial-temporal distributions of low-Mg adakitic rocks, normal granites and mafic dykes in the Bengbu areas, together with previously published chronological data, we propose that partial melting of the pre-existing thickened LCC in the Bengbu area at～130-115Ma might have weakned the LCC and left a denser residue, which was subsequently delaminated into the mantle at some point between 115-112Ma, resulting in removal of the thick lithospheric keel. The lithospheric delamination induced mantle upwelling and melting, in conjunction with extensive crustal melting, to produce basaltic and granitic magmas at～112Ma. In addition, we propose that previous melting of the LCC is an important mechanism that weakened the LCC and caused gravitational instability of the LCC. Foundering of the dense LCC into the mantle is likely a crucial way to generate intraplate basaltic magmatism with a typical LCC affinity.2. High-Mg adakitic rocks from the South Tan-Lu fault zone: Implications for foundering mechanism of the lower continental crustWe newly found four high-Mg adakitic plutons from the South part of the Tan-Lu fault zone (STLF), including Xiaolizhuang, Damaochun, Fangjiangzhuang and Qiaotouji. These plutons were formed at 125,128,129 and 131Ma respectively, and become younger with the distance to the Tan-Lu fault zone. They have high Sr (743-1040ppm) and Sr/Y (43-75), low Y (10.1-18.1ppm) and Yb (0.90-1.48ppm), high Mg# (50-67), MgO (2.0-6.1wt.%) and K20/Na20 (0.8-1.0). The initial Sr-Nd-Pb isotopic compositions show an ancient LCC affinity, with 87Sr/86Sr(i)=0.70569-0.70696,εNd(t)=-17.4 to-11.4,206Pb/204Pb(i)=16.258-16.384,207Pb/204Pb(i)= 15.343-15.398,208Pb/204Pb(i)=36.564-36.900.The geochemical features of the STLF high-Mg adakitic rocks suggest that they were likely derived from partial melting of the delaminated lower continental crust, followed by interaction with the mantle. The four high-Mg adakitic plutons reported here, along with three early Cretaceous high-Mg adakitic plutons from the Dabie orogen reported in previous studies, consistute a high-Mg adakitic rock belt that was closely adjacent to the Tan-Lu fault. This strongly suggests that the movement of the Tan-Lu fault during early Cretaceous might play an important role in triggering delamination of the eclogitic LCC along the fault system. This is further supported by the geographic distribution and age variation among the STLF high-Mg adakites identified here.3. High-Mg adakitic rocks from the Lower Yangtze River Belt:Implications for Cu-Au mineralizationThe Lower Yangtze River Belt (LYRB) in central-eastern China is one of important metallogenic belts in China. The host rocks are generally adakitic rocks. Previous studies concluded that the LYRB adakites were derived from partial melting of delaminated LCC, and proposed that delaminated LCC-derived adakitic rocks can also generate Cu-Au mineralization. This thesis studied adakitic rocks from two representative regions in the LYRB, collected all published data for adakitic rocks from the LYRB, and presented a comprehensive geochemical comparison between the LYRB adakites and the ore-barren high-Mg adakitic rocks from the STLF. We found distinct differences in geochemical compositions between these two groups of adakitic rocks:(1) the LYRB adakites are enriched in Na and Al, while the STLF adakites are relatively enriched in K and depleted in Al; (2) At a given SiO2, the LYRB adakites have MgO contents and Mg# similar to slab-derived adakites in subduction zones, while the STLF adakites display even higher MgO and Mg#; (3) The STLF adakites have high Sr/Y and (La/Yb)N, which are positively correlated, while the LYRB adakites show lower. (La/Yb)N but higher Sr/Y ratios; (4) The LYRB adakites have higher low Sr/La and Ce/Pb than the STLF adakites, the latter are consistent with the LCC and the former are close to the MORB; (5) The LYRB adakites have EM2-like Sr-Nd isotopic compositions indicating sediment involvement in source, while the STLF have Sr-Nd isotopic composition of typical LCC; (6) The LYRB adakites are characterized by highly radiogenic Pb isotopic compositions with 206Pb/204Pb(t) up to 18.8, which are clearly distinct from the STLF adakites with low radiogenic Pb (206Pb/204Pb(t)<16.4); and (7) the LYRB adakites have variable and heavy oxygen isotopic compositions in support of sediment involvement, while the STLF adakites have oxygen isotopic compositions close to the mantle value. Based on these differences, we propose that the LYRB adakites were derived from partial melting of subducted oceanic crust, not supporting an origin from delaminated LCC.It is generally accepted that the subduction zones contain sediments and thus have high oxygen fugacity. In contrast, the LCC contains much less water and thus have low oxygen fugacity. Therefore, although partial melts of the delaminated LCC might have interacted with the mantle peridotites, but they were unable to oxidate mantle sulfides and hence failed to generate significant mineralization. In contrast, reaction of slab-derived melts in subduction zones with the mantle could decompose the sulfides and subsequently produce Cu-Au mineralization. This difference well explains the contrasting Cu-Au mineralization between the STLF and the LYRB high-Mg adakites. We analyzed in-situ trace elemental compositions of magmatic zircons from the two groups of high-Mg adakites, and found that the LYRB adakites have much higher Ce3+/Ce3+and Eu3+/Eu2+ratios than the LYRB adakites, demonstrating that initial magmas of the former indeed have much higher oxygen fagucities.4.Origin of high-Mg adakitic rocks from Xu-Huai area in the North China Craton and evolution of the lower continental crust in this areaPrevious studies proposed that the Xu-Huai adakitic rocks in the southerastern NCC were derived from partial melting of the delaminated LCC of the NCC, and the entrained xenoliths represent source rocks of the host rocks. However, we found that although the Xu-Huai adakitic rocks have high Sr/Y and (Dy/Yb)N, but they are characterized by relatively low (La/Yb)N. In addition, they are enriched in Na and depleted in K. These features are different from those of adakitic rocks derived from partial melting of delamainted LCC, but similar to adakites derived from partial melting of subducted oceanic slab. Moreover, initial Sr-Nd-Pb isotopic compositions of the Xu-Huai adakitic rocks are different from those of the entrained mafic xenoliths: Sr and Nd isotopes of the adakitic rocks show a positive correlation with two endmembers towards the EM-2 and ancient LCC (EM-1) respectively, while the xenoliths display a vertical trend from bulk Earth to EM-1. Although both the Xu-Huai adakitic rocks and the entrained mafic xenoliths have higher radiogenic Pb, they are different in Pb isotopic compositions. The Pb isotope data of the Xu-Huai xenoliths and garnet amphibolites as well as marbles from the Wuhe group (the Archean basement rocks in the Bengbu uplift) defined a perfect Pb-Pb isochron in 207Pb/204/Pb-206Pb/204Pb diagram, while Pb isotopes of the adakitic rocks are similar to MORB and fall below the Pb-Pb isochron defined by the Xu-Huai xenoliths and the Wuhe group metamorphic rocks. These observations indicate that the Xu-Huai adakitic rocks were not derived from partial melting of the ancient LCC of the NCC represented by the xenoliths, as proposed by Xu et al. (2006). In addition, it is difficult to imagine that the delaminated LCC-derived magmas could carry their source rocks (eclogite) to the surface. Also, most of the Xu-Huai xenoliths underwent amphibolite facies retrograde metamorphism, which appears not a result of fast magma ascending. Based on these observations, we propose that the Xu-Huai high-Mg adakitic rocks were likely derived from partial melting of subducted oceanic crust, followed by extensive contamination with the LCC rocks during magma intrusions.Mafic garnet-pyroxenite/garnet-amphibolite xenoliths from the Xu-Huai area and garnet-pyroxene amphibolites as well as the marble wall rocks from the Bengbu area consistute a Pb-Pb isochron with age of～1.95Ga, which is consistent with previously reported high-pressure granulite facies metamorphism time based on zircon U-Pb dating. This implies large-scale Pb isotopic homogenization of the Precambrian basement LCC in the southeastern margin of the NCC caused by the early Proterozoic granulite-facies metamorphism. Substantial surface materials involved in the Wuhe group, such as marbles and graywackes, are likely the main source of the highly radiogenic Pb. The results also imply that using the ancient LCC to balance the terrestrial Pb needs take into account the ancient LCC which has an igneous precursor but are characterized by highly radiogenic Pb.5. Magnesium isotopic studies of granitoids and mafic mineralsThis dissertation reports the first systematic study of Mg isotopic compositions for a suite of granitoids and their associated mafic minerals. Although these granitoids formed through different degrees of partial melting and fractional crystallization with large variations in elemental and mineral compositions, theirδ26Mg values vary from-0.26 to -0.14%o and are indistinguishable within the analytical precision (±0.07‰; 2SD). This suggests limited Mg isotope fractionation during granite differentiation.Coexisting hornblendes and biotites in these granitoids display similar Mg isotopic compositions with△26MgHbl-Bt=-0.06±0.08‰(2SD), further indicating that fractional crystallization of either biotite or hornblende did not significantly change Mg isotopic composition of the host magma, consistent with conclusions based on whole-rock analysis. Therefore, Mg isotopic compositions of granitoids can be used to trace their sources. This provides basis for applying Mg isotopic compositions of adakites to trace their magma sources and thus, magma genesis. Besides, the limited inter-mineral Mg isotope fractionation between coexisting hornblende and biotite agrees with the theoretic prediction that Mg cations in both hornblende and biotite are octahedrally coordinated with oxygen, which restricts the magnitude of equilibrium isotope fractionation at high temperatures. 6. High-temperature inter-mineral Mg isotope fractionation in mantle xenolithsPrevious studies have reported large amounts of Mg isotopic data for mantle minerals (Opx, Cpx, olivine and spinel), but whether there are detectable Mg isotope fractionations between these minerals and whether the isotope fractionation are in equilibrium or not are highly debated. In order to solve these debates, this dissertation measured magnesium isotopic compositions of coexisting Opx, Cpx, olivine and spinel from a suite of mantle xenoliths, which span a wide temperature range (>300℃), and for the first time analyzed phlogopite (Phl).The results show that coexisting Opx and 01 have constant Mg isotopic compositions over the overall temperature range, which are indistinguishable within the analytical precision. In contrast, Mg isotopic compositions of Cpx and Phl are significantly different from coexisting ol, with Cpx and Phl slightly heavier than coexisting 01. In addition, isotope fractionation between Cpx and 01 are related to temperatures, implying equilibrium isotope fractionation. In particular, spinels are much heavier in Mg isotopic composition compared to coexisting silicates, with△26Mgspl-Ol ranging from+0.25 to+0.55%o (n=10). The degree of inter-mineral Mg isotope fractionation among these mantle minerals agrees with theoretical predictions, suggesting that inter-mineral Mg isotope fractionation is primarily controlled by the Mg-0 bond strength, with stronger bonds favoring heavy Mg isotopes. Magnesium is in tetrahedral coordination in spinels and in octahedral coordination in other mantle silicates. Therefore, the theoretical predications confirm that spinels are much heavier than coexisting silicates. Our results demonstrate that the large high-temperature equilibrium Spl-Ol Mg isotope fractionation in peridotite xenoliths can potentially be used as a geothermometry in mantle geochemistry.7. Mg isotopic compositions of adakitic rocks from central-eastern ChinaAfter having demonstrated that there is limited Mg isotope fractionation during granite differentiation, this thesis analyzed Mg isotopic compositions of adakitic rocks that have various Mg#’s and origins from central-eastern China, including low-Mg adakitic rocks from the Dabie orogen, high-Mg adakitic rocks from the LYRB, the STLF and the Xu-Huai area. We found that all of these high-Mg adakitic rocks except one Xu-Huai sample have very homogeneous Mg isotopic composition, with averageδ26Mg=-0.25±0.05‰(2SD), which is indistinguishable from the mantle value (δ26Mg=-0.25±0.05‰; 2SD). These observations suggest that the thickened lower crust in the Dabie mountain root is mainly derived from the mantle by magma under-plating, and they also indicate that although MgO and Mg# of adakitic magmas during interaction with the mantle prominently increase, their Mg isotopic compositions did not significantly change. This reflects that the mafic LCC and MORB themselves have Mg isotopic compositions similar to the mantle, so that the melt-mantle interaction did not significantly modify Mg isotopic composition of the initial LCC/MORB melts. Therefore, Mg isotopes of adakites can not be used to identify origins of adakites and discriminate high-Mg adakites from low-Mg ones. In addition, although Sr-Nd-Pb-O isotopic compositions of the LYRB adakites imply sediment involvement in magma sources, their homogeneous and mantle-like Mg isotopic compositions suggest fast Mg isotope exchange equilibrium between adakitic rocks and the mantle peridotites. The Xu-Huai sample that has relatively light Mg isotopic composition has high CO2 content, suggesting involvement of recycling carbonatite in the magma source, consistent with its high radiogenic Pb signature.更多还原