【作者】 李全忠；

【导师】 陈江峰； 谢智；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2007， 博士

【摘要】 本论文通过对中国东部各构造单元，包括华北的鲁西地区、大别造山带和扬子东北缘的长江中下游地区的早白垩世基性岩以及华北东南缘的胶州大西庄晚白垩世玄武岩中的辉石巨晶的年代学和Sr-Nd-Pb同位素研究，反演白垩世中国东部各构造单元的陆下岩石圈地幔的同位素地球化学特征。作为讨论壳幔相互作用的基础，本论文根据文献资料对华北克拉通、扬子地块和大别造山带的上、下地壳的同位素特征进行了制约。以迁西群麻粒岩相变质岩作为华北下地壳的代表，以泰山群花岗质片麻岩作为华北中、上地壳的代表，以华北古生代金伯利岩包体代表减薄发生前的古生代华北岩石圈地幔。以崆岭群花岗质片麻岩的Sr-Nd同位素组成代表扬子下地壳的Sr-Nd同位素变化范围，以扬子东段中生代铜陵地区花岗岩的Pb同位素组成近似为扬子下地壳的Pb同位素变化范围；以董岭群斜长角闪片岩和皖南浅变质岩及沉积岩的Sr-Nd同位素组成代表扬子中、上地壳的Sr-Nd同位素范围，以安徽沿江沉积岩和板溪群千枚岩Pb同位素组成代表扬子中、上地壳的Pb同位素变化范围。以北大别片麻岩代表大别下地壳，以南大别片麻岩代表大别上地壳。华北鲁西地区的济南和邹平样品包括辉长岩和辉长辉绿岩，其中邹平辉长辉绿岩中的锆石给出128.2±2.0 Ma的SHRIMP U-Pb年龄，代表岩体的形成时代。济南和邹平两岩体的辉长岩均属于拉斑系列，微量元素表现出Rb、Ba等大离子亲石元素（LILE）富集和Pb、Sr正异常而Nb、Ta和Ti等高场强元素（HFSE）亏损的特征，稀土元素均表现出轻稀土（LREE）富集和不同程度的Eu正异常。元素、同位素判别和锆石年代学研究结果表明，其初始岩浆在上升侵位过程中，没有受到地壳物质的显著混染，其同位素组成能够用来示踪地幔源区的性质。济南和邹平辉长岩的初始同位素组成为⁸⁷Sr/⁸⁶Sr（t）=0.7041～0.7055，ε_Nd（t）=-6～-18.7，²⁰⁶Pb／²⁰⁴Pb（t）=16.55～17.00，²⁰⁷Pb／²⁰⁴Pb（t）=15.22～15.35，²⁰⁸Pb／²⁰⁴Pb（t）=36.29～36.95，表明其地幔源区具有同位素富集特征。虽然类似于EMⅠ的特征，但与典型的由大洋玄武岩定义的EMⅠ地幔端元相比，Nd和Pb同位素比值较低，反映了显著的地壳物质贡献。华北古生代金伯利岩包体的Sr-Nd-Pb同位素组成表明华北古生代岩石圈地幔具有EMⅡ型的富集地幔特征，太古代迁西群变质杂岩是华北克拉通代表性的古老下地壳物质，济南和邹平辉长岩的源区物质可能由古老岩石圈地幔和华北下地壳混合而成的。庐枞火山岩出露于扬子地块东北缘的长江中下游地区，是一套包括粗玄岩—玄武粗安岩—粗面岩的富碱钾玄质岩石系列。本论文所研究的双庙旋回玄武岩和浮山旋回粗面岩均表现出富碱富钾、富Al₂O₃，高Fe₂O₃，低MgO、TiO₂的特征，微量元素表现出Th、U、K等大离子亲石元素富集特征，稀土元素表现出LREE富集和没有Eu负异常等特征。双庙组火山岩的锆石给出129.5±2.3 Ma的SHRIMPU-Pb年龄，代表其形成时代。根据元素、同位素判别和锆石年代学的研究结果，其初始岩浆在上升侵位过程中也没有受到地壳物质显著混染。庐枞双庙旋回玄武岩和浮山旋回粗面岩的初始同位素组成为⁸⁷Sr／⁸⁶Sr（t）=0.7057～0.7065，ε_Nd（t）=-3.9～-6.2，²⁰⁶Pb／²⁰⁴Pb（t）=17.88～18.08，²⁰⁷Pb／²⁰⁴Pb（t）=15.50～15.55，²⁰⁸Pb／²⁰⁴Pb（t）=37.93～38.18，介于DMM和EMⅡ型端元之间。Sr-Nd和Pb同位素特征均显示扬子下地壳对庐枞火山岩的地幔源区有一定的贡献。大别造山带中北大别基性岩以椒子岩和沙村岩体为代表，岩性包括辉长岩和辉石岩，大部分属于拉斑系列，少数落在碱性系列范围。具有Nb、Zr、Ti亏损、Rb，Ba，Pb富集和LREE富集、轻微的Eu负异常等特征。北大别基性岩的初始岩浆在上升侵位过程中也没有受到地壳物质的显著混染，其初始同位素组成为⁸⁷Sr／⁸⁶Sr（t）=0.7067～0.7085，ε_Nd（t）=-6.5～-19.1，²⁰⁶Pb／²⁰⁴Pb（t）=16.37～17.40，²⁰⁷Pb／²⁰⁴Pb（t）=15.31～15.45，²⁰⁸Pb／²⁰⁴Pb（t）=37.11～37.98。与华北克拉通基性岩（以济南和邹平辉长岩为代表）相比，有较高的⁸⁷Sr／⁸⁶Sr（t）而较低的ε_Nd（t），较高的²⁰⁷Pb／²⁰⁴Pb和²⁰⁸Pb／²⁰⁴Pb而基本一致的²⁰⁶Pb／²⁰⁴Pb，反映其源区较华北基性岩的源区有较高的μ值和Th／U比值。这些北大别基性岩的Pb同位素表现出类似于EMⅠ的特征，而Sr-Nd同位素特征介于EMⅠ和EMⅡ之间。在Sr_Nd和Pb-Pb同位素图解中，北大别基性岩源区与大别造山带花岗岩的区域重合，并且两者都落在北大别片麻岩的范围内，暗示了以北大别片麻岩为代表的大别下地壳对北大别基性岩的源区有显著贡献。华北克拉通、大别造山带和扬子东北缘的长江中下游地区的早白垩世地幔的同位素特征均暗示地幔源区中有对应的下地壳物质的贡献。不同下地壳物质的加入，是导致不同地质单元早白垩世岩石圈地幔地球化学差异的重要原因之一。在排除了岩石形成过程中地壳混染的可能性后，壳幔物质的源区混合是最主要的过程。本论文倾向认为岩石圈拆沉是下地壳参与基性岩地幔源区的可能的动力学模式，但由于不能为下地壳参与源区的机制提供直接的制约，所以地幔交代作用也不能排除。根据主量元素分析结果，胶州大西庄玄武岩（73 Ma）中的辉石巨晶可能包括两类：单斜辉石巨晶和斜方辉石巨晶。虽然它们的化学组成差别很大，但同位素组成没有差别。辉石巨晶与寄主玄武岩具有几乎完全相同的同位素组成，其⁸⁷Sr／⁸⁶Sr（t）和ε_Nd（t）分别为0.7033～0.7041、+4.5～+6.5和0.7035～0.7038、+7.1～+7.6，暗示辉石巨晶和寄主岩具有相同的地幔源区，即辉石巨晶是玄武岩浆在高压下结晶的产物。亏损的同位素特征表明，在白垩世末期（～73 Ma），胶东地区的岩石圈地幔已经由富集转变为亏损，表现出类似于中国东部新生代地幔的同位素特征，即表明当时岩石圈减薄（或置换）已经完成。更多还原

【Abstract】 The geochronology and Sr-Nd-Pb isotopic compositions of the Early Cretaceous mafic rocks from different tectonic units of the eastern China, including western Shandong province in North China, Dabie orogenic belt and northeast margin of the lower Yangtze region, together with the pyroxene megacrysts within Late Cretaceous basalts from Jiaozhou, are studied to deduce the characteristics of the Early Cretaceous subcontinental lithospheric mantle （SCLM） of eastern China.The isotopic characteristics of the upper and lower crusts of these tectonic units are constrained based on the data from the literatures. The isotopic compositions of the granulites from the Qianxi Group and the granitic gneisses from the Taishan Group are used to represent those of the lower and mid-upper crusts of North China, respectively. The isotopic characteristics of the Paleozoic mantle of North China before thinning are represented by those of Paleozoic kimberlite inclusions. For the Yangtze block, Sr-Nd isotopic compositions of its lower crust are represented by those of the granitic gneisses of the Kongling Group, and Pb isotopic compositions are approximated by those of Mesozoic granites from the Tongling district. While Sr-Nd isotopic compositions of the Yangtze mid-upper crust are represented by those of the gneisses of the Dongling Group and Proterozoic metamorphic rocks from southern Anhui, Pb isotopic compositions are represented by those of Phanerozoic sedimentary rocks along the Yangtze River in Anhui province and those of the Banxi Group. The isotopic characteristics of Dabie lower crust are represented by those of the gneisses from the North Dabie, and those of the upper crust are represented by those of the gneiss from the South Dabie.The samples from Jinan and Zouping intrusions contain gabbro and gabbroic-diorite. Zircons separated from the gabbroic-diorite at Zouping yield a SHRIMP U-Pb age of 128.2±2.0 Ma, which is interpreted as its formation time. Gabbros of both localities belong to tholeiitic series, most of gabbroic samples show significant positive Rb, Ba, Pb and Sr anomalies and negative Nb, Ta and Ti anomalies and are highly enriched in LREEs with positive Eu anomalies. The initial magma of the gabbros from Jinan and Zouping has been little contaminated by curstal materials during the magmas ascending based on geochemical, isotopic and geochronological evidences. Therefore, the isotopic characteristics of the gabbros from the two locations can be used to approximate those of their mantle source. Gabbros from Jinan and Zouping show the range of the initial isotopic ratios as follows: ⁸⁷Sr/⁸⁶Sr （t） = 0.7041～0.7055,ε_Nd（t） = -6～-18.7, ²⁰⁶Pb/²⁰⁴Pb（t） = 16.55～17.00, ²⁰⁷Pb/²⁰⁴Pb（t） = 15.22～15.35 and ²⁰⁸Pb/²⁰⁴Pb（t） = 36.29～36.95, suggesting that the mantle source of the gabbros possesses an enriched isotopic characteristics. Although it is general considered that the Mesozoic subcontinental mantle of the North China shows an isotopic characteristic which is similar to the EMI end member, isotopic character of the Mesozoic mantle of North China is different from the typical EMI end member defined by oceanic basalts. The lower Nd and Pb isotopic ratios suggest a contribution of the lower crust. It is inferred that the mantle source of the gabbros from Jinan and Zouping may be a mixture of the Paleozoic lithospheric mantle with an isotopic character similar to that of the EMII end member and the lower crust of North China.The Luzong volcanoic rocks, located at lower Yangtze regions, the northeastern margin of the Yangtze block, are a suite of shoshonitic rocks, which include trachybasalt-basaltic trachyandesite and trachyte. Both basalts of the Shuangmiao Formation and trachytes of the Fushan Formation show high K₂O, Al₂O₃ and Fe₂O₃, but low MgO and TiO₂, are enriched in LILEs, such as Th, U, K and LREEs, but with no negative Eu anomalies. The zircons separated from the basalt yield an U-Pb age of 129.5±2.3 Ma, which is interpreted as the formation age of the Luzong volcanic rocks. The initial magma of the Luzong volcanic rocks has been little contaminated by curstal materials during ascending based on the results of geochemistry, isotope and chronology, thus, the isotopic characteristics of the volcanic rocks can be reasonably used to indicate their mantle source. Luzong volcanic rocks show the range of the initial isotopic ratios as follows: ⁸⁷Sr/⁸⁶Sr（t） = 0.7057～0.7065,ε_Nd（t） = -3.9～-6.2, ²⁰⁶Pb/²⁰⁴Pb（t） = 17.88～18.08, ²⁰⁷Pb/²⁰⁴Pb（t） = 15.50～15.55 and ²⁰⁸Pb/²⁰⁴Pb（t） = 37.93～38.18. These values are located between the DMM and EMII end members. Furthermore, the isotopic compositions also suggest the contribution of the Yangtze lower crust to the mantle source.The mafic rocks from Jiaoziyan and Shacun. including gabbro and pyroxenite. mostly belong to the tholeiitic series, while a few are plotted in the field of the alkali series. The samples are highly enriched in LREEs. with small negative Eu anomalies and show distinct negative anomalies in Nb, Ti and Zr. and positive anomalies in Rb. Ba and Pb. Similar to the gabbros from Jinan and Zouping and the Luzong volcanic rocks, the initial mafic magma has also been little contaminated by the curstal materials during intrusion. The initial Sr-Nd-Pb isotope compositions of the North Dabie mafic rocks are in the ranges as follows: ⁸⁷Sr/⁸⁶Sr（t） = 0.7067～0.7085,ε_Nd（t） = -6.5～-19.1, ²⁰⁶Pb/²⁰⁴Pb（t）=16.37～17.40, ²⁰⁷Pb/²⁰⁴Pb（t）=15.31～15.45 and ²⁰⁸Pb/²⁰⁴Pb（t） = 37.11～37.98. Comparing with the mafic rocks from the North China, those from the North Dabie show higher ⁸⁷Sr/⁸⁶Sr （t）. but lowerε_Nd （t）; higher ²⁰⁷Pb/²⁰⁴Pb （t） and ²⁰⁸Pb/²⁰⁴Pb （t） at the same ²⁰⁶Pb/²⁰⁴Pb, which suggest that the mantle source of North Dabie has higherμvalue and Th/U ratio than that of the North China Block. In the Pb isotopic evolution diagram, data of the mafic intrusions from the North Dabie located close to EMI, while in ⁸⁷Sr/⁸⁶Sr（t） vs.ε_Nd（t） plot, these samples lie between EMI and EMII end members. On the other hand, in both isotopic diagrams, the mafic intrusions and the gneisses from the North Dabie share the same range, suggesting that the lower crust defined by the gneisses of the North Dabie has the significant contribution to the mantle source.In summary, the isotopic characteristics of Early Cretaceous subcontinental lithospheric mantle of eastern China, including North China Block, Dabie orogenic belt and the lower Yangtze region, suggest the contribution of the corresponding lower crustal material to the mantle source. The involvement of different lower crustal material is one of the main processes which resulted in different isotopic characteristics in the mantle sources of different tectonic units in Eastern China at the Late Mesozoic. When the contamination of the mantle-derived magma by the curstal material during intrusion is eliminated, the source mixing between the crustal and mantle materials is the primary process. The lithosphere delamination may be a possible dynamic model for explaining the incooperation of the lower crustal materials into the mantle source, but metasomatism model cannot be ruled out.The pyroxene megacrysts hosted by basalt （73 Ma） at Daxizhuang, Jiaozhou. contain two types: clinopyroxene megacrysts and orthopyroxene megacrysts based on major element analyses. Although their major element compositions are different, their isotopic compositions are highly homogeneous. Megacrysts and host basalts show uniform isotopic compositions, ⁸⁷Sr/⁸⁶Sr（t）=0.7033～0.7041 andε_Nd（t）= +4.5～+7.6, respectively, which suggest that they were derived from the same mantle source, and the megacrysts were crystallized from the host basaltic magma under high pressure. Moreover, the 73 Ma Daxizhuang basalts and hosted megacrysts show the similar depleted isotopic characteristics with Cenozoic basalts from Eastern China, implying that the transformation of the isotopic characters of the lithospheric mantle from enrichment to depletion in Jiaozhou, and also the lithosphere thinning was completed in Late Cretaceous period （73 Ma）.更多还原