【作者】 杨蔚；

【导师】 李曙光；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2007， 博士

【摘要】 前人对中国东部幔源包体的研究已揭示了在显生宙期间中国东部岩石圈减薄了约100km，然而对岩石圈减薄的时间和机制尚存在争议。辽西地区出露有大量的不同时代的中生代火山岩，它们地球化学性质的演化能够对华北北缘岩石圈减薄的时间和机制给以制约。因此，本文对辽西中生代火山岩进行了年代学和地球化学研究。1．初步建立辽西中生代火山作用的年代学格架本文对辽西中生代四期火山岩和两个含珍稀化石层位火山岩进行了精确的锆石U-Pb SHRIMP和全岩Ar-Ar定年，结果为：兴隆沟组（～176 Ma）、蓝旗组（166-153 Ma）、义县组（125-122 Ma）、张老公屯（～106 Ma）、道虎沟含珍稀化石层（～165Ma）、义县组含珍稀化石层（～125Ma）。结合已发表的年代学数据，初步建立了辽西地区中生代火山作用的年代学格架，辽西地区中生代共有四次岩浆事件，分别是（176Ma）、（166-148 Ma），（132-120 Ma），（～106 Ma）。并确定了辽西热河生物群的时代归属为早白垩世；道虎沟生物群的时代归属为中侏罗世，它对应蓝旗组下部或海房沟组上部层位。2．辽西中生代各期火山岩的地球化学特征及成因兴隆沟组安山岩：属于富镁的埃达克质岩石，除具有典型埃达克岩（adakite）的特征（如：SiO₂％＞56％，Al₃O₂％＞15％，MgO＜3％，Y＜18 ppm，Yb＜1．9 ppm，Sr＞400 ppm，无Eu负异常）外，还表现出高Mg^#（0．57），Cr（156 ppm）、Ni（82．2 ppm）含量的特点，表明其源区有石榴石残留，且岩浆上升过程中与地幔橄榄岩发生过反应。在同位素方面，兴隆沟组安山岩具有中等富集⁸⁷Sr／⁸⁶Sr为0．706597，不太低的ε_Nd（T）为-1．5和高放射成因Pb(²⁰⁶pb／²⁰⁷pb=18．211，²⁰⁷pb／²⁰⁴pb=15．554和²⁰⁸pb／²⁰⁴pb=38．176)，这一特征反映了其源区Sr-Nd-Pb同位素性质，因此其源区应是底垫在华北克拉通之下的古老洋壳。蓝旗组粗面玄武岩和玄武质粗安岩：具有非常低的放射成因Pb(²⁰⁶pb／²⁰⁷pb＜16．5，²⁰⁷pb／²⁰⁴pb＜15．3，²⁰⁸pb／²⁰⁴pb＜36．5)，中等⁸⁷Sr／⁸⁶Sr比值（～0．706）和低ε_Nd（T）值（～-12），富集轻稀土元素（LREE）和大离子亲石元素（LILE），亏损高场强元素（HFSE），低Ce／Pb比值（＜10）的特征，表现出强烈的下地壳特征而并非富集岩石圈地幔的特征，指示了其形成过程中有下地壳物质的加入。同时它们还具有低MgO、Cr、Ni含量，亏损Th、U的特征，指示其在岩浆演化过程中经历了强烈的橄榄石结晶分异和同化混染华北下地壳麻粒岩的过程，反映此时发生了一次玄武岩浆的底侵作用，这一过程使得该区镁铁质下地壳发生增厚。蓝旗组粗面安山岩：不属于埃达克质岩石，它以高Al₂O₃（＞16％），低MgO（＜2．5％），低Cr（＜1 ppm）和Ni（＜7 ppm）含量为特征，与兴隆沟组安山岩有显著不同，具有明显Eu负异常，Sr-Nd-Pb同位素与该组玄武岩相似表现为中等⁸⁷Sr／⁸⁶Sr初始比值（0．704～0．706），低ε_Nd（T）值（-13～-10），低放射成因Pb(²⁰⁶pb／²⁰⁷pb＜16．6，²⁰⁷pb＜²⁰⁴pb＜15．4和²⁰⁸pb＜²⁰⁴pb＜36．8)，表明其是华北麻粒岩相下地壳部分熔融的产物。熔融过程所需热量可能来自底侵玄武岩及其AFC过程。义县组粗面玄武岩和玄武质粗安岩：与蓝旗组粗面玄武岩具有相似的地球化学特征，具有非常低的放射成因Pb(²⁰⁶pb／²⁰⁷pb＜16．7，²⁰⁷pb／²⁰⁴pb＜15．3和²⁰⁸pb／²⁰⁴pb＜36．7)，具有中等⁸⁷Sr／⁸⁶Sr比值（～0．706）和低ε_Nd（t）值（＜-9．7），富集LREE和LILE，亏损HFSE，低Ce／Pb比值的特征，因此表明其形成过程中也有下地壳物质的加入。其与蓝旗组粗面玄武岩的不同之处是具有高MgO、Cr、Ni含量，不亏损Th、U，这一特征表明其混入地壳物质的方式不是AFC过程，而可能是地幔源区混染的是拆离下地壳熔融的富SiO₂熔体。义县组粗面安山岩：与兴隆沟组安山岩地化特征相似，同属富镁的埃达克质岩石（SiO₂％＞56％，Al₃O₂％＞15％，MgO＜3％，低Y＜18ppm，低Yb＜1．9ppm，高Sr＞400ppm，无Eu负异常，Mg^#＞0．54，Cr＞200ppm，Ni＞100ppm），所不同的是具有非常低的放射成因Pb(²⁰⁶pb／²⁰⁷pb＜16．7，²⁰⁷pb／²⁰⁴pb＜15．3和²⁰⁸pb／²⁰⁴pb＜36．7)，具有中等⁸⁷Sr／⁸⁶Sr比值（～0．706）和低ε_Nd（T）值（＜-9．7），表明其源区可能是拆离并进入地幔的华北克拉通下地壳。因此，辽西地区镁铁质下地壳的拆沉应可能发生在早白垩世。张老公屯组玄武岩：与蓝旗组和义县组粗面玄武岩不同，它具有Nb、Ta的正异常和Pb负异常，富集轻稀土元素和大离子亲石元素，不亏损高场强元素，高放射成因Pb(²⁰⁶pb／²⁰⁷pb＞17．8，²⁰⁷pb／²⁰⁴pb＞15．4和²⁰⁸pb／²⁰⁴pb＞37．6)，低⁸⁷Sr／⁸⁶Sr比值（＜0．704）和高ε_Nd（T）值（＞4）的特征，指示其源自一个亏损的软流圈上地幔。源自软流圈地幔的张老公屯玄武岩的出现，说明辽西地区在106Ma时发生了一次岩石圈减薄的重要事件。3．中生代华北克拉通北缘岩石圈的演化模型结合火山岩年代学、地球化学数据和区域构造背景，本文建立了华北克拉通北缘中生代岩石圈演化模型，A三叠纪-早侏罗世：蒙古增生褶皱带向华北克拉通俯冲并与之拼合，蒙古带洋壳平俯冲并底垫在华北克拉通岩石圈之下。Bca．176 Ma：底垫的洋壳在地幔中受到充分加热，发生部分熔融诱发兴隆沟组火山作用。C 166-148 Ma：底垫的蒙古增生褶皱带岩石圈拆离，并扰动上地幔，诱发了大规模的玄武质岩浆底侵作用及低镁粗安岩火山作用。玄武质岩浆底侵作用使该区镁铁质下地壳增厚。D 148-132 Ma：发生在义县组之前的燕山二幕挤压事件，导致地壳的进一步加厚并使加厚镁铁质下地壳相变为榴辉岩。E 132-120 Ma：郯庐断裂系发生大规模走滑运动，诱发了榴辉岩相下地壳发生拆离和拉分引张事件，它导致义县组火山作用。F ca．106 Ma：郯庐断裂系由最初的走滑运动转变为东西向引张运动，这一大规模引张作用导致岩石圈进一步减薄（＜80km）。更多还原

【Abstract】 The studies of mantle xenoliths hosted in the Paleozoic kimberlites and the Cenozoic basalts indicate that about 100 km lithosphere has bee removed during Phanerozoic. However, timing and mechanism for the thinning is still controversial. The Mesozoic volcanic rocks with variable ages widely developed in Westem Liaoning. The geochemical variation of these rocks could provide constraints on timing and mechanism for the thinning. Therefore, this work focuses on the geochronology and geochemistry of these rocks.1. Timing of the each Mesozoic magmatisms in Western Liaoning.The samples collected from four formations including two fossil-bearing beds have been selected for geochronological studies. The results are ca. 176 Ma for the Xinglonggou Formation, 166-153 Ma for the Lanqi Formation, 125-122 Ma for the Yixian Formation, ca. 106 Ma for the Zhanglaogongtun Formation, ca. 165 Ma for the Daohugou bed and ca. 125 Ma for the Yixian fossil-bearing bed. Based on these and previously reported data, four Mesozoic magmatic episodes in Western Liaoning have been recognized, e.g. they occurred in ca. 176Ma, 166-148 Ma, 132-120 Ma and ca. 106 Ma, respectively. The age of Jehol Biota in Western Liaoning is the early Cretaceous And the age of Daohugou Biota is the Middle Jurassic.2. Geochemical characteristics and origin of the volcanic rocks.Xinglonggou andesites: this Magnesian adakitic rock has not only the typical adakitic characteristics （such as SiO₂%＞56%, Al₃O₂%＞15%, MgO＜3%, Y＜18ppm, Yb＜1.9ppm, Sr＞400ppm, without negative Eu anomaly）, but also have high Mg^# （0.57）, and high Cr （156ppm） and Ni （82.2ppm） contents. These characteristics indicate the presence of residual gamet in their magma source and interaction between the adakitic melt and the mantle rocks during ascending of the melt. The moderate ⁸⁷Sr/⁸⁶Sr （0.706597）, slightly lowε_Nd（T）（-1.5） and high radiogenic Pb (²⁰⁶pb/²⁰⁷pb=18.211, ²⁰⁷pb/²⁰⁴pb=15.554 and ²⁰⁸Pb/²⁰⁴pb=38.176) suggest that the Xinglonggou high Mg adakitic rocks were formed by partial melting of subducted oceanic crust. Lanqi basalts and basaltic andesites: They have extremely low radiogenic Pb (²⁰⁶pb/²⁰⁷pb＜16.5, ²⁰⁷pb/²⁰⁴pb＜15.3 and ²⁰⁸pb/²⁰⁴pb＜36.5), moderate ⁸⁷Sr/⁸⁶Sr（～0.706） and lowε_Nd（T）（～-12）. They are enriched in light rare earth dements （LREE）, large ion lithophile elements （LILE）, and Pb with low Ce/Pb （＜10）, while depleted in high field strength elements （HFSE）. These geochemical characteristics suggest significant involvement of low crust materials in their mantle source. They have low MgO, Cr, Ni contents, with depletion of Th and U, indicating granulite wall rock assimilation and large amount olivine fractional crystallization during magma underplating. This basaltic magma underplating may thicken the lower mafic crust.Lanqi andesites: They have high Al₂O₃（＞16%）, and low MgO （＜2.5%）, Cr （＜1ppm） and Ni（＜7ppm） as well as lower Sr and higher Y, with negative Eu anomaly, showing difference from the Xinglonggou andesites and adakites. They have similar Sr-Nd-Pb isotopic compositions to the Lanqi basalts and basaltic andesites. Their moderate ⁸⁷Sr/⁸⁶Sr （0.704～0.706）, lowε_Nd（T）（-13～-10） and low radiogenic Pb (²⁰⁶pb/²⁰⁷pb＜16.6, ²⁰⁷pb/²⁰⁴pb＜15.4 and ²⁰⁸pb/²⁰⁴pb＜36.8) suggest that the Lanqi andesites were derived from lower crust. AFC processes during magmatic underplating may provide the heat needed by partial melting of the lower crust.Yixian basalts and basaltic andesites: They show similar geochemical characteristics to the Lanqi basalts, such as low radiogenic Pb (²⁰⁶pb/²⁰⁷pb＜16.7,²⁰⁷Pb/²⁰⁴Pb＜15.3 and ²⁰⁸pb/²⁰⁴pb＜36.7), moderate ⁸⁷Sr/⁸⁶Sr（～0.706）, lowε_Nd（T）（＜-9.7）, enrichment of LREE and LILE, depletion of HFSE, and low Ce/Pb, suggesting involvement of low crust materials in their mantle source, which could be hybridized by the SiO₂-rich liquids produced by partial melting of delaminated crust. Because high MgO, Cr, Ni contents without depletion of Th and U distinguish them from the Lanqi basalts, indicating that the basaltic magma may derive directly from the mantle source without AFC process.Yixian andesites: They are Magnesian adakitic rocks, similar to the Xinglonggou andesites, such as SiO₂%＞56%, Al₃O₂%＞15%, MgO＜3%, Y＜18ppm, Yb＜1.9ppm, Sr＞400ppm, no negative Eu anomaly, Mg^#＞0.54, Cr＞200ppm, Ni＞100ppm. However, the Yixian andasites are different from the Xinglonggou andesites in Sr-Nd-Pb isotopic compositions. The Yixian andesites have very low radiogenic Pb (²⁰⁶Pb/²⁰⁷Pb＜16.7, ²⁰⁷Pb/²⁰⁴Pb＜15.3 and ²⁰⁸pb/²⁰⁴Pb＜36.7), moderate ⁸⁷Sr/⁸⁶Sr（～0.706） and lowε_Nd（T）（＜-9.7）, indicating that the Yixian adakitic magma was derived from delaminated lower continental crust.Zhanglaogongtun basalts: The Zhanglaogongtun basalts are characterized by positive Nb and Ta anomaly, negative Pb anomaly, enriched in LREEs and LILEs, without depletion of HFSEs, low ⁸⁷Sr/⁸⁶Sr（＜0.704）, and highε_Nd（T）（＞4） and Pb isotopic ratios (²⁰⁶Pb/²⁰⁷Pb＞17.8, ²⁰⁷Pb/²⁰⁴pb＞15.4 and ²⁰⁸Pb/²⁰⁴Pb＞37.6), suggesting derivation from the depleted upper mantle. The abruptly going up ofε_Nd（T） values showing by Zhanglaogongtun basalts indicates that an important lithospheric thinning event happened at about 106 Ma in Western Liaoing.3. The Mesozoic lithosphere evolution model for the North margin of the NCC.Based on the geochronological, geochemical data mentioned above, a model for lithospheric evolution of the North margin of the NCC has been proposed here. A) The Triassic-early Jurassic: the Mongolia slab subducted under the NCC. B) Partial melting of the subducted oceanic crust, which was underplating beneath the lithosphere of the NCC, occurred at ca. 176Ma to produce the Xinglonggou adakitic rocks. C) 166 -148 Ma: The delamination of subducted oceanic crust caused the mantle upwelling and its partial melting at 166 Ma. The upwelling mantle and basaltic magma may result in magma underplating and partial melting of the lower crust to produce the Lanqi basalts and andesites. Accordingly, the mafic lower crust in Western Liaoning area was thickened. D) 148-132 Ma: The regional contraction of the YanshanianⅡbefore the Yixian magmatism further thickened the crust, resulting eclogitic facies trasformation in the thickened mafic lower crust. E) 132-120 Ma: The large scale strike-skip of the Tan-Lu fault may trigger the delamination of thickened mafic lower crust. The adakitic melt （i.e. the Yixian Magnesian andesites） produced by partial melting of the delaminated mafic lower crust may hybridize the upwelling mantle to forme a new enriched lithospheric mantle, which replaced the volume formerly occupied by the delaminated lithospheric mantle, resulting in the "continental" geochemical signatures observed in Yixian basalts. F) ca. 106 Ma: The movement of the Tan-Lu fault zone transferred into extension in the late Cretaceous. This large scale extention resulted in farther lithospheric thinning.更多还原