【作者】 谢文雅；

【导师】 牛漫兰；

【作者基本信息】 合肥工业大学 ， 构造地质学， 2009， 硕士

【摘要】 本论文对发育于华北克拉通东南部蒙阴盆地的一些晚中生代青山群火山岩进行了主量元素、微量元素、Sr-Nd-Pb同位素等地球化学研究和LA-ICP MS法单颗粒锆石U-Pb年代学研究。通过这些研究,明确了火山岩形成的具体时限,查明了其岩石地球化学特征,分析了岩浆源区的性质和深部地质作用过程。在区域对比的基础上,探讨了蒙阴地区火山岩岩浆源区的形成机制。蒙阴青山群火山岩形成于早白垩世晚期,具有相对均一的锆石同位素年龄,具体喷发时期在118.2～122.4Ma之间,与前人对华北东南部岩浆岩的年龄测定相吻合。SiO2含量变化于53.71%～69.08%,在TAS上位于亚碱性系列岩石区域,多数属于粗安岩、粗面英安岩,少数为安山岩,全碱(K2O+Na2O)含量为5.85%～9.35%,岩石属于高钾钙碱性系列、准铝质、硅过饱和类型的岩石。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)富集和重稀土(HREE)亏损的特征,且具有弱的Eu负异常。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。校正到岩浆喷发时间t后,蒙阴青山群火山岩的Sr-Nd同位素初始比值分别为87Sr/86Sr(t)=0.7051～0.7109 , 143Nd/144Nd(t)=0.5117～0.5123 ,对应的εNd(t) =-3.9～-15.3 , Pb初始同位素比值分别为206Pb/204Pb(t)=16.88～18.39 ,207Pb/204Pb(t)=15.31～15.58,208Pb/204Pb(t)=36.63～38.06。以上地球化学特征表明,蒙阴盆地青山群火山岩性质与代表华北克拉通EMⅠ型幔源岩浆的济南、邹平岩浆岩及代表华北下地壳的太古代麻粒岩性质均不相同,类似于方城以及胶莱盆地晚中生代火山岩,样品点分布于EMⅠ和EMⅡ之间,反映该地区岩石圈内部壳-幔相互作用背景下的岩浆混合作用较为普遍,表明其岩浆源区具有同位素富集的特征。结合前人对研究区内晚中生代火山岩,以及华北克拉通东部同期岩浆岩的年代学特征、岩石地球化学特征与同位素地球化学研究,本文认为蒙阴盆地青山群火山岩起源于岩石圈伸展-减薄动力学背景下,可能为俯冲陆壳在地幔源区发生混染和交代作用所形成的富集型地幔部分熔融的产物,并且扬子陆块向华北陆块的俯冲-碰撞作用是造成华北东南缘岩石圈地幔组成改变以及岩石圈减薄的主要驱动力。更多还原

【Abstract】 In this paper, LA-ICP MS zircon U-Pb geochronological, geochemlcal and Sr-Nd-Pb isotopic analyses have been carried out on the Late Mesozoic Qingshan Formation volcanic rocks from Mengyin Basin beneath the southeastern North China Craton. With these study, this paper demarcated formation time of these volcanic rocks, summarized the nature of their petrology and geochemistry, discussed the deep processes of the magmatism, compositions of magma sources, and magmatic evolution features. Based on regional comparsions, this paper also analyzed the patterns of the enriched characteristics in Mengyin area.Qingshan Formation volcanic rocks from Mengyin Basin were erupted during the Early Cretaceous, the eruption ages varied from 118.2Ma to 122.4Ma that anastomosed with primary study in this area. These rocks belong to meta-alkaline series locating in TAS diagram with the SiO2 contents of 53.71%～69.08%and the total alkaline contents of 5.85%～9.35%. Most of the rocks are trachydacite and trachyandesite except a few showing andesite affinity. These volcanic rocks belong to high-K cal-alkaline, metaluminous and silicon-saturation series. And the volcanic rocks are notably rich in LILE and LREE, depleted in HREE and HFSE, detailed showing significant positive Rb, Ba, Pb, and Sr anomalies and negative Nb, Ta and Ti anomalies and negative Eu anomalies weakly. The initial magma of the volcanic rocks has been little contaminated by curstal materials during the magmas ascending based on geochemlcal, isotopic and geochronological evidences. Therefore, the isotopic characteristics of the volcanic rocks can be used to approximate those of their mantle source. The volcanic rocks are rich in Sr-Nd-Pb isotope component, showing the range of the initial isotopic ratios as follows: 87Sr/86Sr(t)=0.7051～0.7109, 143Nd/144Nd(t)=0.5117～0.5123,εNd(t)=-3.9～-15.3, and the radiogenic Pb isotopic ratios: 206Pb/204Pb(t)=16.88～18.39, 207Pb/204Pb(t)=15.31～15.58, 208Pb/204Pb(t)=36.63～38.06. In addition, some geological and geochemical features suggest that the magma sources of Mengyin Basin are different with Jinan, Zouping and lower crust of North China, but similar to Fangcheng and Jiaolai Basin, showing EMⅠand EMⅡlike enriched isotopic characteristics. It is inferred that the mantle source of the volcanic rocks from Mengyin maybe a mixture of the mantle and the crust .Combining with the primary study of geochronological, geochemical and Sr-Nd-Pb isotopic characteristics of volcanic rocks from nearly area in eastern North China Craton during the same period, we consider that the Late Mesozoic Qingshan Formation volcanic rocks from Mengyin Basin originated from the enriched Mesozoic lithospheric mantle, which evolved from its Paleozoic counterpart through extensive interaction with a crust-derived melt. We propose that this melt was generated from the melting of the subducted lower crust of the Yangtze Craton. This geodynamic model is proposed to illustrate such a transform in secular lithosphere evolution and the lower crust was thinned together with lithosphere mantle in eastern North China Block at Mesozoic.更多还原