【作者】 杨德彬；

【导师】 许文良；

【作者基本信息】 吉林大学 ， 矿物学、岩石学、矿床学， 2009， 博士

【摘要】 本文以蚌埠隆起区花岗岩为研究对象,利用锆石SHRIMP和LA-ICP-MS U-Pb定年方法,确定了花岗岩的形成时代;利用花岗岩的主量、痕量元素和Sr-Nd-Pb同位素以及继承锆石和锆石Hf同位素的示踪技术,讨论了蚌埠隆起区不同时代花岗岩的成因类型和岩浆源区性质及其花岗岩形成的构造背景。提出了扬子克拉通与华北克拉通在中生代早期的俯冲碰撞是由南东向北西方向发生的,郯庐断裂带在中生代时期可能并不存在巨大的左行平移的认识。这对揭示华北克拉通东部的构造演化具有重要意义。锆石U-Pb定年结果表明,蚌埠隆起区花岗岩的形成时代为古元古代(2100 Ma～2200 Ma)、中生代的晚侏罗世(160 Ma)、早白垩世早期(130 Ma)和早白垩世晚期(110 Ma)。古元古代以庄子里、磨盘山正长花岗岩为代表;晚侏罗世以荆山正长花岗岩为代表;早白垩世早期以淮光花岗闪长岩和西庐山、女山正长花岗岩为代表;早白垩世晚期以曹山、锥山二长花岗岩和蚂蚁山正长花岗岩为代表。主量、痕量元素和Sr-Nd-Pb同位素以及继承锆石和锆石Hf同位素综合研究表明,(1)古元古代正长花岗岩为“A”型花岗岩,它们的原始岩浆起源于有少量古老地壳物质涉入的新生下地壳的部分熔融,古元古代正长花岗岩形成于伸展环境;(2)中生代花岗岩属于准铝质-弱过铝质岩石,具有“I”型花岗岩的成因特征,其岩浆源区既涉及了遭受超高压变质作用改造的扬子克拉通基底,又涉及了古老的华北克拉通基底。晚侏罗世花岗岩形成于造山后伸展环境,而早白垩世花岗岩形成于强烈的伸展构造背景。蚌埠隆起区深部地壳中扬子克拉通基底物质的存在,暗示扬子克拉通与华北克拉通在中生代早期的俯冲碰撞应首先发生在两陆块的东部,并以郯庐断裂带为拼合带由南东向北西方向俯冲。然后,因扬子克拉通(顺时针方向)与华北克拉通的相对旋转造成大别地区的剪刀式闭合与碰撞。郯庐断裂带中南段应为两陆块碰撞的缝合线,郯庐断裂带在中生代时期可能并不存在巨大的左行平移。更多还原

【Abstract】 This thesis studies the formation time of the granitoids in the Bengbu uplift, the nature of magma sources, and their tectonic setting based on SHRIMP and LA-ICP-MS zircon U-Pb chronology, major- and trace- elements, and Sr-Nd-Pb isotope as well as zircon Hf isotopic geochemistry for these granitoids. These chronological and geochemical data, together with regional tectonic analysis, provide constraints on the petrogenesis of the granitoids in the Bengbu uplift, which is of significant implications for revealing tectonic evolution in the eastern North China Craton (NCC). Main achievements are as follows:1. The granitoids in the Bengbu uplift are mainly composed of syenogranite-monzogranite- granodiorite. Zircons from these granitoids display a typical fine-scale oscillatory zoning, implying a magmatic origin. SHRIMP and LA-ICP-MS zircon U-Pb dating results indicate that the granitoids in the Bengbu uplift formed in the Paleoproterozoic (2100 to 2200 Ma) including the Zhuangzili and Mopanshan syenogranite, Late Jurassic (160 Ma) including the Jingshan syenogranite, early stage of the Early Cretaceous (130 Ma) including the Huaiguang granodiorite, Xilushan and Nushan syenogranite, and late stage of the Early Cretaceous (110 Ma) including the Caoshan and Zhuishan monzogranite, Mayishan syenogranite. They were not previously believed as the Neoarchean granitoids.2. The granitoids intruded into the country rocks (the Wuhe Complex) and display typical granitic textures. Zircons from them show typical fine-scale oscillatory zoning and high Th/U ratios (0.1 to 1.6). Taken together, it is suggested that they formed from crystallization of magma rather than migmatization.3. The Paleoproterozoic granitoids have SiO2=69.65-77.95 %, K2O=4.98-5.17 % and are characterized by enrichment in light rare earth elements (LREEs), Zr, Hf, Rb, Th, U, depletion in heavy REE (HREEs), Ba, Sr, Eu, P and Ti, and obviously negative Eu anomalies. TheirεNd(t) values and Nd two-stage model ages range from -3.4 to +3.2 and from 2.31 to 2.79 Ga, respectively. TheεHf(t) values of zircons and Hf two-stage model ages are between -5.1 and +7.8 and between 2.26 and 2.83 Ga, respectively. Taken together, it is suggested that the primary magma for the Paleoproterozoic granitoids could be derived from partial melting of the Paleoproterzoic and Neoarchean juvenile crust and minor amount of ancient crustal material. Chemically, these Paleoproterozoic granitoids are“A-type”granites, implying that they formed under an extensional tectonic setting.4. The Mesozoic granitoids belong chemically to metaluminous-weak peraluminous series and exhibit characteristics of“I-type”granites. They are enriched in SiO2, LREEs, and large ion lithophile elements (LILEs) (Rb, Ba, U, Sr) and depleted in MgO, HREEs, and high field strength elements (HFSEs) (Nb, Ta). The existence of the Neoproterozoic magmatic (700 to 850 Ma) and the early Mesozoic metamorphic (206 to 231 Ma) inherited zircons from the Late Jurassic Jingshan intrusion and Early Cretaceous Nushan and Xilushan intrusions, together with the high initial Pb isotopic ratios [(206Pb/204Pb)t=17.883-17.905>17.8)] of the Nushan granites and their lowεHf(t) values (-16.1 to -18.4) of zircons, suggests that their primary magma could be derived from partial melting of the Yangtze Craton (YC) basement. In contrast, the occurrence of the Paleoproterozoic (2300 to 2500 Ma, 1800 to 1900 Ma) and Paleoarchean (3400 Ma) inherited (captured) zircons for the Huaiguang and Nushan intrusions, together with low initial Pb isotopic ratios [(206Pb/204Pb)t=16.567-16.633)] of the Huaiguang and Caoshan intrusions, and lowεHf(t) values (-21.1 to -28.1) of zircons from Huaiguang, Caoshan, Zhuishan and Mayishan intrusions, indicate that their primary magmas could be dominantly originated from the partial melting of the NCC basement. The generation of primary magma for the Nushan intrusion could also involved in the NCC basement. 5. Based on association and geochemistry of these Mesozoic granitoids in the Bengbu uplift, it is concluded that that the Late Jurassic granites could form under an post-collsional extensional setting between the YC and the NCC, and that the Early Cretaceous granitoids could formed under an intra-continental extensional environment.6. The existence of the YC basement within the lower continental crust of the NCC in the Bengbu area, combined with the discovery of the eclogite xenoliths in the Early Cretaceous adakitic rocks from the Xu-Huai area, the studies on the Mesozoic high-Mg diorites and the peridotite xenoliths, as well as Fangcheng and Feixian basalts in western Shangdong, implies that the subduction and collision of the YC beneath the NCC could firstly happen in the eastern margin of the NCC oriented in a NW direction in the early Mesozoic, then they progressively scissored together as the YC rotated clockwise relative to the NCC. Subduction of the YC beneath the NCC could have taken place along the Tan-Lu fault zone oriented in a NW direction in the Early Mesozoic and that the huge strike-slip of the Tan-Lu fault zone did not occur in the Mesozoic.更多还原