Chronology And Geochemistry Of Granitoids In Bengbu Uplift, Central China: Constraints On Tectonic Evolution In The Eastern North China Craton

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.