U-Pb-Hf Isotopic Characteristics Of The Jurassic Granitoids In The Jiaodong Area And Their Tectonic Significance

Multiple remarkable geological events occurred in the Jiaodong area, Eastern China in the Mesozoic era:1) the collision between the North China Craton and Yangtze Craton accompanied by ultra-high pressure metamorphism;2) the subduction of ancient Pacific oceanic plate and long-range effects;3) the destruction of the North China Craton; and4) gigantic scale gold mineralization. These overlapping geological events cause the complexity of the tectonic evolution in the area and the arguments on the understanding of the geological history. Yet the profound research work in the area shows significance both in the basic science and for resource exploration.Many Jurasic granitic bodies outcrop in the area. Their genesis are not only related to the crust-mantle interaction and tectonic evolution, but also closely linked with the mineralization in the abundantly developed gold deposits. Therefore, the research on these granitic bodies is a good point to discuss the tectonic evolution in the broader area. In this thesis, we studied the petrography, zircon U-Pb geochronology and zircon Hf isotopic geochemistry of the Jurasic granitic bodies and the mafic dykes intruding them. These data can well characterize the source and origin of the magma, and can also help to understand the tectonic evolution and crust-mantle interaction in the area.Granite and mafic dyke samples were collected from Wendeng pluton, Duogushan pluton and Linglong pluton. Zircon U-Pb dating with LA-ICP-MS shows that the three plutons were all formed in Mid-late Jurasic era (ca160Ma). Inherited zircons with ages of Neoproterozoic and metamorphic zircons with ages of Triassic were found in all three plutons. Hf isotopic features of the Neoproterozoic inherited zircons are similar to those in the orthogneiss and eclogite collected from the CCSD drilling core. The Triassic age of the metamorphic zircons is coeval with the ultra-high metamorphism caused by the collision between the Yangze Craton and the North China Craton. Therefore, the Jurassic granite in the Jiaodong area should have a source including the crust of the Yangtze Craton which was subducted beneath the North China Craton and experienced ultra-high metamorphism. We also obtained age of ca1800Ma and ca2500Ma for the inherited zircons, which are fingerprints of the North China Craton. Moreover, inherited zircon with concordant age of ca2500Ma is also identified in the Wendeng granite. Its εHf(t) is5.7and single stage model age is2623±32Ma, which is roughly consistent with its U-Pb age. This indicates that the age record of ca2500represent the event of crust-mantle differentiation, and could be regarded as a crustal accretion record of the North China Craton. Therefore, the Wendeng pluton, which is located in the Sulu UHP terrane, was also contaminated by the source rocks from the North China Craton.The lamprophyre dyke intruding the Linglong pluton has an age of ca116Ma. Concordant Neoproterozoic age of the inherited zircon indicates a source from the Yangtze Craton. The mafic dyke intruding the Wendeng pluton has a crystallizing age of ca112Ma, and should be a mixture of a more mafic magma and certain crustal source magma. Combined with the data from previous research work, we consider the upper mantle in the Jiaodong area in Cretaceous as enriched mantle, which was transformed by the subduction and metasomatism of the Yangtze Craton in the mantle depth beneath the North China Craton.The εHf(160Ma) values of the4Jurassic granitic pluton (Wendeng, Duogushan, Linglong and Jingshan, respectively) show a rising trend from the east to the west. If this trend can be verified in a larger regional scale, it suggests that the subduction of the ancient Pacific plate did not significantly affect the Jiaodong area in Jurasic, i.e. the transformation of the tectonic system did not start off at that time. Combined with the inherited zircon age pattern and regional εHf(t) trend, the most sounding explanation should be that the dehydration melting of the Yangtze Craton subducted was contaminated in various extents by the overlying lithosphere of the North China Craton.