The Thinning And Uplift Of Eastern Part Of North China Craton In Early Cretaceous

The lithosphere of North China Craton experienced tremendous thinning events in early Cretaceous,but the process and mechanism of thinning are still controversial.The early Cretaceous Xishimen Complex is an important complex related to craton thinning in the eastern North China Craton.Through detailed geological survey and comprehensive study of mineralogy,petrology,geochemistry and chronology,the genesis of the Xishimen Complex is revealed and the lithospheric thinning mechanism in the eastern part of the Cretaceous North China Craton is discussed.The main conclusions are as follows:1.According to the formation age and rock structure characteristics of Xishimen Complex,it can be divided into two sets of rock suite:the early one with equigranular structure and the later one with porphyry structure.The early suite is composed of monzonodiorite-monzonite-syenite series with zircon U-Pb ages of 139.52-131.7 Ma(avg.135.6 Ma),while the latter is composed of porphyry monzonite-porphyry syenite series with ages of 126.1-124.4 Ma(avg.125.3 Ma).In addition,a large number of inherited zircons of about 2500 Ma still exist in these rocks.2.In early suite w(SiO2)=51.72%?68.56%,w(MgO)=0.17%?4.88%,and a few samples show the characteristics of adakite;while in late suite w(SiO2)=57.00%?67.51%and w(MgO)=0.61%? 3.37%.In addition,the trace,REE and isotope compositions of the two sets of rocks are similar,but the zircon in the late suite rocks has a negative ?Hf(t),indicating that they have more crustal material compositions.3.Amphibole thermobarometer indicates that the locating environment of early rock suite is T=883.66 ?,P=194.95 MPa,which is equivalent to 7.36 km deep.The location environment of the late suite is T=642.34 ?,P=26.81 MPa,which is equivalent to 1.01 km deep.Combining the emplacement time difference between the them,it is shown that in about 10.3 Ma,the early rock suite was lifted by 6.35 km,and the average lifting speed is about 0.6 mm/yr.4.The large amount of zircon trapped and inherited in the late suite of the Xishimen Complex reflects that the magma experienced rapid upward intrusion after its formation,and the inherited zircon did not have enough time to exchange material with the melt.Syenite dikes are also widely developed in the Xishimen complex,which further supports the strong uplifting process experienced by the complex.5.At about 135 Ma,the subduction of the Paleo-Pacific Plate began to affect the study area,triggering partial melting and upwelling of the mantle,and prompting the melting and mixing of crustal materials to form early monzonitic-dioritic suites.At about 125 Ma,the continental crust underwent large-scale uplift.It is presumed that large-scale magmatic activity and crustal fold deformation lead to the thickening of the continental crust,which results in large-scale disintegration of the deep lithosphere of the North China Craton,and thus thinning the lithosphere of the North China Craton.