| During the Cenozoic era,volcanic activity was frequent in the eastern domain of China,and the Cenozoic volcanic belt of the Daxing'anling was formed in the northeastern part of North China.This volcanic belt coincided with the Daxing'anling-Taihangshan gravity gradient zone.Previous studies have carried out a lot of researches on the relationship between the Cenozoic volcanic belt and the deep geophysical properties of the Daxing'anling area,which not only reveals the cause of basalt,but also reflects its deep dynamic setting.However,during the formation process of the volcanic belt,the relationship between the changes in deep geophysical properties and new structural geomorphology and its evolution history need to be further discussed.The Cenozoic volcanic rocks placed at the Daxinganling can be divided into four sections from southwest to northeast,which named as Jining,Hannuoba-Chifeng,Abaga-Dongwuqi and Wuchagou-Nuominhe,respectively this study collects Cenozoic basalt samples located in in the Jining and Dongwuqi areas as the main research object,and combines with the previous published results to explore the evolution and genetic mechanism of the Daxing'anling volcanic belt.Through the isotopic chronology and petrochemical study of the Cenozoic volcanic rocks in the Jining and Dongwuqi area.We got the K-Ar dating of the Dongwuqi basalt samples among 3.97-14.90 Ma.New geochemical data suggest that the Cenozoic basalts showe obvious fractionation of rare earth element,all of which are characterized by the enrichment of LREE and the loss of HREE.The Dongwuqi basalt shows obvious Eu anomaly,which is related to be caused by the separation and crystallization of plagioclase.In addition,both of them are enriched in Pb element.The Pb content value in Jining area is about 2.27-5.99 ?g/g,which is much lower than that in Dongwuqi area of 19.2-37.8 ?g/g.The different Pb content indicates that the contaminant effect of the crust is continuously enhanced from the southwest to the northeast,suggesting that the source area is becoming shallower.At the same time,the average Na/Ti ratio in the Jining area is 1.29,which is higher than the average Na/Tiratio in the Dongwuqi area of 1.19.It also shows the fact that the source area is shallow and the lithosphere is thinned.According to the geochronology data,this study suggests the Zhangjiakou-Bohai fault zone,the Xilamulun fault zone and the Taoerhe fault as the boundary,respectively it can be roughly divided into four main eruption periods of 32-34 Ma,20-23 Ma,9-15 Ma and <1 Ma,and the ages are getting younger from the southwest to the northeast.The source area is obviously uplifted and shallow near the fault zone affected by the tensile action and the source zone position gradually migrates to the northeast direction.It is speculated that this is related to the westward subduction of the Pacific oceanic lithosphere starting at 43 Ma.The retreat of the Pacific from 40 Ma caused the North China Craton to suffer from the northwest-south-eastward eastward stress,resulting in the northern margin of North China in a new tectonic setting.At the same time,the demolishing of the oceanic crust further led to the passive upwelling of the mantle,the thinning of the lithosphere and the occurrence of volcanic activity.Under the control of this mechanism,the new generation of Daxing'anling experienced four stages of uplifting from southwest to northeast. |
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