Age And Geochemical Characteristics Of Volcanic Rocks In Bohai Bay Basin-constraints On The Time Limit Of North China Craton Destruction

Craton is the most stable tectonic unit on earth due to its rare magmatic activity,seismic activity and intracrustal ductile deformation.However,North China Craton(NCC)has different craton properties,and craton destruction has occurred.The timing of the cratonic destruction is the key to explore the mechanism and geodynamics of craton destruction.Previous studies on the time limit of the cratonic destruction have made a lot of achievements,but there are still many debates.In this study,we investigate a suite of Mesozoic to Cenozoic volcanic rocks,which are collected from boreholes in the Liaohe Depression of the Bohai Bay Basin the eastern NCC.Based on the previous studies,the geochemical and geochronology characteristics of sample are studied to explore the magmatism and the timing of the cratonic destruction.The results show that,these volcanic rocks have Precambrian basement-like Sr-Nd isotopic characteristics,consistent with derivation from the lower continental crust underneath the NCC.Among them,the Late Jurassic(?165 Ma)intermediate volcanic rocks do not exhibit markedly negative Eu anomalies,which require there is no residual plagioclase in the source area.And the low heavy rare earth elements(HREEs)contents of these samples indicate that their source has garnet as residue.The Early Cretaceous(?122 Ma)felsic volcanic rocks are depleted in HREEs but with remarkable Eu anomalies,suggesting that their source have both garnet and plagioclase.The Cenozoic alkaline basalt have Ocean Island Basalt(OIB)-like trace element characteristics(Sr,Nb,Ta positive anomalies and Pb negative anomalies on the spider diagram).The depleted Sr-Nd isotope,low Hf isotope ratio and high Pb isotope ratio of Cenozoic alkaline basalt also suggest that subducted oceanic crust components are involved in the depleted mantle source area.According to the geochemical characteristics of the intermediate and felsic rocks,the crustal thickness is estimated are ?50 km at Late Jurassic(?165 Ma),and 30?50 km at Early Cretaceous(?122 Ma).The crustal thinning is attributed to lithospheric delamination beneath the NCC.Our results combined with previous studies imply that the large-scale lithospheric removal occurred in the Early Cretaceous,between 140 and 120 Ma.Moreover,the change from the enriched mantle to the depleted mantle in the Cenozoic basalt magma source region represents the final stage of lithospheric thinning.