Early Cretaceous Magmatic Activities And Tectonic Significance In Shangcheng Area,Northern Dabie Orogen

The relationship between the Early Cretaceous magmatism and tectonic regime transition in the Dabie orogenic belt is one of the key issues of the Central Orogen in China.Many scholars believe that transition of tectonic regime and the corresponding tectonomagmatic activities in eastern China during the Late Mesozoic are closely related to the subduction and roll-back of the paleo-Pacific plate.This study area is located in Shangcheng area,Henan Province.We conducted an intergrated investigation involving detailed field obsevations,zircon U-Pb dating,whole rock geochemistry,isotopic studies and synthesis of existing data sets,analyses the magma properties and genesis of Shangcheng pluton,Jingangtai volcanic rock and Daquandian pluton,the composition of the lower crust,the mechanism and tectonic transition time of Cretaceous Dabie orogen.According to the zircon U-Pb dating,the crystallization ages of Shangcheng magmatic pluton are 136.8 ? 129.5 Ma,the formation ages of Jingangtai volcanic rocks are 129.8 ?128.3 Ma,and the crystallization age of Daquandian rock magmatic pluton is 120.9 Ma.The eruption of Jingangtai volcanic rocks is later than the emplacement of Shangcheng pluton,but earlier than the emplacement of Daquandian pluton.The Shangcheng pluton is quasi-aluminous high-K calc-alkaline granite with the geochemical characteristics of adakite formed by melting of thickened lower crust,indicating that it was formed by partial melting of thickened lower crust.There may be garnet residualse but not plagioclase in the source area.The Jingangtai volcanic rocks are characterized by low Sr,high Y,flat distribution of HREE,obvious negative Eu anomaly and high Ga/Al ratio,which are similar to typical Atype granites,and are related to the magma underplating derived from mantle.There may be plagioclase residuals in the source area,which was formed in the transitional environment from thick crust to average crust thickness.Daquandian pluton is quasi-aluminum or aluminum high potassium calc-alkali granite.It has the nature of arc magma,which came mainly from the partial melting of the old lower crust,mixed with mantle-derived magma,and also has A-type granite properties.This pluton is suggested to develop in the transition environment from normal-thickness crust thickness to strongly crustal-extension.According to the magmatic evolution process in the Shangcheng region,we suggest that continental crust has undergone the transition from thicken crust formed by intense compression and to normal-thickness crust formed by crustal extension,and the magma source area was changed from the deep to the shallow.Based on the previous work and this study,it is suggested that the partial melting of the adakite from the thickened lower crust during 143 ? 129.5 Ma is related to the continuous subduction of the Paleo-Pacific oceanic lithosphere.The magmatic rocks with low Sr and Eu negative anomalies were widely exposed during 129.5 ?116 Ma,which was related to the slab break-off and roll-back of the Paleo-Pacific oceanic lithosphere.The transition time of the Paleo-Pacific subducting continental margin to regional extension in the Dabie area is ca.129.5 Ma,and the slab break-off and roll-back of the Paleo-Pacific oceanic lithosphere is the mechanism of the Early Cretaceous magma eruption in the Dabie area.