Petrogenesis And Magma Source Of The Mesozoic Complexes In Western Shandong Province

The Western Shandong Province is located in the southeastern margin of the North China Craton,and has experienced multiple magma-tectonic processes in the Mesozoic.These widely distributed magmatic activities caused the lithospheric thinning and mantle property changing.In this research,by the geochemistry,zircon U-Pb geochronology and Lu-Hf isotopes of the Mesozoic Tiezhai,Jinxingtou and Sanshanyu complexes,the aim is to understand the genesis and dynamic background of them.(1)The main lithology of the complexes is diorite,porphyry monzonite and porphyry diorite.The rocks are characterized by peraluminous,high sodium,low potassium and high Mg#,relative enrichment in light REEs and depletion in heavy REEs absence of obvious Eu and Ce anomalies,positive Sr,Ba and Pb anomalies,and negative Nb,Ta and Ti anomalies,High Sr(433-1107ppm),low Y(6-13.3ppm),Sr/Y(57.26-163.0),which show geochemical features of the adakite-like rocks.(2)LA-ICP-MS zircon U-Pb age of porphyry diorite in the Jinxingtou is 122 Ma.Zircon U-Pb ages of porphyry diorite and porphyry monzonite in the Tiezhai are 123,123 and 125 Ma.Zircon U-Pb ages of porphyry diorite in the Sanshanyu are 123 and 125 Ma.These rocks are all the peak periods of early Cretaceous magmatism.(3)The range of ?Hf(t)values of the new zircons in the samples is wide,ranging from-25.6 to 7.8,indicating that the magma is of multi-source nature;the inherited zircons are concentrated at?2500 Ma,and the ?Hf(t)values are 2.5 to 8.8,which may be the product of the reactivation of the old basic lower crust in North China.(4)Magma source of the complexes is derived from partial melting of the upper mantle contaminated by the lower basic crust materials,with a small amount of bottom-invading asthenospheric materials.The destruction of the NCC is the subduction of the Paleo-Pacific plate,which was related to the asthenospheric upwelling and lithospheric thinning.Dynamic mechanism is characterized by the extension process of decompression and warming,following by the thickening and melting of the lower crust.