Implications From The Hf-B Isotopic Compositions Of The Early Cretaceous Mafic Dykes For Crust-mantle Evolution Of The Jiaodong Area

The Early Cretaceous Mafic dykes in the Jiaodong area has been sampled and studied in this study.Based on field geological survey and whole-rock geochemical characteristics,we carried out the geochronology and Hf isotope studies on magmatic zircon and inherited zircon of the mafic dykes and its whole rock B isotope.The evolution history of the crust and mantle in the Jiaodong area was determined in detail.The geochronology and Hf isotope characteristics?Hf?t?=-39⁸?of the inherited zircons in the mafic dykes indicate that the zircons were mainly derived from the ancient crustal source of the North China craton?NCC?;The two-stage model age(T_DM2)of Hf isotope indicates that the Jiaodong area had begun to crustal growth with magmatic activity at the Early Archean period?3.8-3.6Ga?;The Mesoarchean-Neoarchean?2.9-2.7Ga?period was a large-scale growth stage of the earth's crust,both of which were juvenile crusts,with a peak age of²820Ma.At the same time,a metamorphic event occurred in the early Neoarchean period;In addition,the Neoarchean period?2.7-2.4Ga?is also a stage of crustal growth,which is mainly based on the remelting of the ancient crust,accompanying with a small amount of addition of magma;In the early Paleoproterozoic?2.4-2.1Ga?,?_Hf?t?value of the inherited zircon can reach about-15,which belongs to the formation and development stage of Jiao-Liao-Ji fault zone,moreover the crustal assimilation was obvious,and the continental crust is transformed from immature to semi-mature;High grade metamorphism and series of magmatic thermal events occurred in the late Paleoproterozoic?1.95-1.75Ga?,because the value of?_Hf?t?is 4^-18,and have few positive values.The lithospheric mantle is metasomatized by subduction-related crust-derived fluids/melts.At this time,the mantle has shifted to the enriched mantle;In the Neoproterozoic?800-550Ma?crustal extension and periodic magmatic activity occurred in the Jiaodong;In the Paleozoic?540-252Ma?,both crust and mantle were generally in a stable period.During the Mesozoic,Jiaodong began to undergo structural changes,including that the Sulu orogenic belt was formed by the collision and subduction of the Yangtze Craton?YC?and the NCC during the middle-late Triassic?230-195 Ma?,accompanied by magmatic thermal events;In Jurassic?180-150Ma?,there were a large number of inherited zircons with?_Hf?t?values of about-22,indicating that these inherited zircons derived from ancient continent crust.Meanwhile,asthenosphere beneath Jiaodong began to occur large-scale upwelling,leading to lithospheric thinning and magmatic activity;In the Cretaceous,the lithospheric thinning,and the magmatic activity further were enhanced.The B isotopic composition of the mafic dykes in Jiaodong area(?¹¹B=-21.47‰^-3.6‰)is significantly lower than that of altered oceanic crust,marine sediment or magmatic rock that has undergone seawater alteration,however it is close to the average value of continental crust or continental crust-derived granite.Therefore,it is believed that the source area was affected by the subduction of the YC below the NCC during the Triassic period,and the subduction-related crust derived felsic melt resulted into a change of the lithospheric mantle of Jiaodong area,eastern North China Craton.