Petrology,Geochemistry And Geochronology Of Qiancengbei Intrusion,Yanshan Mt

Yanshan Mt.is located in the northern margin of the NCC.Mesozoic magmatic activityin the areais very strong,forming a series of granitoid.It is very important to do the researchesfor understanding the tectonic-magmatic evolution and the deep mechanism of the NCC.As a Mesozoic granitic rock intrusion,Qiancengbei intrusion spread in the NE direction and was composed by quartz-syenite,syengranite and adamellite with main minerals of Pl,Or,Q and a few Hb and Bi.The major element of the granotoids shows similar signatures that of high SiO₂,high alkalis(Na₂O+K₂O),poor CaO,MgO and Al₂O₃,and comparatively high FeOT/MgO ratios.With regard to trace elements and rare earth element,the quartz-syenite obvious enrichment LILE Ba,K ect.,HFSE Zr and Hf etc.,significant losses of Th,Rb,Sr,P,Ti,?REE content is higher,chondrite-normalized REE patterns is the right type,reflected the characteristics of LREE is comparatively rich and shows weak positive Eu anomalies with?Eu>1.Adamellite have the same signatures with Syengranite,which have a obvious enrichment LILE Rb,Kect.,HFSE Zr,Hf,Th ect.,depletion of Ba,Sr,P,Ti,?REE content is high,chondrite-normalized REE patterns is the right type,reflected the characteristics of LREE is comparatively rich and show negative Eu anomalies with all?Eu<1.Zircon U-Pb chronology study shows that the formation age of the Qiancengbei intrusion was between 140Ma and 126Ma,it is the product of the early cretaceous.Diversity of the rock of the Qiancengbei intrusion was resulted by magma mixing.Quartz-syenite magma was the result of the mixing of a small amount of the alkaline basalt magma formed by the low degree of partial melting of enriched lithospheric mantle and the felsic magma which formed by partial melting of lower crust,the time ofevolution is short,fractional crystallization is not obvious,and emplacement in a relatively short time.Syengranite and Adamellite was the result of the mixing of the alkaline basalt magma formed by the partial melting of enriched lithospheric mantle and the magma formed by partial melting of cal-alkaline granites of the shallow crust,they have experienced in the process of the fractional crystallization of plagioclase,biotite.In the late of the syengranite magma evolution,adamellite magmawas foemed due to the join again of mantle-derived magma graniteQiancengbei intrusion was formed in the crust extension environment,its formation may be related to the,lithospheric thinning,subduction of the Pacific plate.