Geochronology And Geochemistry Characterristics Of The Intermediate-Felsic Instrusive Rocks In Lajishan Mountains,Qinghai

The Caledonian intermediate-felsic intrusive rocks are predominant in the Lajishan Orogenic Belt which is located at the northeast of Tibetan Plateau.This paper is aimed to choose intrusive rocks in the eastern region of Lijishan,Qinghai,as research object.On the base of combining with predecessors’research achievements that are related with this region,this paper carried on the systematic research on petrology,zircon U-Pb chronology and element geochemistry.The eastern region of Lajishan mainly develops four types of intrusive rocks,namely quartz diorite,quartz monzonlite,granodiorite and granite,and their formation time were 446±1Ma,426.5±3.8Ma,467±1Ma and 517.4±1.4Ma,respectively,by the method of zircon U-Pb dating.Depend on the research of major elements,the quartz diorite and quartz monzonlite have Si O₂contents of 58.78%～63.08%,K₂O of 0.69%～4.80%,Al₂O₃of15.00～18.11%,and an A/CNK ratio of 0.858～1.042,belonging to quasi-aluminous rocks in calc alkaline-shoshonite.And they have higher TFe O contents of 3.74%～6.78%and Mg O of 1.71%～3.19%.Comparatively,the granodiorite and granite belong to weakly peraluminous rocks in high k calc alkaline series,with Si O₂contents of 63.91%～72.46%,K₂O of 0.73%～3.81%,Al₂O₃ of 13.12～15.39%,and an A/CNK ratio of 0.902～1.020.And they have lower TFe O contents of 2.27%～5.39%and Mg O of 0.86%～2.42%.The characteristics of REE and trace and minor elements show that all intrusive rocks are rich in LREE and LILE,while poor in HFSE.Quartz diorite,granodiorite and granite show a low Eu negative abnormality and a smooth right-inclined V-shape standard curve with medium slope.But granodiorite shows a low Eu positive abnormality.The comprehensive analyses indicate that the acidic intrusive rocks in eastern region of Lajishan were I-type volcanic arc granites,and might be formed during the convergence of the plates and subduction of the ocean basin,as the mixed product of mantle-derived magma melts and lower crust materials.And the fractional crystallization played a role in magma evolution.