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《Geological Review》 2009-02
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Origin of the Volcanic Rocks in the Donglingtai Formation from Yanhecheng Area,Western Hills of Beijing and Its Geological Implications

WANG Yang1,2),JI Guangyi3),SUN Shanping2),LI Jiazhen2)1) State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing,100083;2) Division of Petrology and Mineralogy,China University of Geosciences,Beijing,100083;3) Geological Institute of Beijing Municipality,Beijing,100011  
The Donglingtai volcanic rocks exposed in Yanhecheng area,Western Hills of Beijing are composed of basaltic trachyandesite in the lowermost segment(1st member of Donglingtai Formation),rhyolitic pyroclastics in the lower portion(2nd member),and trachyte,rhyolite in the upper portion(3rd and 4th member).According to geochemistry of these rocks,the lowermost mafic rocks belong to alkaline series,and exhibit Coombs trend in total alkali vs.silica(TAS) diagram;however,the intermediate and acid rocks belong to high-potassic calc-alkaline series.As a whole,the Donglingtai volcanic rocks exhibit straddle trend in TAS diagram,which evolves from alkali-rich mafic rocks to silica-saturated intermediate and acid rocks.The mafic rocks are derived from fractional crystallization of primitive basaltic magma,which is enrichment of Ba,Sr,LREE,and K,under intermediate pressure;meanwhile,these mafic rocks had been assimilated by lower crust materials during petrogenetic process.The trachytes/dacites in upper Donglingtai Formation are divided into three categories:the aluminous-rich potassic trachyte(KAT),and the aluminous-rich sodic trachyte or dacite(NAT),and the aluminous-poor trachyte or dacite(PAT).The petrogenetic analysis shows that,KATs are produced by melting interaction between basaltic magma and middle crust material;and the fluid-bearing melting of solidified basaltic magma generates NATs;however,PATs are originated from mafic rocks through fractional crystallization accompanying wall-rock assimilation,i.e.AFC process.The acid volcanic rocks in the 2nd member of Donglngtai Formation are generated by low-degree melting of low-potassic rock of lower crust under relative high oxygen fugacity condition.Whereas the low-silica end-members of acid rocks in upper Donglingtai Formation are the products of melting interaction between basaltic magma and middle crust material,and the high-silica end-members of acid rocks is derived from low-silica end-members by fractional crystallization.It is inferred that the large-scale early—middle Early Cretaceous magmatism in Yanshan belt was originated from melting interaction between mantle-derived magma and crustal rocks.Donglingtai volcanism is an exhibition of the crustal heating by intraplated basaltic magma in Yanshan region during middle Early Cretaceous epoch.The increased temperature and partial melting within the lower portion of crust triggered lateral flow of crust;eventually,the Early Cretaceous Yanshan changed from a high relief mountainous chain into a gently-undulated plateau.
【Fund】: 国家自然科学基金资助项目(批准号40572128 40376013 40104003)的成果
【CateGory Index】: P588.1
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