Beijing Yanshan Mesozoic Board The Yanyan Body Characteristics And Causes

The Qipanyan pluton locates in Jiangshuihe village-Qipanyan village,Mentougou area, Beijing, which has a irregular ellipse in plane shape, extension ofnorthwest and trend of northeast. According to the characteristics of outcrop,mineralogy and petrology, the Qipanyan pluton mainly consist of gabbro,olivine-gabbro, (migmatized) gabbro-diorite and monzodiorite. The SHRIMP zirconU-Pb age of the Qipanyan pluton is early Cretaceous. It is the most mafic pluton inthe Yanshan orogenic belt. So it is the best pluton to study property of mantle magmasource.The Qipanyan pluton is mainly a tholeiite series, so the pluton formed in aenvironment of not very high pressure. The LREE and HREE have an obviousfractionation, in which LREE is enriched and HREE is depleted. According to theSHRIMP zircon U-Pb age study of gabbro in the Qipanyan pluton, it has a age of132.90±0.66Ma. The average initial value of ⁸⁷Sr /⁸⁶Sr for the gabro is 0.7045, εNd=-16.54,these characteristics indicate that the source of Qipanyan pluton has adifferent characteristics with EM1 source. Olivine-gabbro has a positive Nbabnormity and a negative Pb abnormity. Rb,Th,Nb,La,U are also have bigdifference with EM1. So we thought the source of olivine-gabbro in Qipanyan plutonis not enriched mantle, but depleted mantle or primitive mantle. The gabbro inQipanyan pluton has a Nb/U=26.4 which is close to the primitive mantle (Nb/U=30). So we concluded that the source of gabbro in Qipanyan pluton is primitivemantle.The thinning mechanism and processes of lithospheric thinning of MesozoicNorth China Craton is the hot spot in the world. There are two models at present: (1)Thermal erosion and/or chemical metasomatism change the lower part of lithospheremantle into asthenosphere. (2) The break off of lithosphere mantle or whlolelithosphere mantle including lowermost part of crust together.According to the study of the Qipanyan pluton, we conclude that the source ofmagma is not enriched mantle (EMI), but asthenosphere mantle. This conclusion isnot consistent with thermal erosion and/or chemical metasomatism mechanismwhich thin lithosphere mantle by changing lithosphere mantle to EM1.So we conclude that the main mechanism for thinning of lithosphere in easternChina is breakoff of lithosphere.