The Geochemical Characteristics, Petrogenesis And Zircon U-Pb Geochronology Of Dashipo-heishantuo Composite Pluton In Beijing

The early Jurassic Dashipo-Heishantuo composite pluton in Beijing, whichconsists of Dashipo syenite and Heishantuo granite, exposed in the western yanshanorogenic belt, eastern north China block. The Dashipo syenite is mgnesian potassicalkaline rock and the trace elements shows an enrichment of large ion lithophileelements such as Rb, Ba, Sr, Pb and LREE, and a relative depletion of high fieldstrength elements such as Nb, Ta, U, Th, Zr, Hf as well as P and Ti, with ISrvalues of0.70506~0.70464and εNd(t) values of12.1~12.2. Heishantuo granite is mgnesianperaluminous high K calc-alkaline rock, with an enrichment of large ion lithophileelements such as Rb, Ba, Th, U and Pb, and a depletion of HREE and high fieldstrength elements such as Nb, Ta, Zr and Hf as well as Sr, P and Ti, with ISr values of0.70516~0.70593and εNd(t) values of15.5~18.0，and is depleted of radioactive Pbisotopes. LA-MC-ICPMS zircon U-Pb analysis show that the ages of syenite andgranite are200±1Maå'Œ201±1Ma, which are early Jurassic. The magma of Dashiposyenite is produced by fractional crystallization of K-rich alkaline enrichedmantle-derived magma under a high pressure. At the same time, the mantle-derivedmagma heat and metasomate the lower crust, leading to the partial melting of thelower crust and produce granitic magma which forms the Heishantuo granite wheninvaded in the center of the unconsolidated syenite, resulting in the appearance of theconcentric composite pluton.