Geochemistry of the Baicha A-type granite in Beijing Municipality:Petrogenetic and tectonic implications.

Located in Changping Distrct of Beijing Municipality,the 127Ma aged Baicha pluton is a vug peralkaline granitic stock intruded into the Badaling Batholith of Yanshan orogenic belt.It obtains high contents of silica,Zr,Ga,Pb,HFSE,and REE,with high Fe/Mg and Ga/Al ratio,but extreme low contents of Ca,Sr,and Eu;meanwhile,the granite exhibits the depleted radiogenic lead isotopes.Therefore,Baicha pluton is the typical reduced A-type granite defined by Dall'Agnol and Oliveira(2007).In geochemistry,Baicha pluton is very similar to the late Mesozoic Houshihushan(Shanhaiguan) and Kulongshan A-type granites in Yanshan belt.The origin of Baicha pluton is inferred to low-degree melting of the lower mafic crust of North China Craton,under the high temperature,low water and oxygen fugacity conditions.Based on regional geology and petrogenetic regime,Baicha granite is considered as a hallmark of the beginning of orogenic collapse in the western segment of Yanshan orogenic belt.Besides Yaokeng pluton in southeastern Zhejiang Province,Baicha pluton is another peralkaline granite reported in China that has Al-type element geochemistry but was not generated in anorogenic tectonic setting.