华北陆块南缘华山、老牛山及合峪花岗岩体Sr-Nd,Pb同位素组成特征及其地质意义

CHARACTERISTICS OF Sr-Nd AND Pb ISOTOPIC COMPOSITION AND ITS GEOLOGICAL SIGNIFICANCE OF GRANITIC PLUTONS INTHE HUASHAN,LAONIUSHAN AND HEYU AREA AT THE SOUTHERN MARGIN OF NORTH CHINA CRATON

华北陆块南缘的华山、老牛山及合峪等燕山期花岗岩体的主量元素、微量元素和Sr-Nd-Pb同位素地球化学研究表明,3个燕山期花岗岩类总体成分呈酸性,岩石主要属于高钾钙碱性系列;它们具有相似的微量及稀土元素组成模式,LREE,HREE分馏明显,轻稀土富集,Eu异常中等或不明显;富集Rb,Sr,Ba,Th,K,Pb和U等元素,亏损Ta,Nb,Zr,P和Ti等元素,岩体高Sr、低Y和Yb,暗示源区有石榴石残留。岩体的锶同位素初始比值ISr=0.706 878~0.717 383,平均0.708 798,εNd(t)=-17.93^-12.29,平均-14.99,二阶段Nd模式年龄TDM集中在1.6Ga^2.1Ga,具有低放射成因铅同位素组成特征,初始铅同位素比值为:206Pb/204Pb=17.337 94~18.158 22,平均17.630 84;207Pb/204Pb=15.457 71~15.530 59,平均15.491 12,208Pb/204Pb=37.811 19~38.278 67,平均38.012 22。元素和同位素地球化学特征表明,华山、老牛山及合峪等燕山期花岗岩体是华北陆块南缘加厚地壳底部熔融的产物,同时有幔源物质的混入。岩体是随着区域构造体制由挤压向伸展转换,软流圈上涌底侵上覆岩石圈,加热加厚地壳熔融而形成。

The study of major,trace elements and Sr-Nd-Pb isotopic geochemistry in the Huashan,Laoniushan and Heyu Yanshanian granites at the southern margin of North China Craton indicates that the compositions of the granites are acid-stage and belong to high-K calc-alkaline series.Those three plutons share the similar composition pattern of the trace and REE elements,with obvious fractionation between LREE and HREE,enrichment of LREE,medium or not obvious abnormity of Eu,enrichment in LILE（e.g.Rb,Sr,Ba,Th,K,Pb,U）and depletion in HFSE（e.g.Ta,Nb,Zr,P,Ti）;and high Sr contents and low Y and Yb contents.This indicates there was garnet residue in magma source area.Three granitic plutons share similar isotopic compositions in Sr,Nd and Pb isotopes,with ISrvalue ranging from 0.706878 to 0.717 383（average：0.708 798）;εNd（t）value from-17.93to-12.29（average：-14.99）;and with TDMage of Nd concentrating in 1.6Ga-2.1Ga,low radiogenic Pb isotopic composition（206Pb/204 Pb range from 17.337 94to18.158 22,average 17.630 84;207Pb/204 Pb value range form 15.457 71 to 15.530 59,average5.491 12;208Pb/204 Pb value ranges from 37.811 19 to 38.278 67,average 38.012 22）.The element and isotope geochemical composition features shows that the Huashan,Laoniushan and Heyu Yanshanian granitic plutons were derived from the partial melting of thickened crust of the southern margin of North China Craton,with involvement of mantle-derived materials.In view of those geochemical results and regional geology,it is proposed that during the transformation regime from collisional shortening to extension,asthenosphere upwelling caused the underplating of overlying lithosphere and heatied the thickened crust resulting in partial melting,which subsequently emplaced in the crust and form the Huashan,Laoniushan and Heyu etc.Yanshanian granite.