【摘要】 本文首次用SHRIMP锆石U-Pb测年法获得房山岩体主期侵入岩-花岗闪长岩的年龄为130．7±1．4Ma,证明房山岩体主体岩石形成于早白垩世。综合该岩体两期侵入宕的常量、微量、稀土元素和同位素及构造环境特征,发现房山岩体侵入岩具有许多与埃达克岩(adakite)极其相似的独特的岩石地球化学特征,但又与Defant和Drummond(1990)定义的典型埃达克岩(O型)有明显差别,与中国东部C型埃达克岩更为接近,或也称之为中国东部燕山期高Sr低Y型中酸性火成岩。本文通过房山岩体Sr、Nd、Pb同位素的系统研究,发现在143Nd／144Nd-87Sr／86Sr图解上,两期岩石投影点均落在EM Ⅰ型富集地幔范围之内,暗示其物质来源与富集地幔有关;钾长石Pb同位素特征也说明房山岩体岩浆来源与EM Ⅰ型富集地幔和下地壳关系密切。结合前人的碳、氢、氧同位素研究成果,认为房山岩体物质来源较深,与上地幔和下地壳有关。此外,发现房山岩体两期侵入岩的εNd(t)值(-13．6--14．2)远高于华北地台区古老下地壳的εNd(t)值(-32--44),而与汉诺坝二辉麻粒岩包体的εNd(t)值(-8--18)近似。由于现有的研究已确证汉诺坝二辉麻粒岩包体是由幔源基性岩浆在晚古生代-中生代时底侵到下地壳底部构成的年青下地壳的一部分,故推测房山岩体的物质来源与华北地台古老下地壳关系不大,而可能与年青下地壳关系密切。在此基础上,提出房山岩体两期岩石形成的两阶段模式:第一阶段可能发生于中生代早期,由于软流体 (层)上涌导致富集岩石圈地幔部分熔融生成带有富集地幔印记的玄武岩浆,该岩浆底侵到下地壳底部,冷却成为年青下地壳的一部分;第二阶段发生于中生代晚期,由于当时软流圈呈蘑菇云状大规模上升,热侵蚀面抬升到壳-幔过渡带,导致早中生代新底侵的玄武质下地壳在榴辉岩-麻粒岩相条件下部分熔融生成C型埃达克质岩浆(也即高Sr低Y型中酸性火成岩浆) 并上侵而成为房山岩体。更多还原

【Abstract】 In this paper zircon SHRIMP U-Pb age (130. 7±1. 4Ma) of granodiorite from Fangshan complex in Beijing is reported for the first time, which suggesting that the main intrusion is a product of early Cretaceous magmatic acrivity. Based on the characteristics of major element, trace element, REE, isotope and tectonic setting of two-stage intrusive rocks from Fangshan complex, they have the similar pelrological and geochemical characteristics to those of adakites, but obviously different from those of typical adakites (O-type) defined by Defant and Drummond (1990). They should belong to C-type adakites in eastern China, or named Yanshanian high Sr and low Y type intermediate-acid igneous rocks in eastern China. Systematical studies on Sr, Nd and Pb isotopic characteristics in this paper, in combination with previous O and C isotopic data, indicate that the source rocks of Fangshan intrusives have a close connection to material from the upper mantle and lower crust. Furthermore, a comparison of εNd(t) values of two-stage rocks from Fangshan complex (- 13.6 - - 14. 2) , granulite xenolith from Hannuoba basalts (- 8 - - 18) and the old Archean lower crust of the North China craton (- 32 --44) shows that the material source of Fangshan intrusive rocks are derived from the young lower crust of the North China Craton instead of the old one. Finally, this paper concludes the two-period genetic mode of Fangshan complex.更多还原