八达岭花岗杂岩的组成、地球化学特征及其意义  

A granitoid complex from Badaling area, North China: composition, geochemical characteristics and its implications.

作　　者：王焰[1] 张旗[1] 

机构地区：[1]中国科学院地质与地球物理研究所

出　　处：《岩石学报》2001年第4期533-540,共8页Acta Petrologica Sinica

基　　金：国家自然科学基金(49872027);中国科学院知识创新工程基金(KZCX 1-07)资助的项目.

摘　　要：北京地区八达岭花岗杂岩是燕山期岩浆侵入活动的典型代表,按照地球化学特征及成因,本文将其分为3类:第1类由辉长-闪长岩组成,富Fe、Ti和P,Th/Ta比值近似等于1(0.7～1.2),ISr和εNd(t)值低(分别为0.705和-8～-11),产于板内环境,推测是大陆岩石圈地幔部分熔融形成的,可能代表了燕山期时底侵到加厚陆壳底部的玄武质岩浆。第2类为八达岭花岗杂岩的主体,由石英闪长岩-花岗闪长岩-二长花岗岩组成,其地球化学特征为:SiO2>57％,K2O>2.7％,Na2O/K2O=0.9～1.7,Al2O3在16％～14％之间,LREE富集,(La/Yb)N=25～41,HREE亏损(Yb<1.32μg/g),无或有弱的负铕异常(Eu/Eu＊=1.0～0.9),富Sr(354～1191μg/g),贫Y(<16μg/g),Sr/Y比值高(45～156)。上述地球化学特征除了K2O含量偏高和Al2O3含量偏低外,大体类似于埃达克质岩石的性质,表明岩浆的源区很深,与石榴石处于平衡,暗示其来源于燕山期华北加厚的陆壳底部,可能是下地壳中基性麻粒岩部分熔融形成的。第3类由碱长花岗岩和石英二长岩组成,Na2O +K2O>9％,贫Sr、Ba,富Rb,LREE富集,有明显的负铕异常(Eu/Eu＊=0.4～0.5),属于A型花岗岩,是由于第2类埃达克质岩石形成之后,引起了岩石圈拆沉,造成地幔上涌至减薄陆壳的底部,引起壳幔过渡带物质部分熔融的结果。Badaling granitoid complex in Beijing area is a typical example of Yanshanian intrusions. According to the geochemical characteristics and genesis, the complex can be divided into three groups. The first group is composed of gabbrodiorites. The rocks are characteristic of enriched Fe, Ti and P, Th/Ta approximate to1 (0.7 similar to1.2), and low I-Sr ratios (0.705 on the average) and low epsilon (Nd)(t) values (-8 similar to -11), probably formed in the interplate environment. The gabbro-diorites are resulted from partial melting of subcontinental lithospheric mantle, probably representing the underplating basaltic magma below the thickened continental crust during Yanshanian period. The second group is the main body of Badaling complexe, consisting of quartz diorite, granodiorite and adamellite. The chemical compositions show that they have SiO2 > 57%, K2O > 2.7%, Na2O/K2O = 0.9 similar to 1.7, Al2O3 between 16 and 14%, LREE enrichment with (La/Yb)(N) = 24 similar to 41, HREE depletion with Yb<1.32 <mu>g/g, no or small negative Eu anomaly (Eu/Eu* = 1.0 similar to0.9), high Sr content (354 similar to 1191 mug/g) and low Y content (<16 <mu>g/g) with high Sr/Y ratios (45 similar to 156). Except for relatively high K2O and low Al2O3 content, they are similar to adakite in composition, suggesting that the magma is equilibrated with garnet in the deep and is derived from the bottom of thickened continental crust of North China block during Yanshanian period, probably the products of partial melting of intermediate-basic granulite in the lower crust. The third group is composed of adamellite and quartz monzonite. They belong to A-type granite and are characteristic of Na2O + K2O > 9%, Sr and Ba depletion and Rb and LREE enrichment, with relatively large negative Eu anomaly (Eu/Eu* =0.4-0.5). Due to the formation of adakite-like rocks of the second group, the lithospheric delamination occurs and cause asthenosphere to upwell to the bottom of thinned continental crust. The granotoid rocks of the third group is therefore f

关 键 词：花岗质岩石 埃达克质岩 地球化学 成因 八达岭 岩石圈拆沉 岩浆活动 

分 类 号：P588.121[天文地球—岩石学] P59[天文地球—地质学]