燕山期中国东部高原下地壳组成初探:埃达克质岩Sr、Nd同位素制约  

Preliminary study on the components of the lower crust in east China Plateau during Yanshanian Period: Constraints on Sr and Nd isotopic compositions of adakite-like rocks.

作　　者：张旗[1] 王元龙[1] 王焰[2] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029 [2]西北大学地质系,西安710069

出　　处：《岩石学报》2001年第4期505-513,共9页Acta Petrologica Sinica

基　　金：国家"973"项目"大规模成矿作用与大型矿集区预测"(G1999043206-05);中国科学院知识创新工程基金(KZCX-07)资助的课题.

摘　　要：大陆地壳组成是地球科学家关注的课题,但困难的是如何确定深部地壳,尤其是下地壳的组成。埃达克质岩的厘定从一个方面有助于这个难题的解决。中国东部“C型”埃达克质岩(adakite-like)的地球化学性质表明其形成的深度大,熔融的温度高,为下地壳中基性麻粒岩部分熔融的产物。因此,可以尝试通过埃达克质岩的地球化学特征来反演下地壳的组成。本文的初步研究表明,中国东部埃达克质岩的Sr-Nd同位素组成有很大的变化,反映埃达克质岩熔融的源区下地壳组成有明显的不同。根据埃达克质岩石的Sr-Nd同位素组成,推测中国东部高原下地壳大体由胶东-大别、北京-辽西和下扬子(长江中下游)3个不同的块体组成。1,胶东-大别区:胶东区的下地壳以富Si、Na2O/K2O低(0.8～1.3)、ISr高(0.709～0.712)、εNd(t)低(-15～-21)和TDM高(1.9～2.1Ga)为特征;大别区的ISr值较低,可能代表了中生代时基底的特征,大体相当于该区下地壳的上限值。2,下扬子区位于扬子地块内,中生代埃达克质岩的Sr初始值、εNd和TDM变化大,说明元古宙的下地壳基底中可能有较多年轻的玄武质岩石底侵加入。相比而言,铜陵区的ISr较高(0.707～0.709)、εNd(t)较低(-11～-16)、TDM高(1.7～2.2Ga),接近扬子下地壳的特征。3,辽西-北京地区的Sr-Nd同位素组成以低的ISr(The composition of continental crust has always been an interesting subject for many geologists. However, the big difficulty is how to determine the composition of the deep crust, especially the composition of the lower crust. The identification of adakite-like rocks within the continental crust may provide a possible way to make sense of the problem. The geochemical characteristics of C-type adakite-like rocks in the eastern China show that they are the products of partial melting of intermediate to mafic granulites in the deep lower crust in high P-T condition, the composition of the lower crust therefore can be traced by these adakite-like rocks. Our preliminary studies show that the geochemical characteristics of adakite-like rocks are different from place to place in the eastern China, suggesting that the lower crust sources from which the adakite-like rocks are derived varying in the compositions from place to place too. Based on the Sr-Nd isotopic compositions of these adakite-like rocks, the lower crust of east China Plateau is argued to be formed by three different parts of Jiaodong-Dabei district, Beijing-Liaoxi district and the lower Yangtze district (the middle-lower reaches of Yangtze River). Following opinions are discussed in the paper. (1) The lower crust of Jiaodong area is characteristic of high SiO2 content, low Na2O/K2O ratios (0.8 similar to1.3), high I-Sr ratios (0.709 similar to0.712), low epsilon (Nd)(t) values (-15 similar to -21) and high T-DM ages (1.9 similar to2.1Ga), whereas the lower crust of Dabie area has relatively low Is, ratios, which is the upper Is, ratio limit of the whole area and may probably represent the composition of Mesozoic basement. (2) The lower Yangtze district lies in the Yangtze Block, in which Mesozoic adakite-like rocks have variable I-Sr ratios, epsilon (Nd) (t) values and T-DM ages, suggesting the underplating of abundant young basaltic magma beneath the lower crust basement of Proterozoic. The adakite-like rocks from Tongling area have relatively high Is. r

关 键 词：燕山期 中国 下地壳 埃达克质岩 SR-ND同位素 锶同位素 钕同位素 

分 类 号：P588.121[天文地球—岩石学] P597[天文地球—地质学]