青藏高原东南木里地区二叠纪苦橄岩的Os-Sr-Nd同位素地球化学研究  

Sr-Nd-Os isotope geochemistry of Permain picrites from Muli area, southeast Tibet.

作　　者：李杰[1] 许继峰[1] 何斌[1] 徐义刚[1] 董彦辉[1,2] 

机构地区：[1]中国科学院广州地球化学研究所同位素年代学与地球化学重点实验室,广州510640 [2]中国科学院研究生院,北京100039

出　　处：《岩石学报》2008年第2期337-347,共11页Acta Petrologica Sinica

基　　金：973项目(2002CB412602);中科院知识创新工程重要方向性项目(KZCX3-SW-143);国家杰出青年基金项目(40425003);基金群体项目(40421303)资助

摘　　要：本文报道了在青藏高原东南木里地区发现的二叠纪苦橄岩和与其共生玄武岩的主微量元素地球化学特征以及Os-Sr-Nd同位素组成。苦橄岩和与其共生玄武岩受地壳混染作用影响较小。根据苦橄岩的Ti/Y比值和初始的Os同位素组成,将木里苦橄岩分为两类:高Ti/Y型苦橄岩和低Ti/Y型苦橄岩,其中高Ti/Y型苦橄岩具有高的γ_(Os)=+5.3～+10.7和ε_(Nd)=+5.9～+6.4,与全球典型洋岛玄武岩的Os和Nd同位素组成接近,代表了地幔柱源区的同位素特征;而低Ti/Y型苦橄岩具有低的γ_(Os)=-4.1～+1.2和ε_(Nd)=+3.2～+5.0,可能表明受到了SCLM(大陆岩石圈地幔)源区物质的混染。与其共生的玄武岩具有低的γ_(Os)=-3.5～-1.6和ε_(Nd)=-0.6～+0.7,表明其来自于不同于低Ti/Y型苦橄岩也有异于高Ti/Y型苦橄岩的地幔源区,但是也可能受到了SCLM物质的混染。基于Nd-Os同位素的地幔柱与SCLM的二端元混合模型显示:低Ti/Y型苦橄岩可能是SCLM物质组分与地幔柱起源的苦橄质原始岩浆混合形成的;与苦橄岩共生的玄武岩可能是由地幔柱来源的玄武质岩浆与SCLM小比例熔融的熔体混合形成的。Permain picrites and associated basahs were recognized in Mull area, Southeat Tibet plateau, This paper presents their elemental and Os, Sr, and Nd isotopic compositions, Magmas of these picrites and associated basahs probably contaminated only minimal amounts of crustal material, Based on Ti/Y ratios and Os isotopic compositions, the Mull picrites can be classified into two types： high-Ti/Y type and low-Ti/Y type： （1） High-Ti/Y type picrites have high Ti/Y and γOs （ ＋ 5. 3 - ＋ 10. 7） and εNd （ ＋ 5.9 - ＋ 6. 4 ）, The Os and Nd isotopic compositions of the high-Ti/Y picrites are similar to those of some modern ocean-island basalts （OIB）, suggesting they were derived from a mantle plume; （2） Low-Ti/Y type picrites have relatively low Ti/Y and γOs （ -4.1 ＋ 1.2） and εNd （ ＋ 3, 2 - ＋ 5,0）, They show εNd values similar to high-Ti/Y type picrites but lower γOs, hinting they were derived from a mantle plume but their magmas experienced interaction with a subcontinental lithospheric mantle （ SCLM ）. The associated basahs have γOs of -3, 5 to - 1,6 and εNd of -0. 6 to ＋ 0. 7, also indicate that the basaltic magmas were probably mixed with a magma derived from SCLM, Using initial Os and Nd isotopic ratios of a plume and SCLM endmember components for mixing model, the mixing diagram shows that the low-Ti/Y type picrites were produced by mixture of plume-type picritic melts with SCLM, and associated basalts were generated by mixing of plume-type basaltic magma with small amount of melts from the SCLM.

关 键 词：苦橄岩 Sr-Nd—Os同位素 二叠纪 地幔柱 木里地区 

分 类 号：P597.3[天文地球—地球化学]