新疆阿尔泰表壳元素地球化学特征

Element geochemical characteristics of the supra-crust of Altay range in China

根据阿尔泰地区1:20万化探均匀采集的1 100余个岩石样品的分析结果,基于全样本统计和迭代剔除方法,得到新疆阿尔泰地区表壳(出露地壳)的39种元素丰度值;同时对其中的SiO2、TiO2、Al2O3等10种常量元素,按氧化物之和为100%进行了归算。阿尔泰地区表壳在化学成分上相当于花岗闪长岩。与全球大陆上地壳元素丰度值相比,明显富集MnO、P2O5和As、Li元素,略富集SiO2和Sb元素,亏损CaO、Na2O和Pb、U以及相容元素Co、Cr、Ni等。阿尔泰地区表壳的K/La、K/Na、K/Th等元素比值与全球大陆上地壳相应元素比值相当;而La/As比值低于全球大陆上地壳,Th/Pb、Th/U、La/Cr比值高于全球大陆上地壳。阿尔泰表壳在成分特征上较典型的岛弧造山带上地壳更为成熟。加里东期、海西期岛弧造山作用以及海西期后的成岩成矿事件是影响和制约阿尔泰表壳元素地球化学特征的重要地质因素。

On the basis of the analysis results of more than 1 100 rock samples colleted evenly during the geochemical exploration of 1:200 000 scale charts covering Altay range in China,the average abundances of 39 elements of the supra-crust(exposed crust)of Altay range were obtained. For calculating the element contents,the total sample statistics and iteration pick-out method were used,the abundances of ten oxides including SiO2,TiO2,Al2O3,etc.,were normalized to 100%. The whole rock chemistry of supra-crust of Altay corresponds to granodiorite composition. In comparison with the average abundances of elements in the upper crust of global continent,the supra-crust of Altay range exhibits the significant enrichment of MnO,P2O5,As and Li,and slight enrichment of SiO2 and Sb. The supra-crust of Altay range shows the depletion of CaO,Na2O,Pb,U and the compatible elements such as Co,Cr,Ni,etc. The element ratios of K/La,K/Na,K/Th,Nb/La,Th/La and Y/La of the supra-crust of Altay are identical to those of upper crust of global continent,but the ratio of La/As is smaller than that of continental upper crust,and the ratios of Th/Pb?Th/U?La/Cr are higher. The supra-crust geochemistry of Altay exhibits more mature characteristics compared with the upper crust of typical island arc orogen. The arc orogenesis of Caledonian and Hercynian cycles and the post Hercynian rock-forming and mineralization events are important geological factors determining the geochemical features of supra-crust exposed in Altay range in China.