中国大陆部分构造单元出露地壳与全球大陆上地壳化学成分的比较——对地壳演化的制约和思考

Comparison of element abundances between the exposed crust of the continent of China and the global average continental upper crust:Constraints on the crustal evolution and some speculations.

根据中国东部上地壳区域元素丰度研究和新疆北部地区1∶20万化探成果,总结了23个二级大地构造单元出露地壳的元素地球化学特征。与全球大陆上地壳元素丰度值的对比显示,中国大陆出露地壳成分演化程度介于岛弧与成熟的加拿大地盾之间。华北、扬子地台由于含大量碳酸盐岩地层,其出露地壳SiO2含量偏低,明显富集CaO和MgO,可见在出露地壳和上地壳元素丰度值研究中不应忽视碳酸盐岩的贡献。相对于最新发表的2种全球大陆上地壳平均成分模型,中国大陆出露地壳系统亏损Au、Hg、Mo、Sn和W,表明现有上地壳模型过高地估计了Au、Hg、Mo、Sn、W等元素丰度值。华北地台及其南缘的秦岭-大别山造山带出露地壳具低μ(238U/204Pb)值(<8)特征,但其他地区μ值较高,暗示华北地台及其周边地区出露地壳的低μ值特点不具备全球意义。鉴于中国大陆内部各构造单元之间出露地壳成分的显著横向差异,以华北地台为主体的上地壳成分模型目前尚无充分理由作为中国和全球大陆上地壳平均值的可靠代表。

Based on the results of the study of regional element abundances in eastern China and 1：200000 geochemical survey in northern Xinjiang, the element geochemical characteristics of the exposed crust in 23 tectonic units of the continent of China are sunmmarized. Comparison with the global average abundance of the upper continental crust, the exposed crust of the continent of China is compositionally more evolved than the upper crust of the island arc but less evolved than the mature Precambrian Canadian shield. The exposed crust of North China and Yangtze platforms has a lower SiO2 content but markedly higher CaO and MgO contents due to the presence of widespread carbonate strata, which suggests that we should not neglect the contribution of carbonate rocks in the study of the exposed crust and the element abundances of the upper crust. In comparison with two recently published average compositional models of the global upper continental crust, the exposed crust of the continent of China is depleted in Au, Hg, Mo, Sn and W, which suggests that the abundances of Au, Hg, Mo, Sn and W in the present global models are over-estimated. The exposed crust of the North China platform and the Qinling-Dabie orogenic belt to its south has lower μ（^238U/^204Pb） values （〈8）, but other regions of China mainland exhibit much higher μ values, which implies that the low μ feature of the North China platform and its adjacent regions does not have global significance. Considering the apparent lateral variation in composition of the exposed crust for the tectonic units of the continent of China, there is no adequate reason to take the average upper crustal compositional model of the North China platform and its adjacent regions as the reliable representative of the Chinese and global continental upper crustal composition.