北羌塘盆地沃若山剖面上三叠统土门格拉组沉积岩地球化学特征与构造背景分析

沉积岩中的微量元素和稀土元素特征对沉积环境、沉积物的物源和构造背景具有十分重要的指示作用(示踪)。对采自北羌塘盆地钙质泥岩样品中微量元素和稀土元素进行了测试与分析。结果表明,研究区土门格拉组微量元素中的深源元素(如N i、Cu、Ga等)普遍低于地壳粘土岩中的平均值,纵向上逐渐减小,而典型陆源元素Zr等的平均含量远高于地壳粘土岩中的平均含量,反映了大量的陆源物质的沉积且随着沉积作用的进行从底部到顶部陆源性质逐渐增强的特征;此外,Cr/Zr、Th/Sc、Th/U、Rb/Sr、Cr/Th等微量元素比值的特征还表明沉积物物源具有浅源、长英质、风化严重的特点。稀土元素分布模式和纵向变化规律表明土门格拉组从底部到顶部,稀土元素地球化学特征上具有一定的差异性,反映了沉积作用分异度低—高—低—高的特点及其沉积环境的还原—氧化—还原—氧化的变化过程。通过沉积岩La-Th-Sc、Co-Th-Zr/10和Sc-Th-Zr/10构造环境判别图、La/Th-H f和Th/Sc-Sc关系图解以及Th/U源岩判断分析,综合说明沃若山剖面土门格拉组沉积物源区主要以大陆岛弧构造背景为主,同时也有类似于被动大陆边缘构造背景的因素。

Geochemical Characteristics and Tectonic Significance of Upper Triassic Tumengela Formation in Woruo Mountains, North Qiangtang Basin

The trace elements of calcareous mudstone in Tumengela Formation of Upper Triassic,north Qiangtang basin,are analyzed for discussing the sedimentary environment,source provenance and tectonic background of those sedimentary rocks.The result shows that the deep derived elements(such as nickel,copper and gallium etc.) contents of Tumengela Formation are lower than average crustal argillite value,and become lesser from the bottom to the top;while the typical continental origin elements(such as zirconium) are much richer than average crustal argillite,which means the materials from the continent are becoming dominant from the bottom to the top of Tumengela Formation.Furthermore,the elements ratios of Cr/Zr,Th/Sc,Th/U,Rb/Sr,Cr/Th of Tumengela Formation indicate that the sediment sources are seriously weathered superficial felsic materials.The distribution pattern and vertical variance character of rare earth elements of Tumengela Formation indicate that from the bottom to the top,the diversity of sedimentation with the feature of low-high-low-high,the change of sedimentary environment with the feature of disoxidation-oxidation-disoxidation-oxidation.The tectonic environment distinguish diagram of La-Th-Sc,Co-Th-Zr/10,Sc-Th-Zr/10,and relation diagram of La/Th-Hf,Th/Sc-Sc,combined with source rock distinguish value of Th/U,all these indicate that source material of Tumengela Formation in Woruoshan area are mainly from continental arc environment,but some material of passive continental margin environment also participate in it.