碎屑沉积岩/沉积物Sm-Nd、U-Pb与Hf同位素的化学地球动力学意义

Chemically geodynamic significances of Sm-Nd isotopic system and U-Pb dating and Hf isotopic compositions of zircons in clastic sedimentary rocks/sediments:Theory,methods and practices

碎屑沉积岩能有效记录它们的源区。碎屑沉积岩的Nd同位素组成及其中锆石U-Pb年龄与Hf同位素组成是解析碎屑物质源区、源区壳-幔相互作用及不同块体间相互作用的关键方法。全岩/全样的Nd模式年龄可以指示其源区地壳从亏损地幔分离的平均时间,而锆石Hf同位素模式年龄能够揭示出所测锆石颗粒其形成时的源区特征。细粒碎屑沉积岩/沉积物化学组成代表了其源区上地壳在风化、搬运、沉积过程中不溶于水的Nb、Ta、Zr、Hf、Sc、Ti、Th、REE等的平均组成。实践表明,华北板块、扬子板块、以大兴安岭为代表的中亚造山带东部具有明显不同的形成与演化历史。大兴安岭的地壳增生主要发生于新元古代—古生代期间。由碎屑沉积岩进行化学动力学分析时,应充分系统考虑地质过程的复杂性,将宏观与微观相结合、局部与整体相结合。

This paper has summarized the theory, methods and practices of clastic sedimentary rocks and sediments for chemicallygeodynamics. Clastic sedimentary rocks/sediments can record their provenances effectively. Nd isotopic compositions and U- Pbdating as well as Hf isotopic compositions of zircons in clastic sedimentary rocks/sediments are critical approaches to decipherprovenances of clastic materials, interactions between the crust and the mantle, different blocks of their derived regions, and formation and evolution history of provenance’s regional crust. Nd model age of a bulk sample indicates an average extraction time of its derived regional crust from the depleted mantle as a whole. Hf model age of a zircon grain shows its source characteristics during its formation. The chemistry of fine- grained clastic sedimentary rocks/sediments represent average compositions of their derived regional upper crust for most insoluble elements during weathering, transportation, and deposition in water, such as Nb, Ta, Th, Sc, Zr, Hf, Sc,Ti and REE. The authors’practice shows that the formation and evolution histories of the North China Plate, the Yangtze Plate, and the Central Asian Orogen represented by the Da Hinggan Mountains are significantly different from each other. Crustal growth occurred predominantly in Neoproterozoic- Paleozoic periods in the Da Hinggan Mountains. During chemically geodynamic analysis of clastic sedimentary rocks/sediments, such factors as complexity of geological processes and combination between micro- and macro-scales as well as between locality and integrity should be considered systematically.