贵州梵净山西北陡山沱组底部白云岩帽地球化学特征及成因探讨

Doushantuo Formation in northwestern Fanjing Mountains, Guizhou

关键词:  梵净山  新元古代  陡山沱组  白云岩帽  地球化学  贵州

Key words:  Fanjing Mountains  Neoproterozoic  Doushantuo Formation  cap dolostone  geochemistry  Guizhou

摘要:

通过对贵州梵净山西北新元古代白云岩帽的研究,发现其δ¹³C_PDB值与全球其他地区冰川混积岩(Marinoan冰期)之上的碳酸盐岩帽中δ¹³C_PDB值一样,发生了一次明显的负偏移,从-1.4‰变化到-8.52‰,平均值为-4.52‰;δ¹⁸O_PDB值从-5.44‰变化到-11.34‰,平均值为-8.68‰。除了白云岩帽顶部的两个数据之外,δ¹³C_PDB值和δ¹⁸O_PDB值均大体表现为随剖面向上升高的趋势,而且两者显示了弱的正相关性。沉积构造和碳同位素值说明,该套白云岩帽的形成与海底永冻土的固态甲烷释放无关,应为南沱冰期后,气候急剧变暖,大量风化的陆源碱性物质因海平面快速的上升被携带到海洋中,与CO₂发生反应,快速沉淀下来的结果,支持了“雪球地球假说”。

Abstract:

Similar to the δ¹³C_PDB values for the cap carbonates overlying the Marinoan glacial diamictites in the other parts of the world, there is a markedly negative excursion of the δ¹³C_PDB values for the Neoproterozoic cap dolostones in northwestern Fanjing Mountains, Guizhou. The δ¹³C_PDB values range from -1.4‰ to -8.52‰, with an average value of -4.52‰, and the δ¹⁸O_PDB values range from -5.44‰ to -11.34‰, with an average value of -8.68‰. Except two data for the cap dolostones at the top, the rest δ¹³C_PDB and δ¹⁸O_PDB values generally increase upsection, and have a slightly positive correlation. The examination of sedimentary structures and carbon isotope shows that the cap dolostones in the study area are not related with the release of solid methane from the submarine permafrost soil. Instead they should be the products originated from the rapid precipitation of the weathered terrigenous alkaline matter following the reaction with CO₂ due to the warmer climates after the Nantuoan glacial stage. This explanation strongly supports the "snowball Earth" hypothesis.