塔里木盆地下寒武统硅质岩地球化学特征及其形成环境

Geochemical characteristics of cherts of Lower Cambrian in the Tarim Basin and its implication for environment

对塔里木盆地北部及塔东1井下寒武统黑色岩系中硅质岩的微量元素和稀土元素分析的结果,硅质岩中As、Sb、Hg、Ba、Sr、V、Ni、Mo、U等元素明显富集,尤其是As、Hg、Ba和U高度富集,Ba/Sr值不大于10×104,U/Th值大于1;稀土元素总量低,具有明显的Ce负异常,δCe值为0.524～0.991,Lan/Cen值为1.112～2.934。地球化学研究表明,该区硅质岩属于海底热水沉积型,其形成环境为大陆边缘深海槽。由于硅质岩与黑色页岩呈互层产出,空间分布稳定,故认为二者形成环境相似;推测该区黑色岩系形成时的缺氧事件可能是海底火山及与之紧密伴生的海底热水流体活动导致的。

In marine sedimentary strata of the Tarim Basin, Northwest China, Yurtusi Formation, Lower Cambrian has been proved to be one of the main source rocks. Laminar cherts intercalated with black shale in outcrops of the Yurtusi Formation. The content of As, Sb, Hg, Ba, Sr, V, Ni, Mo and U are rich in the cherts, especially, As, Hg, Ba and U are much richer. The ratio of Ba/Sr is lower than 10×10^(4), and U/Th ratio is greater than 1. The ∑REE of cherts are lower, with obviously negative anomaly normalized by North America Shale, in which δCe and La_(n)/Ce_(n) values are 0.524-0.991 and (1.112)-(2.934), respectively. The geochemical characteristics suggest that cherts are likely deposited with a hydrothermal origin and formed in continental margin-abyssal environment and developed with the black shale that is deposited in anoxic environment and anoxic event.