上扬子地块北缘灯影组硅质岩系地球化学特征及其成因

通过野外调研, 表明大巴山构造带和米仓山构造带震旦系灯影组发育多套硅质岩系, 并与海相碳酸盐岩相伴产出。微量、稀土元素分析结果表明, 硅质岩系具有热水成因特征。其中, Ba含量较高, Ba/Sr值大于1;Th/U值小于1, Th-U图解中所有样品均投到热水沉积物区;Zr元素较为富集, Zr-Cr图解中样品全部投到热水沉积物趋势线附近;硅质岩系REE总量低, Ce总体为负异常, Eu正异常明显, 硅质岩HREE富集, 硅质白云岩HREE富集不明显。Ce/Ce*、LaN/CeN、V/Y值表明灯影组硅质岩形成于大洋盆地到大陆边缘的过渡地带;Th/Sc、Th/U、V/Cr、V/(V+Ni)值表明硅质岩主要形成于还原环境中, 部分形成于氧化环境中。综合硅质岩稀土、微量元素特征并结合震旦纪区域地质构造演化, 本区硅质岩应形成于复杂大陆边缘富氧的浅海陆棚环境及缺氧的陆内裂陷槽环境中。

Geochemical Characteristics and Genesis of Cherts of Dengying Formation on the North Rim of Yangzi Block

Field survey indicates that a large scale of cherts occur in Dengying formation of Sinian Systemin the tectonic belt of Dabashan and Micangshan, together with carbonatite. The results of trace and rare earth elements analysis indicate that the cherts have the characteristics of hydrothermal origin. Among the trace elements, Ba is enriched and most of the Ba/Sr ratio is greater than 1. Th/U ratio is less than 1 and the values from all samples fall into the hydrothermal sediment area in the Th-U diagram. Zr is also enriched and its content in all samples are neighboring the hydrothermal sediment trend line in the Zr-Cr diagram. The ∑REE of cherts is very low. Ce is negative anomaly in general while Eu is obviously positive. HREE is enriched in the radiolarian cherts, but not distinctly enriched in the siliceous dolostone. The Ce/Ce*, LaN/CeN, V/Y ratios indicate that the cherts are formed in the transition zone from oceanic basin to continental margin. The Th/Sc, Th/U, V/Cr, V/(V+Ni) ratios indicate that the cherts are mostly formed in the reducing environment, with a small number formed in the oxidation environment. Considering the trace and rare earth elements characteristics and regional evolution of tectonics during Sinian period, it is inferred that the cherts are formed in shallow-sea shelf oxygen-enriched environment and anoxic intracontinental taphrogenic trough environmentin the complex continental margin.