Germanium/silica ratio and trace element composition of Early Cambrian siliceous rocks in Keping: implications for the siliceous rocks’ formation and paleoenvironment interpretations

Abstract

This study used the germanium/silica (Ge/Si) ratios, together with rare earth elements and other trace elements to infer the siliceous source and sedimentary environment of the siliceous rocks located at the bottom of Yuertusi Formation in Northwestern Tarim Basin, Keping, China. Previous studies have shown that this siliceous rock stratum formed at the edge of the carbonate platform on the continental shelf. Researchers suggest that these siliceous rocks were formed by hydrothermal activity, but some still draw different conclusions. Understanding the silicon source and depositional environment of these siliceous rocks would help us learn the processes of environmental changes and the causes of biological explosions during this period. The value of germanium/silica ratios of these siliceous rocks is from 0.15 to 0.37 μmol/mol and much lower than above 10 μmol/mol values in siliceous rocks that are known formed by hydrothermal activity. All samples are rich in HREE, which differ from hydrothermal siliceous rocks that are rich in LREE. Most samples lack hydrothermal related elements. All these features show that the source of these siliceous rocks’ siliceous is not hydrothermal fluids. The samples’ Ce/Ce* range from 0.88 to 1, and Th/U ratios range from 0.01 to 0.36. These features suggest these siliceous rocks were formed in an anoxic environment. Considering all the evidence, we conclude that the siliceous rock stratum at the bottom of the Lower Cambrian Yuertusi Formation in northwest Tarim Basin, Keping, was formed in anoxic seawater at the edge of the carbonate platform on the continental shelf. Its silicon source is seawater instead of hydrothermal fluid.