Germanium/silica ratio and trace element composition of Early Cambrian siliceous rocks in Keping: implications for the s
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ORIGINAL ARTICLE
Germanium/silica ratio and trace element composition of Early Cambrian siliceous rocks in Keping: implications for the siliceous rocks’ formation and paleoenvironment interpretations Zixuan Guan1 • Shibiao Deng1 • Peixian Liu1 • Yiqiu Jin2 • Xingchun Cao1
Received: 8 May 2020 / Revised: 10 August 2020 / Accepted: 16 September 2020 Ó Science Press and Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2020
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 lmol/mol and much lower than above 10 lmol/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
& Zixuan Guan [email protected] 1
MOE Key Laboratory od Orogenic Belts and Crustal Evolution, School of Earth and Space Science, Peking University, Beijing 100871, China
2
Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, China
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. Keywords Siliceous rocks Trace elements Ge/Si ratio Tarim Basin
1 Introduction The Ediacaran–Cambrian transition is a primary node in the evolution of the global paleoenvironment, during which major geology events such as ‘‘Snowball Earth,’’ ‘‘Supercontinental Cracking,’’ and ‘‘Cambrian Explosion’’ had occurred. This process is fully recorded in the Yangtze and Tarim platforms in China (Zhang et al. 2016). The Lower Cambrian black rock series (the Black Shales) in the Yangtze platform has important features such as a polymetallic enrichment layer (Ni–Mo–V–Au) and the Oceanic Anoxic Event (OAE), and have received in-d
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