TRACING FORMATION MECHANISM OF CHERT DURING EDIACARAN-CAMBRIAN TRANSITION: INSIGHTS FROM SILICON ISOTOPES AND ELEMENTAL GEOCHEMISTRY

The Ediacaran-Cambrian transition period, also known as the "Cambrian Explosion", was a critical time in geological history marked by significant environmental changes and the evolution of organisms. Particularly, the sedimentary deposits of the Yangtze Block in South China show a considerable occurrence of chert during this transition period (551-526 Ma). To understand the formation mechanism of these widespread Liuchapo bedded cherts in the Nanhua Basin (South China), we employed petrological and geochemical analyses, including isotopic and elemental geochemistry. The study revealed that the cherts are predominantly composed of quartz and carbonate minerals. The poor correlation between redox-sensitive trace elements and thorium (Th) suggests that the enrichment of U, V, Mo, and Mn in the cherts is due to authigenic processes rather than detrital input. This indicates that the Liuchapo cherts are largely constituted by authigenic silica and can preserve the geochemical characteristics of the paleo-ocean. The Th/U and V/V+Ni ratios in the chert samples support the existence of anoxic conditions during deposition, which is further corroborated by the Ce-depletion (Ce/Ce* = 0.61). The REE pattern, characterized by slightly depleted LREE, slightly or no depleted MREE, slightly negative Ce anomaly, and positive Gd and Y anomalies, is consistent with modern anoxic basins. The silicon isotope values provide insights into the processes involved in chert deposition. The most negative δ³⁰Si values (-0.7‰ to -0.3‰) suggest a mixing scenario where volcanic material combines with hydrothermal fluids. Conversely, the positive δ³⁰Si values (0.1‰-0.9‰) indicate that the initial solution for precipitation is a mixture of hydrothermal fluids rich in light Si isotopes, volcanic materials, and seawater with heavy Si isotopes. This chemical deposition occurs after the mixing of cooler seawater with relatively hot hydrothermal fluids. We concluded that the episodic hydrothermal fluid, redox conditions, and volcanic material may have jointly influenced the formation of the Liuchapo bedded cherts during the Ediacaran–Cambrian transition period. This study highlights the effectiveness of elemental geochemistry and silicon isotopes in cherts as valuable tools for reconstructing the evolutionary history of ancient marine basins.