Petrological and micrometer-scale geochemical constraints on chert origins in the Dengying Formation, Yangtze Block, South China: Implications for Late Ediacaran hydrothermal activity and tectonic setting

Abstract

The Ediacaran–Cambrian (E–C) transition was a key interval in geological history. Chert was widely distributed in the Yangtze Block at the end of the Ediacaran and may have recorded important information about the coevolution of earth systems in this period. Five lithofacies of chert were identified in the northern margin of the Yangtze Platform. Breccias with crustiform chert were formed by hydrothermal fracturing of dolomite to form breccias in vents, which were immediately immersed and crusted by a silicon-rich fluid. This silicon-rich fluid was deposited directly onto the inner walls of the conduits to form chert linings. The silicon-rich fluid filled the water-fractured fissures to form spider-veinlet chert and then mixed with residual seawater in primary pores of the bedding to form void-filling cherts. Finally, the dolomite far away from the conduits was replaced by the silicon-rich fluid, forming chert bands. In the vents, there are typical hydrothermal mineral assemblages of quartz, pyrite, barite, Fe–Mn metal microparticles, and saddle dolomite and these assemblages contain abnormally high-temperature inclusions. The rare-earth element (REE-Y) patterns of void-filling chert, crustiform chert, and saddle dolomite show that they originated from normal seawater influenced by a low-temperature hydrothermal fluid, silicon-rich hydrothermal fluid, and high-temperature hydrothermal fluid, respectively. The trace element ratios and Si isotope contents of chert also show that deposition was mainly caused by hydrothermal solutions. All these data indicate that the various types of cherts at the top of the Dengying Formation near the synsedimentary fault zone constitute typical siliceous sinter. There was massive upward migration of silicon-rich hydrothermal fluids from depth to the seafloor during the E–C transition, which resulted in abnormally silicon-rich seawater and a widespread distribution of various cherts in the Yangtze Block. The identification of siliceous sinter in synsedimentary faults indicates an intense extension of the northern margin or even of the entire Yangtze Block during the E–C transition. This may be related to the combined effects of slab pull on the western side and rollback on the northeastern side of the Yangtze Block during the convergence of Gondwana.