THE SOURCE OF SILICON IN CHERTS AND SILICEOUS SHALES FROM THE EDIACARAN THROUGH LOWER CAMBRIAN [SLOPE-FACIES] SEDIMENTARY SUCCESSION OF LIJIATUO, HUNAN PROVINCE, SOUTH CHINA

Ediacaran to Lower Cambrian strata of Central Asia mainly consist of alternating shales, carbonates and phosphorites, which are often associated with remarkable deposits of bedded chert. The origin of these cherts that formed during this crucial phase on Earth when higher life has evolved, remains controversial. Both hydrothermal and continental or biogenic silicon sources have been proposed. The observed enrichment of a range of redox-sensitive metals including Cr, Cu, Ni, Mo, V, U in siliceous shales moreover contributes to the debate about a hydrothermal vs. organo-sedimentary origin of these deposits.

To unravel the source of silicon and the conditions of chert formation, we have investigated the silicon isotope composition and the geochemical composition of chert and siliceous shales of the Ediacaran Liuchapo and the Lower Cambrian Xiaoyanxi Formations at Lijiatuo section, Hunan Province, South China, as well as siliceous sponge spicules from the early Cambrian Niutitang Fm. of Tongpengai section, Guizhou Province (using fs-LA-MC-ICP-MS). The major- and trace element composition of these cherts and siliceous shales strongly supports a non-hydrothermal silicon source. Based on a silicon-isotope-driven mass balance approach, we have estimated the contribution of silicon from different sources. Our results emphasize the importance of continentally-derived and sponge-derived silicon as sources for these cherts.