Hydrothermal alteration of the Ediacaran Doushantuo Formation in the Yangtze Gorges area (South China)

Abstract

The geochemical and fossil record preserved in the Ediacaran age (635–551Ma) Doushantuo Formation of South China has been extensively examined to explore the impact of changing climate and the oxidation state of the oceans on the development and distribution of early multicellular life. In the Yangtze Gorges area, this formation shows many of the geochemical trends and features thought to typify global ocean chemistry in the Ediacaran Period, but there are indications that post-sedimentary processes modified these signals. This study of clay minerals and organic matter builds a more detailed picture of the type and degree of post-sedimentary alteration at different stratigraphic levels of the formation and focuses on how this alteration influenced stable carbon and oxygen isotope records.In the cratonward Jiulongwan and Huajipo sections of the Doushantuo Formation, its lower part (Members 1 and 2) consists largely of dolomitic shale, rich in authigenic saponite that crystallized in an alkaline sedimentary basin. Saponite has been altered to chlorite via corrensite across tens of meters of strata in lower Member 2, with increased alteration downward toward the cap dolostone. The greater chloritization is accompanied by lower δ18O and higher δD values of trioctahedral clays. This pattern of alteration of trioctahedral clays is likely due to hydrothermal fluid activity in the underlying, relatively permeable Nantuo Formation and cap dolostone. A concomitant increase of solid bitumen reflectance toward the base of the formation supports this idea. In the uppermost part of the formation in the Yangtze Gorges area (Member 4), a typical open water marine dolomitic shale rich in illite and organic matter, increases in the methylphenanthrenes ratio index and solid bitumen reflectance correlate with decrease of the bulk rock K/Al ratio upward, providing evidence for hot fluid migration above the nearly impermeable shale.Clay from the upper part of the formation is enriched in 18O, but not in D, relative to clay from the lower parts, indicating progressive 18O-enrichment of hydrothermal fluids that percolated upward and laterally through permeable 18O-rich carbonates. A maximum hydrothermal-alteration temperature of ∼200°C is estimated from a calibration curve for illitization during burial diagenesis, but given that the hydrothermal activity probably occurred in short pulses, the temperature could have been much higher. K–Ar ages are consistent across different size fractions of fine illite from Member 4 shale (∼430Ma) and from a K-bentonite bed near the base of Member 2 in the Jiuqunao section (∼325Ma), ∼25km from Jiulongwan and Huajipo. These age values show that the diagenetic illite of the Doushantuo Formation is a product of either deep burial diagenesis overprinted by spatially limited hydrothermal activity or of two localized hydrothermal events.Patterns of carbonate 13C and 18O depletion in the basal Doushantuo Formation are similar to chloritization trends and 18O variation in diagenetic clay minerals. Given independent evidence for 13C depletion of hydrothermal fluids, these trends indicate carbonate–fluid isotope exchange commensurate with the degree of post-sedimentary alteration, supporting a model of lithologically controlled differential diagenesis induced by hydrothermal fluids as the main control on C and O isotope variability in this stratigraphic interval. This model could potentially explain other notable δ13C excursions higher up in Member 3.