Trace element and strontium isotope constraints on sedimentary environment of Ediacaran carbonates in southern Anhui, South China

Abstract

Two units of sedimentary carbonates in association with Neoproterozoic glaciations occur in the Lantian Formation of the Ediacaran system, equivalent to the Doushantuo Formation elsewhere in South China. Trace elements and strontium isotopes in the carbonates were analyzed by stepwise dissolutions with different strengths of acid. The results show that the carbonate component dissolved in 0.5 M acetic acid (Dissolution I) is principally composed of calcite and almost free from non-carbonate contamination. Thus its element and isotope compositions can be used to unravel the geochemical feature of water from which the carbonates precipitated. The strontium concentrations and isotope ratios, REE + Y concentrations and patterns of Dissolution I from the carbonates are similar to each other between the two units. They exhibit high strontium concentrations and high 87Sr/ 86Sr ratios, depletion of LREE relative to MREE and HREE, minor positive La anomalies, subtle negative Ce, Gd and Er anomalies, and near-chondritic Y/Ho ratios. These indicate that the Lantian carbonates were precipitated from the similar nature of basin water, which is devoid of trace element composition typical of normal marine carbonates but similar to those in freshwater carbonates. The negative Ce anomalies in the carbonates of the two units suggest oxygenation of the precipitation water. Therefore, the Lantian carbonates would form in a continental marginal basin, with predominant contributions from freshwater and high sedimentary rates. Since the Doushantuo–Lantian carbonate secessions may probably correspond to the chemical sediments of post-Marinoan and post-Gaskiers, it is hypothesized that the freshwater would originate from locally deglacial meltwater with significant inputs from continental weathering.