Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China

摘要

Marinoan cap carbonates have been suggested to be primarily deposited in glacial meltwater and upwelled sea-water. However, elemental geochemistry evidence for this depositional model is lacking. Here, we report high-spatial-resolution measurements of major, trace and rare earth elements of the Doushantuo cap carbonates from the Jiulongwan section in the Yangtze Gorges area, South China Our results show that: 1) the basal cap carbonates display slight MREE enrichment, weak positive La anomalies, near-chondritic Y/Ho ratios, and slight negative Ce anomalies; 2) the lower-middle cap carbonates show slight LREE depletion or MREE enrichment, weak positive La and Eu anomalies, supra-chondritic Y/Ho ratios, and slight negative Ce anomalies; 3) the upper-middle cap carbonates have consistent enrichment of P, Fe, and trace metals, slight LREE depletion, and weak positive Ce, La and Eu anomalies; and 4) the upper cap carbonates exhibit LREE enrichment weak positive La and Eu anomalies, supra-chondritic Y/Ho ratios, and mild negative Ce anomalies. These findings indicate that the Doushantuo cap carbonates did not precipitate from normal contemporaneous seawater, rather, the basal cap carbonates were deposited in oxygenated, relatively pure deglacial meltwater; the lower-middle cap carbonates in oxygenated brackish water; the upper-middle cap carbonates in upwelled anoxic brine water; and the upper cap carbonates in oxygenated brackish water. Our depositional model is consistent with the proposed sequence of events after the meltdown of Marinoan giaciation by Shields (2005).