A petrographic and geochemical study of carbonate and silica phases from the Ediacaran Doushantuo Formation in the Three Gorges area of South China: Implications for diagenetic conditions

Abstract

The carbonate of the Ediacaran Doushantuo Formation contains various silica phases in the Three Gorges area of South China. To better understand the diagenetic conditions under which these silica phases formed, we carried out a petrographic and geochemical study of the silica phases and the coadjacent carbonate phases at the Xiaofenghe section (XFH), in the Three Gorges area of South China. Five types of silica phases can be distinguished based on petrographic characteristics: S1 and S2 are composed of microcrystalline quartz, in chert nodules and disseminated in the host dolostone, respectively. S3 is composed of the megaquartz, while S4 and S5 are microcrystalline quartz and fibrous silica, respectively, both replacing calcite cement. Six distinct types of carbonate can be recognized, including the host matrix carbonate (C1), sparry calcite nodules (C2), calcite around chert nodules (C3), dispersed calcite cement within chert nodules (C4), fine micropeloidal calcite cements (C5) and calcite veins (C6). Sedimentary structures suggest that formation of the S1 through S5 silica phases and C2 through C5 carbonate phases occurred during early burial diagenesis, and therefore their formation recorded early diagenetic conditions.Assuming that Ediacaran surface seawater temperature ranged from 28 to 39°C, surface seawater δ18O values can be calculated based on the oxygen isotopic composition of the host dolostone matrix (C1), which range from −6 to 0‰, −4 to 0‰, and −2 to 0‰, for the D2, D3 and D4 subunits of the Doushantuo Formation, respectively. Based on the δ18O values of chert nodules and beds (mainly phase S1), diagenetic temperatures can be constrained to between 27 and 60°C. Under such low diagenetic temperatures, the primary geochemical signatures of the host dolostone, such as REE+Y patterns, δ13CCarb and δ34SPy values, are likely to be preserved.Those silica phases without evidence of contamination of continental silicate detritus show characteristic REE+Y patterns, with minor light REE depletions, positive La, Eu and Gd anomalies, negative Ce anomalies and slightly super-chondritic Y/Ho ratios, implying that the source of silica may be derived from seawater. Negative Ce anomalies in the silica phases in D2 through D4 subunits indicate that the bottom seawater or early diagenetic porewater was oxic at the time of silica precipitation.