A refined recycled carbonatite model for the δ13C negative anomaly in the upper Doushantuo Formation

Abstract

The upper Doushantuo Formation (terminated at 551 Ma) in the Yangtze platform keeps a large carbonate δ13C (carbon isotope composition) negative anomaly, which has been proposed to be related to the Shuram Excursion (SE). However it is still controversial whether such anomaly records the carbon isotope variation of seawater caused by oxidizing dissolved organic carbon (DOC) or the input of a local exotic carbon reservoir. Here, we conducted further chemical and mineralogical studies of samples from the 13C-depleted strata of the Doushantuo Formation at Jiulongwan. The 13C-depleted strata are featured by flat REY patterns and constant δ13Ccarb (δ13C of carbonate) but variable δ13Corg (δ13C of organic carbon), which disagree with the prediction of DOC oxidation model. Euhedral quartz grains separated from these strata show bluish-violet catholuminescence indicating a high temperature origin. Carbonate inclusions are found in some of these quartz grains. Above chemical and mineralogical features of the 13C-depleted strata can be explained by the previously suggested recycled carbonatite model. However, zircons separated from the 13C-depleted carbonates record not only a young age of 567.17 ± 3.57 M, but also two old age clusters of 640 ∼ 960 Ma and 1760 ∼ 2160 Ma. The young age of 567.17 ± 3.57 Ma may record the carbonatite magmatic activity. The age spectrums of old zircons imply that the recycled carbonatite is more likely to be transported and deposited in the intra-shelf area with other clastics through weathering.