Age constraints on the Ediacaran carbonate carbon isotope excursions in the Tarim Block: Evidence from in situ U–Pb dating of dolostone

The Ediacaran period is characterized by a significant negative carbon isotope excursion (N3) of carbonate, followed by a subsequent positive excursion (P1). The timing of the N3 and P1 excursions, particularly the latter, is poorly constrained due to the lack of suitable dating techniques. In this study, we use the newly developed in situ carbonate U–Pb dating technique on Ediacaran carbonates from the Qigebrak Formation at two sections in the Tarim Block, northwestern China, to constrain the timing of P1 for intra- and interbasinal correlation. We also test the applicability of carbonate U–Pb dating coupled with elemental geochemistry to Precambrian carbonates, where post-depositional recrystallization or hydrothermal alteration frequently occurs. Our results indicate that the Tarim carbonate carbon isotope positive excursion (TP1) starts at ∼560 Ma and ends at ∼550 Ma. A compilation of age data from worldwide Ediacaran sections suggests that N3 lasted from ∼570 Ma to ∼560 Ma, and P1 from ∼560 Ma to ∼550 Ma. Finally, we summarized the differences in sedimentary petrology and geochemical characteristics between well-preserved and recrystallized/altered Precambrian carbonates to provide useful criteria for the future geological applicability of U–Pb dating results of Precambrian carbonates. These criteria include: 1. Use fresh samples of microbial dolostone from the penecontemporaneous. 2. Exclude samples with nonplanar-anhedral crystals and retain samples with planar-euhedral/subhedral crystals. 3. Exclude samples with ∑REY relatively higher than in neighboring samples. 4. Exclude samples contaminated by noncarbonate phases, and comprehensively analyze the four methods to exclude samples with diagenetic alteration. This method demonstrates an effective model for the future geological applicability of U–Pb dating results of Precambrian carbonates.