SHRIMP U–Pb ages of K-bentonite beds in the Xiamaling Formation: Implications for revised subdivision of the Meso- to Neoproterozoic history of the North China Craton.

Abstract: We report SHRIMP U–Pb zircon ages from K-bentonite beds from different locations in the Xiamaling Formation near Beijing at the northern margin of the North China Craton (NCC). The 1379±12 Ma and 1380±36 Ma ages obtained in our study correlate with similar data (1368±12 Ma) obtained from recent studies and assign a Mid-Mesoproterozoic (Ectasian Period) age for the formation. In addition, a detailed bed-by-bed study of the K-bentonite sequence indicates that the present dated successions northwest of Beijing can be correlated fairly well with that of the well-known Meso- to Neoproterozoic standard section in Jixian to the east of Beijing. The new data call for a major revision of the previous Meso- to Neoproterozoic chronostratigraphic subdivision of the northern NCC, with a revised calibration into the early to middle Mesoproterozoic Era. Our data suggest that the Tawuia-bearing assemblage in the lower part of the Xiamaling Formation in the NCC could be older than ~1380 Ma, and is not limited to a representative feature of the Neoproterozoic Little-Dal-Group-type macroalgal biota. Combined with other similar discoveries, we conclude that the macro-evolution of Proterozoic eukaryotic biotas, represented by macroalgal assemblages, would have probably been a long, protracted, and somewhat monotonous process. On the other hand, reassessment of the paleomagnetic database for these successions, combined with stratigraphic correlations and similarities in the macroalgal assemblages on the NCC and Laurentia, support a configuration in which the NCC was in close proximity to the Laurentia–Siberia–Baltica grouping during much of the Mesoproterozoic to Neoproterozoic time. Furthermore, our study suggests recent assumptions linking δ ³⁴S_CAS and δ ¹³C_carbonate fluctuations in the Jixian section with the assembly and breakup of Rodinia must now be reconsidered, as these fluctuations may correlate better with the tectonics associated with the Columbia supercontinent cycle. [Copyright &y& Elsevier]