Abstract:

The Dagushi Group in the Dahongshan Area, at the northern margin of the Yangtze Craton, has long been considered as a Mesoproterozoic sedimentary succession unconformably overlain by the Huashan Group. Owing to lacking of accurate and reliable isotopic ages, the correlation between the Dagushi Group and the Shennongjia Group that also occurred on the northern margin of the Yangtze Craton about 260 km to the west of the Dahongshan Area and other relevant strata in China, still remains as an enigmatic issue. In this study, using Sensitive and High Resolution Ion MicroProbe (SHRIMP) and Laser Ablasion coupled with MultiCollector Inductively Coupled Plasma Mass Spectrometer (LAMCICPMS), we analyzed U-Pb ages and LuHf isotopic compositions for zircons from the tuff beds in the Dagushi and Shennongjia groups, respectively. Two precise zircon UPb ages, 1225±19 Ma and 1239±23 Ma, from the tuff beds in the Luohanling Formation of the Dagushi Group were obtained for the first time. On the other hand, zircons from the two tuff samples of the Luohanling Formation show comparable LuHf isotopic compositions with (176Hf/177Hf)i varying from 0.282172 to 0.282397, corresponding to εHf(t) from 4.9 to 12.0 and TCDM from 1150 Ma to 1943 Ma. The LuHf results of the Dagushi Group are consistent with those of the zircons from a tuff sample from the Yemahe Formation of the Shennongjia Group. Integrating the data obtained in this study with the available regional geological data, we suggest that the Dagushi Group and the Upper Shennongjia Subgroup are coeval sedimentary succesion formed during the breakup of the Columbia Supercontinent, possibly geodynamically related to a super plume.

Key words: Dagushi Group, Shennongjia Group, tuff, zircon SHRIMP-U-Pb ages, Lu-Hf isotope, Yangtze Craton