Abstract:Exposed in the Lüliang area, North China, the Hangaoshan Group is dominated by terrigenous clastic deposits and bear an ideal candidate section for the stratotype of the lower Mesoproterozoic or upper Paleoproterozoic sequence, especially for the boundary between them. The determination of formation age is of great significance for understanding the Precambrian geological evolution of the North China Craton, such as emergence of the initial cover sequence of the craton, assembling and breakup of the Nuna supercontinent. To date, there are still great contro- versies on the age, subdivision, correlation and geological implication of the Hangaoshan Group. In this study, zircon U-Pb isotopes of the andesites from the Hangaoshan Group have been analyzed with SHRIMP technique, giving a weighted mean 207Pb/206Pb age of 1 769±9 Ma ( 2σ) . This is the first direct measurement of the zircon U-Pb age of the Hangaoshan Group. In addition, weighted mean 207Pb/206Pb zircon ages of the volcanic rocks in three different parts of the Xiaoliangling Formation of the same stratohorizon are 1 792±18 Ma, 1 773±19 Ma and 1 778±20 Ma, respectively, which are consistent with the age of Hangaoshan Group within the error range. Therefore, the ages of the Hangaoshan Group and Xiaoliangling Formation are determined at 1 769±9 Ma and 1 792 ~ 1 773 Ma, respectively. In combination with previous research results, formation time of the Hangaoshan Group and Xiaoliangling Formation is ca. 1 780 Ma. As regards sedimentology, the section strata of the Hangaoshan Group in Lüliang Mts area are continuous, and a horizon of andesite is stably and widly distributed in the region. So the horizon is an excellent regional isochronous marker in the central and southern part of the North China Craton. Furthermore, rift deposits and volca- nic rocks in this period occur in the western Yangtze Block, the northwestern Laurentia, northern Australia, the São Francisco, Congo and Siberian cratons, and all can be well correlated, showing the global distribution of intracontinental rift sediments during initial breakup of Nuna supercontinent in the same period. We propose here that the international Mesoproterozoic bottom boundary should be moved down, from 1.6 Ga to 1.8 Ga, and the Hangaoshan section can be used as a new candidate stratigraphic type section of the international Mesoproterozoic bottom boundary.
