An analysis of formation age of Jining Group in Shandong Province

The Jining Group is concealed low-grade metamorphosed strata in western Shandong Province. Based on isotopic ages and micropaleoplants analysis, combined with regional stratigraphic correlation and iron formations correlation, this paper discusses formation age of Jining Group. Results of isotopic age determination show that the genesis of zircon grains in dacite is complex, the upper intercept age of 7 points from 16 zircon analyzed points is 2522±7 Ma and the youngest age is 2487±7 Ma. In 30 pieces of rock samples, 11 samples were found to contain the microplants, which are mainly lower diversity simple spherical acritarchs, such as Leiosphaeridia, Trachysphaeridium and Stictosphaeridium. After comparison, it is considered that stratigraphic characteristics and metamorphic grade of Jining Group are similar to features of Proterozoic and Mesoproterozoic strata in North China, and are different from things of Neoarchean strata. Characteristics of seam of iron ore in Jining Group are unlike Neoarchean BIF in western Shandong Province and North China Craton; nevertheless, they are very similar to features of Yuanjiacun BIF in Shanxi Province. Synthetic analysis shows that Jining Group was formed in the transitional period from Archaean to Proterozoic. The thorough study is helpful to understanding the evolution of the regional geological environment in Archaean-Proterozoic period.