山东省济宁群形成时代探讨

An analysis of formation age of Jining Group in Shandong Province

济宁群是在铁矿勘查过程中发现的埋藏于地下近千米深度以下的全隐伏浅变质地层。通过对济宁群中变质火山岩同位素年龄测试和千枚岩中微体古生物分析,结合区域地层和含铁建造对比,探讨了济宁群的形成时代。结果表明,翟村组变英安岩中的锆石成因比较复杂,在15颗锆石上进行了16个数据点分析,其中7个数据点构成不一致线的上交点年龄为(2 522±7)Ma,最年轻锆石的年龄值为(2 487±7)Ma;在30件岩石样品中,发现11件样品含有微古植物化石,为类型分异度不高的疑源类化石组合,以简单球形的疑源类为主,如光面球藻(Leiosphaeridia)、鮫面球藻(Trachysphaeridium)、线脊球藻(Stictosphaeridium)。比较发现:济宁群特征及变质程度,与华北古-中元古代地层的相似程度大于与太古宙地层的相似程度;含铁岩系特征与鲁西及华北克拉通其他新太古代BIF有明显差异,而与山西袁家村BIF相似。综合分析认为,济宁群是新太古代—古元古代过渡期的岩石地层单位,对其深入研究可以揭示太古宙—元古宙过渡期的地质环境演变。

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, suchas 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.