鲁西山区与早、中前寒武系有关的几个地质问题的新认识

作者介绍了广布于鲁西山区的早、中前寒武纪地层:泰山群中的雁翎关组、山草峪组、柳杭组及济宁群的岩石组合、原岩性质、时代及上、下接触关系。泰山群生成年龄通过与新甫山奥长混合花岗岩接触关系及蒙山英云闪长岩的U-Pb法测定的年龄对比,推测它应早于2628Ma。 本文讨论了比泰山群更早的地壳部分——表壳岩的组成成分、演变过程及其上限等问题,并有了一些新认识。对蒙山地区花岗质岩石是地层还是岩体?长期以来一直是悬而未决的问题,通过矿物成分化学成分、分析资料及野外观察,结合镜下鉴定,基本肯定了它是古老的英云闪长岩体。

NEW RECOGNITION ON SOME GEOLOGICAL PROBLEMS RELATED TO THE EARLY-MIDDLE PRECAMBRIAN IN THE WESTERN SHANDONG MOUNTAINOUS REGION

The western Shandong mountainous region, situated to the west of the Tancheng-Lujiang fault zone, cover an area of about 40,000 km2. Two meta-volccano-sedimentary series occur in this region. One is the Taishan Group, formed more than 2,450 Ma age, and the other is the Jining Group with an age older than l,753Ma.The Taishan Group in the Yanlingguan area, which was studied elaborately by Cheng Yuqi et al. in 1960's was formerly divided into two formations. The lower one, named the Yanlingguan Formation, is of meta-volcano-sedimentary origin, composed chiefly of amphibolitic rocks intercalated with biotite granulitite and also with conglomeratic hornblende granulitite of detrital origin; besides some ultrabasic intrusives and probably lavas are also present. The thickness of this formation is up to 1400m. The upper one is named the Shancaoyu Formation, which is composed chiefly of biotite-granulitite, with a much less amount of amphibolite and hornblende schist. Their protolith equivalents are mainly siltstones of greywacke character and also some basic- intermediate-acidic volcanic rocks. Rhythmic layering and cross-bedding structures can be seen occasionally. The lithology of the rocks is less variable. The thickness of the formation is greater than 2000m.In addition, sections composed of amphibolitic sequences have been observed in three areas, which have quite different characteristics as compared with each other and could hardly be correlated with the typical Yanling-guan succession. However, this indicates the presence of a rather widespread basic volcanic activity occurring in many parts of this region probably during 2628-2450 Ma.The recently-recognized "Liuhang Formation", which overlies the Shancao-yu Formation, is a volcano-sedimentary sequence varying from basic composition upwards to chiefly acidic, and has a thickness of 450m around Tai'an. It should also belong to the Taishan Group.The Taishan Group, which is usually ascribed to the late Archaean, may be correlated to the upper Anshan Group in Liaoning and the Dengfeng Group in Henan and appears to be similar to the rocks of the Archaean greenstone belt in other parts of the world in some aspects. The U-Pb age of 2628 Ma recently determined on zircon from the tonalite of the Mengshan Mountain, which intrudes into the Taishan Group, indicates a still older age for the Taishan rocks. The chemical composition, mineral assemblenges, and REE pattern of the tonalite suggest its anatectic origin.A series of phillitic rocks with martite and magnetite layers more than 120m thick and lying unconformably on the Archaean metamorphics, were discovered in 1977 by drilling in the Jining-Dongping district. The rocks give a Middle Proterozoic isochron age of 1753 Ma; they were derived from clayey and sandy rocks intercalated with some intermediate to acidic volcanics.