泥河湾盆地沉积物的岩石磁学特征及其与大同火山活动的关系探究

Rock magnetic characteristics of sediments in the Nihewan Basin and its relationship with Datong volcanic activity

泥河湾盆地沉积物中具有丰富的古人类遗址和哺乳动物动物群,因此在盆地内开展地层年代学和古气候学研究对于研究人类演化和哺乳动物群落演变具有重要意义.而岩石磁学能够识别沉积物中的磁性矿物种类、含量和颗粒大小,因此是开展磁性地层学定年和利用环境磁学进行环境演化研究的基础.因此本文依托中国科学院地球环境研究所CZ01科研钻孔(孔深305.83 m),开展了详细的实验室测试和系统的岩石磁学研究工作,通过等温剩磁获得曲线、磁化率随温度变化曲线、磁滞回线、FORC图、低温剩磁和磁化率等岩石磁学实验以及XRF,多参数扫描仪进行分析,旨在探讨大同盆地火山活动历史及沉积物磁性物质来源.结果表明:泥河湾沉积物中磁性矿物主要以低矫顽力的磁铁矿为主,部分层位含有高矫顽力赤铁矿,地层沉积后受后期成岩作用影响较小;根据磁化率异常高层位和黄土沉积剖面中的高磁化率-低Rb/Sr比值层位,推断泥河湾盆地第四纪以来至少经历了十数次火山活跃期.最后证实大同火山活动远多于前人研究的4个火山活跃期,另外通过火山灰地层对比,认为CZ01钻孔中钻孔中120 m左右层位大概对应其认为的第一活跃期,即0.9 Ma.

The Nihewan layer is a huge,non-dated Quaternary lacustrine sedimentary layer deposited in the Nihewan Basin of China,and is also an internationally recognized Quaternary standard strata.In recent years,a large number of paleoanthropological sites and mammal fauna have been found in the Nihe Bay layer;At the same time,frequent volcanic activity in the late Pliocene and late Pleistocene in the basin formed the famous Datong Quaternary volcanic group,leaving rich traces in the lacustrine sedimentary layer and loess sedimentary layer.Therefore,stratigraphic chronology and paleoclimatology research in the Nihewan Basin are of great significance for the study of human evolution,mammalian community evolution,regional geological evolution and environmental change.Petromagnetism can identify the type,content and particle size of magnetic minerals in sediments,which is the basis for magnetic stratigraphy dating and environmental evolution research using environmental magnetism.Therefore,relying on the CZ01 scientific research hole of the Institute of Earth Environment,Chinese Academy of Sciences(hole depth 305.83 m),this paper carried out detailed laboratory tests and systematic rock magnetism research,and analyzed the rock magnetism experiments such as isothermal residual magnetism acquisition curve,magnetic susceptibility change curve with temperature,hysteresis loop,FORC diagram,low temperature residual magnetism and magnetic susceptibility,aiming to explore the history of volcanic activity and the source of sediment magnetic material in Datong Basin.The specific research content and research results are as follows:(1)Detailed analysis of the composition and characteristics of magnetic minerals in the sediment of Nihewan showed that magnetite with low coercivity was the mainstay,and some layers contained high coercivity hematite,which was less affected by late diagenesis after formation deposition.(2)The analysis and study of the source of magnetic minerals was carried out,and the results showed that the magnetic minerals in the basin mainly came from exogenous detrital input,and autogenic magnetite was only generated in a few locations,and the proportion was very low.(3)According to the high level of magnetic susceptibility anomalies and the high susceptibility-low Rb/Sr ratio layer in the loess sedimentary profile,it is inferred that the Nihewan Basin has experienced at least a dozen volcanic active periods since the Quaternary,and each active period has had multiple intensive volcanic eruptions,which provides valuable information for the history of regional volcanic activity and confirms that the Datong volcanic activity is much more than the four volcanic active periods studied by previous studies.