含菱铁矿砂岩磁性的初步研究

作为煤田盖层产出的中国内蒙古平庄J₃砂岩,其平均磁化率为60×10^-5（SI）;平均剩磁为5×10^-3A/m.经热退磁后,在磁化率、剩余磁化、等温剩磁曲线、粘滞剩磁及磁组构等方面都发生了明显的变化,出现一系列奇异磁特征.岩石透射光与反射光鉴定、物相热红外分析、差热分析、X射线衍射分析表明,该砂岩的胶结物为弱磁性的菱铁矿,它在逐步热退磁过程中逐渐变为磁铁矿,部分变为赤铁矿.并大致确定,该砂岩胶结物含量近50％.此外,对这种砂岩样品仔细地进行了常规方法的退磁试验,得到其特征剩磁方向,确定其J₃古地磁极位置为198.8°E,84.6°N(α₉₅=4.8,k=94.5).

PRIMARY MAGNETIC STUDY ON THE SIDERITE-BEARING SANDSTONE

7 sites （52 cores in total） were collected from the sandstone of J3-K1 which as an overlying formation on the coal field has outcropped at the Pingzhuang, Inner Mongolia, China. Noticeable and unusual changes in susceptibility, magnetic remanence, IRM curve, viscous re-manence as well as magnetic fabric have been found after some steps of thermal demagnetization although these specimens were originally associated with fairly weak remanence and low susceptibility (mean values:J0=5×10^-3A/m; K = 60×10^-5 （SI）. It is shown by a further examination of the petrology and minerals, i.e. the thin section analysis through transmission and reflection lights, ultrared-spectrometry analysis, differential thermal analysis （DTA） and X-ray diffraction （XRD） analysis that the siderite with weak magnetization which as a cement was appearing within the natural sandstone has been gradually transferred into magnetite and partly into hematite during the process of stepwise thermal demagnetization. It is estimated that the content of cement is about 50% of the whole rocks. The characteristic remanent component has been obtained after thermal demagnetization. The corresponding VGP for J₃-K1 is thus found 10 be at 198.8°E, 84.6°N （α₉₀5 = 4.8 ° k = _94.5）.