【作者】 白凌燕；

【导师】 张世红；

【作者基本信息】 中国地质大学（北京） ， 构造地质学， 2008， 硕士

【摘要】 地质时期碳酸盐岩分布面积很广。其中,碳酸盐矿物本身及其微量元素和同位素特征与碳酸盐岩形成的环境密切相关,较详细而准确地记录了古温度、古降水、古盐度以及古生产力等多方面环境要素的变化历史。这些变化也反映在磁性矿物组合和岩石磁性变化方面。因而,利用岩石磁学方法研究碳酸盐岩剖面所记录的古环境变化是一个重要的研究方向。但由于地质作用的复杂性,影响磁参数变化的因素很多,因而,对磁参数影响因素的研究是环境磁学解释的重要前提。本次研究对陡山沱组第三岩性段上部20.72m的地层连续采样,制作环境磁学样品2562个。样品的岩石磁学测量结果表明,11.18m—11.65m为一段磁性增强的异常层,其磁化率(χ)、非磁滞剩磁磁化率(χARM)、饱和等温剩磁(SIRM)等均高于其上下层位的样品。而且,该异常层上下层段的磁性特征不同,S-ratio、等温剩磁获得曲线及其退磁曲线的测量表明,异常层之下的硬磁性矿物比异常层之上的硬磁性矿物含量多。根据野外和薄片的镜下观察,按照碳酸盐岩结晶粒度的大小,将陡山沱组第三岩性段中上部碳酸盐岩分为粉屑碳酸盐岩和砂屑碳酸盐岩两类。两类碳酸盐岩的非磁滞剩磁(ARM)等磁学参数与碳酸盐颗粒的结晶粒度有很好的相关性。风化作用使样品在King Plot上的投影位置向右下方偏移,总体呈现ARM减小的趋势。粉晶X-射线衍射结果表明,宜昌陡山沱组第三岩性段中上部碳酸盐岩中的矿物主要为方解石、铁白云石、石英、钾长石和少量黄铁矿。对部分样品进行了弱酸溶解处理。溶解之后,样品的磁化率随温度变化曲线(χ-T曲线)及粉晶X-射线衍射结果能很好地揭示黄铁矿的存在。但溶解之前,样品的χ-T曲线不能揭示黄铁矿的存在,推测可能样品中大量的方解石、铁白云石等抗磁性矿物,掩盖了碳酸盐岩中铁硫化物的热磁特征。更多还原

【Abstract】 Carbonate rocks distribute widely through geological history. The mineralogy of carbonate rocks and the character of their trace elements and isotopic element are controlled by the carbonate depositional environment. Thus carbonate sediment may well preserve paleoenvironmental changes, including paleotemperature, paleo-precipitation, paleosalinity, paleoproductivity and so on. These changes can also be traced by the character of magnetic composition in the carbonate sediment and thus its magnetism. Therefore, it is potentially valuable to study a carbonate profile by using rock magnetic methods. But because of the complexity of geological processes, various factors may lead to changes of rock magnetic parameters. It is necessary to find out how these factors effect changes of magnetic parameters before deciphering paleoenvironmental changes.20.72 meters carbonate profile was sampled continuously from the upper part of the third member of Doushantuo Formation. 2562 specimens were made for rock magnetic measurements. The profile of magnetic parameters indicates that there exists a special magnetic-enhanced section (SMES) of 11.18~11.65m. Values of mass susceptibility (χ), anhysteretic remanent magnetization susceptibility (χARM) and saturation isothermal remanent magnetization (SIRM) of the SMES are higher than that of the other sections. The magnetic character of the section upon the SMES (the upper section) are different from that of the section below the SMES (the under section). Variations of S-ratio, isothermal remanent magnetization acquision curve and its demagnetic curve suggest that more hard magnetic minerals exist in the upper section than in the under section.Two types of carbonate, find-grained and coarse-grained, can be recognized combined with the investigation in the field and the thin observation. Magnetic measurements show that crystal size of carbonate grains correlates well with values of magnetic parameters such as ARM. Weathering diminishes ARM value of specimens. Thus weathered specimens distribute in the under area in King Plot.X-ray diffraction spectrum shows the carbonate rocks under study mainly contain calcite, ankerite, quartz, feldspar and a little pyrite. In order to get more detailed mineralogical information, several specimens were selected to be dissolved with 10% HCl. Measurements of temperature-dependence susceptibilities (χ-T curves) and results of X-ray diffraction of the acid-dissolved specimens suggested the existence of pyrite. However, these pyrites can not be discovered in the specimens before dissolved from theχ-T curves. It is supposed that diamagnetic minerals, that are carbonate and ankerite, may cover the mineralogical changes of pyrite during heating.更多还原