泥河湾盆地大长梁剖面河湖相沉积物的岩石磁学性质

Rock magnetic properties of the fluvio-lacustrine sediments from the Dachangliang section in the Nihewan Basin,northern China

以热磁分析为主,对中国北方泥河湾盆地更新世河湖相地层中灰绿色粉砂和灰黄色粉砂/细砂两种典型沉积物进行了详细的岩石磁学研究,有效确定了这两类沉积物中磁性矿物的种类、粒度特征以及加热过程中磁性矿物的变化过程和产物,并对其包含的古环境意义进行了初步探讨.灰绿色粉砂样品主要含有磁铁矿和赤铁矿两种磁性矿物。磁性相对较弱,颗粒相对较细;灰黄色粉砂/细砂主要含有磁铁矿和磁赤铁矿.磁性相对较强,颗粒相对较粗.在氩气环境中经700℃加热处理后,这两种沉积物中的绿泥石都分解。并生成超细粒(处于超顺磁和单畴颗粒区间)的磁铁矿,导致磁化率大幅升高.因此泥河湾盆地沉积物的热磁特征可以用来检测样品中绿泥石的相对含量,进而反映该地区化学风化作用强度的变化.此外,灰绿色粉砂样品中绿泥石含量比灰黄色粉砂/细砂样品的含量高.在氩气中加热后,磁化率升高幅度也较高,可能反映了化学风化相对较弱的沉积环境.

The fluvio-lacustrine sequence of the Nihewan Basin in northern China consists mainly of alternating grayish-green silts and grayish-yellow silts/fine-grained-sands. A multi-parameter investigation was conducted on the two types of representative sedimentary facies from the Dachangliang section in the eastern Nihewan Basin, using environmental magnetic approaches coupled with X-ray diffraction analysis. The magnetic mineralogy, magnetic grain size and thermal-magnetic characteristics of the two types of sediments were determined. The grayish-green silts have a magnetic mixture of magnetite and hematite, characterized by weak magnetism and relatively smaller magnetic particles. The grayish-yellow silts/fine-grained-sands have a magnetic mixture of magnetite and maghemite, characterized by strong magnetism and relatively coarser magnetic particles. The clay mineralogy of the two sediments is dominated by chlorite and illite. Chlorite in both types of sediments decomposes after 700℃ thermal treatment in an argon atmosphere, providing iron source for the neoformation of fine-grained （superparamagnetic/ single-domain） magnetite, leading to increasing magnetic susceptibility. Thus, the thermally- induced rock magnetic characteristics （in particular magnetic susceptibility） may be used to determine the relative contents of chlorite in the Nihewan sediments, which probably further signal the variations of chemical weathering intensity in this basin. In addition, the grayish-green silts generally have higher contents of chlorite and more enhanced magnetic susceptibility after heating than the grayish-yellow silts/fine-grained sands, implying that the former sediments were possibly deposited in an environment of weaker in situ chemical weathering.