太行山红层磁学性质及古环境意义初探

环境磁学研究磁性矿物特征（磁性矿物类型、含量和颗粒大小）及其转化与环境关系，在不同地质时期古气候和古环境重建中得到广泛应用。本文将磁学方法应用于太行山中元古界红层研究，并从其磁学性质角度初步探讨太行山红层的古环境。对太行山红色石英砂岩和紫红色砂质泥岩样品进行了高温磁学和常温磁学系统测试。结果显示，太行山红层主要以硬磁性的赤铁矿为主，包含少量磁铁矿，部分样品赤铁矿是唯一磁性矿物；磁性颗粒大小以单畴为主；红色石英砂岩不同样品间磁性差别显著，反映沉积环境的不稳定性。紫红色砂质泥岩中的青灰色层表现为顺磁性特征，原因可能是紫红色砂质泥岩中的赤铁矿在后期还原环境下溶解或转化为弱磁性矿物所致，说明紫红色层并非形成于长期的水下还原环境。太行山红层中普遍存在的红色波痕和泥裂说明水环境对其有短暂影响，但是红层及赤铁矿的富集说明其长时间处于氧化环境，推断太行山红层可能形成于高度氧化的陆地环境。

Magnetic properties of red beds in Taihang Mountains and paleoenvironmental implications

Iron-bearing magnetic minerals,such as ferrimagnetic oxides,are universal in various sediments.Their formation,conversion and stability are closely related to environmental conditions.Environmental magnetism is dedicated to study environments by characterizing magnetic minerals including magnetic assemblies,grain-size (i.e. magnetic domains) and the proportion of each mineral.It has been widely used in Quaternary paleoclimate reconstructions.Herein,environmental magnetism was applied to Mesoproterozoic red beds in Taihang Mountains,north of China,which was conventionally regarded as marine deposition mainly based on widespread mud cracks and ripple marks.However,few similar red mud cracks and ripple marks occur in modern marine environments.The magnetic properties of red beds in Taihang Mountains potentially provided new insights of its paleoenvironment.Red beds in Taihang Mountains are mainly compose of red quartz sandstones and include other colors such as greyish white,dark purple and light yellow.Two red bed sections were investigated:Guoliang section (35°06.9'N,113°36.3'E;1700m) of red quarz sandstones,140km away northwest of Xinxiang City,and Zhangshiyan section (37°28.9'N,114°06.6'E;1490m) of purple sandy mudstones,110km away southwest of Shijiazhuang City (Fig.1a).12 samples (GL01~GL12) were collected from Guoliang section (140m thick) along hanging road on the cliff (Fig.1b).Zhangshiyan section (5m thick) is located in upper part of the red sandstones with a 40cm cinerous mudstone on its top (Fig.1c).Several cinerous bandings and mottles appear among the purple mudstones.22 purple sandy mudstones (averaged as GL13) and 4 cinerous mudstones (averaged as GL14) were collected from Zhangshiyan section.Cross bedding,ripple marks and mud cracks are very universal in the red beds (Fig.1d).Rock magnetic measurements were carried out on all samples.Temperature dependent magnetic susceptibility curves (κ-T),temperature dependent magnetization curves (M-T) and hysteretic loops of representative samples were used to determine magnetic assemblies.Room temperature magnetic properties including χlf,χfd%,χARM,SIRM,HIRM,S-ratio and Bcr were measured as well for characterizing magnetic minerals.κ-T and M-T curves displayed that hematite (TN=675℃) was the dominant magnetic minerals.In some samples (e.g. GL12) hematite is the only magnetic mineral.Small proportion of magnetite appeared in some samples (e.g. GL02,GL05 and GL10) indicated by the slight decrease of κ at 580℃.Wide-waisted,goose-necked and wasp-waisted hysteretic loops with Bcr>400mT suggested the evident influence of hard magnetic minerals (e.g. hematite) and co-existence of magnetite and hematite.Magnetic domains inferred by Day plot demonstrated SD and MD groups.Magnetic properties of quartz sandstones varied dramatically due to different proportion of hematite,diverse domain size and non-magnetic minerals (e.g. quartz).The purple sandy mudstone GL13 was dominated by fine grained hematite,while the cinereous banding sample GL14 in the purple mudstones was strikingly dominated by paramagnetic minerals.It was likely that GL14 was transformed from GL13 in reducing condition caused by water logging along cracks.Accordingly,it suggested that the purple mudstone likely formed in non-marine environment.Because hematite is formed and stable in well-drained oxic conditions,the dominant role of hematite in red beds indicated that terrestrial oxic environments were likely prevailing during its formation,while the mud cracks and ripple marks in red beds only implied short duration under water level.