亚热带地区花岗岩风化壳上发育红土的磁性矿物转化机制——基于非磁学指标和岩石磁学的综合分析

MECHANISM OF MAGNETIC MINERALS TRANSFORMATION OF SUBTROPICAL RED SOILS DERIVED FROM GRANITE WEATHERING CRUST： COMPREHENSIVE ANALYSIS BASE ON NON-MAGNETIC INDICATORS AND ROCK MAGNETISM

选取亚热带地区发育于花岗岩风化壳上的两个典型红土剖面（福州剖面和泉州剖面）作为研究对象，在前期系统岩石磁学研究的基础上，结合常量地球化学元素、色度、粒度指标，分析磁学特征与环境条件的内在联系，重点探讨亚热带湿热气候条件下红土中磁性矿物转化途径及机制。结果显示：1）两个剖面硅（SiO2）、铝（Al2O3）、铁（Fe2O3）等常量地球化学元素含量及其随剖面深度变化规律基本一致；衡量风化强度的化学蚀变指数（CIA）、风化淋溶指数（ba指数）以及元素比率（如硅铝率和硅铝铁率）在数值上非常相近。常量地球化学参数表明两个剖面土壤均处于脱硅富铝化的成土阶段。2）两个剖面红度和黄度值差异较大，表明福州剖面土壤中赤铁矿和针铁矿含量相对较高；色度参数在两个剖面中随深度变化规律具有较好的一致性。3）两个剖面粒径均随风化程度增强而变细，强风化层位粘粒含量最高；相较于泉州剖面，福州剖面粒度更细。综合分析野外土壤发生特征及上述参数，认为福州红土剖面由于成土时间较长，因此经历了更强的后期风化作用。在此基础上，分析土壤磁性转化影响因素及转化机制，得出如下结论：1）亚热带地区湿热的气候条件是磁性矿物总体特征变化的主导因素；2）土壤母质和成土时间是亚热带地区不同红土剖面磁性矿物特征差异的主控因子；3）亚热带地区土壤磁性矿物转化途径不同于温带地区：湿热气候条件对原生磁性矿物改造作用更为明显，并且影响次生磁性矿物的赋存形式。

Red soils, extensively distributing in Southern China, are considered to be good archives of paleoenvironment and paleoclimate. Although the red soil environmental magnetism has been studied for a long time, the relationship between magnetic properties and their environmental implications is still unclear. The fundamental and crucial work to address this question is the understanding of the magnetic minerals transformation of red soil in subtropieal- tropieal region, which is the focus of this study. Base on the systematic rock magnetism research, major elements, ehorma and grain size measurements of two typical subtropical red soil sections, which are Fuzhou section （FZ, 26°13. 938′N, 119°21. 622′E; 58m a.s.l.） and Quanzhou section （QZ, 25°22. 801′N, 118°10. 096′E; 291m a.s.l.） , derived from granite were carried out. The thickness of Fuzhou and Quanzhou section are 1.90m and 2.85m, respectively. Total 31 powder samples were collected from Fuzhou section at 5em intervals from 0 - 1.2m, 10cm intervals from 1.3- 1.6m, and the last sample is at the depth of 1.9m. Total 16 powder samples were collected from Quanzhou section at 20cm intervals from 0 - 3m. We analyzed the connections among the magnetic properties and the parameters above in order to discuss the transformation of red soils magnetic minerals in the subtropical region which the climatic characteristic is humid and warm. The results show that： （1） The eontents of major elements, such as SiO2, Al2O3, Fe2O3, and their sequential variations are similar between two sections. The average values of chemical index alteration （CIA） , ba, the ratio SiO2/Al2O3, SiO2/ （Al2O3＋F2O3） , are very similar. These chemical parameters also suggest that two sections undergo strong chemical weather/pedogenesis. （2） The differentiations of color index [ a＊（redness）, b＊（yellowness） ] are the same between Fuzhou section and Quanzhou section. B horizon has higher a＊ and b＊ values in both sections. The average a＊ and b＊ values of FZ section are higher than QZ section, suggest that FZ section contains more hematite and goethite. （3） The grains of red soils become finer because of the chemical weather in both seetion. Samples of B horizon have the highest clay fraction. The grain sizes of FZ seetion are much finer than Qz section. By combining the pedogenesis characteristics and differentiations of these parameters, we suggest that FZ section undergo stronger post ehemical weather due to longer pedogenesis time. The influence of soil forming factors （including climate, parent material, topography, vegetation and time） on soil magnetism is discussed. The conclusion with respect to the factors of magnetic minerals transformation and the mechanism can be summarized as： （1） The magnetic susceptibility and S1RM decrease with degree of pedogenesis, which differs from Chinese Loess Plateau in temperate zone. It is interpreted that this difference is due to the faet that more preeipitation and higher temperature in subtropical areas strengthen the transformation and dissolution of lithogenic minerals. （2） Parent material and time are the main factors on the differences of magnetic properties in two sections. （3） The transformation pathway of secondary soil magnetic minerals in the subtropical areas is different from which in temperate zone. The iron ions released during pedogenesis are not form fine strongly magnetic minerals （e.g. magnetite, maghemite） , but weakly magnetic minerals （e.g. hematite, goethite）.