我国南方花岗岩风化壳磁性和元素变化特征研究

The origin and transformation of magnetic minerals are closely related to the migration and enrichment of elements during the formation of weathering crust. They are both controlled by the climate and environment, thus the weathering crust is good carrier of paleoclimate. In the hot and humid southern China, rare earth ore is formed through strong weathering of some granites. However, both dissolution and leaching are strong during intense chemical weathering, thus the generation and transformation of magnetic minerals is very complicated and it is difficult to extract paleoclimate information. The relationship between the occurrence state of rare earth elements and magnetic minerals is still highly controversial. This proposal intends to conduct a comprehensive magnetic and elemental geochemistry study of three granite weathering crusts under different climatic conditions in southern China. CBD dissolution and particle size separation method are used, combined with magnetic and geochemical analysis. The purpose is to clarify the relationship among the occurrence of rare earth elements, magnetic minerals, and clay minerals. Finally, the relationship between the formation and transformation of magnetic minerals and the accumulation and migration of elements is explored during intense chemical weathering, and its impact on climate and environmental changes. At the same time, the feasibility of magnetic parameters as an indicator for the concentration of rare earth elements was verified. This study is important for understanding evolution of climate change and the formation mechanism of rare earth ore in southern China.

风化壳形成过程中磁性矿物生成转化和元素的迁移富集有密切的关系，同时均受气候环境控制，是古气候良好载体。在我国南方湿热气候条件下花岗岩经强烈风化作用能形成具有战略意义的风化淋积型稀土矿。但在强烈风化过程中溶解和淋滤作用均较强，磁性矿物的生成转化非常复杂，提取古气候信息存在困难，稀土元素的赋存状态特别是与磁性矿物的关系目前仍存在较大争议。针对这一问题，本项目拟对我国南方不同气候条件下三个花岗岩风化壳开展磁学和元素地球化学的综合研究，利用包括CBD溶解和粒径分离等手段，结合磁学和地球化学分析，厘清稀土元素的赋存与磁性矿物、粘土矿物等的关系，探讨强烈化学风化过程中磁性矿物生成、转化和元素迁移累积之间的关联，剖析其控制机制，特别是其与气候环境变化之间的关系。同时验证磁性参数对风化淋积型稀土矿集中稀土元素含量进行示踪的可行性。该研究对了解对南方气候环境变迁和风化过程中稀土矿形成机制有重要的意义

南方风化壳形成过程中铁元素会迁移富集，但其对成土磁性矿物生成转化的影响知之甚少。通过对三个典型风化壳剖面详细的磁学和地球化学元素特征研究发现。在强烈的风化作用下，花岗岩风化壳和玄武岩风化壳剖面中钾、钙和钠等易迁移元素在风化壳的上部几乎全部被淋溶掉；难迁移的铁元素在花岗岩风化壳剖面的上层富集，在下层几乎没有变化。由于玄武易被风化，铁元素在玄武岩风化壳中发生了明显迁移富集。根据玄武岩风化壳剖面磁性增强机制的不同，可以分为七层。铁元素在1、3、4和5层迁移，在2、6和7层富集。玄武岩风化壳中铁元素的迁移富集对其成土磁性矿物的生成转化产生了重要的影响。铁迁移会导致成土生成的细粒磁赤铁矿的粒径分布变窄，铁富集使得其粒径分布变宽。此外，由于赤铁矿粒径主要取决于其前置磁性矿物水铁矿的含量，因此，铁迁移有利于生成细粒的赤铁矿，铁富集有利于生成粗粒的赤铁矿。“硬”等温剩磁（HIRM）在赤铁矿粒径均一时，可以用来指示赤铁矿含量。因而在南方的风化壳或土壤中，在强烈的风化作用下，铁元素的迁移富集导致赤铁矿粒径不均一，HIRM不再是指示赤铁矿含量的可靠指标。铁元素的迁移富集还会影响成土磁性矿物的转化，即水铁矿→粗粒赤铁矿→细粒磁赤铁矿→细粒赤铁矿，铁迁移使得转化速度变快，铁富集使得转化速度变慢。花岗岩风化壳的铁元素在强烈风化的条件下，由于易迁移元素的迁移导致其在上部风化层富集，但成土磁赤铁矿的粒径分布没有发生改变；由于花岗岩母质铁含量较低且难风化，释放的铁离子较少，生成细粒赤铁矿。铁迁移富集对磁性矿物的形成和转化产生了重要影响，对其的深入研究有利解释南方强风化环境中形成的红土磁性变化。