发育完整的灰岩风化壳及其矿物学与地球化学特征

MINERALOGICAL AND GEOCHEMICAL CHARACTERISTICS OF THE LIMESTONE WEATHERING PROFILE IN JISHOU, WESTERN HUNAN PROVINCE, CHINA

对于碳酸盐岩上覆土壤成因 ,尽管碳酸盐岩风化残积成土说被多数学者认同 ,但由于碳酸盐岩中酸不溶物含量极低 ,在风化成土过程中会伴随巨大的体积缩小变化 ,原岩结构和半风化带无法保留 ,从而缺失了探索上覆土壤物质来源的重要中间环节 ,使得这种观点缺乏野外宏观证据的支持。最近 ,我们在贵州、湖南等地发现了数个以泥质灰岩和泥质白云岩为基岩的碳酸盐岩风化壳剖面 ,尚保留有较好的原岩结构 ,具有明显的风化壳分带和过渡现象。这些风化壳剖面的发现为深入研究碳酸盐岩风化成土过程提供了良好的研究场所。本文选取了较为典型的吉首泥灰岩风化壳剖面 ,从矿物学及地球化学的角度来探讨碳酸盐岩风化壳的形成过程和发育特征 ,结果表明该风化壳既遵循非碳酸盐岩 (主要是结晶岩类 )风化壳的发育特征 ,也具有自己独特的地球化学演化规律。风化壳总体特点受碳酸盐岩中的酸不溶物矿物组合及化学成分的影响甚至控制 ,风化作用早期半风化带基本保留了酸不溶物的各种矿物学及地球化学特征 ;风化作用后期 ,风化壳发育显示了和非碳酸盐岩风化壳相似的发育特征。吉首泥灰岩风化壳剖面的发育特征和作者早先提出的碳酸盐岩风化成土的两阶段模式是一致的 ,即以碳酸盐矿物大量淋失、酸不溶物逐渐堆积或残积为特征的早期阶?

In virtue of the low content of insoluble residues of carbonate rocks, the remarkable volume decreasement will happen during the weathering and soil formation processes of carbonate rocks. The structure and texture of the original rocks cannot be conserved in the weathering crusts of carbonate rocks under the most conditions, and the boundary between soil and rock is usually sharp. It is very hard to find the transitional weathering residues because of the absence of saprolite. So, up to now, the origin and development of the soil over the carbonate rocks cannot be understood clearly. Recently, some carbonate rock weathering crusts whose original structures were conserved in the saprolite have been found developing on marlite at some places in Guizhou and Hunan. In this paper, mineralogy and element geochemistry are studied and the whole development characteristics are concluded in the Jishou limestone weathering crust. In addition to goethite and montmorilonite, other minerals in the weathered zone are the same as those in the bedrock, which indicates the minerals in the weathered zone were derived from the original limestone whose mineral composition is similar to that of the bedrock. With increasing weathering, the increase of clay minerals and the decrease of feldspar can be observed in the crust, which is in agreement with the characteristics of mineral development in crystalline rock weathering profiles. The geochemical behaviors and development characteristics of major elements, trace elements and rare earth elements are also similar to those in crystalline rock weathering crusts. But some characteristics suggest that the carbonate rock weathering crusts have many special characteristics which are not in agreement with those of other types of weathering crusts, such as chemical weathering intensity, mineralogical and chemical compositions, and so on. All evidence suggests that the weathering and differentiation of minerals and elements of the profile are not strong, which may be controlled by the original composition of limestone. The results of the study support the two stage weathering model put forward by the authors.