宜昌三斗坪地区南华系化学蚀变指数特征及南华系划分、对比的讨论  

The Character of the Chemical Index of Alteration and Discussion of Subdivision and Correlation of the Nanhua System in Yichang Area

作　　者：王自强[1] 尹崇玉[2] 高林志[2] 唐烽[2] 柳永清[2] 刘鹏举[2] 

机构地区：[1]中国地质大学,北京100083 [2]中国地质科学院地质研究所,北京100037

出　　处：《地质论评》2006年第5期577-585,共9页Geological Review

基　　金：中国地质调查局地质调查项目(编号1212010511607);中国地层委员会<中国南华系及其层型剖面的综合地层学研究>项目的成果。

摘　　要：化学蚀变指数（chemical index of alteration）最早作为判别源岩区化学风化程度的指标而提出，随后又应用于对沉积物沉积环境的判定。化学蚀变指数的表达式为CIA=n（Al2O3）／[n（Al2O3）＋n（CaO^＊）＋n（Na2O）＋n（K2O）]×100。CaO^＊代表硅酸盐中的CaO，n（CaO^＊）=n（CaO）-n（CO2，方解石）-O．5·n（CO2，白云石）-10／3·n（P2O5）。化学蚀变指数研究样品的选取极为重要，最佳岩性为细碎屑岩，需要清除成岩过程中钾交代作用的影响。用成分变异指数（ICV）来判别沉积再循环作用和沉积物成分被改造的程度。用A—CN—K三角图解来判别物源区的源岩性质和样品钾交代的特征及其风化趋势。宜昌三斗坪地区南华系CIA研究表明：该区南华系下统莲沱组下部的CIA值在50～65之间（干燥寒冷），上部为65～75之间（温暖潮湿）和顶部为55～60之间（干燥寒冷）；南华系上统南沱组的CIA值基本在60～65之间（干燥寒冷），近顶部两个样品的CIA值达70（温暖潮湿）。上述CIA值变化表明本区南华纪经历自老到新由冰期干燥寒冷-间冰期温暖潮湿-冰期干燥寒冷沉积环境的变化过程。本文据此提出了新的扬子古陆南华系新的划分和对比方案。The chemical index of alteration（CIA）is first proposed as a marker to distinguish the chemical weathering degree of source rocks, and later it is used to distinguish the depositional environment of sediments. The formula of the chemical index of alteration is CIA=n（Al2O3）/[n（Al2O3）＋ n（CaO^＊）＋n（Na2O）＋n（K2O）]× 100, here, CaO^＊ represents CaO in silicates, n（CaO^＊）=n（CaO）-n（CO2,calcite）-0.5·n（CO2 ,dolomite）-10/3·n（P2O3）. It is very important to select samples for analyses of the chemical index of alteration. Following two factors have to be considered when sampling in the field. First, the best rocks are the fine clastic rocks. Second, the influence of diagenetic potassium replacement should be avoided. The index of compositional variability（ICV）is used to distinguish circulation and reformed degree of the composition of sediments. At the same time, A-CN-K diagram is used to distinguish the feature of source rock and character of potassium replacement and its weathering degree. Our results show that the value of CIA is at 50-65 in the lower part, 65-75 in the upper part and 55-60 in the top part of the Liantuo Formation, Lower series of Nanhua. The value of CIA varies between 60-65 in the Nantuo Formation, but is up to 70 at the top of the Nantuo Formation at Sandouping, Yichang area. Variation of the value of CIA indicates separately the circle of the arid-frigid low chemical weathering environment to warm-damp medium chemical weathering environment and to arid-frigid low chemical weathering environment from base to the top in the section. The results demonstrates that the lower part of the Liantuo Formation represents an arid-frigid cold ice period, the upper part of the Liantuo Formation represents a warm-damp interglacial period, and the top Liantuo Formation and the Nantuo Formation represent an arid-frigid cold ice period again. According to the present result, a new division and correlation of the Nanhua System in the Yangtze Platform are suggeste

关 键 词：化学蚀变指数(CIA) 钾交代 成分变化指数(ICV) 新元古代 南华系 莲沱组 

分 类 号：P532[天文地球—古生物学与地层学]