黔南—桂北地区南华系化学地层特征

Chemostratigraphic Characteristics of the Nanhua System in Southern Guizhou-Northern Guangxi Area

化学蚀变指数（chemical index of alteration）（CIA）最早作为判别源岩化学风化程度而提出，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）-n0．5×n（CO2白云石）-10／3xn（P2O5）。化学蚀变指数样品的选取极为重要，最佳岩性为细碎屑岩，并需清除成岩过程中钾交代作用的影响，以及采用成分变异指数（ICV）来判别沉积再循环作用和沉积物成分被改造的程度。使用A—CN—K三角图解判别物源区的源岩性质和样品钾交代的程度及其风化趋势。黔南-桂北地区南华系的CIA研究表明：该地区南华系下部的长安组其下部CIA值为60～65之间，中部为70～5，上部回落到65—70之间。南华系中部富禄组CIA值高，在85-95之间。南华系上部的南沱组CIA值再次回落到60—65的范围。上述CIA值的变化表明本区南华纪时期自老至新经历多次由干燥寒冷-温暖潮湿气候期的变化。本文据此提出扬子地块南华系新的划分、对比方案。

The chemical index of alteration （CIA） was first proposed as a marker to distinguish chemical weathering degrees of source rocks, and later it was used to distinguish depositional environments. 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, and n（CaO＊）=n（CaO）-n（CO2,calcite）-0.5Xn（CO2dolomite）-10/3 x n（P2O5）. It is very important to select samples for the analysis of the chemical index of alteration. The following two factors have to be consid- ered when we perform sampling in the field： the best rocks are fine clastic rocks, and the influence of diagenetic potassium replacement should be avoided. The index of compositional variability （ICV） is used to distinguish the circulation and reformation degree of the composition of sediments. At the same time, A-CN-K diagram is used to distinguish features of source rocks and characteristics of potassium replacement and its weathering degrees. The results of the authors＇ study show that the value of CIA is at 60-65 in the lower part, 70-75 in the middle part, and 65-70 in the upper part of Changan Formation of Nanhua System. CIA value is at 85-95 in Fulu Formation of Nanhua System, and CIA value is at 60-65 in the Nantuo Formation of Nanhua System. The repeated variation of the CIA value indicates separately the circle from the arid-frigid weathering environment to the warm-damp weathering environment from old to young in the Nanhua Period. Based on these results, the authors suggest a new division and correlation of the Nanhua System in the Yangtze Platform.