化学蚀变指数(CIA)及其在新元古代碎屑岩中的应用  

CIA (CHEMICAL INDEX OF ALTERATION)AND ITS APPLICATIONS IN THE NEOPROTEROZOIC CLASTIC ROCKS

作　　者：冯连君[1] 储雪蕾[1] 张启锐[1] 张同钢[1] 

机构地区：[1]中国科学院地质与地球物理研究所中国科学院矿物资源研究重点实验室,北京100029

出　　处：《地学前缘》2003年第4期539-544,共6页Earth Science Frontiers

基　　金：国家自然科学基金(49973008;40172015);中国科学院创新项目(KZCX2-116)

摘　　要：化学蚀变指数(CIA)首先是作为一个判断源区化学风化程度的化学指标被广泛应用的,它的表达式通常表示为:CIA={x(Al2O3)/[x(Al2O3)+x(CaO*)+x(Na2O)+x(K2O)]}×100,主成分均指摩尔分数,CaO*仅为硅酸盐中的CaO.利用CIA分析湖南石门杨家坪剖面的新元古代地层,发现冰期沉积物(南沱组、东山峰组)的CIA值(60～70)和间冰期沉积物(湘锰组)CIA值(70～75)分别反映了冰期和间冰期的化学风化特点,并且根据渫水河组CIA值(55～70)可以判断渫水河组可能也是冰期的沉积产物.另一方面,可以利用A-CN-K图解判断沉积岩源岩成分,因为样品点在三角图解中投影点的趋势线与Pl-Ksp连线的交点代表的是源岩斜长石和钾长石的比率.据此初步判断渫水河组源岩成分是南秦岭-扬子地区元古宙TTG岩石.此外,钾交代作用在前寒武纪普遍发育,在A-CN-K图解中表现为样品点的趋势线向右偏离预测的风化趋势线,渫水河组的样品点就表现出这样的特征,可以判断渫水河组钾交代作用发育.样品的选取是进行CIA研究的一个重要的步骤.首先样品的岩性最好是细屑岩,这样可以排除分选作用对源岩成分的改造;其次,需要考虑去除成岩作用过程中加入的K2O;再次,利用ICV这个化学指标来判断沉积再循环作用对沉积物成分的改造,ICV大于1意味着沉积再循环作用的影响很小,所以选取ICV大于1的细屑岩样品作为我们的研究对象.The chemical index of alteration (CIA) was widely used as an index ascertaining the degree of chemical weathering, this index can be calculated using molecular proportions: CIA = {x(Al2O3)/ [x(Al2O3)+x(CaO* )+x(Na2O)+x(K2O)]} ×100,where CaO* is the amount of CaO incorporated in the silicate fraction of the rock. We used CIA to analyze the Neoproterozoic strata from the Yangjiaping section in the Shimen County of Hunan Province, the values of CIA from glacial sediments (Nantuo Formation and Dongshanfeng Formation) ranging from 60 to 70 and interglacial sediments (Xiangmeng Formation) ranging from 70 to 75 respectively reflect the characteristic of the chemical weathering during the glacial and interglacial period, and the values of CIA from the Xieshuihe Formation ranging from 55 to 70 indicate that the sediments from the Xieshuihe Formation belong to glacial materials. On the other hand, the A - CN - K triangle also can be used to constrain the initial composition of source rocks, since the extrapolation of the sample points back to the P1-Ksp line in the A - CN - K diagram suggests a pla-gioclase to K-feldspar ratio in the provenance. According to the rule, the source rocks from the Xieshuihe Formation is probably the Proterozoic TTG from the South Qinling-Yangtze areas. In addition. K-metasomatism is very pervasive in Precambrian, which represents the right deviation relative to the predicted weathering trend in the A - CN - K diagram. The deviation displayed by the Xieshuihe Formation data indicates that K-metasomatism effect extensively exists. It is an important step to take some proper samples in research. First, we would better take some samples like argillaceous rocks in order to avoid sorting effect changing the composition of the source rocks; second, remove the additional K2O during diagenesis; third, index of compositional variability(ICV) is a useful index to estimate whether the recycling of previously weathered detritus would change the composition of the source rocks or not. When ICV is more than

关 键 词：化学蚀变指数(CIA) 钾交代 源岩成分 化学风化程度 新元古代 杨家坪剖面 

分 类 号：P588.21[天文地球—岩石学]