利用泥质岩化学蚀变指数分析物源区风化程度时的限制因素

摘要

近年来,根据泥质岩中常量元素的摩尔数计算出的化学蚀变指数(CIA:Chemical index of alteration)、化学风化指数(CIW:Chemical index of weathering)和斜长石蚀变指数(PIA:Plagioclase index of alteration)被广泛用来反映物源区的风化程度及物源区古气候,这些指数在应用时都有一些严格的限制因素,应给予足够重视.CIA计算公式未排除成岩作用过程中钾交代作用的影响,需要采用A-CN-K[A12O3-(CaO*+Na2O)-K2O]三角图进行判断,对发生钾交代作用的样品利用A-CN-K三角图或CIAcorr计算公式进行校正.CIW计算公式中去掉了K2O,但没有排除钾长石中的Al元素.PIA计算公式中考虑了钾长石中的A1元素,但只适用于判断母岩中仅含有斜长石而不含钾长石的物源区风化程度.综合分析表明,在判断物源区风化程度及古气候时,CIA的干扰因素相对较少,值得推广.但即使利用从泥质岩常量元素获得的CIA值来判断物源区的风化程度时,仍需要考虑沉积分异作用、再旋回作用、沉积区进一步风化作用以及成土作用、成岩期的钾交代作用的影响,建议首先依据泥质岩常量元素的摩尔数计算出成分变异指数(ICV:Index of compositional variability),然后对ICV＞1的样品进行CIA的计算,并利用A-CN-K三角图或CIAcorr计算公式对CIA值进行钾交代作用的校正,该校正过的CIAcorr计算值可用来判断物源区的风化程度.%In recent years the chemical index of alteration (CIA),chemical index of weathering (CIW) and plagioclase index of alteration (PIA),have been widely used in attempts to quantify the degree of weathering in the provenance areas and to infer paleoclimatic conditions in provenance areas.There are,however,some limiting factors in the application of these indices which merit careful consideration.The CIA does not exclude the effect of potassium metasomatism so that it is necessary to use the A-CN-K [Al2O3-(CaO* +Na2O)-K2O] triangle to identify it and make corrections,or to utilize the CIA formula.The CIW provides a correction for K2O,but no allowance is made for Al in potassium feldspar.The PIA considers the A1 in potassium feldspar,but is only useful to judge changes involving plagioclase feldspars.Comprehensive analyses have shown that use of the CIA is the most effective way of obtaining an estimate of weathering severity and provenance.Before utilizing CIA values to estimate the degree of weathering in the provenance area,the influences of sedimentary differentiation (grain size differences),sediment recycling,further weathering in sedimentary region,pedogenesis and potassium metasomatism should be taken into account.We recommend a procedure,that is,(1) analyzing elemental compositions of mudstones,(2) calculating an index of compositional variability (ICV),(3) selecting samples with ICV＞1 and using these values for the CIA calculation,(4) using the A-CN-K ternary diagram or CIA formula to correct for potassium metasomatism.The corrected CIA values can then be used to estimate the degree of weathering to which the provenance area has been subjected.