安徽沿江地区晚石炭世黄龙组白云岩地球化学特征及其成因研究

安徽沿江地区晚石炭世黄龙组白云岩广泛发育,主要为晶粒白云岩,其次是角砾白云岩和残余颗粒白云岩。地球化学分析表明,安徽沿江地区黄龙组白云岩中Fe、Mn、Sr、Na含量分别为:2 567×10^-6(1 600×10^-6～4 400×10^-6)、353×10^-6(166×10^-6～742×10^-6)、109.7×10^-6(69.0×10^-6～176.5×10^-6)、288×10^-6(100×10^-6～700×10^-6),稀土元素总量为7.29×10^-6(2.56×10^-6～20.82×10^-6),ΣLREE/ΣHREE比值平均为3.07(1.16～10.17),经海水标准化后,显示LREE富集,HREE相对亏损,δCe平均为4.53(2.82～7.78),正异常显著,δEu平均为1.85(0.94～2.92),配分模式曲线显示白云岩化流体来自海水。白云岩中可见蒸发岩类或蒸发构造。综合分析认为该地区白云岩主要为准同生成因,部分为埋藏成岩成因,其中角砾白云岩为准同生成因,怀宁地区白云岩δEu平均为2.38(1.91～2.92),正异常显著,表明后期受到热液流体影响。

Geochemistry characteristics and origin of Upper Carboniferous Huanglong Formation dolomites in the Anhui segment of the Lower Yangtze area

Dolomites distribute widely in the lower part of the Late Carboniferous Huanglong Formation in the Anhui segment of the Lower Yangtze.Dolomites include three rock types,mostly rystalline dolomites,secondly breccia dolomites and remnant grain dolomites.According to geochemical analysis,Average concentrations of Fe,Mn,Sr and Na of dolomites of the Huanglong Formation are 2 567×10^-6(1 600×10^-6～4 400×10^-6), 353×10^-6(166×10^-6～742×10^-6), 109.7×10^-6(69.0×10^-6～176.5×10^-6)and 288×10^-6(100×10^-6～700×10^-6); The average concentrations of REE is 7.29×10^-6(2.56×10^-6～20.82×10^-6)and ratios of ΣLREE/ΣHREE is 3.07(1.16～10.17),After normalizing to standard seawater,it shows that dolomites are all rich in LREE and depleted in HREE.The average values of δCe and δEu are 4.53(2.82～7.78)and 1.85(0.94～2.92),distributions of REE_SWN of dolomites indicate that dolomitization fluids come from seawater.Evaporative rocks and sedimentary structures could be found in dolomites in Anhui segment of the Lower Yangtze,so we think that dolomites of Huanglong Formation are mainly penecontemporaneous and some are formed by burial dolomitization,breccia dolomites are penecontemporaneous.Besides,positive anomalies of Eu(the average of δEu is 2.38(1.91～2.92))of dolomites in Huaining area indicates that dolomites are effected by later hydrothermal activity.