老挝中部沙湾拿吉盆地晚白垩世钾盐蒸发岩:非海相输入的地球化学证据  

Late Cretaceous potash evaporites in Savannakhet Basin of middle Laos: Geochemical evidences of non-marine inputs

作　　者：张西营[1] 程怀德[1] 谭红兵[2] 袁小龙[1] 李永寿[1,3] 苗卫良[1,3] 李廷伟[1] 马海州[1] 

机构地区：[1]中国科学院青海盐湖研究所盐湖资源与化学重点实验室青海省盐湖地质与环境重点实验室,西宁810008 [2]河海大学地球科学与工程学院,南京210098 [3]中国科学院大学,北京100049

出　　处：《岩石学报》2015年第9期2783-2793,共11页Acta Petrologica Sinica

基　　金：国家重点基础研究计划(973)项目(2011CB403004);国家自然科学基金项目(40903014;41073034;41173049)联合资助

摘　　要：老挝中部沙湾拿吉盆地上白垩统农波组钾盐蒸发岩是研究区域成矿卤水演化的重要材料。盐类矿物中微量元素的含量及其变化特征共同揭示了成盐物质应该主要来自海水,但深部水的补给也具有重要作用。研究结果表明,所谓的含盐系"基底"硬石膏只是水-岩作用下下盐层底部溶滤后的残余物。光卤石中偏负的δD、δ18O值及Br的海相特征则指示了该矿物是浓缩海水混合大气水后蒸发的产物,陆相水对成矿卤水演化过程产生了一定影响。农波组下盐层蒸发岩可分为3个不同的演化阶段。(1)海相输入阶段:海水进入成盐盆地,石盐大量沉积,海侵作用早期具有明显的波动性而后期较稳定;该阶段末期发生了海相输入向陆相输入的转变,陆相输入在水文封闭和收缩的成盐盆地中逐渐占据主导地位,卤水组分因深部水输入而发生变化。(2)非海相输入及混合卤水蒸发阶段:混合卤水蒸发形成钾盐,光卤石沉积过程中发生了一次轻微的卤水淡化事件。(3)沉积后改造阶段:已沉积的光卤石岩在陆相水淋滤作用下转变为钾石盐岩,钾石盐是光卤石被交代后的次生矿物。系统的微量元素和氢氧同位素分析表明,非海相输入在成矿卤水演化及沉积后的改造过程中扮演了重要角色。Upper Cretaceous Nongbok Formation potash evaporites in Savannakhet Basin of middle Laos are the important materials for studying the regional evolution of ore-forming brines. The contents of trace elements in salt minerals together with their change characteristics reveal that the source of these salts was from the seawater but the supply of deep-seated water was also a significant source. The study results show that the so-called＂basal＂anhydrite only were the residues of primary halites which were leached by the late water-rock reaction. Slightly negative δD and δ18O values of hydration water and marine nature of Br in carnallites indicate that these minerals were the products evaporated from the concentrated seawater mixed with meteoric water and that the continental water exerted influence to some extent on the evolution of ore-forming brine. These evaporites in the lower section of Nongbok Formation are divided into three different evolutionary stages：（ 1） Stage of marine input： the seawater entered into salt basin and voluminously halites deposited; the transgression is characterized by the early obvious fluctuation and the late stabilization; the transition of marine to nonmarine input occurred at latest stage and then continental inputs was gradually dominant for the hydrologically closed and restricted salt basin; the components of ore-forming brine were changed by input of deep-seated fluid.（ 2） Stage of nonmarine input and mixed brine evaporation： the potash deposits formed during progressive evaporation; a slightly freshening event of brine happened at the early stage.（ 3） Stage of post-depositional reworking： the buried carnallitite was translated into the sylvitite by leaching of ground water and therefore the sylvite is the secondary mineral which substituted for precursor carnallite. Systematical analyses of trace element,oxygen and hydrogen isotope show that the nonmarine inputs played an important role during the evolution of ore-forming brine post-depositional reworkin

关 键 词：钾盐蒸发岩 非海相输入 微量元素 氢氧同位素 老挝沙湾拿吉盆地 呵叻高原 

分 类 号：P588.247[天文地球—岩石学]