准噶尔盆地玛湖凹陷早二叠世风城组沉积时期古湖盆卤水演化及碳酸盐矿物形成机理

Brine evolution of ancient lake and mechanism of carbonate minerals during the sedimentation of Early Permian Fengcheng Formation in Mahu Depression,Junggar Basin,China

准噶尔盆地西北缘玛湖凹陷风城组沉积时期古湖泊为典型的碳酸盐型湖泊,发育丰富的碳酸盐矿物。碳酸盐矿物是风城组主要的盐类矿物,可以按照金属阳离子划分为Mg、Ca碳酸盐(方解石、白云石)、Na碳酸盐(碳酸钠、碳酸氢钠)、过渡型碳酸盐(碳钠钙石、碳钠镁石、氯碳钠镁石)。3类碳酸盐矿物平面分布具有"牛眼"特征,表明蒸发浓缩是直接的成盐诱导因素。方解石、白云石主要分布在古湖盆的浅水平台区、湖盆的周缘浅水区。碳钠钙石、碳钠镁石、氯碳钠镁石主要分布在与碱矿互层的暗色沉凝灰岩层段。碳酸钠和碳酸氢钠主要分布在湖盆的凹陷中心。充足的HCO_3^-阴离子控制了玛湖凹陷古湖盆盐类矿物的沉淀,碳酸盐析盐序列表现为Mg^(2+)、Ca^(2+)首先与HCO_3^-结合,消耗殆尽后碱金属阳离子Na+与富余的HCO_3^-结合形成碱矿。早二叠世强烈的火山活动以及火山物质的矿物组成对碳酸盐矿物的形成起了决定性的作用。岛弧型火山活动脱气提供大量的CO2,增加了大气的CO_2分压,进一步提高了沉积水体HCO_3^-浓度。HCO_3^-的含量是控制碳酸盐矿物沉淀的决定性因素。另外,火山喷发的CO_2气体增加了降雨的酸度,并加速了对基岩的风化以及对火山碎屑物质的水解。准噶尔盆地西北缘石炭纪-早二叠世形成的火山岩富含Na长石分子及Ca长石分子,K长石分子含量低。风城组火山碎屑物质也以富含Na、Ca为主。基岩的风化以及凝灰质的水解提供了大量Na^+、Ca^(2+)、Mg^(2+)金属阳离子,与HCO_3^-结合形成了风城组大量的碳酸盐沉积。

Saline minerals in Early Permian ancient Mahu lake,Junggar Basin were mainly carbonate minerals.Carbonate minerals in Fengcheng Formation covered 3 types according to positive metal ions,Mg-Ca carbonate minerals,calcite and dolomite,Na carbonate minerals,sodium carbonate,sodium bicarbonate,transitional carbonate minerals,shortite,eitelite,and northupite.The different kinds of carbonate minerals had regular horizontal distribution as bull＇s-eye,which demonstrated that evaporation and concentration induced the deposition of carbonate minerals.Calcite and dolomite developed in shallow platform zone of Xiazijie area mainly and shallow lakemargin.Shortite,eitelite,and northupite developed in dark tuff layers which interbedded with alkali layers.Sodium carbonate and sodium bicarbonate developed in the center area of basin.Sufficient HCO_3^- negative ion controlled the evolution of brine of ancient Mahu lake.The Mg and Ca ions synthesized with HCO_3^- firstly and Na ions synthesized with residual HCO_3^- after calcite and dolomite precipitated.Intense volcanic activities and chemical compositions of Carboniferious-Early Permian igneous rocks and volcanic eruption materials played a decisive role for carbonate minerals precipitation.Island-arc volcanic eruptions delivered massive CO_2,and increased P_（CO_2） in atmosphere which improved HCO_3^- content in sedimentary water.In addition,CO_2 in the atmosphere could promote bedrock weathering through rainfall and promote hydrolysis of tuff in sedimentary water with high HCO_3^- content.Igneous rocks around and basement of ancient Carboniferious-Early Permian Mahu lake,Northwest Junggar Basin were composed by albite molecules and Calcium feldspar molecules mainly,with low potassium feldspar molecules content.Volcanic clastic materials also had high Na and Ca content.Weathering of around Carboniferious-Early Permian igneous rocks and tuff hydrolysis offered plenty of Na,Ca,Mg ions,which synthesis with HCO_3^- and form various carbonate minerals.