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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。
The ancient lakes during the deposition of the Fengcheng Formation in the Mahu Sag of the Northwestern Junggar Basin are typical carbonate lakes with abundant carbonate minerals. Carbonate minerals are the main salt minerals in Fengcheng Formation. They can be divided into Mg, Ca carbonate (calcite, dolomite), Na carbonate (sodium carbonate, sodium bicarbonate), transitional carbonic acid Salt (sodalite, stannous, stannane). The distribution of carbonate minerals in the three classes has the feature of “bull’s eye”, indicating that evaporation concentration is a direct salt-inducing factor. Calcite and dolomite are mainly distributed in the shallow platform area of the ancient lake basin and the peripheral shallow water area of the lake basin. The sodalite, diabase, and carbonated magnesia are mainly distributed in the dark-colored sedimentary limestone layer interbedded with the alkali mine. Sodium carbonate and sodium bicarbonate are mainly distributed in the depression center of the lake basin. Sufficient HCO_3 ~ - anions control the sedimentation of salt minerals in the Paleohu basin of the Mahu Sag. The carbonate salt sequence is Mg ~ (2 +). Ca ~ (2+) first binds with HCO_3 ~ After that, the alkali metal cation Na + is combined with the surplus HCO 3 - to form an alkali mine. The intense volcanic activity of the Early Permian and the mineral composition of volcanic material played a decisive role in the formation of carbonate minerals. The island arc-type volcanic activity degassing provides a large amount of CO2, increases the atmospheric CO 2 partial pressure, and further increases the concentration of HCO 3 - in the sedimentary water. The content of HCO 3 ~ - is the decisive factor controlling the precipitation of carbonate minerals. In addition, CO 2 gas from volcanic eruptions increases the acidity of rainfall and accelerates the weathering of bedrock and the hydrolysis of pyroclastic matter. The Carboniferous-Early Permian volcanic rocks in the northwestern margin of the Junggar Basin are rich in Na feldspar and Ca feldspar, while K feldspar is low in molecular weight. Fengcheng group pyroclastic material is also rich in Na, Ca-based. The weathering of bedrock and the hydrolysis of tuff provide plenty of Na ~ +, Ca ~ (2 +) and Mg ~ (2+) metal cations, which combined with HCO_3 ~ form a large amount of carbonate deposition in Fengcheng Formation.