柴西下干柴沟组湖相白云岩特征及成因分析

对柴达木盆地西部地区下干柴沟组湖相白云岩的岩石组构、矿物成分、主微量元素地球化学及碳氧同位素特征进行研究，并分析白云岩成因。研究区白云岩沉积环境为浅水相对封闭湖相环境，白云岩以泥晶白云岩为主，层状、块状分布，见藻云岩、膏质白云岩及颗粒云岩，有序度普遍较低。主量元素中Mg2+/Ca2+比值低，Mn、Fe、Na、Sr元素含量高，指示沉积环境水体盐度较高和相对封闭；微量元素中稀土元素具有轻稀土相对富集、Eu负异常特征，反映低温浅埋藏的成岩环境；氧同位素为较低的负值，反映整体咸化水体特征，碳同位素为较低的正值或负值，主要是受到大气水影响和生物的影响。研究认为白云石形成于准同生期相对封闭咸化环境中，白云石化流体中Mg2+主要来源于大气水的淋滤和深部卤水，少数来自于藻类的分解；Mg2+通过蒸发、盐类矿物沉淀而富集，富Mg2+流体通过回流渗透交代方解石发生白云石化。

Characteristics and Origin Analysis of Lacustrine Dolomites in the Lower Ganchaigou Formation,Western Qaidam Basin

The rock fabric,mineral composition,main trace-element geochemistry and carbon and oxygen isotopic characteristics of lacustrine dolomite in the Lower Ganchaigou Formation of the Qaidam Basin were studied,and the origin of the dolomite was analyzed. The sedimentary environment of dolomite in the study area was shallow water and a relatively closed lacustrine environment. The dolomite is predominantly micritic,and occurs in layers and blocks. It is seen as algal,gypsiferous,and grain dolomite. The degree of ordering is generally low. The ratio of Mg2+ to Ca2+ is low. The Mn,Fe,Na and Sr contents are all high in the major elements,indicating that the water body was highly sa-line and that it was relatively closed. Light rare earth elements among the trace elements are relatively enriched,with negative europium anomalies,reflecting a low-temperature diagenetic environment and shallow burial. Oxygen iso-topes have a low negative value,indicative of salinized water overall. Carbon isotopes are low-positive or negative, mainly due to atmospheric water and biological effects. The study suggests that the dolomite was formed penecontem-poraneously in a relatively closed saline water environment. The Mg2+ in the dolomitic fluid is mainly derived from leaching by atmospheric water and deep brine,with a small amount from algal decomposition. The Mg2+ was enriched by evaporation and salt mineral precipitation,and the enriched fluid was dolomitized by seepage-reflux infiltration of calcite.