准噶尔盆地玛湖凹陷下二叠统风城组含碱层段韵律特征及成因

Characteristics of alkaline layer cycles and origin of the Lower Permian Fengcheng Formation in Mahu sag,Junggar Basin

准噶尔盆地玛湖凹陷早二叠世风城组沉积时期为典型的碳酸盐型咸化湖盆,湖盆咸化过程中析出大量的碱矿(钠碳酸盐类矿物)。通过含碱层段主量元素、碳氧同位素分析,结合岩心宏观及微观矿物岩石特征,探讨了含碱层段韵律特征及形成机理。研究层段碱矿与沉凝灰岩互层,形成明暗相间的咸化韵律。浅色碱矿层质地较纯,主要为苏打和小苏打,主量元素表现为Na_2O含量高,CaO、MgO、SiO_2和Al_2O_3含量低;暗色层段主要为含斑点状碳钠钙石的沉凝灰岩,主量元素表现为Na_2O含量低,CaO、MgO、SiO_2和Al_2O_3含量高。浅色碱矿层和暗色沉凝灰岩段碳氧同位素值均为正,表明其沉积水体具高盐度特征。浅色含碱层段具有更高的δ^(18)O值,反映了碱矿层沉积时期强烈蒸发的气候条件。根据δ^(18)O/δ^(13)C值的波动将研究层段划分为炎热干旱和相对湿润的气候条件,并据此解释不同韵律层盐类矿物形成机理。暗色的沉凝灰岩沉积代表地表流水注入量大于蒸发量,沉凝灰岩中盐类矿物含量少或者呈斑点状从颗粒间的卤水中析出;随着蒸发量的增高以及古气候由温暖潮湿转变为干旱炎热,地表流水注入量急剧减少,强烈蒸发控制了碱矿的沉积,形成浅色质地较纯的碱矿层。气候的频繁交替形成了风二段碱矿与沉凝灰岩频繁互层的特征。

Ancient Mahu lake was a typical carbonate saline lake in the Early Permian where abundant carbonate minerals（ mostly with positive ion of sodium） were deposited. Based on the data of major elements,oxygen and carbon isotope,petrology and mineralogy characters of systematic samples of study core section,rhythm characteristics and their mechanism were discussed. The study core section alternated with soda layers and sedimentary tuff layers. Light soda layers are relatively pure nahcolite and sodium,and have major elements characteristics as follows： High content of Na_2O,low content of CaO,MgO,SiO_2 and Al_2O_3. Dark sedimentary tuff layers developed white spot shaped minerals of shortite,and have major elements characteristics as follows： Lower content of Na_2O,high content of CaO,MgO,SiO_2 and Al_2O_3. Both soda layers and sedimentary tuff layers have high values of δ-（18）O and δ-（13）C,which indicate that sedimentary water has high salinity during the depositional period of whole layers. Light soda layers have higher δ-（18）O value,which indicate intense evaporation when soda deposited. The study core section can be explained into multiple stages of hot and dry climate and relative humid climate according to the value ofδ-（18）O / δ-（13）C. Origin of different carbonate minerals formed in different layers could achieve. Dark layers formed when palaeoclimate was warm and humid that water input was more than water output,and shortite crystaled inter-particles when palaeoclimate transformed into hot and dry. When palaeoclimate transformed into hot and dry,calcium ion and magnesium ran out,nahcolite and baking soda crystaled with strong evaporation. Frequent changes of palaeoclimate have produced saline cycles of soda layers and sedimentary tuff layers.