准噶尔盆地西北缘乌尔禾—风城地区二叠系风城组白云岩地球化学特征及成因分析

Dolomite genesis and geochemical characteristics of permian Fengcheng formation in Wuerhe-Fengcheng area,northwestern Junggar basin

以白云岩的地球化学方法为主,对准噶尔盆地西北缘乌尔禾-风城地区二叠系风城组广泛发育的白云岩化岩类进行详细的研究.通过对乌尔禾-风城地区二叠系风城组白云岩化岩类的元素分析、碳氧同位素和古盐度、古温度的研究,得出研究区白云岩类以白云质泥岩、泥质白云岩和凝灰质白云岩三种形态存在.白云质岩类富含陆源碎屑(泥质)或凝灰岩组分;白云质岩类中c(Mg)/c(Ca)分布范围较宽,主要分布于0.636～4.785,平均值为1.532,大部分分布于1～2,说明研究区有丰富的Mg^(2+)供应;白云质岩类中w(Sr)较高,主要为95.9×10^(-6)～783.6×10^(-6),平均值为464.95×10^(-6),w(V)/w(Ni)为0.8～4.8,主要为2～3;研究区二叠系白云质岩类的δ^(18)O和δ^(13)C值分布范围较大,δ^(13)C为-1‰～5.2‰,平均值为3.1‰,δ^(18)O为-14.8‰～3.2‰,平均值为-2.8‰.由地球化学元素分析结合古温度和古盐度分析认为研究区风城组白云岩形成于一盐度较高、温度偏低的环境中,火山喷发的热液活动对白云岩的形成也有一定的影响.白云质岩类形成大部分发生为成岩早期,少部分白云岩化发生为成岩中晚期的埋藏时期,总的来说白云质岩类形成于早成岩阶段的陆相咸水湖泊中.

By means of dolomite geochemical methods,an elaborate study was carried out on dolomitized rocks which are of widely distributed permian Fengcheng formation at Wuerhe-Fengcheng area in the northwest margin of the Junggar basin.According to element analysis,carbon and oxygen isotopic analysis,and paleogeotherm and paleosalinity of the permian Fengcheng formation dolomitized rocks,we have drawn the conclusions that dolomite rich in terrigenous clastic（muddy） or tuff component;c（Mg）/c（Ca） is between 0.636 and 4.785,with the average being 1.532 and most of them ranging between 1 and 2;w（Sr） is distributed 95.9×10^（-6）～783.6×10^（-6）, with the average being 464.95×10^（-6）;w（Vi）/w（Ni） is between 0.8 and 4.8,with the most being between 2 and 3. However,δ^（18）O andδ^（13）C are very scattered,the mainδ^（13）C is between—1‰and 5.2‰,with the average being 3.1‰,and the mainδ^（18）O ranges from—14.8‰to 3.2‰,the average being—2.8‰.Based on the geochemical element analysis combined with the temperature and paleosalinity analysis,it was concluded that the Fengcheng formation dolomite formed on a higher salinity and low temperature environmentl volcano hydrothermal activities also have had a certain influence on the dolomite formation;dolomitic rock formation occurred mostly in the low diagenetic stage;little occurred in the late diagenesis burial period and overall dolomitic rocks formed in the early diagenetic stage of continental saline lakes.