新疆准噶尔盆地吉木萨尔凹陷二叠系芦草沟组云质岩地球化学特征

Geochemistry of the dolomitic rocks from the Permian Lucaogou Formation in the Jimusar depression, Junggar Basin, Xinjiang

研究区芦草沟组云质岩主要包含泥晶白云岩、凝灰质白云岩和白云质凝灰岩等类型,对准噶尔盆地吉木萨尔凹陷二叠系芦草沟组广泛发育的云质岩进行详细的地球化学研究,分析了芦草沟组云质岩的物质来源和沉积环境。云质岩的c(Mg)/c(Ca)比值0.03~1.44,磷含量262×10^-6~17283×10^-6,Th/U比值0.58~2.00,V/(V+Ni)比值0.60~0.89,V/Cr比值1.25~6.38,Sr/Ba比值0.55~8.25,反映了云质岩形成于生物繁盛的还原性咸水湖泊环境,Mg离子主要来源于深部岩浆热液。碳、氧同位素研究反映芦草沟组主要形成于温度较高的咸水环境中,且由早至晚芦草沟组沉积期具有温度升高而盐度降低的趋势。温度的升高引起的蒸发量增大并没有带来湖水盐度的增加,这种碳酸盐碳、氧同位素反向变化关系可能由于芦草沟组沉积后期不断有外源淡水的输入造成。

The dolomitic rocks from the Permian Lucaogou Formation in the Jimusar depression,Junggar Basin,Xinjiang mainly consist of micritic dolostone,tuffaceous dolostone and dolomitic tuff. In the present paper,the highlights are concentrated in the geochemical signatures,provenance and sedimentary environment of the dolomitic rocks widespread in the study area. These dolomitic rocks have P contents ranging from 262×10 ^-6 to 17283×10 ^-6 (with an average of 427.70×10^-6 ,very close to that of North America Shale Composite),c(Mg)/c(Ca) ratios of 0.03-1.44,Th/U ratios of 0.58-2.00,V/(V+Ni) ratios of 0.60-0.89,V/Cr ratios of 1.25-6.38,V/Sc ratios of 3.65-16.19,and Sr/Ba ratios of 0.55-8.25. The analytical results outlined above suggest that the dolomitic rocks in the Lucaogou Formation were formed in the salt-water reducing lake environments with abundant organisms,and magnesium ions are inferred mainly to be originated from deep-seated magmatic hydrothermal solutions. The carbon and oxygen isotopic data also demonstrate that the Lucaogou Formation was laid down in the continental salt-water lake environments with higher temperatures. There is a tendency of gradual increase of temperatures and gradual decrease of salinities from the early to later stages of the deposition of the Lucaogou Formation. However,there is no evidence in favour of salinity increase in the highly-evaporated lake water due to temperature increase. The abnormal changes of the above-mentioned palaeotemperatures and palaeosalinities based on carbon and oxygen isotopic data of the carbonate minerals may be caused by the input processes of allochthonous fresh water in the later stages of the deposition of the Lucaogou Formation.