四川盆地西缘地区中侏罗统沙溪庙组地球化学特征及其环境意义

Geochemical Characteristics and Environmental Implications of Middle Jurassic Shaximiao Formation,Western Margin of Sichuan Basin

沉积岩的微量元素及稀土元素蕴含了大量的地质信息,它对沉积环境的水文变化有着较高的敏感度,是研究沉积物沉积时的古气候、古环境的有效手段。选取四川盆地西缘地区中侏罗统上、下沙溪庙组的17件样品进行了详细的微量元素及稀土元素的测定,应用了能够敏感反应沉积环境的微量元素指标及特征微量元素、稀土元素比值,对四川盆地西缘地区中侏罗统上、下沙溪庙组沉积时期的古环境、古气候进行了研究。结果表明:研究区上、下沙溪庙组Sr/Ba比值均小于1,说明沙溪庙组沉积期湖水为淡水;V、U、Mo、V/Cr、Ni/Co、U/Th、(Cu+Mo)/Zn、V/(V+Ni)、δCe、δEu、∑REE、La/Yb微量元素及稀土元素特征值及比值,说明整个沙溪庙组沉积时期湖泊底层水体总体上为中等分层的含氧环境;根据Sr/Cu比值及前人研究成果认为下沙溪庙组沉积期整体表现为温暖干旱,局部时期转凉,此后上沙溪庙组沉积期又转变为类似于下沙溪庙沉积期的温暖干旱的气候环境,且干旱程度逐渐加强。

Trace elements and REE of sedimentary rocks contain abundant information of geology. Having a high sen- sitivity on hydrological changes of sedimentary environment, trace elements and REE can be used as an effective method on the study of paleoclimate and paleoenvironment during deposition period. Choosing the trace elements and REE measured value of 17 samples of middle Jurassic Shaximiao Formation of western margin of Sichuan basin. Re- sults indicate that, in the study area, both the Sr/Ba ratios of the upper and lower Shaximiao Formation are less than 1, which illustrate that there was a fresh water environment during deposition of Shaximiao Formation. The character- istic values and ratios of trace elements and REE such as V, U, Mo, V/Cr, Ni/Co, U/Th, （ Cu ＋ Mo）/Zn, V/（ V ＋ Ni）, δCe, δEu, δ REE, La/Yb, suggest that the bottom water of the lake was moderate oxidation environment during the whole time Shaximiao Formation deposition. According to the Sr/Cu ratios and the research findings before, we suggest that there paleaoclimate was warm and arid with cool interphases during the whole time of lower Shaximiao Formation, and after then it turned to worm and aridity again during the upper Shaximiao period, which is as the lower Shaximiao Formation, and the aridity strengthened gradually.