准噶尔盆地南缘下白垩统呼图壁河组沉积地层地球化学特征及其物源分析

Geochemical characteristics and provenance of Lower Cretaceous Hutubihe Formation in southern margin of Junggar Basin,NW China

在野外调查和剖面测量的基础上,对采自准噶尔盆地南缘下白垩统呼图壁河组岩石样品进行了沉积岩石学、岩相学和地球化学研究,探讨了下白垩统呼图壁河组沉积及物源特征。结果表明:研究区主要岩性由一套粒度较细的粉砂岩和泥岩组成,沉积构造以水平层理为主,反映湖泊相的沉积特征;样品化学蚀变指数(CIA)平均为58.72,表明物源区经历了弱风化作用;成分变异指数(ICV)平均为1.37,表明沉积物成分成熟度低,代表构造活动带内初次旋回的产物,物源区性质基本未受成岩作用的影响;流纹岩岩屑、La/Sc和Th/Sc比值以及La/Th-w(Hf)和w(Ni)-w(TiO_(2))判别图指示呼图壁河组源岩来自于上地壳的长英质火山岩;此外,安山岩岩屑的出现及Th/Co和Th/Cr的比值特征表明有少量中-基性源岩的存在;清水河组碎屑锆石物源U-Pb年龄约束了呼图壁河组物源年龄主要集中分布在472.7~277.7 Ma之间,与邻区岩浆锆石年龄对比分析表明整体上其物源区以北天山为主。V/(V+Ni)、V/Cr、Ni/Co及气候指数“C”指示古沉积环境以富氧的氧化环境为主,间或有贫氧过渡相,古气候整体为干燥环境。地球化学特征及La-Th-Sc、Th-Sc-Zr/10和Th-Co-Zr/10判别图显示物源区主要为大陆弧构造背景。

Sedimentary petrology,petrography and geochemistry of rock samples from the Lower Cretaceous Hutubihe Formation in the southern margin of Junggar Basin were studied on the basis of field investigation and profile measurement.Sedimentary and provenance characteristics of the Lower Cretaceous Hutubihe Formation were discussed.The results show that the main lithology in the study area is composed of a set of fine siltstone and mudstone,and the sedimentary structure is mainly horizontal bedding,reflecting the sedimentary characteristics of lacustrine facies.The chemical index of alteration(CIA)mean value of the samples is 58.72,indicating that the provenance experienced weak weathering.The index of compositional variability(ICV)mean value of the samples is 1.37,indicating that the sediment composition maturity is low,and the samples represent the product of the first cycle in the tectonic activity zone,whose source area is basically not affected by diagenesis.Rhyolite debris,La/Sc and Th/Sc ratios,and La/Th-w(Hf)and w(Ni)-w(TiO_(2))discrimination diagrams indicate that the source rocks of the Hutubihe Formation are from the felsic volcanic rocks in the upper crust.In addition,the occurrence of andesite debris and the ratio characteristics of Th/Co and Th/Cr indicate the existence of a small amount of intermediate-basic source rocks.The U-Pb age of the detrital zircon from the Qingshuihe Formation restricts that the provenance age of the Hutubihe Formation is mainly distributed between 472.7 and 277.7 Ma.The comparison with the magmatic zircon ages of the adjacent area shows that the provenance is mainly from the North Tianshan.V/(V+Ni),V/Cr,Ni/Co and climate index‘C’indicate that the sedimentary environment is dominated by oxygen-rich oxidation environment,and there may be an oxygen-poor transition phase,and the paleoclimate is dry.Geochemical characteristics and La-Th-Sc,Th-Sc-Zr/10 and Th-Co-Zr/10 discriminant diagrams show that the provenance is mainly in a continental arc tectonic background.