鄂西南来凤地区纱帽组碎屑岩地球化学特征及其物源区和构造背景分析

Geochemical Characteristics and Analysis of Provenance&Tectonic Setting of Sedimentary Rocks of Shamao Formation in Laifeng Area,Southwestern Hubei

针对鄂西南来凤地区志留纪纱帽组碎屑岩样品的主量、微量及稀土元素地球化学特征的分析,总结出其物源区及构造背景特征。粉砂岩中SiO 2含量由下至上有降低趋势,TFe_(2)O_(3)+MgO含量相对较高,K_(2)O/Na_(2)O比值变化范围较大。微量元素蛛网图表现为“三峰四谷一平坦型”。REE配分模式图显示轻稀土富集,轻重稀土分异较大,δEu亏损明显和δCe异常较弱。样品ICV和CIA指数特征以及Th/U、Rb/Sr比值表明物源区母岩成熟度较低,主要受温润条件下的中等化学风化作用,且经历过复杂的沉积演化。部分元素比值(Al_(2)O_(3)/TiO_(2)、Sm/Nd、Th/U)及判别图(TiO_(2)-Ni、La/Th-Hf、Co/Th-La/Sc、Th/Sc-La/Sc)显示沉积物母岩是来源于上地壳的长英质岩石。样品的三变量图解(La-Th-Sc、Th-Sc-Zr/10和Th-Co-Zr/10)和双变量图解(La/Y-Sc/Cr及La-Th)特征表明鄂西南来凤地区纱帽组碎屑岩的物源区构造背景以大陆岛弧和被动大陆边缘为主。

Taking the sedimentary rocks of Silurian Shamao formation in Laifeng area as the research object,the whole rock analysis has been made.Based on the result,the geochemical characteristics of rocks are discussed,and the characteristics of provenance and tectonic background are analyzed.The SiO_(2)content of siltstone decrease from bottom to top,TFe_(2)O_(3)+MgO content is relatively high,but the K_(2)O/Na_(2)O ratio changes greatly.The normalized arachnoid map of trace elements shows“three peaks,four valleys and one flat type”.The REE distribution pattern map shows enrichment of LREE,the differentiation of LREE and HREE is large.Obvious negative anomaly ofδEu and weak negative anomaly ofδCe.ICV and CIA of the samples and Th/U,Rb/Sr reflect that the maturity in the source area are relatively low,and suffered the moderate chemical weathering under warm and moist conditions,and underwent complex sedimentary evolution.Al_(2)O_(3)/TiO_(2),Sm/Nd,Th/U and TiO_(2)-Ni,La/Th-Hf,Co/Th-La/Sc,Th/Sc-La/Sc discriminant charts show that the sediment source come from felsic rock of upper crust.La-Th-Sc,Th-Sc-Zr/10,Th-Co-Zr/10 Trivariate diagram and La/Y-Sc/Cr,La-Th bivariate diagrams indicate that the provenance tectonic setting of the sedimentary rocks of Shamao formation in Laifeng,western Hubei Province is dominated by the continental island arc and passive continental margin tectonic setting.