川西昌台勉戈组沉积岩系稀土特征

CHARACTERISTICS OF REE IN THE SEDIMENTARY ROCKS FROM MIANGE FORMATION OF UPPER TRIASSIC IN CHANGTAI OF WESTERN SICHUAN

川西昌台地区上三叠统勉戈组中酸性火山岩之上主要为一套灰黑色至黑色沉积岩石组合,包括板岩、页岩和泥岩等。这套岩石组合可分为两类:底部为存在热水活动参与形成的沉积岩,厚度较薄;底部之上为无热水活动参与形成的沉积岩,厚度较大。本文取样上述两类岩石测得的稀土元素特征基本相似,无明显区别:两类样品稀土含量普遍偏高,与球粒陨石标准化的的配分曲线呈右倾型,LREE富集,基本无Ce正/负异常,具有明显的Eu负异常。有热水活动参与的沉积岩样品与太平洋SER地区现代海底典型热水沉积物的稀土元素特征明显不一致;与该区域内呷村矿区的典型热水沉积岩重晶石和硅质岩的稀土元素特征也不一致。但与秦岭志留系部分热水沉积岩样品和湘黔寒武系牛蹄塘组底部碳质伊利石页岩样品的稀土特征一致,呈现正常碎屑岩的稀土元素特征。综合分析认为,川西勉戈组碎屑页岩出现Eu负异常特征的主要原因是所测样品中的热液组分含量较少甚至无。

The mid-acidic lavas of Miange Formation of Upper Triassic in Changtai area of West- ern Sichuan are overlain by a suit of black colored sedimentary rocks,including shale and mud- stone. These sedimentary rocks can be classified into two types, the first type was at the low part with relative small tickness was affected by hydrothermal activity and the second type lies at the upper part with large thickness was not affected by hydrothermal activity. The characteristics of REE in the two kinds of rocks indicate that they are similar to each other. Their total REE con- tents are very high. The curves of Chonodrite-normalized patterns lean right and possess enriched LREE and negative Eu anomalies. The REE patterns of the first type are different from those of the modern hydrothermal sediments of the Pacific ocean and evidently different from those of the hydrothermal rocks（barite and siliceous rock）in Changtai area. However, the REE patterns are comparable with those of some hydrothermal sedimentary rocks in Silurian system of Qinling area and black rock series of the lower Niutitang Formation in Hunan-Guizhou, similar to the characteristics of REE in the normal clastic sedimentary rocks. It is considered that the main reason for the occurrence of negative Eu anomalies is that the hydrothermal constituent is too little in clastic sedimentary rocks from Miange formation in this area.