右江盆地泥盆系硅质岩地球化学特征及地质意义

Geochemical characteristics of Devonian siliceous rocks in Youjiang Basin of Guangxi and its implications for tectonic evolution

探讨右江盆地泥盆纪盆地演化。对盆地东南缘六强、那洋地区泥盆纪硅质岩地球化学特征研究表明,硅质岩的SiO_2质量分数为88.40%～97.89%,具有较高的硅化程度。多数样品具有较低的Eu/Eu~*值和较高的Al/(Al+Fe+Mn)值,指示硅质岩主要为非热液成因。上泥盆统榴江组有较明显的Ce负异常(Ce/Ce~*值为0.37～0.57),中下泥盆统平恩组和坛河组具有中等Ce负异常值(Ce/Ce~*值分别为0.55～0.57和0.55～0.71)。3组样品均具有明显的Y正异常。除去SiO_2稀释作用的影响后,榴江组硅质岩具有较高的稀土元素含量(∑REE+Y含量相当于PAAS组成的2～5倍),暗示其形成于相对远离陆源供应的环境。综合其他地区硅质岩的地球化学特征,认为泥盆纪右江盆地具有逐渐伸展的演化趋势。

Study of geochemical characteristics of Devonian siliceous rocks from the Liuqiang and Nayang areas in the Youjiang Basin of Guangxi indicates that the mass fraction of SiO 2 of the rock is 88.40%~97.89%.Most samples have lower Eu/Eu^*values and higher Al/(Al+Fe+Mn)values,characteristic of non-hydrothermal origin.The Upper Devonian Liujiang Formation siliceous rocks in Liuqiang area have obvious lower Ce/Ce^*values(0.37~0.57),while the siliceous rocks in Pingen and Tanhe Formations possess moderate level values(0.55~0.57 and 0.55~0.71,respectively)and all analyzed samples display distinct positive Y anomalies.After removing the influence of SiO 2 dilution,the siliceous rocks of the Liujiang Formation exhibit higher REE+Y contents(REE+Y values are equivalent to 2~5 times of PAAS composition),indicating that these siliceous rocks were deposited in a basin far away from the terrigenous supply environment.On the basis of above geochemical study,combined with the geochemical characteristics of siliceous rocks from other regions,it is considered that the Devonian Youjiang Basin displays an evolutionary trend of gradual extension.