广东东莞地区中侏罗统塘厦组凝灰质沉积物的元素地球化学特征及构造背景

Elemental geochemical characteristics of tuffaceous sediments and tectonic setting of Tangxia Formation of Middle Jurassic in Dongguan,Guangdong Province

东莞地区中侏罗统塘厦组为富含凝灰质的湖泊沉积。对样品进行全岩分析和ICP-MS微量元素分析,并计算Sr/Ba、B/Ga和Ca/Mg比值,其比值变化与古盐度变化成正相关,特征显示塘厦组为咸水湖泊沉积,由下至上古盐度逐渐降低,水体深度也逐渐变浅。对样品进行ICP-MS稀土元素分析,计算各样品δCe值和(La/Yb)_N比值,δCe值介于0.83～1.31之间,(La/Yb)_n值均明显大于1或小于1,其特征反映湖泊沉积物沉积速率较低,只在塘厦组底部和上部出现相对高的沉积。元素P和Sr/Cu值反映其富集与迁移,受到气候的控制。元素P平均值在0.205×10^(-2);Sr/Cu平均值大于4.0×10^(-6),在中晚期出现最低值1.42×10^(-6),这是塘厦组以干热气候为主,中晚期出现了短暂温湿气候所造成的。塘厦组为干热气候下缓慢沉积的咸水湖泊沉积。Zr-Th、La-Th-Sc判别图解、稀土元素特征及区域构造分析,显示中侏罗世东莞地区构造背景为活动大陆边缘的弧后盆地。受伸展作用影响,本区发育弧后裂谷型湖盆。泥岩La/Yb-∑REE源岩判别图解显示,源岩为玄武岩、花岗岩和沉积岩。

The Tangxia Formation of Middle Jurassic in Dongguan,Guangdong Province,is a kind of lacustrine sediments in rich of tuffaceous materials.The changes of Sr/Ba,B/Ga and Ca/Mg have a positive correlation with the changes of paleosalinity.The result of the ratios is based on the whole rock analysis and ICP-MS trace element analysis of the samples.The characteristics of those shows that sediments of Tangxia Formation ares salty lacustrine sediments.The paleosalinity of Tangxia Formation descends gradually from bottom to top,and the lake depth reduces as well.The value ofδCe and the ratio of(La/Yb)_N to the samples are got through the ICP-MS trace element analysis.The value ofδCe is between 0.81 and 1.31.The ratio of(La/Yb)_N is obvious larger than 1 or smaller than 1.The value ofδCe and(La/Yb)_N reflects that the depositional rate of lake sediments was slower during the main stage of development of Tangxia Formation except the initial and final stages with a relatively faster rate.The values of P and Sr/Cu reflect their enrichment and migration,controlled by the climate.The average of P is 0.205×10^(-2) and the average of Sr/Cu which achieved lowest value 1.42×10^(-6) in middle and late stages is larger than 4.0×10^(-6).The averages are caused by the climate of Tangxia Formation,which was mainly hot-arid climate and the warm-humid climate only in middle and late stages.The Tangxia Formation is salty lacustrine sediments with slow depositional rate and hot-arid climate.The discrimination diagrams of Zr-Th and La-Th-Sc,REE and the regional tectonic analysis show that the tectonic setting of Dongguan area of Middle Jurassic was back-arc basin of active continental margin.Affected by the extension,Dongguan area developed into the rift lake basin in the back-arc.La/Yb-ΣREE source rock discrimination diagram of the mudstones shows that source rock were basalt,granite and sedimentary rocks.