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

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.