重庆云阳地区中侏罗世新田沟组底部细粒碎屑岩地球化学特征及意义

THE GEOCHEMISTRY CHARACTERISTICS AND SIGNIFICANCE OF FINEGRAINED CLASTIC ROCKS FROM THE XINTIANGOU FORMATION IN YUNYANG COUNTY,CHONGQING

重庆云阳地区中侏罗统新田沟组是四川盆地新发现恐龙化石的重要埋藏层位之一。野外地质调查表明,研究区内新田沟组下部岩性以深灰色薄层泥质粉砂岩及粉砂质泥岩为主,夹中层状岩屑石英砂岩,偶含紫红色粉砂岩,向上砂岩增多。本次研究连续采集重庆云阳地区新田沟组底部细碎屑岩,通过主、微量元素地球化学分析,探究沉积物物源特征及沉积时期古风化和古气候条件。分析结果显示:样品w(SiO_2)为57.41%~66.53%,w(Al_2O_3)介于15.66%~20.01%;全碱含量w(Na_2O+K_2O)约3.21%~4.85%,地化特征说明新田沟组底部细碎屑岩主要属富石英岩屑杂砂岩。样品轻稀土元素(LREE)含量为163.6~216.3×10^(-6),重稀土元素(HREE)含量为18.1~25.0×10^(-6),轻重稀土比值(LREE/HREE)为7.71~10.10,说明样品轻重稀土分异明显。各项岩石地球化学比值表明其源岩为长英质火山岩,源区构造背景为大陆岛弧环境。剖面底部4件样品的化学蚀变指数(CIA)均在80以上,向上逐渐减小(平均73),指示中侏罗纪早期气候条件存在由炎热潮湿到温暖湿润的变化趋势,化学风化作用经历了由极强到中等风化的转变。

The Middle Jurassic Xintiangou Formation in Yunyang area of Chongqing is a newly discovered burial beds of dinosaur fossils in Sichuan Basin.Field geological investigation shows that the lithology of the lower part of Xintiangou Formation in the study area is mainly composed of dark gray colored,thin bedded argillaceous siltstone and silty mudstone,intercalated with mid bedded lithic quartz sandstone,which occasionally contains purplish red siltstone,and increased upward sandstone.The fine clastic rocks from the bottom of Xintiangou Formation in Yunyang area of Chongqing are collected continuously,and the provenance characteristics,paleo-weathering and paleoclimate conditions during the sedimentary period are explored through the geochemical analysis of major and trace elements.Geochemical analysis shows that w(SiO2)ranges from 57.41%to 66.53%,w(Al2O3)from 15.66%to 20.01%,w(Na2O)from 0.20%to 1.69%,and w(K2O)ranges from 2.47%to 3.97%,also total alkali content(Na2O+K2O)is in the range of 3.21%-4.85%,indicating that the fine clastic rocks at the bottom of Xintiangou Formation are mainly quartz-rich lithic graywacks.The contents of light rare earth elements(LREE)and heavy rare earth elements(HREE)range from 163.6 to 216.3×10^(-6),18.1 to 25.0×10^(-6),and the ratios of light and heavy rare earth elements(LREE/HREE)range from 7.71 to 10.10,which indicates that the differences of light and light rare earth elements are obvious.The geochemical ratios of the fine classic rocks indicate that the source rocks are felsic volcanic rocks and the tectonic setting of the source area is continental island arc environment.Chemical alteration index(CIA)of four samples from the bottom of the section are all over 80 and gradually decreased upward(mean value 73),indicating that the climatic conditions of the early Middle Jurassic had a changing trend from hot and humid to warm and humid,and the chemical weathering underwent a change from extremely strong to moderate weathering.