摘要
贺兰山—六盘山地区是研究华北板块西部大地构造背景的关键区域。在研究区内中侏罗统直罗组地层采集了20件样品,应用全岩分析和ICP-MS方法对采集的样品进行主、微量及稀土元素测试并对其地球化学特征做出分析。测试结果显示,样品中Al_2O_3/SiO_2值介于0.17~0.31之间,平均值为0.25;K_2O/Na_2O值介于0.77~73.77之间,平均值为9.78;Al_2O_3/(Na_2O+CaO)值介于0.49~96.00之间,平均值为15.68。上述含量和比值与样品成分成熟度、元素迁移、热液流体影响和风化程度有关。部分元素之间存在良好的相关性,能够反映物源及环境的变化。稀土元素值总和(ΣREE)变化较大(132.16~394.47μg·g^(-1)),其球粒陨石标准化配分模式与大陆上地壳(UCC)极为相似,表现为轻稀土富集、重稀土平坦、Eu负异常,Ce异常不明显等特征。结合UCC标准化曲线、La/Yb值、GdN/YbN值、La/Th值和Hf含量,认为直罗组源岩主要为长英质岩石,混杂了少量古老沉积岩和基性岩或者特殊矿物,且后太古界地层提供主要物源。微量元素构造背景判别图揭示研究区中侏罗统直罗组地层的物源区具有大陆岛弧和活动大陆边缘的特征,物源可能来自于兴蒙造山带和祁连造山带。化学蚀变指数(CIA)值和A-CN-K图反映源岩经历了较强的风化作用,但不同区域之间的风化程度存在差异。
Helan Mountain- Liupan Mountain area is the key location to study the tectonic background in the west of North China Plate. We collected 20 samples of mud stone and silty mud stone and 1coal sample in the study area. Then the testing on major,trace and rare earth elements were carried out by bulk rock analysis and ICP-MS,which could help understand the geochemistry characteristics of these samples. The results show： The ratio value of Al2O3/ SiO2 stays steady from 0. 17 to 0. 31 with the average value 0. 25; the ratio value of K2O / Na2O has a big fluctuation from 0. 77 to 73. 77 with the average value9. 78; the ratio of Al2O3/（ Na2O＋CaO） ranges from 0. 49 to 96 with the average value 15. 68. These ratio values are related to compositional maturity,element migration,hydrothermal fluid and weathering. Strong correlation among some elements is a natural advantage to determine provenance area. The total content of rare earth elements in the fine clastic sediments changes rapidly from 132. 16×10μg·g^-1to 394. 47μg·g^-1with the average value 198. 68 μg·g^-1. Meanwhile chondrite-normalized REE patterns reveal a deficit of light rare earth elements while the enriched heavy ones,negative Eu anomal and weak Ce anomaly. All of these characteristics together with UCC normalized REE patterns,ratios of La / Yb,Gd / Yb,La / Th and Hf indicate that the major source rock is felsic rock with slight elder sedimentary rock and basic rock or special minerals,meanwhile,the post-Archeozoic strata provided most source rocks. The tectonic discrimination diagrams of trace element show the provenance of Middle Jurassic is close to the continental island arc and active continental margin,Mongol-Hinggan and Qilian orogenic belt maybe the provenance of the study area. The ratios of CIA and A-CN-K plots reflect the source rocks had experienced strong weathering with varying degrees in various regions.
出处
《古地理学报》
CAS
CSCD
北大核心
2016年第6期1030-1043,共14页
Journal of Palaeogeography：Chinese Edition
基金
国家自然科学基金项目(编号:41330315)
中国地质调查局专项项目(编号:12120113039900,12120114009201)
西北大学研究生自主创新项目(编号:YZZ15018)联合资助~~
关键词
构造背景
地球化学
物源
风化作用
直罗组
侏罗系
贺兰山—六盘山地区
tectonic background
geochemistry
provenance
weathering
Zhiluo Formation
Jurassic
Helan Mountian-Liupan Mountain area
