塔里木盆地塔北隆起奥陶系——侏罗系泥岩稀土元素地球化学特征

Rare Earth Elemental Characteristics of Ordovician-Jurassic Mudstone in Tabei Uplift, Tarim Basin

分析了塔里木盆地塔北隆起奥陶系、三叠系和侏罗系泥岩的稀土元素丰度。不同时代泥岩都表现出轻稀土富集、重稀土含量均一和具有Eu负异常特征;但它们∑LREE/∑HREE、LaN/YbN、(La/Yb)UCC、Eu/Eu和Ce/Ce等参数则显示这些泥岩形成时的物源和环境有差别。奥陶纪时,研究区处于相对稳定的沉积环境,物源为成熟度高的大陆上地壳;二叠纪到三叠纪构造—火山活动强烈,部分沉积物来自火山岩的风化;侏罗纪塔北隆起区则进入稳定构造沉降阶段。这一差别对认识塔里木盆地及周边造山带形成演化研究有指示意义。

Rare earth element （REE） abundance of Ordovician, Triassic and Jurassic mudstone in the Tabei uplift, Tarim basin has been analysed. All the mudstones show the characteristics of high LREE content, uniform HREE and negative Eu anomaly. The ratios such as ∑ LREE/∑HREE, LaN/YbN, （La/Yb）ucc, Eu/Eu ^＊ and Ce/Ce ^＊ , however, are obviously different among them, and thus reflecting the different sources and environments when they were formed. The mudstones formed in different ages display variable degrees of REE differentiation and negative Eu anomaly. The Ordovician mudstones have high ratios of ∑ LREE/∑ HREE and LaN/YbN （ 12.1 and 19.8 ） and significant negative Eu anomaly （ Eu/Eu^＊ = 0.59 ）. The Triassic mudstones, however, show unusual low ratios of LREE/∑ HREE and LaN/YbN （5.9 and 7.7 ） and small negative Eu anomaly （ Eu/Eu ^＊ = 0.78 ）. The Jurassic mudstones are similar to the Ordovician samples except less negative Eu anomaly （ Eu/Eu^＊ = 0.72）. Similarly, UCC - normalized REE patterns of samples also show significant changes within different ages. In general, the Ordovician and Jurassic mudstones show slight enrichment of LREE relative to UCC, whereas the Triassic ones show strong LREE depletion. These characteristics indicate that the Ordovician mudstones were formed under a relatively stable sedimentary environment. The sources should be the upper continental crust with high maturity. During Permian -Triassic times, the north of the Tarim basin was tectonic and volcanic, which provide abundant volcanic materials. It stepped into the stable sinking period in Jurssic. The REE difference among mudstones has indicating significance for studying the formation and evolution of the Tarim basin and surrounding orogenic belt.