摘要:
对准噶尔盆地东南缘中二叠统芦草沟组上、中、下三段黑色泥页岩进行稀土元素地球化学特征研究,并讨论其地质意义。结果表明,芦草沟组黑色泥页岩的稀土元素总量∑REE为(51.39~138.27)×10-6,平均值为94.23×10-6,较全球平均大陆上地壳成分(UCC)和北美页岩(NASC)的平均值均偏低。∑LREE/∑HREE、(La/Yb)N、(Ce/Yb)N等地化参数及稀土元素配分模式均反映轻、重稀土元素的明显分异,呈现LREE富集、HREE亏损的特征,趋势曲线较为平坦。黑色泥页岩具有明显的δEu负异常和弱的δCe负异常。Ce异常值均大于-0.1,反映研究区在中二叠世水体为较强且稳定的还原环境。REE的明显分异指示沉积速率总体较低,呈现逐渐变慢的趋势,反映了沉积时期水体加深的过程,这也与滨浅湖-半深湖-深湖的沉积相吻合。根据δEu的异常情况和(La/Yb)N与∑REE的关系,认为芦草沟组沉积物的母岩为沉积岩和花岗岩的混合。黑色泥页岩物源主要来自南部的伊连哈尔比尕山。
Abstract:
REE geochemistry and geological implications are examined for the black mudstones and shales from the Lucaogou Formation on the southeastern margin of the Junggar Basin, Xinjiang. The ΣREE range from 51.39×10-6 to 138.27×10-6 with an average of 94.23×10-6, lower than those of the global average upper crust compositions (UCC)and North American Shale Composites (NASC). The geochemical parameters such as ΣLREE/ΣHREE ratios, (La/Yb)N and (Ce/Yb)N, and REE distribution patterns reflect a marked differentiation of light and heavy rare earth elements, light rare earth element enrichment, heavy rare earth element depletion and slightly declined REE distribution patterns. The black mudstones and shales display noticeably negative δEu anomalies and slightly negative δCe anomalies. The Ce anomalous values of more than-0.1 suggest a highly stable reduction environment of the study area during the Middle Permian. The highly differentiation of REE exihibits a general low and slowing trend of depositional rates, and progressively deepening of waters during the deposition of the Lucaogou Formation, which is represented by the littoral-shallow lake, bathyal and abyssal lake facies. Judged from the δEu anomalies and the relationship between (La/Yb)N and ΣREE, the parent rocks of the black mudstones and shales in the Lucaogou Formation are interpreted as the mixture of sedimentary rocks and granites, and may be derived from the Ilianhaerbga Mountain in the south.
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