塔里木北部二叠纪长英质火山岩年代学及地球化学特征

大面积分布于塔里木盆地的二叠纪玄武岩构成了面积250000km²的大火成岩省(LIP),长英质火山岩的发现为塔里木二叠纪火山作用的研究打开了新的窗口。本文从塔北地区约5000m深的钻井中收集了4件二叠纪长英质火山岩的样品。通过对其进行锆石U-Pb同位素测试,得出其形成时代为274~282Ma,为塔里木大火成岩省晚期岩浆作用的产物。岩石具有高钾的特征K₂O+Na₂O=7.29%~8.34%,K₂O/Na₂O>1,大部分属于高钾钙碱性系列,且属于过铝质(A/CNK=1.32~1.53)。具有富集LREE和Zr、Hf、Y,亏损Sr、P、Ti、Nb、Ta等特征,微量元素分布曲线形态与地壳相近,具有右倾的稀土元素配分曲线,且显示出一定的负铕异常。通过Sr-Nd-Pb同位素的分析得出其源区有大量地壳物质,这与其具有较高的Th、U含量和与地壳平均值相似的Nb/La、Nb/U、Th/Ta相一致。综合年代学、地球化学特征及构造环境的判断,认为塔北地区二叠纪长英质火山岩形成于地幔柱活动背景下的地壳物质的部分熔融。

Felsic volcanic rocks from northern Tarim, NW China: Zircon U-Pb dating and geochemical characteristics

The widely distributed Early Permian magmatism in Tarim Basin, extending about 250,000km², is considered to be a new large igneous province (LIP). The research about the felsic volcanic rocks in Tarim basin provides a new way to understand the Permian magmatism of Tarim. We report zircon U-Pb dating, major and trace element composition and Sr-Nd isotopic data for felsic volcanic rocks from northern Tarim uplift. All the samples were collected from the drill holes which have depths about 5km. Zircon U-Pb dating of these samples revealed an eruption age span of 274Ma to 282Ma, which is supposed to be post major eruption magmatism of Tarim LIP. These samples are enriched in potassium, K₂O+Na₂O=7.29%~8.34%, K₂O/Na₂O>1, and belong to high-K calc-alkaline series and peraluminous (A/CNK=1.32~1.53). Most of the samples are enriched in LREE, Zr, Hf, Y, with low Sr, P, Ti, Nb and Ta. The primitive-mantle normalized incompatible element spidergrams are similar to those of continental crust and they have right-dipping type chondrite-normalized REE patterns with minor negative Eu anomalies. The study of Sr-Nd-Pb isotope shows that the origin of the northern Tarim felsic rocks may suffer contamination by continental rocks. This is also supported by the high Th, U, Nb/La, Nb/U and Th/Ta ratios which are affinity with the crust. Based on the data indicated above we suppose that the felsic volcanic rocks from northern Tarim uplift are the products of anatexis of the continental crust associated with the mantle plume activity.