阿尔金山拉配泉地区元古宙裂谷火山岩系及其构造意义

THE PROTEROZOIC RIFT VOLCANIC SERIES IN THE LAPEIQUAN AREA, THE ALTUN MOUNTAINS AND ITS TECTONIC SIGNIFICANCE

阿尔金山拉配泉之北,拉配泉断裂和阿尔金山北缘断裂夹持的火山岩片为一套典型的双峰式大陆裂谷火山岩系。基性端员属碱性系列和亚碱性系列,亚碱性系列又可分为钙碱性系列和拉斑玄武岩系列。酸性端员均属钙碱性系列。基性火山岩Sm-Nd同位素年龄为1793±270(2σ)Ma,ε_(Nd)=6.4±1.4。稀土元素地球化学特征表明,基性火山岩以轻稀土富集型配分型式为主,个别样品呈MORB型平坦配分型式;酸性火山岩以轻稀土高度富集型配分型式和Eu负异常为特征。以岩相学、地球化学结果为基础,推测发育这套火山岩的古裂谷经历了较彻底的演化,最终可能达到局部大洋化阶段。阿尔金山东西分段、南北分片,充斥外来构造岩片的现实构造格局表明,阿尔金山可能是通过东西走滑,南北挤压、断隆,拼接而成的一条独特山系。

North of the Lapeiquan area of the Altun Mountains, there is a volcanic series between the Lapeiquan fault in the south and the Altun northern boundary fault in the north. Geochemically and petrographically, the volcanic series belongs to typical continental rift bimodal volcanics. The basic end-member of the bimodal volcanics can be divided to alkaline and sub-alkaline series, and the subalkaline series can be subdivided to calc-alkaline and tholeiitic series. The acid end-mem-ber belongs to calc-alkaline series. A Sm-Nd whole-rock isochron age of 1793±270(2σ) Ma with εNd = + 6. 4±1. 4 has been yielded from the basalts. In rare earth element (REE) geochemistry, most basic volcanic rocks are characterized by the light-REE-enriched pattern, and a few samples have the flat REE pattern similar to that of the MORB. The acid volcanic rocks are all character-ized by strong light-REE-enriched pattern and negative Eu anomaly. Based on the petrographical and geochemical results, the paleorift which was represented by the volcanic series of the Lapeiquan area underwent relatively strong evolution. In the peak period, it could partially be an o-cean. The present tectonic framework of the Altun Mountains, which could be divided to a series of sections from east to west and also from north to south, suggest that the Altun is a very special orogenic belt that was probably formed by E-W strike-slip, N-S compression, upwarping faulting and finally tectonic collage.