东天山板块缝合带石炭纪火山岩地球化学和Sr-Nd同位素特征及其大地构造意义

摘要

对东天山沿康古尔塔格缝合带分布的企鹅山群和梧桐窝子组火山岩的地球化学特征及Rb - Sr和Sm - Nd同位素进行了系统研究.结果显示:企鹅山群火山岩富集LREE,Eu异常变化较大(从明显负异常到微弱正异常),富集Sr、Ba和U等大离子亲石元素,相对亏损Nb、Zr、Ti等高场强元素,Sr - Nd同位素特征是Isr=0.703 97～0.704 26,εNd (t) =5.61～6.99,具有亏损地幔性质,表明其岩浆源区可能来自古生代期间被古亚洲洋俯冲板片释放的流体/熔体交代过的岩石圈地幔楔；梧桐窝子组玄武岩富集轻稀土元素(LREE)和大离子亲石元素,而高场强元素没有明显负异常,样品的εNd值都在9以上,Isr (330 Ma)为0.704 71～0.705 33,表明梧桐窝子组玄武岩可能来自亏损的软流圈地幔的部分熔融.企鹅山群和梧桐窝子组火山岩在Zr - Zr/Y判别图解中分别落在岛弧拉斑玄武岩(IAB)和板内玄武岩(WPB)范围内,表明企鹅山群可能为准噶尔洋盆向南俯冲形成的石炭纪岛弧火山岩,而梧桐窝子组火山岩可能形成于在弧后盆地的基础上拉张为裂陷槽的伸展环境.%Geochemistry, Rb - Sr and Sm - Nd isotopic compositions of the volcanic rocks, from the Qi'eshan Group and Wutongwozi Formation in the eastern Tianshan plate suture zone have been studied in this paper. The basalt from the Qi' eshan Group is highly enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) such as Sr, Ba and U, and depleted in high field strength elements (Nb, Ti and Zr) , with variable Eu anomalies (varying from significant negative Eu anomalies to minor positive Eu anomalies) in the chondrite-normalized REE patterns. It is charactered by isotopic signatures of depleted mantle source, with εNd(t) = 5. 61 -6. 99, and Isr(330 Ma)=0. 703 97 -0. 704 26. All these geochemical characteristics suggest that the basalt in Qi'eshan Group probably was derived from the lithospheric mantle wedge previously metasomatized by fluids/melts released from the down-going paleo-Asian oceanic slab in the Paleozoic period. The basalt from the Wutongwozi Formation is characterized by enrichment of LREEs and LILEs, without obvious negative anomalies of high field strength elements. High and positive Nd values (＞ + 9) and relatively low Isr ratios (0. 704 71 -0. 705 33) suggest that the basalt in the Wutongwozi Formation was derived from partial melting of the depleted asthenospheric mantle. In the Zr-Zr/Y discrimination diagram, samples from the Qi'eshan Group are mainly plotted in the field of island arc basalt (IAB), while, those from the Wutongwozi Formation in the field of within plate basalt (WPB). It's suggested that the Qi'eshan Group formed in an island arc setting owing to the southward subduction of the Junggar oceanic slab, while the Wutongwozi Formation formed in a rifting setting that was developed upon the extensional back-arc basin.