中文摘要：

内蒙古西乌旗地区出露一些石英闪长岩，其LA-ICP-MS锆石U-Pb年龄为（304.64±0.82）Ma和（309.84±0.86）Ma，表明这套岩石形成于晚石炭世。地球化学分析表明，西乌旗地区石英闪长岩属于低TiO2和高Al2O3钙碱性玄武岩系列；富集Rb、Ba、K大离子亲石元素（LILE），亏损Nb、Ta、Ti、P等高场强元素（HFSE），具有典型弧岩浆岩的特征，显示岩浆源区曾经历过俯冲带流体的交代作用；稀土元素丰度偏低，呈LREE富集HREE亏损的右倾模式；强不相容元素比值（La/Nb、La/Ta等）及判别图解表明晚石炭世石英闪长岩类似大陆边缘弧玄武岩，岩浆来源于浅部岩石圈地幔，系石榴石+尖晶石二辉橄榄岩源区中等程度(10%~20%)部分熔融的结果。综合研究区样品的地球化学特征，结合区域地质资料及对比分析前人成果，认为早石炭世末期西乌旗地区在强烈拉张的裂谷环境下形成了有限洋盆，由于洋盆发育不成熟或边伸展边向两侧俯冲—消减，形成了西乌旗地区类似大陆边缘弧玄武岩特征的晚石炭世石英闪长岩。

英文摘要：

Late Carboniferous quartz diorites are distributed in the Xi Ujimqin Banner of Inner Mongolia. The results of LA-ICPMSzircon U- Pb dating of the quartz diorite samples indicate ages of 304.64 ± 0.82 Ma and 309.84 ± 0.86 Ma. In addition, newgeochemical data from six quartz diorite samples are presented. All six samples show high Al2O3 values but low TiO2 values, and arethus classified as calc- alkali basalt series. These samples are characterized by low REE abundance and slight LREE enrichment.They also show LILE enrichment, HFSE depletion and distinctly negative Nb and Ta anomalies. All of these characteristics resemblethose of arc magmatic rocks. Furthermore, as shown in the correlation plots of La/Ba versus La/Nb, Ba/La versus Ce/Pb, and Nb/Yversus La/Yb, the magma source has experienced contamination and metasomatism from the subduction fluid. According to the Zr/Nb, La/Nb, La/Ta ratios and the diagram of Sm/Yb versus La/Sm, the magma was derived from the shallow lithospheric mantle andformed by moderate (10% - 20% ) partial melting of spinel- garnet lherzolites. Based on geochemical characteristics of thesesamples in this study, combined with regional geological data and previous research results, the authors hold that, in the late Early-Carboniferous, there was a new limited ocean basin under an intensely extensional rift setting. Consequently, as the newly formedocean basin was still immature or experienced subduction- related contamination and metasomatism, the Late Carboniferousquartz diorites of Xi Ujimqin Banner exhibited the continental margin-arc-like enriched signatures.