内蒙古固阳地区埃达克质花岗岩的发现及其地质意义

内蒙古固阳地区西营子花岗岩体属于钙碱性系列, 具有岛弧花岗岩的特征.其岩石地球化学具有与adakite岩相似的特征.这些岩石的SiO₂含量均大于56 (57.28~66.63), Al₂O₃含量均大于15 (15.93~18.04), MgO含量为1.14~3.47, 富钠、贫钾, Na₂O/K₂O在1.16~1.97之间, Sr含量高(446~582μg/g), 均大于400μg/g, Sr/Y比值为31.32~103.74, 大于20~40, 低Y (5.61~17.4μg/g, 小于18μg/g)和Yb (0.42~2.06μg/g, 仅1个样品大于1.9μg/g), 具弱Eu正异常, 无明显的Eu负异常; 稀土元素分异强烈, 属轻稀土元素富集型; Sr具明显的正异常而Nb的负异常亦较为明显.利用激光等离子体质谱仪(LA-ICP-MS) 对固阳地区埃达克质花岗岩中的锆石进行了U-Pb定年, 其同位素年龄为281.9±3.1Ma (MSWD=3.3), 形成于早二叠世.该地区埃达克岩的发现, 为古亚洲洋的闭合时限提供了新的线索, 对研究中亚造山带晚古生代构造演化、地球动力学特征具有十分重要的意义, 为解决内蒙古及古亚洲洋研究中长期争论的地质问题提供了年代学和地球化学制约. 

Foundation and Geological Significance of Adakitic Granite at Guyang of Inner Mongolia

Xiyingzi granite-body in Guyang area, Inner Mongolia, belongs to calc-alkaline series, bearing the characteristics of island arc affinity.Its petrological geochemistry characteristics are almost similar to those of adakite: SiO₂ content > 56% on average (57.28%-66.63%); Al₂O₃ > 15% (15.93%-18.04%); MgO=1.14%-3.47%, enriched in Na and depleted in K; Na₂O/K₂O=1.16-1.97;Sr content (446-582 μg/g) > 400 μg/g on average and Sr/Y ratio =31.32-103.74, > 20-40;low Y content (5.61-17.4 μg/g, less than 18 μg/g); low Yb (0.42-2.06 μg/g, only one sample's Yb content > 1.9 μg/g); with small positive Eu anomaly and without significantly negative Eu anomaly; REE has a strong differentiation, which belongs to the strong LREE enrichment type; Sr has obvious positive anomaly, while Nb has significant negative anomaly.The laser-plasma mass spectrometry (LA-ICP-MS) was used for the zircon U-Pb dating of adakitic granite in Guyang area and the isotope age was obtained to be 281.9±3.1 Ma (MSWD=3.3), indicating an Early Permian activity.The discovery of adakite in this region provides new clues on the closure time of Paleo-Asian Ocean, and is of great significance to the study of Central Asia Late Paleozoic orogenic belt's tectonic evolution and geodynamics characteristics.It also provides geological and geochemical constraints on resolving the long-term debates on the Inner Mongolia and Paleo-Asian Ocean.