Foundation and Geological Significance of Adakitic Granite at Guyang of Inner Mongolia
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摘要: 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.Abstract: 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.
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Key words:
- adakitic rock /
- granite /
- Paleo-Asian Ocean /
- LA-ICP-MS /
- Guyang area
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图 1 内蒙古固阳地区地质简图
1.第四系冲积物; 2.第四系洪积物; 3.新近系宝格达乌拉组; 4.白垩系固阳组; 5.渣尔泰山群增隆昌组; 6.渣尔泰山群书记沟组; 7.新太古界色尔腾山岩群柳树沟岩组; 8.古太古界兴和岩群; 9.晚三叠世花岗闪长岩; 10.晚三叠世二长花岗岩; 11.晚三叠世二长花岗岩; 12.晚二叠世二长花岗岩; 13.早二叠世闪长岩; 14.早二叠世石英闪长岩; 15.早二叠世英云闪长岩; 16.早二叠世花岗闪长岩; 17.早泥盆世角闪二长岩; 18.新太古界片麻状英云闪长岩; 19.新太古代片麻状石英闪长岩; 20.区域性断裂; 21.华北板块北缘; 22.白云鄂博凹陷带; 23.包尔汗-温都尔庙褶皱带; 24.索伦山-西拉木伦河褶皱带断裂
Fig. 1. Sketch map showing the geological characteristics of Guyang area in Inner Mongolia
图 4 固阳地区花岗岩类的(La/Yb)v-Yb、和Sr/Y-Y图解(肖庆辉等, 2002)
Fig. 4. (La/Yb)y-Yby and Sr/Y-Y diagrams of granites in Guyang area
表 1 固阳地区埃达克质花岗岩的主量元素(%)、微量和稀土元素(10-6) 分析及有关参数
Table 1. Bulk (%), trace and rare earth element (10-6) compositions of adakitic granites in Guyang area
表 2 固阳地区埃达克质花岗岩错石LA-ICP-MS同位素分析结果
Table 2. LA-ICP-MS U-Th-Pb analytical data of zircon from the adakitic granites in Guyang area
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