内蒙古横峦山组类高镁安山岩年代学及地球化学特征

横峦山组火山岩主要分布在红柳河—牛圈子—洗肠井缝合线南部红山头一带。通过锆石同位素测年及岩石地球化学分析等方法对横栾山组火山岩进行研究,结果显示: 该组火山岩富集LREE,亏损HREE,无明显的负铕异常,稀土配分形式与日本Setouchi火山岩带中典型高镁安山岩类似。此外,该火山岩具较高的MgO含量(2.85%~4.13%)和较低的FeO^T/MgO比值( FeO^T/MgO=0.75~1.2<1.5),Al₂O₃含量介于5.59%~17.15%,CaO含量小于10% (5.97%~8.30%)。其岩石地球化学具有类高镁安山岩的特征,因此认为横峦山组火山岩形成于北山洋早期洋壳俯冲环境。横峦山组火山岩的锆石U-Pb测年结果为(453.5±2.7) Ma,代表着北山地区在晚奥陶世早期北山洋已经开始发生俯冲。

Geochronological and geochemical characteristics of analogy high-magnesium andesite in Henluanshan Formation of Inner Mongolia

Hengluanshan Formation volcanic rocks mainly distribute in the Hongshantou area, which is located in the south of Hongliuhe-Niujuanzi-Xichangjing suture belt. The authors researched Hengluanshan Formation volcanic rocks through zircon isotopes dating and geochemical analysis. The results show that the volcanic rocks are rich in LREE, lack HREE, and have not significantly negative europium anomaly. The REE patterns of the volcanic rocks are similar to the typical high-magnesium andesite of the Setouchi volcanic belt in Japan. There is higher MgO content (2.85% to 4.13%) and lower FeO^T/MgO value (0.75 to 1.2, less than 1.5) in the volcanic rocks. The Al₂O₃ content is from 5.59% to 17.15%, and the CaO content is less than 10% (5.97% to 8.30%). The volcanic rocks have the geochemical characteristics of analogy high-magnesium andesite. So, the authors considered that Henluanshan Formation volcanic rocks formed at the early stage of Beishan oceanic crust subduction. The zircon U-Pb age of Henluanshan Formation volcanic rocks is (453.5±2.7) Ma, and it indicates that Beishan ocean subducted at the early stage of Late Ordovician.