内蒙古北山地区三个井辉长岩LA-ICP-MS锆石U-Pb年龄及地球化学特征

内蒙古北山地区位于中亚造山带南缘中段,岩浆活动强烈。为了厘清北山地区的构造演化,对北山地区三个井辉长岩开展了年代学和地球化学研究。结果表明: 其LA-ICP-MS锆石U-Pb年龄为(404.9±1.6) Ma,形成年代为早泥盆世; 岩石地球化学特征显示,岩石Al₂O₃含量高,为18.99%~21.12%(大于17%),岩石属高铝玄武岩类,轻、重稀土分馏明显,(La/Yb)_N为7.16~9.07,大离子亲石元素Rb、Ba、K和Sr明显富集,而高场强元素Ta、Nb和元素P亏损明显。上述特征表明三个井辉长岩的形成与俯冲带相关,结合区域地质特征,该辉长岩是沿红石山一带的主洋盆向南俯冲形成的早期岩浆弧的产物。

LA-ICP-MS zircon U-Pb ages and geochemical characteristics of Sangejing gabbro in Beishan area of Inner Mongolia

Beishan area, located in the central segment of the southern margin of Central Asian Orogenic Belt, has undergone intense magma activities. In order to ascertain the tectonic evolution in Beishan area, the authors carried out the research of geochronology and geochemistry for Sangejing gabbro of Beishan area. The results show that the LA-ICP-MS zircon U-Pb age of the gabbro is (404.9 ± 1.6) Ma, and the formation age is Early Devonian. The geochemical characteristics show that the gabbro has high Al₂O₃ content(18.99% ~ 21.12%, above 17%) and belongs to high-alumina basaltic rock. There is evident fractionation between LREE and HREE, and (La/Yb)_N value is 7.16 ~ 9.07. The large ion lithophile elements (e.g., Rb, Ba, K, Sr) are enriched and the high field strength elements (e.g., Ta, Nb, P) are depleted. The above characteristics indicate that the formation of Sangejing gabbro is related with the subduction belt. Combined with the regional geological features, the authors thought that Sangejing gabbro could be the products of the early arc magmatism and formed during the main ocean basin southward subduction in Hongshishan area.