内蒙古北山小黑山辉长岩LA-ICP-MS锆石U-Pb年龄及地球化学特征

内蒙古北山地区泥盆纪岩浆作用的研究对于探讨晚古生代早期的构造演化具有十分重要的意义。小黑山辉长岩的LA-ICP-MS锆石U-Pb定年结果显示,其就位年龄为(404.9±1.6) Ma,为早泥盆世。岩石学和地球化学分析结果表明: 岩石SiO₂含量低,富Al、Ca,低K、P, Mg^#值大,属于钙碱性岩石系列; 岩石稀土元素总量低,富集轻稀土,轻、重稀土元素之间分馏相对明显,具有正Eu异常(δEu=1.70~1.71); 岩体富集大离子亲石元素Ba、Sr、K等和强不相容元素U,亏损高场强元素Nb、Ta、Ti等。综合区域地质、地球化学特征,认为该岩体形成的构造环境为火山弧,岩浆上升过程中遭受到地壳物质的混染,进而认为本区泥盆纪仍处于造山作用过程中。

Geochemical characteristics and LA-ICP-MS zircon U-Pb age of the gabbro in Xiaoheishan of Beishan area,Inner Mongolia

Research on Devonian magmatism has important significance for the structural evolution of early Late Paleozoic in Beishan area of Inner Mongolia. LA-ICP-MS zircon U-Pb dating results show that the emplacement age of the Xiaoheishan gabbro is (404.9±1.6) Ma, which is Early Devonian. Petrological and geochemical analysis shows that the gabbro belongs to cal-alkaline igneous rock, and is characterized by the shortage of SiO₂, K, P, enrichment of Al and Ca, and high Mg^# value. The total amount of rare-earth elements (REE) is low, and that of light rare-earth elements (LREE) is enriched. Fractional distillation is obvious between LREE and HREE with positive Eu anomalies (δEu=1.70~1.71). The large ion lithophile elements (Ba, Sr, K) and the strong incompatible element(U) are enriched in the gabbro, while the high field strength elements (Nb, Ta, Ti) are relatively depleted. Combined with the regional geology and geochemical characteristics, the authors conclude that the tectonic setting is volcanic arc, and that the lava is polluted by the earth crust during the upward process, which means the Devonian period in this area is still in orogeny period.