Abstract: 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.