青海省阿什贡含镍矿镁铁-超镁铁岩体形成时代及其对成矿机制的启示

通过对阿什贡岩体的野外实地调查,发现其是由橄榄岩相、辉石岩相、角闪石岩相及辉长岩相组成的镁铁-超镁铁岩体,并且在辉长苏长岩中发现有原生硫化镍矿体。利用LA-ICP-MS方法对岩体进行测年,获得锆石U-Pb年龄为(436.1±1.2)Ma(MSWD=0.13),为加里东期。岩石地球化学特征分析及区域对比研究揭示,阿什贡镁铁-超镁铁岩体以低Ti、亏损Nb和Ta、富集轻稀土元素和大离子亲石元素为特征。结合带内其他同时代镁铁-超镁铁岩体地球化学特征及其同位素组成,启示我们包括阿什贡在内的化隆镁铁-超镁铁岩带的形成与祁连山及其邻区460~440 Ma时期俯冲向碰撞转换的作用有关,其成矿作用与岛弧岩浆作用相关。这对于青海省化隆一带区域铜镍找矿具有重要指示意义,同时对丰富发展完善早古生代镁铁-超镁铁岩体及所含的岩浆铜镍硫化物矿床成因研究和形成机制具有借鉴作用与研究价值。

Formation Age of Agong Ni-bearing Mafic-ultramafic Intrusion in Qinghai Province and Its Enlightenment to Metallogenic Mechanism

This paper reports partial achievements in the evaluation of magmatic nickel ore resources potential in Hualong County, Qinghai Province, mainly on the basis of field survey and study of Agong mafic-ultramafic intrusion, which is composed of peridotite, pyroxenite, hornblendite and meta-gabbro. The authors obtained the zircon U-Pb age of (436.1±1.2) Ma by LA-ICP-MS. Geochemical studies of the mafic-ultramafic intrusion closely associated with the deposit show that it is characterized by low Ti, depletion of Nb and Ta, and enrichment of LREE and large lithosphile elements. These features, combined with the geochemical characteristics and isotope composition of other mafic-ultramafic intrusions in Hualong rocks belt, have led the authors to believe that the magma occurred during the conversion from the subduction to the collision in 460~440 Ma in the Qilian Mountain and its adjacent areas, and mineralization was the result of arc magmatism. These achievements are not only of great significance for regional prospecting of Ni-Cu ore deposits in Hualong area but also of reference value for the enrichment of studies of Early Paleozoic mafic-ultramafic intrusions and their magmatic Ni-Cu sulfide deposits and formation mechanism.