Longshou mountains at the southern margin of Alxa block are famous for hosting the Neopaleozoic Jinchuan Cu-Ni sulfide deposit,prospecting for Cu-Ni sulfide deposits surrounding the Jinchuan ore deposit has long received much attention.The Xiaokouzi mafic-ultramafic intrusions,located in the eastern segment of Longshou Moutnrain,consist mainly of serpentine rock,augite peridotite,olivine pyroxenite.Star-shaped sulfide can be found locally.Sulfide contains pyrite and chalcopyrite,and no Ni-and Cu mineralization but parital mgnetitization were found in orebodies.The previous study suggests that the Xiaokouzi intrusions were likely the product of the same magma event with the Jinchuan mafic-ultramafic intrusions.The litho-geochemical analysis in this study shows that the intrusions have fairly high MgO/FeOT values with m/f=5.31～8.74,which is higher than that of average Ni-bearing intrusions.The chondrite-normalized REE diagrams and primitive-mantle normalized trace elements diagram of the samples display a flat distribution pattern with low total REEs content,indicating olivine accumulation in the intrusions.Sr and Nd isotopic study yields εNd (t) values of 6.09～7.77,indicating the magma source is characterized by depleted mantle.Trace element ratios and modeling results also suggest that parental magma experienced weak contamination of middle-upper crust.Single zircon U-Pb age of augite peridotite from the intrusions is 284.4±3.5 Ma.Compared with regional geology,the Permian igneous rocks may be related to the closure of the Paleo-Asian Ocean,or an extensional regime after subduction.%阿拉善地块南缘龙首山地区以产出新元古代的金川铜镍硫化物矿床而著名,围绕金川矿床外围寻找铜镍矿—直备受关注.小口子镁铁—超镁铁质岩体位于龙首山东段,主要由蛇纹岩、单辉橄榄岩、橄榄辉石岩组成,岩体中局部发现了星点状硫化物,硫化物由黄铁矿、黄铜矿组成,没有发现铜镍矿体,仅有部分磁铁矿化,前人研究认为小口子岩体可能是金川铜镍矿床同期的岩浆事件产物.本次研究通过岩石地球化学研究显示,小口子岩体镁铁比值为5.31～8.74,高于一般赋存铜镍矿岩体的镁铁比值.样品球粒陨石标准化的稀土元素配分曲线为平坦型,稀土元素总量低,暗示岩石具有橄榄石堆晶特征;Sr,Nd同位素研究显示:εNd(t) =6.09～7.77,指示岩浆源区为亏损型地幔特征;微量元素比值与同位素模拟计算结果指示母岩浆经历了微弱的中上地壳物质混染.岩体中单颗粒锆石U-Pb年龄为284.4±3.5Ma,结合区域地质对比,小口子镁铁—超镁铁质岩体的形成背景可能与古亚洲洋的闭合或俯冲之后的伸展作用相关.
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