金川超镁铁质岩元素及稀有气体同位素地球化学特征

ELEMENT AND NOBLE GAS ISOTOPIC GEOCHEMISTRY OF ULTRAMAFIC ROCKS IN JINCHUAN

金川超镁铁质岩是金川铜镍硫化物矿床的赋存岩体,采自金川镍矿 矿区露天矿第2期侵入岩体的样品其岩石类型主要为含二辉橄榄岩和二辉橄榄岩,见镍黄铁矿、磁黄铁矿、黄铁矿呈成条带状出现,表明其形成时低氧逸度的成岩环境。运用ICP-MS等离子质谱仪和MM5400稀有气体质谱仪分别对全岩样品的稀土元素、微量元素和包体稀有气体同位素进行测定后得到一些认识:金川超镁铁质岩的微量元素丰度与球粒陨石相比,变化曲线基本呈水平分布,但是地壳中高度亏损Cr,Ni元素含量较高,主要富集于超基性的辉石和橄榄石中,说明其成因类型具有明显的幔源特征。岩石中稀土元素总量较高,为轻稀土富集型,重稀土亏损。其中由深至浅稀土元素对La/Yb,Ce/Yb,La/Sm,(La/Sm)N的比值均基本呈现增大趋势,说明其中稀土元素分馏程度呈增大趋势,轻稀土的分馏尤为明显。超镁铁质岩的3He/4He值分布于0.847Ra～1.625Ra之间,虽远低于上地幔的3He/4He值,但仍存在地幔初始氦的踪迹。研究结果表明,该岩体在侵入过程中发生了明显的分异作用,并与地壳发生了的混染作用和热液交代作用,进一步证明了该岩体为上地幔局部熔融的超基性岩浆上侵形成的。

Copper-nickel sulfide deposits of Jinchuan occurred in the ultramafic rocks.Samples were collected from the No.2 period intrusive mass of the open pit of the No.Ⅰ nickel mining site in Jinchuan area, and the major types of rocks are two-pyroxene-bearing peridotite and buchnerite. Pentlandite, pyrrhotine and pyrite in the rocks appear in the shape of belts, and this suggests that these ores deposited in the rock-forming environment with low oxygen fugacity. Rare earth elements (REEs), trace elements and noble gas isotopes in enclosure of the whole-rock samples were measured with ICP-MS plasma mass spectroscopy and MM5400 noble gas mass spectroscopy, respectively.Comparing trace elements with chrondrite,the changing curves of trace elements in the rocks basically show horizontal distribution.The total REEs are relatively high and characteristic of obvious redchment of light REEs and depletion of heavy REEs.The study shows that an obvious differentiation occurred during the rock intrusive process,and the rocks were suffered obvious hybridism and hydrothermal metasomatism.For all that,the helium isotope composition in Jinchuan ultramafie rocks still remains the initial helium characteristics of mantle.The study further proved that the rocks were formed by intrusion of ultrabasic magma resulted from the partial melting of upper mantle.