新疆富蕴科克别克提基性杂岩体锆石U-Pb年代学、地球化学及其地质意义

Zircon U-Pb geochronology and geochemistry of the Kekebieketi basic complex in Fuyun,Xinjiang and the geological significance

科克别克提基性杂岩体位于新疆额尔齐斯成矿带南部,于2016年发现铜镍矿体,具有良好的成矿潜力。本文报道了科克别克提基性杂岩体的岩相学、岩石地球化学及年代学特征,以期了解该岩体的形成时代、构造环境及岩石成因。科克别克提岩体受北西、北西西向构造控制,由角闪辉石岩、辉长岩及少量闪长岩组成。主要含矿岩相为角闪辉石岩,发育黄铁矿及黄铜矿,多呈星点状、细脉状及浸染状分布。科克别克提岩体镁铁比值介于0.8~2.0,为铁质基性岩,微量元素相对亏损相容元素(Cr,Co,Ni)及高场强元素(Nb,Ta,Zr,Hf,Th),富集大离子亲石元素(Ba,U,K,Pb,Sr)。稀土元素总量(∑REE含量)为(29.77~148.02)×10^-6,随岩石基性程度增高而降低。各岩相稀土配分曲线大致平行,均为轻、重稀土分馏较强的右倾型。岩体硫化物中硫同位素δ^34S值变化于-0.6‰^-0.4‰,呈幔源硫特征。本文测得科克别克提辉长岩和闪长岩的锆石U-Pb年龄分别为(330.3±0.7)Ma与(328.9±0.6)Ma,结合区域地质背景,本文认为科克别克提岩体形成于早石炭世中晚期碰撞后的伸展环境。另外,本文对矿体的成因做了初步的推测,认为科克别克提岩体是原生岩浆经过适度演化的产物,其深部经历以橄榄石为主的分离结晶作用,之后中、上部岩浆上涌至现存空间并发生以斜方辉石为主的分离结晶。在此过程中,上地壳中硅的加入促使岩浆体系中硫达到饱和从而分溶出铜矿体。

The Kekebieketi basic complex is located in the south of Xinjiang Erqisi metallogenic belt.The copper-nickel ore body was discovered in 2016,with good metallogenic potential.In this paper,we report the petrographic,geochemical and chronological characteristics of the complex in order to understand its formation age,tectonic environment and petrogenesis.The complex is mainly controlled by NW and NWW trending structures.The main petrofacies are hornblende pyroxenite,gabbro and a few diorite.Hornblende pyroxenite is mainly ore-bearing facies,and the pyrite and chalcopyrite are stellate,veined and disseminated.The complex had a magnesia ferro ratio between 0.8 and 2.0,containing mainly iron basalts.Trace elements results showed that the complex is depleted in compatible(Cr,Co,Ni)and high field strength(Nb,Ta,Zr,Hf,Th)elements,and enriched in large-ion lithophile element(Ba,U,K,Pb,Sr)elements.The total rare earth elements(ΣREE)content was(29.77-148.02)×10^-6 and decreased with increasing basicity.The distribution patterns of REE in each petrofacies were roughly parallel,and fractionation between light and heavy REE showed strong right leaning.Sulfur isotopeδ^34S of sulfide varied from-0.6‰to-0.4‰,with typical mantle-sourced sulfur characteristics.The results of La-ICP-MS U-Pb dating of zircon showed that the ages of Kekebieketi gabbro and diorite were 330.3±0.7 and 328.9±0.6 Ma,respectively.Combined with the regional geological background,we considered the magmatic emplacement and crystallization occurred in the mid-and late-stage of the Early Carboniferous in a post-collisional extension setting.In addition,we speculated that the Kekebieketi basic complex is the product of moderately evolving magma,and it may have experienced fractional crystallization of olivine in the deep magma chamber.After that,the middle and upper magma rose and experienced fractional crystallization of orthopyroxene.At the same time,the addition of silicon in the upper crust led to magmatic sulfur saturation to form chalcopyrite.