吐哈盆地早二叠世玄武岩原始岩浆性质:来自熔融包裹体成分的制约

摘要

The Permian basalts in the eastern Tianshan,NE Xinjiang,mostly distribute along the ENE fault in this area.The Ar-Ar age of the basalts in the Tuha basin is 298Ma.This confirms that the Tuha basalt is coeval with former basalts and sulfide-bearing mafic-ultramafic intrusion in the Tianshan region.The Tuha basalts have similar compositional features to island arc tholeiites and continental arc basalts,LREE enrichment and negative Nb and Ta anomaly,and are probably related with mantle metasomatized by subduction slab.The Tuha basalts have fresh olivine and plagioclase phenocrysts and in olivine phenocrysts which occurred many melt inclusions (MI).The opaque material in the melt inclusion are magnetite,suggest high Fe and oxygen fugacity when the formation of melt inclusions.The contents of melt inclusions show two types of high and low MgO.Furthermore,MI with high MgO content has low SiO2,trace elements and REE abundance;maybe indicate the characteristics of high partial melting and low degree evolution of mantle.The high MgO melt show depleted of Nb and Ta and the N-MORB type trace element and REE normalized patterns,reveal the melt had been metasomatized by subducted slab-derived fluid or melt.The low Th and Ta abundances,weaker Nb and Ta negative anomaly of MI,relative to Tuha basalts,implied the weaker metasomatism and less segregation.The Cu contents (12.4 × 10-6 ～ 299 × 10-6) of the melt inclusions are similar with the basaltic magma,while the Ni contents (236 × 10-6 ～ 697 × 10-6) of them are higher than the high Mg komatiitic and oceanic ridge basalts.A small number of sulfide segregation in deeper and more depleted of Cu than Ni due to higher partition coefficient of Cu can explain the decoupling of Cu and Ni content in the melt.Ni contents in parental magma are relatively higher than Cu may be the key factor which controls the total Ni/Cu ratios of sulfide ores is greater than 1.0 in Eastern Tianshan mafic-ultramafic intrusion related magmatic sulfide deposits.%东天山地区的二叠纪玄武岩沿着区域的北东东向断裂呈脉状分布,吐哈盆地玄武岩的40 Ar-39 Ar坪年龄为298.2±3.8Ma,为早二叠世,与前人的玄武岩年龄结果在误差范围内一致.可能与东天山地区二叠纪岩浆铜镍矿床镁铁-超镁铁岩有密切的成因联系.吐哈玄武岩的主微量成分显示其为岛弧拉斑、大陆弧玄武岩,轻稀土富集和Nb、Ta负异常,指示源区可能经历过俯冲作用的改造.吐哈盆地二叠纪玄武岩含有新鲜的橄榄石和长石斑晶,橄榄石斑晶中熔融包裹体较发育.熔融包裹体为玻璃质、气相和玻璃质、气相、固相两种类型.包裹体中不透明矿物主要为磁铁矿,说明捕获包裹体时岩浆的氧逸度和Fe含量较高.熔融包裹体分为高MgO和低MgO含量两种.高MgO含量的包体同时具有低Si02、低微量和稀土元素含量的特征,可能为地幔高部分熔融的产物,且经历过深部演化程度较弱.该高MgO熔体的微量元素显示Nb、Ta亏损的特征,具有N-MORB特征的微量和稀土元素分配模式,预示该熔体为受到俯冲交代的地幔熔融形成.熔融包裹体相对玄武岩具有低的Th和Ta含量、相对弱的Nb和Ta的负异常的特征,指示熔融包裹体的成分经受改造程度低于玄武岩,暗示可能为经历过较少后期作用改造的相对原始的熔体.熔体中Cu含量(12.4×10-6～ 299×10-6)在正常玄武质岩浆含量范围内,而Ni含量(236× 10-6～697×10-6)高于高镁溢流科马提岩和洋中脊玄武岩.该Cu、Ni含量略显解耦的熔体可能代表了经历过深部少量的硫化物熔离,带走小部分Cu和Ni等成矿元素之后所捕获的岩浆.如果将该熔体视为东天山地区二叠纪岩浆铜镍硫化物矿床的母岩浆,该母岩浆中Ni含量相对较高可能是岩浆铜镍硫化物矿床中矿石的Ni/Cu比值大多大于1.0的主要因素.