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

发布时间：2018-11-21 11:55

【摘要】：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40Ar-39Ar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SiO_2、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。
[Abstract]:The Permian basalts in the East Tianshan area are distributed in vein form along the east-west direction of faults in the region. The 40Ar-39Ar plateau age of basalts in Turpan-Hami Basin is 298.2 卤3.8 Ma, which is the early Permian epoch, which is consistent with the results of previous basalt ages in the error range. It may be closely related to the Magnesium-ultramafic rocks of the Permian magmatic copper-nickel deposits in the East Tianshan area. The main trace compositions of Tuha basalt indicate that it is island arc tholeiite, continental arc basalt, LREE enrichment and Nb,Ta negative anomaly, indicating that the source area may have undergone subduction transformation. Fresh olivine and feldspar porphyry are found in Permian basalts in Turpan-Hami Basin, and melt inclusions are developed in olivine porphyry. The melt inclusions are vitreous, gaseous and vitreous, gas-phase and solid-phase. The opaque minerals in the inclusions are mainly magnetite, which indicates that the oxygen fugacity and Fe content of the magma are higher when the inclusions are trapped. Melt inclusions can be divided into two types: high MgO and low MgO content. The inclusion with high MgO content has the characteristics of low SiO_2, and low trace and rare earth elements, which may be the product of high partial melting of the mantle, and has experienced a weak degree of deep evolution. The trace elements of the high MgO melt show the characteristics of Nb,Ta depletion and the distribution pattern of trace and rare earth elements with the characteristics of N-MORB, which indicates that the melt is formed by subducted metasomatic mantle melting. The melt inclusions have lower Th and Ta contents than basalts, and relatively weak negative anomalies of Nb and Ta, indicating that the composition of melt inclusions is less reformed than basalt. The implication may be relatively primitive melts that have undergone less later-stage modification. The Cu content (12.4 脳 10 ~ (-6) ~ 299 脳 10 ~ (-6) in the melt is in the range of normal Black Tortoise magma content, while the Ni content (236 脳 10 ~ (-6) ~ 697 脳 10 ~ (-6) is higher than that of high-mafic overflow Komatite and mid-oceanic ridge basalt. The slightly decoupled melt with Cu,Ni content may represent the magma trapped after a small amount of deep sulfide melting and removal of a small part of the Cu and Ni ore-forming elements. If the melt is regarded as the parent magma of the Permian magma copper-nickel sulfide deposit, the relatively high Ni content in the parent magma may be the main factor that the Ni/Cu ratio of the ore in the magmatic copper-nickel sulfide deposit is more than 1.0.
【作者单位】：中国科学院矿产资源研究重点实验室中国科学院地质与地球物理研究所;中国科学院大学;地质过程与矿产资源国家重点实验室中国地质大学;岩石圈演化国家重点实验室中国科学院地质与地球物理研究所;
【基金】：国家自然科学基金项目(41472075、41002026、41030424)资助
【分类号】：P588.145

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