宾川-永胜地区二叠纪火山岩及其成矿潜力评价

【作者】 朱士飞；

【导师】 钱壮志；

【作者基本信息】 长安大学 ， 构造地质学， 2004， 硕士

【摘要】 大陆溢流玄武岩(CFB)代表主要的岩浆事件，有的与重要硫化物矿床(如西伯利亚CFB与Noril·sk矿床)有关。它是由岩石圈扩张引发而来还是由深部地幔柱就位而来争论不己。峨眉山大陆溢流玄武岩广布于扬子地块西缘，是目前我国地学界关注的地区之一。前人依据岩石组合、岩相学特征，将峨眉山玄武岩划分为西、中、东三大岩区。通过对构造背景及古地理环境分析，峨眉山玄武岩主要形成于地幔上涌和广泛的大陆伸展作用过程中。 本文主要选择具有代表性的宾川-永胜及丽江地区的峨眉山玄武岩及其中的拉马古和昔腊坪岩体作为研究对象，在前人研究成果的基础上，对峨眉山玄武岩岩石地球化学及玄武岩浆活动特征进行了系统地阐述，并运用现代岩石成因理论初步分析了该区二叠纪火山岩及其相关超基性侵入岩体岩石的地幔源区特征、岩浆过程及其可能的形成机制。在此基础上同国内外铜镍铂钯岩浆硫化物矿床产出的地质背景和成矿条件作比较，探讨该区成矿的潜力。 取得主要进展如下： 1．根据MgO含量和前人的资料，将研究区：二叠纪火山岩可划分为三个系列类型：高镁、低镁和过渡系列火山岩。它们的岩石地球化学性质呈现递变性。富集轻稀土和大离子亲石元素，具有与洋岛玄武岩相同或相似的地球化学特征，为板内拉张环境玄武岩。 2 高镁系列岩石接近原始岩浆的组成，岩浆源区为下地幔，其岩石地球化学特征主要受到源区控制，形成深度大，熔融比例小，代表了原始岩浆的性质；低镁系列受到较强的地壳混染作用的影响，显示出富集岩石圈地幔或地壳物质的参与，形成深度浅，熔融比例大，属混合岩浆；过渡系列介于前两者之间。 3．超基性侵入岩体与高镁系列岩石MgO含量均大于12％，前者Mg／值介于0.71-0.82，后者绝大多数样品Mg~#介于0.70-0.80，它们可能代表了原始岩浆性质，为同源异相的产物。 4．结合研究区大地构造背景分析，认为岩石成因与地幔柱活动有关，与前人研究一致。但本文提出的地幔柱活动不是大陆伸展裂陷-地幔柱上涌作用，而是地幔柱撞击作用导致大陆伸展裂陷，起源于地幔柱的岩浆与岩石圈相互作用，经历不同程度的壳源混染，形成了不同系列的玄武岩及其相关的侵入体。在此研究基础上建立了可能的成岩模式。 5．对本区及其邻区已有的成矿事实及成矿背景的研究，同时与国外典型的大陆溢流玄武岩型Nori1’sk铜镍铂岩浆硫化物矿床的地质背景与成矿条件及成矿事实相对比，认为本区具一定的区域成矿潜力，并指出今后应加强该区与高镁系列火山岩及其同源异相的超基性侵入体方面的研究和找矿工作。更多还原

【Abstract】 Continental flood basalts (CFB) represent major magmatic events, and some are associated with important sulphide mineralization, for example, the Norils’k deposits. About CFB formation there is an active debate both over the extent to which one modle is applicable to all CFB, and whether melting is triggered by lithosphere extension or deep-seated mantle plumes. Emeishan basalts, widely spreading out in the west margin of the Yangzi block, are one of many regions in which Geo-science circles are interested. According to characteristics of its petro-assemblage and petrography, Emeishan basalts can be divided into three-big rock regions- west, middle and east. These basalts are mainly formed in the course of mantle upwelling and widely continental extension through analyzing tectonic setting and pale-geographical setting.Based on the previous study, in this paper the most completely outcropped geological section, the basalts of Binchuan and Yongsheng County with intrusions of Lamagu region and Xilaping region are chosen for Major-Element, Trac-Element and Isotopic Composition study. Applying modem theory of petrology formation, the paper describes systemically some characteristics of Emeishan basalts’ petro-geochemistry and magma activation and discusses on mantle source characteristics and magma evolution and magmatism of the late Permian Emeishan CFB. On the basis of this paper’s research results, the problem whether the deposits can be formed in Binchuan and Yongsheng County, having compared with the geological setting and viscosity of the Norils’k deposits, is discussed. Through a comprehensive study and comparative analysis of existing datum, some conclusions can be drawn as follow:1. CFB occurring in Binchuan and Yongsheng County can be compositionally subdivided into three types: the high-Mg (MgO=l2.3-27.2%), transitional-Mg (MgO=l 0.4-11.3%) and low-Mg groups (MgO=2.48-7.53%). All three groups show continuous shift in major and trace elements, enriching LREE and LILE. They exhibit similar geochemical features to OIB and are contributed to tholeiitic basalts.2.The high-Mg series approach the primitive mantle compositions, and their magmatic source is derived from the lower mantle, showing similar petro-geochemical features to primitive mantle. So they can be ascribed to lower degrees of partial melting and greater melting depths. On the contrary, the low-Mg volcanic rocks have the lowest (Ce/Yb)cN with significant "crustal source" signatures, indicating the largest melting degrees and shallowest melting depths. So their magmatic source is generated from hybrid magma. The trans-Mg series have transitional characteristics between the high-Mg and low-Mg rocks.3.The ultra-basic intrusions and high-Mg volcanic rocks have higher content of MgO with high Mg# value. The Mg# value is respectively from 0.71 to 0.82 and from0.70 to 0.80, representing primitive magma. But they have dissimilar patterns.4.On the basis of the geological background, in research region the generation of CFB is somewhat temporally related to mantle plume activities, which consistent with what the predecessors make. But this paper think that the mantle plume activities are not resulted from lithosphere extension but result in occurring of the latter. The magma arisen from the mantle plume interacts with lithospheric materials, undergoes different degrees mixing of lithospheric mantle and crustal components and develops a variety of basalts and intrusions. Based on particular researches, this paper makes an approach to the formatting mechanism of high-Mg volcanic rocks.5.Compratived with the metallogenic realities, settings and viscosities at home and abroad, especially with Noril’sk region, this paper estimates the perspective of nickel-copper sulfilde deposits in Binchuan and Yongsheng County and suggests further assignment be carried through in order to expand perspective targets.更多还原