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高校地质学报 ›› 2023, Vol. 29 ›› Issue (6): 847-861.DOI: 10.16108/j.issn1006-7493.2022006

• 岩石·矿物·地球化学 • 上一篇    下一篇

喜马拉雅东部拉隆岩体伟晶岩中的铌铁矿族矿物特征及两期铌钽成矿作用

李雪姣   

  1. 南京大学 内生金属矿床成矿机制研究国家重点实验室, 南京大学 地球科学与工程学院,南京 210023
  • 出版日期:2023-12-20 发布日期:2023-12-18

Columbite-group Minerals from the Pegmatite in the Lalong Pluton, Eastern Himalaya and the Two-staged Nb-Ta Mineralization

LI Xuejiao   

  1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
  • Online:2023-12-20 Published:2023-12-18

摘要: 喜马拉雅造山带内广泛分布着淡色花岗岩,近期研究表明多个岩体与稀有金属成矿作用密切相关。喜马拉雅东部拉隆岩体中花岗质伟晶岩发育有铌钽成矿作用,主要赋存在铌铁矿族矿物中。本次研究考察了该岩体的伟晶岩,观察了这些铌铁矿族矿物的产状,并对它们的主量、微量元素成分进行了分析。根据岩相学特征的差异,拉隆伟晶岩被分为边部带、过渡带和核部带。铌铁矿族矿物大多出现在过渡带中,具有明显的核边结构,根据产状和Ta# [Ta/(Ta+Nb)]可将铌铁矿族矿物分为两类,第一类矿物成分均一,自形程度好,晶体内部缺少内部结构。第二类矿物自形程度差,分布于第一类矿物的边部、填充于第一类矿物的裂隙或交代其核部,并发育有一系列的亚显微结构(包括韵律环带结构、交代结构、溶蚀孔洞结构和裂隙填充结构)。第一类铌铁矿族矿物是Ta#较低(0.08~0.34)的铌铁矿,而第二类为Ta#相对较高(0.45~0.60)的铌铁矿和钽铁矿。这两类铌铁矿族矿物的稀土配分模式相近,Eu呈强烈的负异常,δEu的范围为0.001~0.020。配分曲线总体呈海鸥型,四分组效应不明显;LREE/HREE值范围为0.016~0.044,反映出重稀土元素富集而轻稀土元素相对亏损的特征。根据铌铁矿族矿物的矿物学研究表明,拉隆岩体存在两期铌钽成矿作用,第一期与岩浆作用相关,形成了第一类低Ta#铌铁矿族矿物;第二期成矿受到岩浆—热液共同作用的影响,结晶出第二类铌铁矿族矿物,热液流体对第一类矿物进行了交代。铌铁矿族矿物的稀土成分特征对岩浆—流体过程指示不显著。铌铁矿U-Pb同位素年龄为22.3±0.3 Ma,为该岩体第一期铌钽成矿时代,指示了新喜马拉雅期(25~14 Ma)的淡色花岗岩体具有铌钽成矿的潜力。

关键词: 拉隆淡色花岗岩, 稀有金属成矿作用, 高喜马拉雅, 洛扎

Abstract: Leucogranites are widely distributed in the Himalayan orogenic belt and are closely related to rare-metal mineralization. This study focuses on the Nb and Ta mineralization and columbite-group minerals (CGMs) from the granitic pegmatite in the Lalong pluton, eastern Himalayas. Three zones (i.e., wall zone, intermediate zone, and core zone) can be identified from the Lalong pegmatite based on their lithology. The CGMs mainly exist in the intermediate zone (quartzfeldspar- muscovite zone). Major element, trace element, and U-Pb isotopic compositions of CGMs were systemically analyzed. Two types of CGMs are distinguished by occurrences and chemical compositions. Type I CGMs have euhedral occurence and uniform chemical compositions with low Ta# [Ta/(Nb+Ta)] ranging from 0.08 to 0.34. Type II CGMs mainly distribute around crystals of the type I CGMs or as tiny veins cutting through the type I CGMs with higher Ta# ranging from 0.45 to 0.60. Several structures (i.e., oscillatory structure, metasomatic structure, fissure-filling, and porous) are well developed in the type II CGMs. The normalized REE patterns of these CGMs are similar, showing a strong negative Eu anomaly with δEu ranging from 0.001 to 0.020. The LREE/HREE ratio of CGMs ranges from 0.016 to 0.044. Our results indicate a two-staged Nb-Ta mineralization in the Lalong pluton. The first-staged mineralization is related to the magmatic process with the crystallization of the type I CGMs, while the second-staged process is late-staged hydrothermal fluidrich magmatism with the formation of the type II CGMs. The REE pattern of the type II CGMs, however, is not convincing enough to indicate the magmatic-hydrothermal mineralization. The CGMs U-Pb isotopic analysis reveals the age of Nb- Ta mineralization at 22.3±0.3Ma, implying a rare-metal resource potential of the leucogranites formed in the Neohimalayan period (25-14 Ma).

Key words: Lalong leucogranite, rare-metal mineralization, Higher Himalaya, Lhozhag

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