Comparative studying on zircon, cassiterite and coltan U-Pb dating and <sup>40</sup>Ar/<sup>39</sup>Ar dating of muscovite rare-metal granitic pegmatites: A case study of the northern Tugeman lithium-beryllium deposit in the middle of Altyn Tagh

Abstract

稀有金属花岗伟晶岩年代学是花岗伟晶岩型稀有金属矿床研究的重要内容。目前测年方法较多，但不同方法测试结果的对比研究亟待开展。我们选择吐格曼北花岗伟晶岩型锂铍矿床3条含矿伟晶岩开展锆石、锡石、铌钽铁矿及白云母四种矿物不同方法的测年对比研究。结果显示：1）ρ31白云母-锡石伟晶岩中锡石²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb谐和年龄为468±8.7Ma（MSWD=1.1，N=39）、白云母-钠长石-锂辉石伟晶岩中锆石²⁰⁶Pb/²³⁸U谐和年龄为458.7±2.3Ma（MSWD=7.2，N=16）；2）ρ38白云母-钠长石-锂辉石伟晶岩中锆石²⁰⁶Pb/²³⁸U谐和年龄为454.7±4.0Ma（MSWD=8.0，N=10）、白云母⁴⁰Ar/³⁹Ar坪年龄为350.2±1.6Ma（MSWD=4.7）；3）ρ87含铌钽铁矿-白云母-石英伟晶岩中铌钽铁矿²⁰⁶Pb/²³⁸U谐和年龄为464.1±2.7Ma（MSWD=5.2，N=39）。可以看出，铌钽铁矿与锡石的U-Pb年龄在误差范围内一致，可能代表花岗伟晶岩岩浆结晶的年龄；2件样品的蜕晶化锆石U-Pb年龄也可以对比，可能代表岩浆锆石蜕晶化后经流体交代作用及重结晶作用导致U-Pb同位素系统重置的时间。白云母Ar-Ar年龄明显晚于铌钽铁矿、锡石和锆石的U-Pb年龄，鉴于ρ38白云母-钠长石-锂辉石伟晶岩脉中白云母与锂辉石发生了强烈变形与蚀变，认为变形的白云母记录的是叠加变形与热液蚀变的时间。由此推断吐格曼北锂铍花岗伟晶岩形成于468~454Ma，这意味着阿尔金山地区中-晚奥陶世可能存在持续时间较长的稀有金属成矿事件。基于花岗伟晶岩矿物成因与吐格曼北锂铍花岗伟晶岩不同方法测年对比结果，可以得出以下结论：铌钽铁矿与锡石的U-Pb年龄可代表伟晶岩岩浆结晶的年龄，蜕晶化的锆石U-Pb年龄记录的是岩浆锆石蜕晶化后经流体交代作用及重结晶作用导致U-Pb同位素系统重置的时间，含钾矿物的⁴⁰Ar/³⁹Ar年龄能够约束伟晶岩的变形与蚀变年龄；多种定年方法的联合约束可以更好地限定稀有金属花岗伟晶岩的各个阶段成矿事件时间。;Geochronological dating is a vital part of the study of granitic pegmatite type rare-metal deposits. There are many dating methods; however, comparative study on dating results obtained by different methods hasn't been performed. In this study, we report new dating results of zircon, cassiterite, coltan (short for columbite-tantalite) and muscovite from three pegmatites of the northern Tugman lithium-beryllium deposit. The results show that: 1) the ρ31 muscovite-cassiterite pegmatite has a concordant U-Pb age of 468±8.7Ma (MSWD=1.1, N=39) of cassiterite, whereas the muscovite-albite-spodumene pegmatite has a concordant U-Pb age of 458.7±2.3Ma (MSWD=7.2, N=16) of zircon; 2) the deformed ρ38 muscovite-albite-spodumene pegmatite has a concordant U-Pb age of 454.7±4.0Ma (MSWD=8.0, N=10) of zircon and ⁴⁰Ar/³⁹Ar plateau age of 350.2±1.6Ma (MSWD=4.7) of muscovite; and 3) the ρ87 coltan-bearing muscovite-quartz pegmatite has a concordant U-Pb age of 464.1±2.7Ma (MSWD=5.2, N=39) of coltan. The U-Pb ages of coltan and cassiterite are consistent within the error range, they may represent the crystallization age of granitic pegmatite at the magmatic stage. The U-Pb ages of the metamict zircons of the two samples can also be compared, they may represent the reset time of U-Pb isotope system caused by fluid metasomatism and recrystallization of magmatic zircon after metamictization. The ⁴⁰Ar/³⁹Ar plateau age of muscovite is obviously later than the U-Pb age of coltan, cassiterite, and zircon. Considering the strong deformation and alteration of muscovite and spodumene in the ρ38 muscovite-albite-spodumene pegmatite, it is suggested that the ⁴⁰Ar/³⁹Ar age of deformed muscovite may decipher the superposition deformation and alteration time. It is concluded that the lithium-beryllium granitic pegmatites in the northern Tugman area were formed in a period of 468~454Ma. Therefore, we consider that an intensive rare metal mineralization during the Middle-Late Ordovician might occur in the Altyn Tagh area. Moreover, it is also suggested that the U-Pb ages of coltan and cassiterite might represent the crystallization time of a pegmatite during the magmatic stage, whereas the U-Pb age of metamict zircon could record a reset time of U-Pb isotope system by fluid metasomatism and recrystallization, while the ⁴⁰Ar/³⁹Ar age of deformed potassium-bearing minerals might represent the deformation and alteration time of the pegmatites. The combination of different dating methods may constrain the integrated formation epoch of the rare-metal pegmatite deposit.