关键词: 准铝质—过铝质花岗岩, 铌钽, 矿物温压计, 氧逸度计

Abstract: A series of metaluminous-peraluminous rare-metal granites formed at around 130 Ma is distributed in the Jiangnan Orogenic Belt in China. They are usually characterized by multiple phases with (amphibole-bearing) biotite granite as the early one and the biotite granite and zinnwaldite albite granite as the later phases. Their whole-rock chemical compositions are commonly rich in niobium (Nb), zirconium (Zr), and rare-earth elements (REEs), and columbite is the typical ore mineral. They are different from the peraluminous rare-metal granites formed at around 150 Ma (such as Yichun pluton) widely distributed in South China, which are mainly composed of muscovite granite and zinnwaldite and/or lepidolite granite, and are mainly rich in lithium (Li), tantalum (Ta), rubidium (Rb), and cesium (Cs). Three key factors govern rare-metal mineralization in granitic systems: the primary condition of magma, the fractional crystallization and evolution, and the effects of fluids hydrothermal fluids. Most studies have concentrated on the late-staged lithologies of the pluton, thereby providing significant insights in to the latter two factors. In order to better understand the influence of the primary condition of magma on late-stage mineralization, this study focuses on the early lithologies of the Lingshan pluton in the Jiangnan Orogenic Belt, including coarse-grained porphyritic amphibole-bearing biotite granite (AmBG) and the newly discovered fine-grained porphyritic amphibole-bearing biotite granite (F-AmBG) contacting with AmBG. Based on detailed and systematic petrographic and mineralogical results, this study estimated the primary physicochemical condition of the magma of the Lingshan metaluminous granites, including the temperature, pressure, oxygen fugacity, water content, and melt composition (especially Nb and halogen content) characteristics. Meanwhile, a series of calculation methods suitable for this system were suggested. The results show that the pressure of 300-330 MPa, the oxygen fugacity (ΔNNO) of -1.53 to -1.19, the water content of around 5 wt%, and the main crystallization temperature of minerals of around 720-800 ℃ . The clinopyroxene and columnar amphibole in the matrix found in the F-AmBG formed at 830-890 ℃ , as the products of rapid crystallization. The estimated Nb content of 27×10-6 -362×10-6 indicates the Nb enrichment in the initial magma. The highest Nb value in the melt composition is correlated to the Nb composition in titanite, which may be associated with the local enrichment of rare-metal caused by the volatile-rich components. 

Key words: metaluminous-peraluminous granite, niobium-tantalum, mineral thermobarometer, oxybarometer