华南基底变质岩的铌钽赋存状态及对铌钽富集和分异的指示
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南京大学地球科学与工程学学院

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国家自然科学基金项目(面上项目,重点项目,重大项目)


Occurrence of niobium-tantalum in basement metamorphic rocks in South China block and indication for niobium-tantalum enrichment and differentiation
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1.Nanjing Unniversity;2.Nanjing University

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    摘要:

    华南板块东南部的华夏地块和江南造山带是我国前寒武纪基底和显生宙铌钽等稀有金属矿床重点分布地区。显生宙铌钽等稀有金属成矿作用是否与前寒武纪基底成分有关,以及基底变质岩中铌钽等稀有金属的赋存状态如何影响部分熔融产生的岩浆中铌钽含量和比值是重要的基础科学问题。本文详细分析了华夏地块和江南造山带各种基底变质岩中变质矿物的Nb-Ta含量。结合全岩地球化学、矿物成分、矿物含量、变质温度和部分熔融模拟,本研究揭示了各种变质矿物的Nb-Ta含量和变化的规律,讨论了不同条件下不同矿物熔融对熔体Nb-Ta富集的影响。分析结果显示,黑云母是华南变质岩中最富集Nb-Ta的造岩矿物,Nb和Ta平均含量为64.1×10-6和4.93×10-6,对Nb和Ta的相容性相似。白云母的Nb-Ta富集能力略弱,在固相线下更相容Nb,平均Nb/Ta比值为16.9,是潜在的高Nb/Ta储库。辉石、角闪石、石榴子石和长石等造岩矿物的Nb-Ta含量很低,不会对体系Nb-Ta富集和分异产生明显的影响。固相线下黑云母和白云母的Nb-Ta含量、Nb/Ta比值以及与全岩的分配系数主要受控于寄主岩成分、矿物组合、矿物晶体化学成分以及变质温度。云母中的Nb-Ta含量和与全岩的分配系数与变质温度呈良好的线性正相关,与矿物含量呈反相关,体现了矿物含量效应(modal abundance effect)。当云母发生部分熔融后,其化学成分和分配系数发生变化,Nb-Ta含量快速下降,不再受控于固相线下的影响因素,这种转变被称之为“熔融效应”(melting effect)。根据对南岭地区一个二云母片岩的部分熔融模拟,可以得出熔体的Nb-Ta富集和分异程度主要取决于原岩Nb-Ta含量、熔融矿物组合、氧逸度以及熔融程度。高的氧逸度和压力能有效促进Nb-Ta在熔体中的富集。模拟结果显示在0.6 GPa和FMQ+2条件下,熔体的Nb和Ta含量最高达45.1×10-6和3.44×10-6,相比于原岩富集了2.65倍。分离结晶模拟指出正常的分离结晶对熔体的Nb-Ta富集作用非常有限,极端的分异,达到富流体阶段,才能导致Nb-Ta在熔流体中明显的富集。

    Abstract:

    The Cathaysia Block and the Jiangnan Orogenic Belt in the southeastern part of the South China Block are important areas for the distribution of Precambrian basements and Phanerozoic Nb-Ta and other rare-metal deposits. Whether the rare-metal mineralization is related to the Precambrian basement composition, and how the occurrence of rare metals in the basement metamorphic rocks affects the Nb-Ta contents and Nb/Ta ratios in the magma generated by partial melting are important fundamental scientific problems. In this study, the Nb and Ta contents of different minerals of the metamorphic rocks in the Cathaysia Block and Jiangnan Orogenic Belt are analyzed in detail. Combining whole-rock geochemistry, mineral compositions, mineral proportions, metamorphic temperature, and partial melting modelling, this study reveals the critical factors controlling the Nb-Ta contents and variations in biotite, muscovite and other metamorphic minerals, and discusses the influence of melted minerals on the Nb-Ta contents in melts in different conditions. Analytical results show that biotite is the most enriched Nb-Ta rock-forming mineral in the metamorphic rocks of South China, have mean Nb content of 64.1×10-6 and mean Ta content of 4.93×10-6, with similar compatibility between Nb and Ta. Muscovite is another metamorphic mineral containing high Nb-Ta contents, slightly lower than biotite. Niobium is more compatible in muscovite than Ta in subsolidus conditions, with an high Nb/Ta ratio of 16.9, being a potential high Nb/Ta reservoir. Pyroxene, amphibole, garnet and feldspar have low Nb and Ta contents, and make little effect on the enrichment and differentiation of Nb-Ta in the system. The Nb-Ta contents and Nb/Ta of biotite and muscovite, and partition coefficients with bulk rocks are mainly controlled by the host-rock composition, mineral assemblage, mineral composition, and metamorphic temperature. The Nb-Ta contents and partition coefficients of biotite and muscovite show positive correlation with metamorphic temperature, and inverse correlation with mineral proportions, reflecting the modal abundance effect. When partial melting occurs and melt is extracted, the Nb-Ta contents in mica sharply decrease owing to low Nb-Ta contents in rocks and changes in mineral compositions and partition coefficient, which is not controlled by those factors in subsolidus conditions, but by the degree of melting, i.e. "melting effect". Based on the partial melting modelling of a two-mica schist from the eastern Nanling region, it is concluded that the Nb-Ta enrichment and differentiation of melts mainly depend on source compositions, melted mineral assemblage, oxygen fugacity and melting degree. High oxygen fugacity and pressure can effectively promote the Nb-Ta enrichment in melts. The modelling results suggest that at pressure of 0.6 GPa and oxygen fugacity of FMQ+2, partial melting may produce the melt with Nb-Ta contents up to 45.1×10-6 and 3.44×10-6, respectively, 2.65 times higher than the source. Fractional crystallization modelling indicates that normal crystallization differentiation can’t lead to significant Nb-Ta enrichment in residual melt. Extreme differentiation, even reaching the fluid-rich stage, must be needed to promote significant Nb-Ta enrichment.

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  • 收稿日期:2024-04-10
  • 最后修改日期:2024-05-13
  • 录用日期:2024-05-21
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