Trachytic rocks widely occur to the north of the Hongchunba–Zengjiaba Fault in the North Daba Mountain of South Qinling. These trachytic rocks contain relatively large quantities of Nb‐ and rare earth element (REE)‐bearing minerals. This study presents the zircon U–Pb ages, REE content, Hf isotope data, and whole‐rock compositions of the Nb‐REE mineralized trachytes from the Pingli area. U–Pb geochronological data reveal the ages of zircon grains can be divided into three groups: 431–395, 270–255, and 241–240 Ma. Cathodoluminescence images and REE compositions confirm a magmatic origin for the zircons with age of 431–395 and 270–255 Ma. Zircon grains with ages of 431–395 Ma have the characteristics similar to relict zircons. The 270–255 Ma age group is interpreted to represent the diagenetic age of the trachyte. Zircon grains with a younger age (241–240 Ma) are distinguishable from magmatic zircons with regards to their cathodoluminescence patterns and REE signatures, indicating a hydrothermal origin. The 241–240 Ma age group reflects the Nb–REE mineralization age in the trachyte. Geochemically, the trachytes have high SiO₂ contents but low MgO contents and Mg# values. They can be classified as alkaline‐series and belong to the metaluminous–peraluminous rocks, with A/CNK ratios between 0.81 and 1.11. The trachytes are characterized by enrichments in the light rare earth elements (LREEs) and large‐ion lithophile elements (LILEs), but are depleted in the heavy REEs (HREEs), Ti, Sr, and P, with negligible Eu. These geochemical features, together with the highly negative εHf(t) (−8.25 to −11.25) values of the zircon grains, indicate that the primary magma associated with these trachytes likely originated from the partial melting of ancient lower crust. The presence of a positive εHf(t) (4.12) value suggests that this process may have involved a small amount of juvenile basaltic crust material. This is different from other rare metal‐hosting trachytes worldwide, whose parental magmas are mantle‐derived, analogous to basalts generated in an intraplate setting. Nevertheless, trachytes are specifically Nb‐REE mineralized by primary enrichment processes (fractional crystallization) or secondary upgrading processes (hydrothermal alteration). The Nb‐REE mineralization is closely related to Early Triassic alkaline activity, such that the identification of highly fractionated Early Triassic trachyte is important for targeting regional Nb–REE deposits. The transport and facilitation of the precipitation of Nb and the REEs require volatile F components, where the provenance of fluorite can be determined and used during geochemical exploration.

中文翻译：

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北达巴山平利地区的层状岩浆作用和Nb-稀土元素矿化：来自年代学和地球化学的见解

在南秦岭北大巴山的红春坝—曾家坝断层以北，广泛分布有滑行性岩石。这些曲折的岩石含有相对大量的Nb和稀土元素（REE）矿物。这项研究显示了平利地区锆石的U–Pb年龄，REE含量，Hf同位素数据以及Nb‐REE矿化的长粒小粒岩的全岩成分。U–Pb地质年代学数据显示锆石的年龄可分为三组：431–395、270–255和241–240 Ma。阴极发光图像和稀土元素组成证实了锆石的岩浆成因，年龄为431–395和270–255 Ma。年龄在431–395 Ma的锆石晶粒具有类似于遗骸锆石的特征。270至255 Ma年龄组被解释为代表了小行星的成岩年龄。较年轻的锆石晶粒（241-240 Ma）在阴极发光特征和稀土元素特征上与岩浆锆石有明显区别，表明其为热液成因。241–240 Ma年龄组反映了苗条岩中Nb–REE的矿化年龄。从地球化学上看，这些小粒球具有较高的SiO₂含量，但MgO含量和Mg＃值低。它们可以归类为碱性系列，属于金属-高铝质岩石，A / CNK比在0.81至1.11之间。茶粒的特点是富含轻稀土元素（LREEs）和大离子嗜石元素（LILEs），但富含重稀土元素（HREEs），Ti，Sr和P，而Eu却可以忽略不计。这些地球化学特征以及锆石晶粒的εHf（t）（-8.25至-11.25）高度负值，表明与这些小摆线相关的原生岩浆可能起源于古代下地壳的部分熔融。正εHf（t）（4.1​​2）值的存在表明此过程可能涉及少量的少年玄武岩地壳材料。这与世界上其他稀有金属寄托的长毛绒不同，其父母的岩浆是幔源的，类似于板内环境中产生的玄武岩。然而，小粒状球特别是通过一次富集过程（部分结晶）或二次升华过程（水热蚀变）而被矿化的Nb-REE。Nb-REE矿化与早三叠世的碱性活动密切相关，因此，确定高分馏率的早三叠世的松散粒对于确定区域性Nb-REE矿床很重要。Nb和REE的沉淀的运输和促进需要挥发性F成分，可以在地球化学勘探中确定和使用萤石的来源。菱形铁矿特别是通过初级富集过程（部分结晶）或次级提质过程（水热蚀变）矿化的Nb-REE。Nb-REE矿化与早三叠世的碱性活动密切相关，因此，鉴定高度分馏的早三叠世的曲奇岩对确定区域Nb-REE矿床很重要。Nb和REE的沉淀的运输和促进需要挥发性F成分，可以在地球化学勘探中确定和使用萤石的来源。菱形铁矿特别是通过初级富集过程（部分结晶）或次级提质过程（水热蚀变）矿化的Nb-REE。Nb-REE矿化与早三叠世的碱性活动密切相关，因此，确定高分馏率的早三叠世的松散粒对于确定区域性Nb-REE矿床很重要。Nb和REE沉淀的运输和促进需要挥发性F成分，可以在地球化学勘探中确定和使用萤石的来源。因此，鉴定高分馏的早三叠世松露体对于靶向区域Nb-REE矿床很重要。Nb和REE沉淀的运输和促进需要挥发性F成分，可以在地球化学勘探中确定和使用萤石的来源。因此，鉴定高分馏的早三叠世松露体对于靶向区域Nb-REE矿床很重要。Nb和REE沉淀的运输和促进需要挥发性F成分，可以在地球化学勘探中确定和使用萤石的来源。