Trachytic magmatism and Nb–rare earth element mineralization in the Pingli area, North Daba Mountain: Insights from geochronology and geochemistry

Abstract

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.