Of zircons and zircons: The tumultuous story of Zr-Hf and REE during cooling of peralkaline granites

Abstract

Understanding the behavior of trace elements such as Zr-Hf, REE, and Th-U during magmatic and hydrothermal processes is crucial for unraveling the evolution of rare-metal peralkaline granites. These elements play key roles in geochemical cycles, mineral formation, and economic mineral deposits. The Evisa intrusion, a rare-metal peralkaline granite, is an ideal setting to study such processes due to its diverse zircon population, which records a continuous transition from early igneous to late hydrothermal precipitation and alteration. This evolution spans temperatures from more than 600 °C to approximately 150 °C, with zircon demonstrating exceptional diversity in textures and compositions. In-situ U-Pb dating of both igneous and hydrothermal zircon suggests they precipitated coevally at ∼ 270 Ma, indicating the synchronous formation and alteration of zircon during the crystallization and cooling of the Evisa granite. Hf isotopic compositions vary from 0 to + 10 εHf(t) for igneous zircon and from −5 to + 5 εHf(t) for hydrothermal zircon, reflecting a shift from a mantle-derived to a more crustal signature. Both U-Pb and Hf isotopic data support a rift-related context for the Corsica terrane during the late Permian. Zircon in this granite is highly enriched in REE (median value > 2 wt% for Y + REE) and variably enriched in U and Th (up to several wt.%), with igneous zircon exhibiting higher Ce anomalies and HREE/LREE ratios than hydrothermal zircon. Alteration of zircon results in the formation of pores (ranging from nm to µm scale) and the redistribution of trace metals between newly-formed mineral inclusions, newly-crystallized zircon, and the aqueous fluid. This redistribution, combined with the breakdown of other REE-minerals and availability of suitable ligands in the fluid, controlled the cyclic enrichment of REE in hydrothermal zircon and influenced their fate in the Evisa granite. Hydrothermal remobilization of Hf at low temperature led to significant Hf isotopic variations (up to > 20 εHf(t) unit) in newly crystallized zircon, likely due to mass-dependent kinetic fractionation. Discrepancies in the Zr/Hf ratio (from ∼ 0.5 to 2.5) in successive growth zones of hydrothermal zircon were interpreted as resulting from fractionation due to mass-independent effects on bond strength during transport, probably in the form of fluoride complexes. This study not only provides new insights into the behavior of Zr-Hf, Th-U and REE in rare-metal peralkaline granites but also underscores the need for caution when interpreting zircon compositions, particularly when using commonly employed geochemical tools, such as Ti-based thermometers.