Zircon Megacrysts from Wenchang, Hainan Island, South China: Weak Metamictization and Heat-Induced Color Modification

Zircon megacrysts from Wenchang, Hainan Island, South China, were investigated to characterize their mineralogical and geochemical properties, U-Pb age, radiation-damage state, and heat-induced spectral and color responses. We integrated electron probe microanalysis (EPMA), laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb and trace-element analyses, Fourier transform infrared (FTIR), Raman and ultraviolet–visible (UV-Vis) spectroscopy, powder X-ray diffraction (XRD), and controlled heating. This approach extends previous occurrence-based descriptions of the Wenchang deposits through an integrated mineralogical, chronological, structural, and thermal-response assessment. The zircons show HREE-enriched patterns, positive Ce anomalies, Eu/Eu* values close to unity, and Th/U ratios of 0.42–1.26, supporting a magmatic affinity without identifying a specific parental melt. A weighted mean 206Pb/238U age of 3.78 ± 0.10 Ma (1σ) was obtained. FTIR spectra retain relatively sharp SiO4 bending features, and five representative spectra show no resolvable broad OH-related feature at the displayed scale. Raman spectra show a narrow ν3(SiO4) band near 1008 cm−1 with an average corrected full width at half maximum of 5.5 cm−1. XRD patterns of two unheated grains contain sharp zircon reflections without a conspicuous diffuse-scattering halo. Together, these observations support retained crystalline order and weak radiation damage in the analyzed samples. Within the measured low-dose dataset, ν3(SiO4) full width at half maximum (FWHM) increases with accumulated alpha dose (Dα), whereas Raman band position shows no statistically significant dose-related trend. Heating weakens the broad 450–550 nm absorption as the brown samples become lighter, while treatment at 1100 °C under nitrogen produces a bluish-gray hue accompanied by broad red-region absorption. The data constrain the age, radiation-damage state, and heat-induced responses of the analyzed Wenchang zircons; their primary host, source region, and transport history remain unresolved.