Gem-quality zircon megacrysts occur in placer deposits in the Central Highlands, Vietnam, and have euhedral to anhedral crystal shapes with dimensions of ~3 cm in length. These zircons have primary inclusions of calcite, olivine, and corundum. Secondary quartz, baddeleyite, hematite, and CO₂ fluid inclusions were found in close proximity to cracks and tubular channels. LA-ICP-MS U-Pb ages of analyzed zircon samples yielded two age populations of ca. 1.0 Ma and ca. 6.5 Ma, that were consistent with the ages of alkali basalt eruptions in the Central Highlands at Buon Ma Thuot (5.80–1.67 Ma), Pleiku (4.30–0.80 Ma), and Xuan Loc (0.83–0.44 Ma). The zircon geochemical signatures and primary inclusions suggested a genesis from carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source, but not from the host alkali basalt. Chondrite-normalized rare earth element patterns showed a pronounced positive Ce, but negligible Eu anomalies. Detailed hyperspectral Dy³⁺ photoluminescence images of zircon megacrysts revealed resorption and re-growth processes.

The Higo metamorphic terrane in west–central Kyushu, southwest Japan, consists of a sequence of greenschist– to granulite–facies metamorphic rocks. This study reports the U–Pb Zrn ages and rare earth elements (REE) characteristics of Zrns and Grts from pelitic gneisses in the Higo metamorphic terrane to constrain its timing of high–temperature metamorphism. Based on the morphology, internal textures, occurrences, Th/U ratios, REE patterns and ages of Zrn, and the heavy REE (HREE) partition coefficient between Zrn and Grt, the results indicate that the prograde metamorphism occurred at ~ 120 Ma immediately after precursor deposition at 120–130 Ma and peak metamorphic conditions were attained during the period between 110 and 120 Ma. The ~ 110 Ma age might be indicative of the onset of cooling and exhumation after the peak metamorphic stage. Retrograde metamorphism continued at least to 105 Ma as obtained from Zrns in the granitic intrusion.