No abstract

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 greenschistto 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.

The metamorphic and plutonic rocks within a serpentinite mélange in the Nagato Tectonic Zone, Yamaguchi prefecture, southwest Japan have been known as a part of the early Paleozoic rock record in Japan. Thus, they play an important role in deciphering the tectonic evolution of proto-Japan. This study determined their U-Pb zircon ages to deduce the origin and geological significance of the Nagato Tectonic Zone. The results of the petrographic observation and LA-ICP-MS U-Pb zircon dating revealed the occurrences of the massive amphibolite originated from the 460 Ma mafic igneous rock, two-mica tonalites formed at 470-460 Ma in association with some xenocrystic zircon grains, and low-grade metasedimentary rocks including detrital zircon grains of 2460-400 Ma. The lithology, petrographic characteristics, and U-Pb zircon ages of these constituent rocks in the Nagato Tectonic Zone are well comparable with the blocks within serpentinite mélange in the Kurosegawa Tectonic Belt, rather than the Hida Gaien Belt which has been considered to form a single geological unit with the Nagato Tectonic Zone as the Sangun-Renge metamorphic belt or Nagato-Renge belt. It implies the Nagato Tectonic Zone could be regarded as an isolated fragment of the Kurosegawa Tectonic Belt. The features of detrital age spectra and Ordovician magmatism in the Nagato Tectonic Zone and Kurosegawa Tectonic Belt provide the possibility of the development of their granitoids and protoliths of metamorphic rocks at the active continental margin of the South China craton.