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Minerals 2019, 9(2), 89; https://doi.org/10.3390/min9020089

Gem-Quality Zircon Megacrysts from Placer Deposits in the Central Highlands, Vietnam—Potential Source and Links to Cenozoic Alkali Basalts

1
Graduate School of Integrated Sciences for Global Society, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
2
Division of Earth Sciences, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
3
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria
4
Australian Research Council (ARC) Centre of Excellence for Core to Crust Fluid Systems (CCFS) and National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
*
Author to whom correspondence should be addressed.
Received: 30 December 2018 / Revised: 28 January 2019 / Accepted: 29 January 2019 / Published: 1 February 2019
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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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. View Full-Text
Keywords: zircon megacrysts; placer deposits; rare earth elements (REE); carbonatite-dominant melts; Central Highlands; Vietnam; hyperspectral photoluminescence imaging; LA-ICP-MS zircon megacrysts; placer deposits; rare earth elements (REE); carbonatite-dominant melts; Central Highlands; Vietnam; hyperspectral photoluminescence imaging; LA-ICP-MS
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Bui Thi Sinh, V.; Osanai, Y.; Lenz, C.; Nakano, N.; Adachi, T.; Belousova, E.; Kitano, I. Gem-Quality Zircon Megacrysts from Placer Deposits in the Central Highlands, Vietnam—Potential Source and Links to Cenozoic Alkali Basalts. Minerals 2019, 9, 89.

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