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

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.

Age and Geochemistry of Zircon Megacrysts from Alluvial Deposits in the Huadian Area, Northeastern China

“…The partial melting of metasomatized peridotites would produce melts with carbonatitic compositions at a low degree, and carbonated silicate melts with further melting. Some recent studies have invoked such a model (partial melting of a carbonatitic-metasomatized mantle to a variable degree) to illustrate the generation of 'carbonatitic signature' zircon megacrysts, which are associated with alkali basalts found in Vietnam and Cambodia [10,14]. However, it appears difficult to simply reconcile the formation of zircon megacrysts from Huadian with crystallization from melts produced by the partial melting of a carbonatiticinfluenced mantle to a variable degree.…”

“…Megacrystic zircons (which are typically centimeter-sized) are found within intraplate alkali basalt deposits around the world; their derivatives are formed as a result of secondary processes, especially those seen in the countries along the western Pacific continental margins [6][7][8][9]. These basalts commonly contain abundant mantle xenoliths, along with a variety of high-pressure megacrysts such as corundum, pyroxene, garnet and spinel [10][11][12][13], offering scientists an opportunity to elucidate the composition and evolutionary history of the deep lithosphere. Some species of megacrysts (e.g., corundum and zircon) have also been commercially exploited for several centuries to supply the global market with gemstones [14].…”

Zircon Megacrysts from the Kawisigamuwa Carbonatite, Sri Lanka – A Potential Reference Material for In SituU‐Pb and Hf Isotope Measurement

“…For the in situ analysis of zircon, fragments of isotopically well-characterised zircon megacrysts are sought-after. Zircon megacrysts are exceptionally large crystals ( [ 5 mm) that are mainly associated with mantle-derived kimberlites, carbonatites, alkali basalts, syenitic pegmatites or marbles (e.g., Hoskin and Schaltegger 2003, Belousova et al 1998, Piilonen et al 2018, Sutherland et al 2016, Sinh et al 2019, Schaltegger et al 2015b, Cavosie et al 2011. Since they form during relatively short time spans in magmatic, hydrothermal and metamorphic environments, zircon megacrysts are commonly used as reference material for U-Pb geochronology (e.g., Mud Tank, Gain et al 2019, Penglai, Li et al 2010, 91500, Wiedenbeck et al 1995, M257, Nasdala et al 2008, GZ7 and GZ8, Nasdala et al 2018, BB zircons, Santos et al 2017, M127, Nasdala et al 2016, CZ3, Pidgeon et al 1994, GJ1, Jackson et al 2004, Ple sovice, Sl ama et al 2008.…”

Editorial for Special Issue “Mineralogy and Geochemistry of Gems”

“…Zircon is another important mineral geologically [27,28], which can be of gem-quality. Studies on gem-quality zircons from two occurrences are also included in this Special Issue [29,30]. Data by Piilonen et al [29] on zircon xenocrysts from alkali basalts in Ratanakiri Province (Cambodia) suggest that their genesis involved zirconium-saturated, aluminium-undersaturated, carbonatitic-influenced, low-degree partial melting (<1%) of peridotitic mantle at ca.…”

“…60 km beneath the Indochina terrane. Data by Bui Thi Sinh et al [30] on zircon crystals (up to 3 cm long) from placer deposits in the Central Highlands of Vietnam suggest a genesis from carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source as well as resorption and re-growth processes.…”