Silicate melt inclusions in gem-quality zircons from the Jizerka placer deposits in Jizera Mts. as indicator of the cogenetic alkaline magmatism in basement of the Krkonoše-Jizera granite massif

Geoscience Research Reports 58, 2025, pages 97–106

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Published online: 2026-05-19

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Abstract

The silicate melt inclusions in zircons represent autonomous relics of cogenetic magma carrying information on their age, composition and physico-chemical conditions of formation. Such zircons with other mineral inclusions of apatite, fergusonite, thorite and baddeleyite have been found in aluvial placers of the Cenozoic alkaline basalt area of the Eger Rift in the Bohemian Massif.

This is the first description of silicate melt inclusions in gem quality zircons from the well-known Jizerka Quaternary placer in the Jizerské hory Mts. The earlier published U/Pb ages of these zircons correspond ca. 5.63 Ma. The melt inclusions we examined are almost rounded, ranging from 5 up 40 µm in size. Majority of them contains gas-liquid bubbles of spherical shape of sub-micron size. The largest melt inclusion in the Jizerka gem-quality zircon contains gas-liquid bubbles up to 18 µm in size and daughter phenocrysts of Na-K amphibole. The silicate melt inclusions range from nepheline syenite to quartz monzonite in composition and may represent a melt residuum of the parental rocks in the basement of the Krkonoše-Jizera granite massif. Jizerka gem-quality zircons were transported to the surface by local Cenozoic alkaline basalts. This is evidenced by the presence of baddeleyite at their rims.

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