Fluid-driven destabilization of REE-bearing accessory minerals in the granitic orthogneisses of North Veporic basement (Western Carpathians, Slovakia)

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• 作者：M. Ondrejka ; M. Putiš ; P. Uher ; I. Schmiedt ; L. Pukančík…
• 刊名：Mineralogy and Petrology
• 出版年：2016
• 出版时间：October 2016
• 年：2016
• 卷：110
• 期：5
• 页码：561-580
• 全文大小：2,448 KB
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Mineralogy
  Geochemistry
  
• 出版者：Springer Wien
• ISSN：1438-1168
• 卷排序：110

文摘

A variety of rare earth elements-bearing (REE) accessory mineral breakdowns were identified in granitic orthogneisses from the pre-Alpine basement in the Veporic Unit, Central Western Carpathians, Slovakia. The Ordovician granitic rocks were subjected to Variscan metamorphic-anatectic overprint in amphibolite facies. Chemical U-Th-Pb dating of monazite-(Ce) and xenotime-(Y) reveal their primary magmatic Lower to Middle Ordovician age (monazite: 472 ± 4 to 468 ± 6 Ma and xenotime: 471 ± 13 Ma) and/or metamorphic-anatectic Variscan (Carboniferous, Visean) age (monazite: 345 ± 3 Ma). Younger fluid-rock interactions caused breakdown of primary magmatic and/or metamorphic-anatectic monazite-(Ce), xenotime-(Y), fluorapatite and allanite-(Ce). Fluid-induced breakdown of xenotime-(Y) produced numerous tiny uraninite inclusions within the altered xenotime-(Y) domains. The monazite-(Ce) breakdown produced secondary egg-shaped coronal structures of different stages with well-developed concentric mineral zones. Secondary sulphatian monazite-(Ce) (up to 0.15 apfu S) occasionally formed along fluorapatite fissures. Localized fluorapatite and monazite-(Ce) recrystallization resulted in a very fine-grained, non-stoichiometric mixture of REE-Y-Fe-Th-Ca-P-Si phases. Finally, allanite-(Ce) decomposed to secondary REE carbonate minerals (members of the bastnäsite and synchysite groups) and calcite in some places. Although the xenotime alteration and formation of uraninite inclusions is believed to be the result of dissolution-reprecipitation between early magmatic xenotime and late-magmatic granitic fluids, the monazite, apatite and allanite breakdowns were driven by metamorphic hydrothermal fluids. While earlier impact of post-magmatic fluids originated probably from Permian acidic volcanic and microgranitic veins crosscutting the orthogneisses, another fluid-rock interaction event most likely occurred during Late Cretaceous metamorphism in the Veporic basement and covering rocks. This stage indicates carbon-bearing fluids precipitating the carbonate minerals.