Hellandite-(Y)–hingganite-(Y)–fluorapatite retrograde coronae from breakdown of xenotime-(Y) at Fabova Hola (Slovakia)

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Marco E. Ciriotti
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Hellandite-(Y)–hingganite-(Y)–fluorapatite retrograde coronae from breakdown of xenotime-(Y) at Fabova Hola (Slovakia)

Messaggio da Marco E. Ciriotti » sab 15 gen, 2022 11:08

Referenza:
▪ Ondrejka, M., Molnárová, A., Putiš, M., Bacik, P., Uher, P., Voleková, B., Milovská, S., Mikuš, T., Pukančík, L. (2022): Hellandite-(Y)–hingganite-(Y)–fluorapatite retrograde coronae: a novel type of fluid induced dissolution-reprecipitation breakdown of xenotime-(Y) in the metagranites of Fabova Hoľa (Western Carpathians, Slovakia). Mineralogical Magazine, 86, (in press).

Abstract:
Two contrasting reaction coronae were developed around REE accessory phosphates in Variscan granitic rocks, which had been overprinted by Alpine blastomylonitization from the Fabova Hoľa Massif (the Veporic Unit, Western Carpathians, Central Slovakia). The Th-U-Pb total EPMA dating of primary magmatic monazite of the investigated metagranite provided the Carboniferous (Mississippian, Tournaisian) age of 355 ± 1.9 Ma. While the monazite-(Ce) breakdown resulted in impressive, but common fluorapatite ± Th-silicate + allanite-(Ce) + clinozoisite coronae, the alteration of xenotime-(Y) produced a novel type of secondary coronal microtexture consisting of a massive fluorapatite mantle zone and tiny satellite crystals of hellandite-(Y) and hingganite-(Y) ∼1–5 μm; occasionally ≤10 μm in size. The localized occurrence of Y-B-Be silicates, which are closely associated with other secondary minerals, suggests the involvement of B and Be during the dissolution-reprecipitation transformation of xenotime-(Y). The granitic rocks underwent Early Cretaceous burial metamorphism in the greenschist to lower amphibolite facies P-T conditions. Subsequently, Alpine post-collisional uplift and exhumation of the Veporic Unit, starting from the Upper Cretaceous epoch was accompanied with a retrograde tectonometamorphic overprint and activity of external fluids, which likely caused a breakdown of monazite-(Ce) and xenotime-(Y), as well as the production of secondary coronae minerals. A part of B (± Be) should have been liberated from the metagranite feldspars, micas or xenotime-(Y) enriched in (Nb,Ta)BO4 (schiavinatoite or béhierite) components. However, a principal source of B and Be in fluids necessary for the production of hellandite and hingganite was probably of an external origin, either from adjacent magmatic, metamorphic, or sedimentary rocks (Permian granites, rhyolites and sedimentary rocks, Palaeozoic metapelites).
Marco E. Ciriotti

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