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

Martin Ondrejka,Libor Pukančík,Marián Putiš,Alexandra Molnárová,Bronislava Voleková,Stanislava Milovská,Pavel Uher,Tomáš Mikuš,Peter Bačík

doi:10.1180/mgm.2022.7

Abstract

AbstractTwo contrasting reaction coronae were developed around rare earth element (REE) accessory phosphates in Variscan metagranitic rocks, which have been overprinted by Alpine blastomylonitisation from the Fabova Hol'a Massif, in the Veporic Unit, Western Carpathians, Central Slovakia. The Th–U–Pb total EPMA age determination of primary magmatic monazite-(Ce) from the metagranite indicates a Carboniferous (Mississippian, Tournaisian) age of 355 ± 1.9 Ma. Monazite-(Ce) breakdown resulted in impressive, though common, fluorapatite ± Th-silicate + allanite-(Ce) + clinozoisite coronae. The alteration of xenotime-(Y) produced a novel type of secondary coronal micro-texture consisting of a massive fluorapatite mantle zone and tiny satellite crystals of hellandite-(Y) [(Ca,REE)4Y2Al□2(B4Si4O22)(OH)2] and hingganite-(Y) [Y2□Be2Si2O8(OH)2] of ~1–5 μm, and rarely ≤10 μm in size. The localised occurrence of Y–B–Be silicates, which are associated closely with other secondary minerals, suggests the involvement of B and Be during the metasomatic alteration transformation of xenotime-(Y). General reactions for monazite-(Ce) and xenotime-(Y) decomposition, including the fluids involved, can be written as follows: Mnz + (Ca, Fe, Si, Al and F)-rich fluid → FAp + Ht + Aln + Czo; Xtm + (Ca, Fe, Si, Al, F, B and Be)-rich fluid → FAp + Hld + Hin + Czo.The granitic rocks underwent Early Cretaceous burial metamorphism under greenschist- to lower amphibolite-facies P–T conditions. Subsequently, Alpine post-collisional uplift and exhumation of the Veporic Unit, starting from the Late Cretaceous epoch, was accompanied by a retrograde tectono-metamorphic overprint; the activity of external fluids, caused the formation of secondary coronae minerals around monazite-(Ce) and xenotime-(Y). A portion 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, the principal source of B and Be in fluids necessary for the production of hellandite and hingganite, was probably of external origin from adjacent magmatic, metamorphic, or sedimentary rocks (Permian granites, rhyolites and sedimentary rocks, and Palaeozoic metapelites).

Highlights

Together with allanite-(Ce), monazite-(Ce) and xenotime-(Y) are the most importantrare earth elements (REE) carriers in common magmatic rocks and metamorphic rocks with granitic and metapelitic protoliths (e.g. Bea, 1996; Förster, 1998a,b; Spear and Pyle, 2002; Gieré and Sorensen, 2004)
A detailed study of their chemical composition, stability, and breakdown processes provides us with an understanding of REE systematics and processes that occur during petrogenesis and evolution of the parental rock (Wark and Miller, 1993; Bea, 1996; Gratz and Heinrich, 1997; Heinrich et al.,1997; Förster, 1998a,b; Poitrasson et al, 2002; Pyle et al, 2001; Spear and Pyle, 2002; Wing et al, 2003; Kohn and Malloy, 2004; Ondrejka et al, 2012, 2016; Berger et al, 2008; Janots et al, 2008; Uher et al, 2009, 2015; Harlov et al, 2011 and references therein)
A similar reaction coronae around xenotime-(Y) in granitoids and metapelites (Broska et al, 2005; Majka and Budzyń, 2006; Janots et al, 2008; Majka et al, 2011; Broska and Petrík, 2015; Budzyń et al, 2018; Hentschel et al, 2020), or the partial compositional alteration of xenotime-(Y) via dissolution-reprecipitation resulting in a texture characterised by thorite (ThSiO4) and uraninite (UO2) inclusions (Hetherington and Harlov, 2008; Ondrejka et al, 2016), are less frequent

Summary

Introduction

Together with allanite-(Ce), monazite-(Ce) and xenotime-(Y) are the most importantREE carriers in common magmatic rocks and metamorphic rocks with granitic and metapelitic protoliths (e.g. Bea, 1996; Förster, 1998a,b; Spear and Pyle, 2002; Gieré and Sorensen, 2004). Broska and Siman, 1998; Finger et al, 1998, 2016; Budzyń et al, 2010; Krenn et al, 2012; Ondrejka et al, 2012, 2016; Upadhyay and Pruseth, 2012; Lo Pò et al, 2016; Ji et al, 2021) This microtexture is mostly documented in low- to medium-temperature metagranitoids with primary magmatic monazite(Ce). Experiments suggest the significant dissolution, etching, and alteration of monazite-(Ce) and xenotime-(Y) by common metamorphic and igneous fluids and the formation of porous textures and various secondary phases are dependent on the P-T-X conditions (Hetherington et al, 2010; Harlov et al, 2011; Harlov and Wirth, 2012; Budzyń et al, 2011, 2017), as well as by low-T metamorphic or post-magmatic-hydrothermal fluids in granitic systems (Budzyń and KozubBudzyń, 2015)

Methods

Conclusion