REE Minerals As Geochemical Proxies of Late-Tertiary Alkalic Silicate ± Carbonatite Intrusions Beneath Carpathian Back-Arc Basin

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Article REE Minerals as Geochemical Proxies of Late-Tertiary Alkalic Silicate ± Carbonatite Intrusions Beneath Carpathian Back-Arc Basin

Vratislav Hurai 1,* , Monika Huraiová 2 and Patrik Koneˇcný 3

1 Earth Science Institute, Slovak Academy of Sciences, 840 05 Bratislava, Slovakia 2 Department of Mineralogy and Petrology, Comenius University, 842 15 Bratislava, Slovakia; [email protected] 3 State Geological Institute of Dionýz Štúr, 817 04 Bratislava, Slovakia; [email protected] * Correspondence: [email protected]

Abstract: The accessory mineral assemblage (AMA) of igneous cumulate xenoliths in volcanoclastic deposits and lava flows in the Carpathian back-arc basin testifies to the composition of intrusive complexes sampled by Upper Miocene-Pliocene basalt volcanoes. The magmatic reservoir beneath Pinciná maar is composed of gabbro, moderately alkalic to alkali-calcic syenite, and calcic orthopyroxene granite (pincinite). The intrusive complex beneath the wider area around Fil’akovo and Hajnáˇckamaars contains mafic cumulates, alkalic syenite, carbonatite, and calc-alkalic granite. Both reservoirs originated during the basaltic magma underplating, differentiation, and interaction with the surrounding mantle and crust. The AMA of syenites is characterized by yttrialite-Y, Citation: Hurai, V.; Huraiová, M.; britholite-Y, britholite-Ce, chevkinite-Ce, monazite-Ce, and rhabdophane(?). Baddeleyite and REE- Koneˇcný,P. REE Minerals as zirconolite are typical of alkalic syenite associated with carbonatite. Pyrochlore, columbite-Mn, and Geochemical Proxies of Late-Tertiary Ca-niobates occur in calc-alkalic granites with strong peralkalic affinity. Nb-rutile, niobian ilmenite, Alkalic Silicate ± Carbonatite and fergusonite-Y are crystallized from mildly alkalic syenite and calc-alkalic granite. Zircons with Intrusions Beneath Carpathian increased Hf/Zr and Th/U ratios occur in all felsic-to-intermediate rock-types. If rock fragments are Back-Arc Basin. Minerals 2021, 11, 369. absent in the volcanic ejecta, the composition of the sub-volcanic reservoir can be reconstructed from https://doi.org/10.3390/ the specific AMA and zircon xenocrysts–xenolith relics disintegrated during the basaltic magma min11040369 fragmentation and explosion.

Academic Editors: Ján Spišiak, Jaroslav Pršek, Jiˇrí Sejkora and Keywords: yttrialite; britholite; zirconolite; chevkinite; syenite; carbonatite; basalt; Western Carpathians Nigel J. Cook

Received: 27 January 2021 Accepted: 27 March 2021 1. Introduction Published: 31 March 2021 Direct information about the composition of the deep lithosphere is encoded in rock fragments (xenoliths) ejected by basaltic volcanoes. The vast majority of smaller rock frag- Publisher’s Note: MDPI stays neutral ments scavenged from the walls of volcanic conduits is resorbed in the hot basalt, whereas with regard to jurisdictional claims in the larger xenoliths are disintegrated and ground during the magma fragmentation and published maps and institutional affil- phreatomagmatic eruption triggered by the magma-water interaction and explosive release iations. of rapidly expanding gas bubbles. Consequently, only specific accessory mineral assemblage (AMA) is preserved, consisting of phenocrysts crystallized from the magma and xenocrysts derived from the disintegrated xenoliths. Detailed knowledge of morphology, composition, zoning, and elemental substitutions in individual accessory minerals is neces- Copyright: © 2021 by the authors. sary to decipher the information on the composition of rocks sampled by the ascending Licensee MDPI, Basel, Switzerland. magma [1,2]. This article is an open access article Upper Miocene–Quaternary basaltic pyroclastic deposits in the northern part of the distributed under the terms and Pannonian Basin (Carpathian back-arc basin) contain a unique assemblage of igneous conditions of the Creative Commons xenoliths [3], and significantly more abundant spinel peridotite and wehrlite xenoliths Attribution (CC BY) license (https:// transported from the metasomatized upper mantle [4]. Gabbroic and syenite xenoliths rep- creativecommons.org/licenses/by/ resent gravitational and flotation cumulates, respectively, scavenged from stratified, partly 4.0/).

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solidified magmatic intrusions [5]. Occasional granite fragments range in composition from barren magnesian and calcic orthopyroxene granite (pincinite) to productive, HFSE- and REE-rich ferroan calc-alkalic granite [6–8]. Despite the diverse compositions, trace elements and radiogenic isotopes define the whole suite of igneous xenoliths as a cogenetic assemblage of an Upper Miocene–Pliocene A1-type intra-plate continental magmatism [9]. Here we investigate the AMA of igneous xenoliths and tuff from one classical (Pinciná maar) and one newly discovered (Hajnáˇckadiatreme) locality. We compare the AMAs of various syenite types with productive calc-alkaline granites. We furthermore demonstrate that the composition of sub-volcanic magmatic intrusive complexes can be inferred from the specific AMA recovered from pyroclastic deposits. The information obtained from AMA can be supplemented by the chemical composition of crystal-melt inclusions enclosed in zircon xenocrysts, which represent refractory relics of igneous xenoliths disintegrated during explosion.

2. Geological Setting The volcanoes studied belong to the Late Miocene–Late Pleistocene alkali basalt province of the Pannonian Basin (Figure1), which is an intra-Carpathian back-arc basin characterized in the northern part by a moderate crustal thickness (26–28 km) and a 70–90 km thick lithospheric mantle [10,11]. Spatially isolated monogenetic volcanoes of the alkali basalt province originated from the post-rift thermal subsidence superimposed on Middle Miocene, Alpine-type, gravity-driven subduction coincidental with the collision of the ALCAPA (ALpine-CArpathian-PAnnonian) and Tisza-Dacia microplates with the European plate [12]. The Miocene subduction was compensated by the diapiric rise of the asthenosphere and the decompression melting of the lithospheric mantle [12,13], giving rise to basanite, basalt-trachybasalt, and tephrite-phonotephrite magmas erupted in the time interval between 8 and 0.2 Ma [14,15]. The Pinciná maar (48◦21052.47” N, 19◦46017.04” E, Figure2), 760 × 930 m in area, was created by phreatomagmatic eruptions in the ﬂuvial-lacustrine environment of the Pontian Poltár formation of the LuˇcenecBasin—the northernmost promontory of the Pannonian Basin. The burial beneath the sediments preserved the Pinciná maar from erosion and de- struction [16]. The southern part of the maar is overlain by marine littoral and shallow shelf sediments of the Egerian Széczény Schlier. According to indirect geological evidence [17], the Pinciná maar was afﬁliated with the earliest Late Pannonian (~7–8 Ma) volcanic pulse. However, U-Pb-Th and (U-Th)/He geochronology applied to zircon, monazite, and apatite fragments isolated from volcanoclastic material and igneous xenoliths yielded consider- ably younger ages, 5.2–5.4 Ma, overlapping the Upper Miocene–Pliocene boundary [1,18]. Syenite xenoliths and detrital zircons with REE minerals mentioned in this study have been extracted from friable tuff deposits exposed in the south-western wall of the maar structure. The Hajnáˇckadiatreme (48◦1305.25” N, 19◦57018.59” E, Figure2) in the Cerov á Upland is a feeder conduit of a basaltic maar, towering about 75 m above the surrounding Quater- nary river terrace. The diatreme has been assigned to the early Romanian third volcanic phase [15], according to a 2.7 ± 0.5 Ma K–Ar whole-rock age of intersecting basalt dike [16]. The (U-Th-Sm)/He dating of apatite recovered from the block of friable basaltic tuff yielded an age of 2.1 ± 0.2 Ma [1]. The carbonatite syenite xenolith studied, about 2 × 3 cm in size, was isolated from the welded tuffaceous breccia exposed atop the diatreme. Minerals 2021, 11, 369 3 of 20 Minerals 2021, 11, x FOR PEER REVIEW 3 of 21

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Figure 1. Schematic drawing of the Carpathian arc and the intra-Carpathian back-arc basin (Pan- Figure 1. Schematic drawing of the Carpathian arc and the intra-Carpathian back-arc basin (Pannon- nonian Basin) modified after [19]. Explanations: 1. European (W) and Skythian (E) platforms; 2. ian Basin) modiﬁed after [19]. Explanations: 1. European (W) and Skythian (E) platforms; 2. foredeep, foredeep,Figure 1. Schematic3. Tertiary drawingaccretionary of the wedge, Carpathian 4. Klippen arc and belt, the 5. intra-CarpathianPalaeogene back-arc back-arc basin, basin 6. Variscan (Pan- 3. Tertiary accretionary wedge, 4. Klippen belt, 5. Palaeogene back-arc basin, 6. Variscan basement basementnonian Basin) with modified pre-Tertiary after sedimentary [19]. Explanations: cover and 1. European Mesozoic (W)nappe and units Skythian (Cretaceous (E) platforms; accretionary 2. withwedge),foredeep, pre-Tertiary 7. 3. Neogene Tertiary sedimentary andesites accretionary and cover wedge, rhyolites, and 4. Mesozoic Klippen8. Upper belt, Miocene–Quater nappe 5. Palaeo unitsgene (Cretaceousnary back-arc alkali basin, accretionarybasalt, 6. 9. Variscan Major wedge), 7.normal,basement Neogene strike-slip, with andesites pre-Tertiary and and thrust rhyolites,sedimentary faults. Inset 8. Uppercover shows and Miocene–Quaternary state Mesozoic boundaries. nappe Rectangle units alkali (Cretaceous marks basalt, the 9. accretionary south- Major normal, strike-slip,Slovakianwedge), 7. volcanic Neogene and thrust province andesites faults. enlarged and Inset rhyolites, shows in Figure state8. Upper 2. boundaries. Miocene–Quater Rectanglenary marks alkali thebasalt, south-Slovakian 9. Major volcanicnormal, strike-slip, province enlarged and thrust in faults. Figure Inset2. shows state boundaries. Rectangle marks the south- Slovakian volcanic province enlarged in Figure 2.

Figure 2. Enlarged view of the study area with volcanic phases inferred from geochronology

[1,14–16,18–20]. FigureFigure 2.2. EnlargedEnlargedview view of of the the study study area area with with volcanic volcanic phases phases inferred inferred from from geochronology geochronology [1,14 –16,18–20]. [1,14–16,18–20].

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3. Methods Chemical compositions of minerals and silicate glass were determined using a wave- length-dispersive (WDS) mode of CAMECA SX-100 electron probe micro-analyzer main- tained in the State Geological Survey of Dionýz Štúr, Bratislava, Slovakia. An accelerating voltage of 15 kV was used to optimize the spatial resolution and matrix correction factors, and minimize the sample surface damage. Beam diameters ranged between 2 µm and 5 µm for minerals and 20 µm for silicate glass (if possible). Excitation lines, crystals, cali- brants, average detection limits, and average standard deviations are summarized in the Table S1. Matrix effects were resolved using the X-PHI correction method [21]. Spectral lines unaffected by interferences were selected. When peak overlaps existed, empirical correction factors [22,23] were applied.

4. Results 4.1. Petrography and Mineralogy Syenite xenoliths in the pyroclastic deposits of Pinciná maar are either encapsulated in basalt bombs or occur as angular fragments and blocks up to 30 cm in size in friable lapilli tuffs. The syenites are almost monomineral feldspar-rich rocks devoid of maﬁc minerals, showing an equi-granular texture composed of feldspar interlocked with less than 10 vol.% of opaque brown to translucent yellow intergranular glass. The investigated samples, PI-12 and HP-3, belong to the group of less-oxidized syenite xenoliths, having 16 ± 1% Fe3+ relative to total iron in the silicate glass [5]. Whole-rock chemical analyses of the xenoliths are listed in the Table S2. The feldspar composition spans the interval from Na-sanidine, through anorthoclase, to albite. The silicic intergranular glass is quartz-normative peraluminous, occasionally metaluminous, trachyte to alkalic rhyolite. The opaque brown domains in the glass are enriched in FeOtot relative to the translucent parts by 2–3 wt.% and often contain an immiscible Fe-oxide phase [5]. Dark red to pink zircons, up to 1 mm in size, occur in the intergranular glass. All zircons exhibit simple combinations of prismatic (100) > (110) and pyramidal (101) forms characteristic of high-temperature alkaline series granites. Zircon fragments recovered from the friable white tuff (sample P-4) have the same typomorphic characteristics and represent refractory relics of smaller syenite xenoliths disintegrated during phreatomagmatic eruption. REE-bearing silicates and phosphates occur as daughter phases of silicate melt inclusions in zircons and crystal aggregates in feldspar-silicate melt matrix intimately intergrown or replaced by a hydrous Ce-phosphate (Figure3). Apart from zircon, the AMA of syenites from Pinciná maar also involves Mg-magnetite, corundum, apatite, titanite, monazite-Ce (Table S3), uranothorianite, uranothorite, and fergusonite (Table S4). The silicate glass enclosed in inclusions corresponds to potassic trachyte (Table S5). Syenite xenolith HA-8 recovered from the Hajnáˇckadiatreme is composed of K- feldspar, quartz, and interstitial silicate-carbonate glass. The xenolith texture is reminiscent of a hydraulic breccia with chaotically distributed, partially melted quartz, and feldspar grains cemented by the silicate glass and amoeboid Mg-calcite ocelli (Figure4). The feldspar compositions span the range from anorthoclase to Na-sanidine, the latter occurring along rims and contacts with the silicate-carbonate glass matrix. Rare maﬁc minerals are represented by the aluminian-ferroan aegirine-augite replaced by ferrian hedenbergite (Figure4a). Minerals 2021, 11, 369 5 of 20 Minerals 2021, 11, x FOR PEER REVIEW 5 of 21

FigureFigure 3.3.Back-scattered Back-scattered electron electron (BSE) (BSE) images images of of yttrium yttrium silicates silicates and and associated associated minerals minerals in in syenite sye- xenolithsnite xenoliths and volcanicand volcanic tuff ejectedtuff ejected in the in Pincinthe Pincináá maar. maar. (a) Part(a) Part of aof large a large zircon zircon crystal, crystal, 1 mm 1 mm in size,in size, in syenitein syenite xenolith xenolith showing showing numerous numerous inclusions inclusions of of silicate silicate glass glass (M) (M) and and crystal crystal aggregates aggre- composedgates composed of yttrium of yttrium silicate silicate (Y-Si), (Y-Si), magnetite magn (Mt),etite and (Mt), uranothorianite and uranothorianite (Ur). Inset (Ur). shows Inset shows close-up ofclose-up one of theof one crystal–glass of the crystal–glass aggregates. aggregates. (b) Magniﬁed (b) Magnified view of the view inclusion of the frominclusion the previous from the image, previ- showingous image, euhedral showing yttrium euhedral silicate, yttrium magnetite, silicate, and magnetite, silicate glass. and silicate (c,d) Zir0con glass. ( isolatedc,d) Zir0con from isolated volcanic from volcanic tuff with two bright yttrium silicate crystals embedded in silicate glass (M). The tuff with two bright yttrium silicate crystals embedded in silicate glass (M). The close-up documents close-up documents an elongated silicate glass inclusion with vapour bubble (V) and bright Ca-Y an elongated silicate glass inclusion with vapour bubble (V) and bright Ca-Y silicate (Y-Ca). Another silicate (Y-Ca). Another yttrium silicate (Y-Si) embedded in silicate melt is documented in (d). (e,f) yttriumBSE images silicate documenting (Y-Si) embedded the close in silicate spatial melt relationship is documented of REE-Ti in (d silicate). (e,f) BSE(Re-Ti), images Y-Ca documenting silicate (Y- theCa), close and spatialCa-Ce phosphate relationship (Ca-Ce) of REE-Ti in syenite silicate xeno (Re-Ti),lith HP-3. Y-Ca Dark silicate matrix (Y-Ca), is composed and Ca-Ce of phosphate potas- (Ca-Ce)sium feldspar in syenite (Kfs) xenolith and silicate HP-3. glass Dark (M). matrix is composed of potassium feldspar (Kfs) and silicate glass (M). The silicate glass domains that appear dark in BSE contrast represent molten feldspars,The as silicate is documented glass domains by the that almost appear identical dark in chemical BSE contrast compositions represent moltenof both, feldspars, and neg- asligible is documented amounts of by Fe,Ti,Mg-oxides. the almost identical Brighter chemical domains compositions with increased of both, Fe, Ti, and and negligible Mg and amountslower Si ofcontents Fe,Ti,Mg-oxides. correspond Brighter to potassic domains trachyte with (Table increased S5). Fe, Ti, and Mg and lower Si contentsThe correspond carbonate ocelli to potassic show trachyte polygonal (Table fibrous S5). texture in transmitted light, and fine domain zoning in back-scattered electron images (Figure 4a,c,d). Some ocelli contain arag-

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onite crystals growing from the silicate glass-carbonate and feldspar-carbonate boundaries. Small aragonite crystals appear freely suspended in the ocelli without any preferred Minerals 2021, 11, 369 orientation (Figure 4d). X-ray and Raman mapping [24] revealed an incremental oscilla-6 of 20 tory growth zoning of aragonite, contrasting with an irregular patchy domain microtexture of the associated Mg-calcite.

FigureFigure 4.4. Back-scatteredBack-scattered electronelectron imagesimages ofof accessoryaccessory mineralsminerals inin carbonatite-syenitecarbonatite-syenite xenolith HA-8HA- 8 from Hajnáčka diatreme. (a) Microscopic view documenting rounded calcite blebs embedded in a from Hajnáˇckadiatreme. (a) Microscopic view documenting rounded calcite blebs embedded in a silicate glass matrix and partly resorbed grains of titanite and aegirine replaced by hedenbergite. (b) silicate glass matrix and partly resorbed grains of titanite and aegirine replaced by hedenbergite. Coronal rutile replacing resorbed ilmenite crystals. (c) Apatite globule embedded within silicate (bglass,) Coronal partly rutile encompassed replacing by resorbed carbonate. ilmenite Enlarged crystals. view (c in) Apatite the inset globule documents embedded a growth within zoning silicate of glass,the resorbed partly encompassed crystal. Deformation by carbonate. of calcite Enlarged around view the inglobular the inset apatite documents testifies a growthto the carbonate zoning of theliquid. resorbed (d) Aragonite crystal. crystals Deformation grown of from calcite the around interfac thee between globular silicate apatite and testiﬁes carbonate to the phases. carbonate One liquid.of the two (d) Aragonite larger aragonites crystals was grown detached from the from interface the original between positi silicateon during and carbonate the carbonate phases. liquid One ofmovement. the two larger Perfect aragonites euhedral wasshapes detached indicate from the theequilibrium original positioncrystallization during from the carbonatethe carbonatite liquid movement.liquid. (e) Zirconolite–baddeleyite Perfect euhedral shapes aggregate indicate in the the equilibrium interstitial glass crystallization matrix. (f) from Domain-zoned the carbonatite zirconolite embedded in silicate glass. Brighter domains are enriched in Th and REE, whereas less liquid. (e) Zirconolite–baddeleyite aggregate in the interstitial glass matrix. (f) Domain-zoned bright domains are enriched in Ti and Ca. Abbreviations: Aeg—aegirine-augite, Ap—apatite, Arg— zirconolitearagonite, embeddedBdy—baddeleyite, in silicate Cal—calcite, glass. Brighter Hed—hedenbergite, domains are enriched Kfs—eldspar, in Th andM—silicate REE, whereas glass, lessTn—titanite, bright domains Qz—quartz, are enriched Zrn—zirconolite. in Ti and Ca. Abbreviations: Aeg—aegirine-augite, Ap—apatite, Arg—aragonite, Bdy—baddeleyite, Cal—calcite, Hed—hedenbergite, Kfs—eldspar, M—silicate glass, Tn—titanite, Qz—quartz, Zrn—zirconolite.

The carbonate ocelli show polygonal ﬁbrous texture in transmitted light, and ﬁne domain zoning in back-scattered electron images (Figure4a,c,d). Some ocelli contain aragonite crystals growing from the silicate glass-carbonate and feldspar-carbonate boundaries. Small aragonite crystals appear freely suspended in the ocelli without any preferred orientation (Figure4d). X-ray and Raman mapping [ 24] revealed an incremental oscillatory Minerals 2021, 11, 369 7 of 20

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growth zoning of aragonite, contrasting with an irregular patchy domain microtexture of the associatedThe AMA Mg-calcite. of syenite xenolith HA-8 consists of ilmenite, rutile, titanite, barite, baddeleyite,The zircon, AMA apatite, of syenite and xenolith zirconolite. HA-8 Manganoan consists of ilmenite,to magnesian rutile, ilmenite titanite, with barite, 1–2 badde- wt.% leyite, zircon, apatite, and zirconolite. Manganoan to magnesian ilmenite with 1–2 wt.% Nb2O5 is replaced along grain boundaries by Nb-rutile symplectite (Figure 4b) with 2.3– Nb O is replaced along grain boundaries by Nb-rutile symplectite (Figure4b) with 7.32 wt.%5 Nb2O5 (Table S6). 2.3–7.3Euhedral wt.% Nb fluorapatite2O5 (Table S6).is enclosed in feldspars. Resorbed apatites occur in the intersti- Euhedral ﬂuorapatite is enclosed in feldspars. Resorbed apatites occur in the interstitial silicate glass. Both morphological types exhibit growth zoning with the brighter core tial silicate glass. Both morphological types exhibit growth zoning with the brighter core enriched in REE and darker margins depleted in REE (Figure 4c, Table S7). enriched in REE and darker margins depleted in REE (Figure4c, Table S7). Titanite creates rounded resorbed grains in the silicate-carbonate glass matrix. BSE Titanite creates rounded resorbed grains in the silicate-carbonate glass matrix. BSE images revealed complex titanite zoning. Nb-poor cores with primary magmatic oscilla- images revealed complex titanite zoning. Nb-poor cores with primary magmatic oscillatory zoning are replaced from grain boundaries by at least four metasomatic zones with tory zoning are replaced from grain boundaries by at least four metasomatic zones with gradually increasing (Nb,Ta)2O5 and ZrO2 contents, reaching the maximum concentra- gradually increasing (Nb,Ta) O and ZrO contents, reaching the maximum concentrations tions of up to 6.9 and 1.0 wt.%,2 5 respectively,2 in the marginal zone. The total YREE oxide of up to 6.9 and 1.0 wt.%, respectively, in the marginal zone. The total YREE oxide content content attains 4.5 wt. % (Table S8). attains 4.5 wt. % (Table S8). Zirconolite (Figure 4e,f) forms irregular opaque grains with brown translucent mar- Zirconolite (Figure4e,f) forms irregular opaque grains with brown translucent margins enclosedgins enclosed in the in interstitialthe interstitial glass glass or rock-formingor rock-forming feldspar. feldspar. Some Some zirconolite zirconolite grains grains are are intergrown with baddeleyite (Figure 4e,f), which contains 1.3–2.3 wt.% Nb2O5 + Ta2O5 and intergrown with baddeleyite (Figure4e,f), which contains 1.3–2.3 wt.% Nb 2O5 + Ta2O5 and 1.4–1.8 wt.% HfO2 (Table S9). The zirconolites enclosed in feldspars are closely spatially 1.4–1.8 wt.% HfO2 (Table S9). The zirconolites enclosed in feldspars are closely spatially associated with zircon with an HfO2 content up to 1.6 wt.% (Table S9). associated with zircon with an HfO2 content up to 1.6 wt.% (Table S9).

4.2.4.2. SubstitutionsSubstitutions inin REE-BearingREE-Bearing SilicatesSilicates andand PhosphatesPhosphates 4.2.1.4.2.1. ChevkiniteChevkinite ElectronElectron probeprobe micro-analyses of of REE-Ti-silicates REE-Ti-silicates from from Pinciná Pincin á(Table(Table S10) S10) recalcu- recal- culatedlated on on the the basis basis of of 22 22 (O,F,Cl) (O,F,Cl) correspond to anan almostalmost idealideal chevkinitechevkinite formulaformula 2+ 3+ 2+ (REE,Ca,Th)(REE,Ca,Th)44FeFe2+(Fe3+,Fe,Fe2+,Ti,Al,Mn),Ti,Al,Mn)2Ti2Ti2Si2Si4O4O22,22 with, with perfect perfect substitution substitution in in the crystallo-crystallo- 3+ graphicgraphic sitessitesA Aand andC Caccording according to to the the scheme: scheme: (Ca (Ca + + Sr) Sr)AA+ + (Ti (Ti + + Zr) Zr)CC == (Y(Y ++ REE)AA + ( (MM3+ + 2+ 3+ 2+ +MM2+)C), Cwhere, where MM3+ andand M2+M referrefer to tri- to and tri-and divalent divalent cations, cations, respectively respectively [25] [(Figure25] (Figure 5). The5). Thetotal total sum sum of cations of cations is greater is greater than than 13, 13, thus thus indicating indicating the the presence presence of ferric iron,iron, withwith calculatedcalculated concentrations concentrations between between 0.3 0.3 and and 1 apfu,1 apfu, along along with with ferrous ferrous iron. iron. Lower Lower analytical analyt- totalsical totals testify testify to the to the presence presence of eitherof either OH OH or or metamictization. metamictization. However, However, FeO FeO and and CaO CaO variationsvariations coincide coincide with with those those in in primary primary magmatic, magmatic, non-altered non-altered chevkinite-Ce chevkinite-Ce (Figure (Figure6a), and6a), theand same the same is indicated is indicated by the by negativelythe negatively correlated correlated TiC andTiC and Th contentsTh contents (Figure (Figure6b). 6b).

FigureFigure 5.5.Correlation Correlation betweenbetweenA A-- andandC C-site-site cations cations (apfu) (apfu) in in chevkinite-Ce chevkinite-Ce from from Pincin Pinciná.á.

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Figure 6. (a,b) Chevkinite-perrietite discrimination diagrams [26,27] with fields for non-altered chevkinite (1), perrierite FigureFigure 6.6.( a(a,b,b)) Chevkinite-perrierite Chevkinite-perrietite discrimination discrimination diagrams diagrams [26 [26,27],27] with with fields fields for for non-altered non-altered chevkinite chevkinite (1), perrierite(1), perrierite (2), (2), and hydrothermally altered chevkinite and perrierite (3) from A-type syenites and granites of Brasil [28]. (c) Correla- and(2), hydrothermallyand hydrothermally altered altered chevkinite chevkinite and and perrierite perrierite (3) from (3) from A-type A-type syenites syenites and and granites granites of Brasil of Brasil [28]. [28]. (c) Correlation(c) Correla- tion between titanium and thorium contents (apfu), illustrating compositional trends in post-magmatic (2) and primary betweention between titanium titanium and thorium and thorium contents contents (apfu), illustrating(apfu), illustrati compositionalng compositional trends in trends post-magmatic in post-magmatic (2) and primary (2) and magmatic primary magmatic (1) chevkinite [28]. Squares are projection points of chevkinites from Pinciná. (1)magmatic chevkinite (1) [chevkinite28]. Squares [28]. are Squares projection are pointsprojec oftion chevkinites points of chevkinites from Pincin fromá. Pinciná. 4.2.2. Yttrialite–britholite-Y 4.2.2.4.2.2. Yttrialite–britholite-YYttrialite–britholite-Y Two groups of yttrium silicates with contrasting Ca and Th contents have been dis- TwoTwo groupsgroups ofof yttriumyttrium silicatessilicates withwith contrastingcontrasting CaCa andand ThTh contentscontents havehave beenbeen dis-dis- cerned in the Piciná maar. The first group is typical of high Th + U (13–20 wt.% oxide) and cernedcerned inin thethe PicinPicináá maar.maar. TheThe firstfirst groupgroup isis typicaltypicalof ofhigh highTh Th+ + UU (13–20(13–20 wt.%wt.% oxide)oxide) andand low CaO (0.5–2 wt.%) contents. Analytical totals close to 100% indicate a non-metamict lowlow CaOCaO (0.5–2(0.5–2 wt.%)wt.%) contents.contents. AnalyticalAnalytical totalstotals closeclose toto 100%100% indicateindicate aa non-metamictnon-metamict structure without molecular water or hydroxyl groups (Table S11). The crystal formula structurestructure withoutwithout molecularmolecular waterwater oror hydroxylhydroxyl groupsgroups (Table(Table S11).S11). TheThe crystalcrystal formulaformula calculated on the basis of seven oxygen atoms, (Y0.97–1.13REE0.34-0.40(Fe,Ca,Mn,Mg)0.22– calculatedcalculated onon the the basis basis of sevenof seven oxygen oxygen atoms, atoms, (Y0.97–1.13 (YREE0.97–1.130.34–0.40REE0.34-0.40(Fe,Ca,Mn,Mg)(Fe,Ca,Mn,Mg)0.22–0.340.22– 0.34(Th,U)0.23–0.32(Si,Ti)1.98–2O7, corresponds to yttrialite-(Y). The mineral is characterized by (Th,U)0.34(Th,U)0.23–0.320.23–0.32(Si,Ti)(Si,Ti)1.98–21.98–2OO77, corresponds to yttrialite-(Y).yttrialite-(Y). TheThe mineralmineral isis characterizedcharacterized byby the dominant coupled substitution M2+2+ + (U,Th)4+ 4+↔ (Y,REE)3+. 3+The chemical analyses are thethe dominantdominant coupledcoupled substitution M2+ ++ (U,Th) (U,Th)4+ ↔↔ (Y,REE)(Y,REE)3+. The. The chemical chemical analyses analyses are almost ideally aligned along the tie-line connecting the pure yttrialite (Y,REE)2Si2O7 and a arealmost almost ideally ideally aligned aligned along along the tie-line the tie-line connecting connecting the pure the pure yttrialite yttrialite (Y,REE) (Y,REE)2Si2O27Si and2O 7a hypothetical M2+(U,Th)Si2+ 2O7 endmember, the latter reaching 15–35 mol.% in the investi- andhypothetical a hypothetical M2+(U,Th)SiM (U,Th)Si2O7 endmember,2O7 endmember, the latter the reaching latter reaching 15–35 mol.% 15–35 in mol.% the investi- in the gated mineral (Figure 7). The high-Th yttrium silicates from Pinciná are similar to those investigatedgated mineral mineral (Figure (Figure 7). The7). high-Th The high-Th yttrium yttrium silicates silicates from fromPinciná Pincin are similará are similar to those to identified in zircon megacryst from the Fiľakovo maar [1], which are also displayed in thoseidentified identified in zircon in zircon megacryst megacryst from from the Fi theľakovo Fil’akovo maar maar [1], [which1], which are are also also displayed displayed in Figure 7 for comparison. inFigure Figure 7 7for for comparison. comparison.

Figure 7. Molecular proportions in high- and low-Th yttrium silicates (open squares and black Figure 7. Molecular proportions in high- and low-Th yttrium silicates (open squares and black diamonds,Figure 7. Molecular respectively) proportions recalculated in high-from formulae and low-Th based yttrium on seven silicates oxygen (open atoms. squares Re and and Ac blackrefer diamonds, respectively) recalculated from formulae based on seven oxygen atoms. Re and Ac refer todiamonds, Y + REE and respectively) Th + U, respectively. recalculated M from2+ are formulae large divalent based cations on seven (F oxygene, Ca, Mn, atoms. Mg).Re Dataand fromAc refer to Y + REE and Th + U, respectively. M2+ are large divalent cations (Fe, Ca, Mn, Mg). Data from Fitoľ Yakovo + REE maar and (shaded Th + U, squares) respectively. are fromM2+ [1].are large divalent cations (Fe, Ca, Mn, Mg). Data from Fiľakovo maar (shaded squares) are from [1]. Fil’akovo maar (shaded squares) are from [1]. The second group of yttrium silicates is characterized by lower ThO2 (1.2–3.8 wt.%), The second group of yttrium silicates is characterized by lower ThO2 (1.2–3.8 wt.%), medium-to-high CaO concentrations (2.9–4.9 wt.% in syenite PI-12, 9.2–10.9 wt.% in sye- medium-to-high CaO concentrations (2.9–4.9 wt.% in syenite PI-12, 9.2–10.9 wt.% in sye-

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The second group of yttrium silicates is characterized by lower ThO2 (1.2–3.8 wt.%), medium-to-high CaO concentrations (2.9–4.9 wt.% in syenite PI-12, 9.2–10.9 wt.% in syenite HP-3),nite HP-3), and elevated and elevated amounts amounts of halogens of halogens (0.5–1.3 (0.5–1.3 wt.%) (Tablewt.%) S12).(Table Molecular S12). Molecular proportions pro- recalculatedportions recalculated using seven using oxygen seven atoms oxygen yield atom excessives yield excessive cations (>4), cations and (>4), a different and a differ- slope ofent the slope exchange of the exchange vector in vector the M in2+ the–(U,Th) M2+–(U,Th)4+–(Y,REE)4+–(Y,REE)3+ correlation3+ correlation diagram diagram (Figure (Figure7) compared7) compared to theto the first first group. group. Atomic Atomic proportions proportions based based on on eight eight cations cations plotted plotted inin thethe MM2+2+ ++ (U,Th) (U,Th)4+4+ versusversus (Y,REE) (Y,REE)3+ 3+coordinatescoordinates show show almost almost ideal ideal correlation correlation (r2 =(r 0.98)2 = 0.98)indi- indicatingcating the theheterovalent heterovalent substitution substitution Re3+Re =3+ 4 =− 40.5(− M0.5(2+ M+ 2+Ac+4+)Ac (Figure4+) (Figure 8) contrasting8) contrasting with withthe exchange the exchange vector vector of Re of3+ Re= 3+2 −= (M 2 −2+ +(M Ac2+4++) inAc the4+) inTh-rich the Th-rich yttrialite. yttrialite. The observed The observed trend trendreflects reflects the probable the probable solid solidsolution solution of two of yttrialite two yttrialite molecules molecules and one and “britholite-Y” one “britholite-Y” mol- moleculeecule (i.e., (i.e., 2Y2Si 2Y2O2Si7 ↔2O Y7 3↔CaTh(SiOY3CaTh(SiO4)3(OH)4)33).(OH) The3 ).77.3–86.7 The 77.3–86.7 mol.% of mol.% the yttrialite of the yttrialite compo- componentnent defines defines the yttrialite-Y the yttrialite-Y in the in syenite the syenite PI-12, PI-12, contrasting contrasting with with 50.4–60.6 50.4–60.6 mol.% mol.% of the of thebritholite-Y britholite-Y component component in inthe the syenite syenite HP-3. HP-3. Th Thee yttrium yttrium silicate silicate from zirconzircon xenocrystxenocryst extractedextracted fromfrom tufftuff (sample(sample P-4)P-4) alsoalso correspondscorresponds toto britholite-Y,britholite-Y, withwith 68.168.1 mol.%mol.% ofof thethe britholite-Ybritholite-Y component.component.

FigureFigure 8.8. SubstitutionsSubstitutions inin low-Thlow-Th yttriumyttrium silicatessilicates fromfrom PincinPincináá maar inferred from atomic propor- 2+ tionstions recalculatedrecalculated using eight cations. cations. ReRe andand AcAc referrefer to to Y Y+ REE + REE and and Th Th + U, + respectively. U, respectively. M2+ M designatesdesignates largelarge divalentdivalent cationscations (Fe,(Fe, Ca,Ca, Mn,Mn, Mg). Mg). 4.2.3. Britholite-Ce 4.2.3. Britholite-Ce Representative analyses of silicates dominated by light rare earth elements as- Representative analyses of silicates dominated by light rare earth elements associated sociated with chevkinite-Ce are listed in Table S13. Atomic proportions recalculated with chevkinite-Ce are listed in Table S13. Atomic proportions recalculated from formulae from formulae based on eight cations plot along the britholite-apatite exchange vec- torbased(Ca on + P)eight = ~7.86 cations− ( Replot+ along Si) [29 the]. The britholite-apatiteRe + Si values exchange between vector 6.40 and (Ca 6.64+ P) indicate= ~7.86 − 82–85.1(Re + Si) mol.% [29]. Thebritholite-Ce Re + Si values component between in the6.40 Ca-Ce-silicate and 6.64 indicate from 82–85.1 syenite HP-3mol.% (Figure britholite-9a). Ce component in the Ca-Ce-silicate from syenite HP-3 (Figure 9a). The La-Nd silicates in the syenite PI-12 contain more ThO2 (1.1–7.3 wt.%) and have The La-Nd silicates in the syenite PI-12 contain more ThO2 (1.1–7.3 wt.%) and have variable P2O5 (6.6–26.8 wt.%), SiO2 (1.5–20 wt.%), and CaO (7.3–13.2 wt.%) contents com- paredvariable to theP2O syenite5 (6.6–26.8 HP-3. wt.%),Re + SiSiO values2 (1.5–20 between wt.%), 2.9 and and CaO 5.8correspond (7.3–13.2 wt.%) to 37.5–74.6 contents mol.% com- britholite-Ce.pared to the syenite The perfect HP-3. 2Ca Re =+ ThSi values + U substitution between 2.9 contrasting and 5.8 correspond with unsystematic to 37.5–74.6 Ca versusmol.% Pbritholite-Ce. ratios and the The position perfect of 2Ca projection = Th + U points substitution between contrasting the huttonite–monazite with unsystematic and yttrialite–britholite-YCa versus P ratios and exchange the position vectors of in projection the (M2+ + pointsAc) versus betweenRe diagram the huttonite–monazite (Figure9b) indi- cateand ayttrialite–britholite-Y rather complex interplay exchange of monazite, vectors huttonite, in the (M britholite-Ce,2+ + Ac) versus and Re apatite diagram molecules. (Figure The9b) intersectionindicate a rather of yttrialite–britholite-Y complex interplay of and monazite, the britholite-Ce–apatite huttonite, britholite-Ce, exchange and vectors apatite (Figuremolecules.9a) indicativeThe intersection of the involvement of yttrialite–britholite-Y of the yttrialite and molecule the britholite-Ce–apatite in the complex REE- exchange vectors (Figure 9a) indicative of the involvement of the yttrialite molecule in the

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silicate-phosphatecomplex REE-silicate-phosphate are also noteworthy. are also A notewo more preciserthy. A analysismore precise of the analysis mixing of trends the mix- is precludeding trends by is theprecluded insufﬁcient by the number insufficient of analyses number caused of analyses by the caused mineral by scarcity the mineral and small scar- dimensionscity and small of mineral dimensions grains, of making mineral the grains, EPMA making analyses the tricky, EPMA as the analyses position tricky, of electron as the beamposition changes of electron with switching beam changes between with two switching different between sample two currents. different sample currents.

FigureFigure 9. 9.Substitutions Substitutions inin britholite,britholite, yttrialite, and mo monazitenazite from from Pinciná Pinciná mmaar,aar, and and apatite apatite from from HajnHajnááˇckadiatreme,čka diatreme, inferredinferred fromfrom atomicatomic proportionsproportions recalculated using using eight eight cations. cations. ReRe andand AcAc 2+ referrefer to to Y Y + + REE REE and and Th Th + + U, U, respectively. respectively.M M2+ are are large large divalent divalent cations cations (Fe, (Fe, Ca, Ca, Mn, Mn, Mg). Mg). (a ()aRe) Re+ Si+ Si versus Ca + P correlation. (b) M2+ + Ac versus Re correlation, showing the arrangement of mona- versus Ca + P correlation. (b) M2+ + Ac versus Re correlation, showing the arrangement of monazites zites from Pinciná maar along the monazite-huttonite tie-line. from Pinciná maar along the monazite-huttonite tie-line.

4.2.4.4.2.4. Ca-CeCa-Ce Phosphate Phosphate RepresentativeRepresentative analysesanalyses of hydrous Ca-Ce Ca-Ce phosphates phosphates asso associatedciated with with britholite-Ce britholite- Ceand and chevkinite-Ce chevkinite-Ce are arelisted listed in Table in Table S14. S14.A crystalchemical A crystalchemical formula formula based basedon four on oxygen four oxygenatoms returned atoms returned approximately approximately two cations, two cations, thus resembling thus resembling a rhabdophane-like a rhabdophane-like phase. phase.However, However, low analytical low analytical totals (81–88 totals (81–88wt.%) in wt.%) the analyzed in the analyzed phosphates phosphates indicate indicate 2–3 wa- 2–3ter watermolecules molecules compared compared to ideal to idealrhabdophane rhabdophane CePO CePO4.H2O4.H with2O with7.12 7.12wt.% wt.% H2O. H The2O. Thehigh highCaO CaOcontents contents (5.3–7.2 (5.3–7.2 wt.%, wt.%, 0.23–0.37 0.23–0.37 apfu) apfu)are also are unusual, also unusual, possibly possibly indicating indicating a greyite a greyitecomponent component (Ca,Ce,Th)PO (Ca,Ce,Th)PO4.H2O. However,4.H2O. However, the charge the of charge divalent of divalent cations is cations not counter- is not counterbalancedbalanced by a sufficient by a sufficient amount amount of large of tetravalent large tetravalent cations cations (Th and (Th U). and In U). addition, In addition, up to up1 wt.% to 1 wt.% halogens halogens may may be associated be associated with with structurally structurally bound bound OH. OH. Hence, Hence, identification identification of ofthe the hydrous hydrous Ca-Ce Ca-Ce phosphate phosphate remains remains only only provisional, provisional, demanding demanding support support from fromaddi- additionaltional spectroscopic spectroscopic methods. methods. 4.2.5. Zirconolite 4.2.5. Zirconolite The zirconolites from Hajnáčka diatreme (Table S15) are characterized by high Y + The zirconolites from Hajnáˇckadiatreme (Table S15) are characterized by high Y REE contents, 0.20–0.41 apfu, accommodated in the eight-fold-coordinated A-site by the + REE contents, 0.20–0.41 apfu, accommodated in the eight-fold-coordinated A-site by incorporation of charge-balancing divalent (mostly Fe2+) and pentavalent (Nb + Ta) cations the incorporation of charge-balancing divalent (mostly Fe2+) and pentavalent (Nb + Ta) cationsin the infive- the or five- six-fold-coordinated or six-fold-coordinated C-site.C -site.Up to Up 0.13 to 0.13apfu apfu of actinides of actinides in the in theA-siteA-site are 4+ areaccommodated accommodated by substituting by substituting charge-bal charge-balancingancing ferrous ferrous iron irontogether together with withTi in Ti the4+ in C- thesite.C -site. TheThe chemical chemical substitutions substitutions in zirconolitesin zirconolites are characterizedare characterized by the by following the following exchange ex- vectors,change reflectingvectors, reflecting heterovalent heterovalent substitutions substitutions in several in sites several (Figure sites 10 (Figure): 10): 1. Re3+ + M5++M2+ ↔ Ca2+ + 2Ti4+ (r2 = 0.938), 2. Re3+ + M5+ ↔ Zr4+ + Ti4+ (r2 = 0.9325), 3. Re3+ + 2Ti4+ ↔ Zr4+ + M5++M2+ (r2 = 0.8387), 4. M5+ + M2+ ↔ Ti4+ + M3+ (r2 = 0.892), 5. Ac4+ + Ti4+ ↔ Re3+ + M5+ (r2 = 0.8608), 6. Ac4+ + M2+ ↔ Ca2+ + Ti4+ (r2 = 0.720), 7. M5+ + M3+ ↔ 2Ti4+ (r2 = 0.679),

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8. Ac4+ + Ti4+ + M2+ ↔ Ca2+ + M5+ + M3+ (r2 = 0.5483). Minerals 2021, 11, 369 11 of 20 The exchange vectors define the compositional space bound by AcZrTiM2+O7, CaZrTi2O7, ReZrM5+M2+O7, and CaZrM5+M3+O7 endmembers [30].

Figure 10. Figure 10. ImportantImportant substitutionssubstitutions inin zirconoliteszirconolites fromfrom HajnHajnááˇcka.čka. Substitution numbers refer to exchangeexchange vectors vectors defined defined in in [27 [27]] and and Figure Figures 11a,b. 11a,b. 1. Re3+ + M5++M2+ ↔ Ca2+ + 2Ti4+ (r2 = 0.938), The Fe3+/Fetot ratios recalculated from stoichiometry based on four cations and the 2. Re3+ + M5+ ↔ Zr4+ + Ti4+ (r2 = 0.9325), cationic charge equal to 14 [31] range between 0.10 and 0.45, showing the prevalence of 3. Re3+ + 2Ti4+ ↔ Zr4+ + M5++M2+ (r2 = 0.8387), 2+ ferrous4. M over5+ + Mferric2+ ↔ iron,Ti4+ and+ M the3+ (r general2 = 0.892), prevalence of other than Ca + Sr M cations over 3+ 3+ 2+ 3+ M cations5. Ac4+ +(Al, Ti4+ Bi,↔ FeRe).3+ Owing+ M5+ (rto2 the= 0.8608), prevalence of M over M and the high correlation 2 coefficients6. Ac4+ +ofM corresponding2+ ↔ Ca2+ + Ti 4+exchange(r2 = 0.720), vectors (r = 0.72–0.94), the Hajnáčka zirconolites can be7. Mclassified5+ + M3+ using↔ 2Ti triangle4+ (r2 = 0.679),(b) in Figure 11. 8.TheAc 4+following+ Ti4+ + end-membersM2+ ↔ Ca2+ + orderedM5+ + M by3+ the(r2 =decreasing 0.5483). abundance occur in Hajnáčka 2+ zirconolites:The exchange 1. vectors(Y,REE)Zr(Nb,Ta)(Fe,Mg,Mn)O define the compositional space7, bound2. (Ca,Sr)ZrTi by AcZrTiM2O7,O 7, 3. 5+ 3+2+ 5+ 3+ CaZrTi(Ca,Sr)Zr(Nb,Ta)(Fe2O7, ReZrM M, Al,Bi)OO7, and7, CaZr4. (U,Th)ZrTi(Fe,Mg,Mn)OM M O7 endmembers7, and [30]. 5. (Ca,Sr)Ti3O7. The first 3+ fourThe components Fe /Fetot areratios typical recalculated of natural from zirconolites. stoichiometry The CaTi based3O on7 component four cations known and the only cationicas synthetic charge zirconolite equal to 14[30] [31 attains] range up between to 16.6 0.10mol.%, and and 0.45, is showing locally the the third prevalence most abun- of ferrousdant component. over ferric Proportions iron, and the of generalother theo prevalenceretical endmembers of other than are Ca below + Sr 2.7M2+ mol.%.cations over MThe3+ zirconolitecations (Al, compositions Bi, Fe3+). Owing from to Hajná the prevalencečka diatreme of Mproject2+ over withinM3+ theand triangle the high area 2 correlationbounded by coefficients REE-rich members of corresponding of the zirconolite exchange group—that vectors (r = 0.72–0.94),is, steffanweissite, the Hajn laachite,áˇcka zirconolitesand nöggerathite can be classified(Figure 11c). using One triangle outlier (b) falls in Figure outside 11 the. field, close to the Ca-apex diagnostic of the common REE-depleted zirconolite CaZrTi2O7. Other Hajnáčka zirconolites differ from the holotypes described from the Tertiary Laacher See volcano by lower Mn,

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The following end-members ordered by the decreasing abundance occur in Hajnáˇcka 3+ zirconolites: 1. (Y,REE)Zr(Nb,Ta)(Fe,Mg,Mn)O7, 2. (Ca,Sr)ZrTi2O7, 3. (Ca,Sr)Zr(Nb,Ta)(Fe , Al,Bi)O7, 4. (U,Th)ZrTi(Fe,Mg,Mn)O7, and 5. (Ca,Sr)Ti3O7. The ﬁrst four components are typical of natural zirconolites. The CaTi3O7 component known only as synthetic zirconolite [30] attains up to 16.6 mol.%, and is locally the third most abundant component. Proportions of other theoretical endmembers are below 2.7 mol.%. The zirconolite compositions from Hajnáˇckadiatreme project within the triangle area bounded by REE-rich members of the zirconolite group—that is, steffanweissite, Minerals 2021, 11, x FOR PEER REVIEWlaachite, and nöggerathite (Figure 11c). One outlier falls outside the ﬁeld, close12 to of the21

Ca-apex diagnostic of the common REE-depleted zirconolite CaZrTi2O7. Other Hajnáˇcka zirconolites differ from the holotypes described from the Tertiary Laacher See volcano by lowerNb, and Mn, Ce, Nb, and and higher Ce, Th and + higherU contents Th + (excep U contentst for laachite). (except forThe laachite). contents of The dominant contents ofcations dominant are similar cations to nöggerathite, are similar to despite nöggerathite, the proportions despite of the eight-fold-coordinated proportions of eight-fold- cat- coordinatedions, except Ti, cations, project except between Ti, project stefanweissite between and stefanweissite laachite. and laachite.

FigureFigure 11. ClassificationClassiﬁcation diagrams for M 3+-dominated-dominated (a (a) )and and M M2+2+-dominated-dominated zirconolites zirconolites (b ()b based) based onon endmember proportions proportions (mol.%) (mol.%) calculated calculated from from crystal crystal formulae formulae based based on onfour four cations cations and and a a cationiccationic charge equal equal to to 14, 14, with with projection projection points points of zirconolites of zirconolites from from Hajná Hajnčkaá (blackˇcka(black squares). squares). Numbers on tie-lines refer to the substitutions documented in Figure 10. Boundaries are con- Numbers on tie-lines refer to the substitutions documented in Figure 10. Boundaries are constructed structed using the dominant valency rule [32]. (c) Classification diagram based on atomic propor- usingtions of the eightfold-coordinated dominant valencyrule cations, [32]. with (c) Classiﬁcationprojection points diagram of zirconolites based on from atomic Hajná proportionsčka (black of eightfold-coordinatedsquares), steffanweissite, cations, laachite, with and projection nöggerathite points holotypes of zirconolites from the from Tertiary Hajná Laacherˇcka(black See squares), vol- steffanweissite,cano of the Eifel laachite,region, Germany and nöggerathite [33–35]. Bounda holotypesries are from constructed the Tertiary using Laacher the dominant See volcano compo- of the Eifelnent region,rule [32]. Germany [33–35]. Boundaries are constructed using the dominant component rule [32].

4.3.4.3. Chemical Composition of of Silicate Silicate Glass Glass Zircon-hosted glass inclusions large large enough enough to to be be analyzed analyzed by by the the defocused defocused electron electron beambeam yielded a peraluminous alkalic alkalic trachyte trachyte co composition,mposition, similar similar to to that that of of interstitial interstitial glassglass (Figure 1212,, Table S5). The The silicate silicate glass glass a associatedssociated with with yttrialite yttrialite and and britholite-Y britholite-Y was was aa metaluminous alkali-calcic alkali-calcic trachyte. trachyte. The The inters interstitialtitial glass glass associated associated with withzirconolite zirconolite cor- correspondedresponded to strongly to strongly alkalic alkalic metaluminous metaluminous trachyte. trachyte. In summary, summary, silicate silicate glasses glasses from from Pinciná Pincin andá and Hajná Hajnčkaá ˇckaareare dominantly dominantly potassic potassic and andferroan ferroan trachytes trachytes with withvariable variable alkalinity alkalinity and aluminosity. and aluminosity. RatherRather peculiar peculiar are the arepres- the presenceence of quartz of quartz in the in strongly the strongly alkaline alkaline syenite syenite xenolith xenolith from from Hajná Hajnčkaá andˇckaand the absence the absence of ofquartz quartz in inthe the quartz-normative quartz-normative syenite syenite xenolith xenolith from from Pinciná. Pinciná Compositions. Compositions of of silicate silicate meltmelt inclusions indicate trachytic trachytic parental parental melts, melts, which which are are more more alkaline alkaline in in Hajná Hajnčákaˇcka comparedcompared to PincinPinciná.á.

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FigureFigure 12.12. ClassificationClassification diagrams diagrams of of magmatic magmatic rocks rocks based based on onmajor major elements elements [36–38], [36– 38with], with projec- projec- tiontion pointspoints ofof silicate glass glass from from Pinciná Pincin á(PI,(PI, HP, HP, P) P) and and Hajná Hajnčákaˇcka(HA, (HA, MKD) MKD) (Table (Table S5). S5). A/NKCA/NKC = = Al2O3/(CaO + Na2O + K2O–1.67P2O5), NK/A = (Na2O + K2O)/Al2O3, MALI = Na2O + K2O–CaO. Black Al O /(CaO + Na O + K O–1.67P O ), NK/A = (Na O + K O)/Al O , MALI = Na O + K O–CaO. and2 open3 symbols 2refer to2 silicate glass2 5 inclusions in zircons2 and2 interstitial2 3 glass, respectively.2 2 Ab- Blackbreviations and open of rock symbols names: refer TePh—tephrip to silicate glasshonolite, inclusions TB—trachybasalt, in zircons and A—andesite. interstitial glass, respectively. Abbreviations of rock names: TePh—tephriphonolite, TB—trachybasalt, A—andesite. 5. Discussion 5. Discussion 5.1.5.1. GeneticGenetic Constraints from from Silicat Silicatee Glass Glass and and Bulk Bulk Rock Rock Compositions Compositions ProductiveProductive syenites enriched in in REE-bearing REE-bearing minerals minerals correspond correspond to to ferroan, ferroan, potas- potassic, andsic, and locally locally sodic sodic types, types, typically typically representing representing flotation flotation cumulates cumulates from from fractionated fractionated alkali basalt.alkali basalt. Variations Variations in the in parental the parental basalt basalt composition composition resulted resulted in in the the variable variable alkalinity alkalin- of theity residualof the residual melts, rangingmelts, ranging from strongly from strongly alkaline-to-peralkaline alkaline-to-peralkaline (HA-8), (HA-8), through through medium alkalinemedium (PI-12), alkaline to (PI-12), alkali-calcic to alkali-calcic compositions compositions (HP-3, P-4).(HP-3, P-4). TheThe productive granites granites enriched enriched in in HFSE- HFSE- and and REE-minerals REE-minerals are areferroan ferroan and andcalc- calc- alkalic,alkalic, whereaswhereas barren orthopyroxene orthopyroxene granite granite (p (pincinite),incinite), practically practically devoid devoid of accessory of accessory mineralsminerals exceptexcept for zircon zircon and and monazite, monazite, is is magnesian magnesian and and calcic calcic [9]. [ 9All]. All granite granite xenoliths xenoliths areare peraluminous.peraluminous. TheThe melt aluminosity aluminosity does does not not correlate correlate well well with withthe alkalinity, the alkalinity, which whichis rather is het- rather heterogeneouserogeneous within within a single a single xenolith. xenolith. For instance, For instance, while the while interstitial the interstitial glass in the glass syenite in the syeniteHP-3 is HP-3metaluminous, is metaluminous, silicate glass silicate inclusions glass inclusionsin zircon and in the zircon bulk and xenolith the bulk composi- xenolith compositiontion are both areperaluminous. both peraluminous. The peraluminous The peraluminous melts are traditionally melts are traditionally interpreted as interpreted either asanatectic either anatecticcrustal melts crustal or those melts resulting or those from resulting extensive from crustal extensive assimilation crustal assimilation of mantle- of mantle-derivedderived melts. Despite melts. the Despite strong the peralumino strong peraluminositysity of the productive of the productive syenite and syenite granite and granitexenoliths, xenoliths, superchondritic superchondritic εHf(t) valuesεHf(t) in values accessory in accessory zircons from zircons both fromrock bothtypes rocktestify types testifyto mantle-derived to mantle-derived parental parental magmas magmas unaffected unaffected by the crustal by the assimilation crustal assimilation [7]. [7]. TheThe increasedincreased Ca Ca content content in in the the productive productive calc-alkalic calc-alkalic granite granite from from ČamovceCamovceˇ [7,9] [7 ,9] withwith strongstrong peralkaline affinity affinity indicated indicated by by AMA AMA could could result result from from the the assimilation assimilation of of crustal limestones. This hypothesis is indirectly supported by the abundant anorthite-Ca- Tschermak clinopyroxene-forsterite-carbonate skarnoid xenoliths in the same lava flow.

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Despite the variegated major element compositions, the whole-rock trace element signatures based on Y/Nb, Rb/Nb, and Y/Ce/Nb ratios in the granites and syenites from the northern Pannonian Basin are diagnostic of an A1-type anorogenic igneous assemblage from intra-plate tectonic settings [7,9,18].

5.2. Accessory Mineral Assemblages (AMA) A total of 20 accessory minerals have been detected in felsic and intermediate igneous xenoliths ejected by Pliocene basalts in the northern Pannonian Basin (Table1).

Table 1. AMAs identiﬁed in syenite and granite xenoliths.

Locality Pinciná Hajnáˇcka Camovceˇ Syenite- Xenolith Syenite Granite Carbonatite Aluminosity Peraluminous Metaluminous Metaluminous Peraluminous Moderately to Alkalic to Strongly Alkalinity Calc-Alkalic Strongly Alkalic Alkali-Calcic Alkalic Apatite + + + + Baddeleyite + Britholite-Ce + + Britholite-Y + + Columbite-Mn + Chevkinite-Ce + Fergusonite-Y + + Fersmite/viggezite + Ilmenite-Nb + + Monazite-Ce + Pyrochlore + Rhabdophane (?) + Rutile-Nb + + Samarskite + + Titanite + + Uranothorianite + ? Uranothorite + ? + Yttrialite-Y + + Zircon + + + + Zirconolite + Notes: +: present; ?: uncertain; empty space: absent.

Zircon occurs in all xenolith types, though it is preferably concentrated in syenites. Zr/Hf ratios in Pliocene zircon xenocrysts in maar sediments and syenite xenoliths are scattered within an interval between 40 and 90 [1]. The Zr/Hf ratios > 60 reﬂect a late Zr saturation in alkaline parental melts [39]. Th/U ratios between 0.5 and 8.1 in the xenolith zircons overlap the range of 0.2–4.6 determined in Pliocene xenocrystic zircons [1]. Such high Th/U ratios are also diagnostic of evolved alkaline syenite or phonolite parental melts [40–42] similar to those detected in silicate glass inclusions (Figure 12). The baddeleyite–titanite–zirconolite assemblage from Hajnáˇckadiatreme testiﬁes to the reduced silica activity in the parental magma caused by the interaction with the carbonatite melt. Baddeleyite rims around zircon xenocrysts in pyroclastic deposits of a neighboring Hajnáˇcka-Kostná Dolina maar [43,44] also indicate the interaction with a silica-poor, most probably carbonatitic melt. The high Zr/Hf ratio in the zircon (74–80) is indicative of evolved, alkaline silicate parental melt. The AMA with chevkinite-Ce and britholite is diagnostic of syenites from Pinciná, which lack the evidence of the silicate magma interaction with carbonates or carbonatites. Britholite-Ce from alkalic syenite has substantially higher ThO2 content (7.2 wt.%), lower La,Ca, and higher Nd and Sm contents compared to that in less alkalic to alkali-calcic syenite. The excessive P in the alkalic syenite triggered the crystallization of monazite-Ce associated with minerals of the britholite-apatite solid solution series. Fergusonite-Y occurs in syenite and as well as productive calc-alkalic granite. The syenite-hosted fergusonite is enriched in Ce2O3 (up to 4 wt.%) and Nd2O3 (up to 5.2 wt.%) Minerals 2021, 11, 369 15 of 20

compared with the maxima of 1.5 and 1.8 wt.%, respectively, determined in the fergusonite from granite. Silicate glass inclusions in fergusonite from granite correspond to potassic, ferroan, peraluminous, sub-alkalic granite [7], being thus similar to the bulk rock composition. Pyrochlore, columbite, subordinate Ca-niobate (fersmite/viggezite), Nb-ilmenite, and Nb-rutile also occur in the productive calc-alkalic, peraluminous granite. The AMA is nor- mally diagnostic of peralkaline magmas [45,46]. The relatively Ca-rich granite composition is mirrored in the crystallization of oxycalciopyrochlore and Ca-niobates. The inhibited allanite crystallization is most likely due to the insignificant water dissolved in the parental melt, which precluded the precipitation of OH-bearing minerals besides fluorapatite. Nb-bearing ilmenite replaced by niobian rutile is typical for strongly alkalic syenite as well as calc-alkalic granite. Significant concentrations of HFSE in Fe,Ti-oxides are regarded as diagnostic of their magmatic origin and close genetic relationship with alkaline rocks and carbonatites [47]. Yttrialite-Y is a diagnostic mineral of syenites from Pinciná maar. Identical yttrialite-Y inclusions in zircon xenocrysts in the Fil’akovo maar [1] unequivocally testify to the syenite reservoir or intrusion beneath both maars. Compositions of syenite xenoliths accompanying mafic cumulates, granites, and skarns are fairly similar to the xenolithic assemblage found in volcanic ejecta of the dormant Colli Albani volcano in Central Italy [48]. Carbonatite syenites interpreted as solidified crystal-liquid mush have also been described from the Laacher See volcano in the Eifel region, Germany [41,49]; phonolitic volcanoes of the Kula volcanic province of Western Turkey [50]; and Tenerife, Canary Islands [51–53].

5.3. Geochemical Proxies Additional information regarding the composition of parental magmas can also be obtained from the chemistry of accessory minerals. For instance, zirconolites from Hajnáˇcka plot within the syenite ﬁeld and along the syenite-carbonatite boundary in the discrimina-

Minerals 2021, 11, x FOR PEER REVIEWtion diagram correlating HFSE and REE (Figure 13), being thus reliable16 genetic of 21 indicators of alkaline syenite-carbonatite parental magmas.

FigureFigure 13. 13. REE–(ThREE–(Th + U)–(Nb + U)–(Nb + Ta) +diagram Ta) diagram [28] with [28 proj] withection projection points of zirconolites points of from zirconolites Hajnáčka from Hajnáˇcka (blue(blue circles) and and compositional compositional fields fields for forzirconolites zirconolites from frommetasomatic metasomatic rocks, skarns, rocks, skarns,syenites, syenites, mafic mafic and ultramafic rocks, and carbonatites compiled from literature. Violet squares are projection and ultramafic rocks, and carbonatites compiled from literature. Violet squares are projection points points of laachite, stefanweissite, and nöggerathite from an alkaline syenite-carbonatite intrusive ofcomplex laachite, subjacent stefanweissite, to the Laacher and See nöggerathite volcano, Eifel fromvolcanic an region, alkaline Germany syenite-carbonatite [33–35]. intrusive complex subjacent to the Laacher See volcano, Eifel volcanic region, Germany [33–35].

Figure 14. Discrimination diagram [27] correlating chevkinite/perrierite compositions with their host rocks. Black squares are projection points of chevkinite-Ce from Pinciná. 1—Si-oversaturated evolved magmatic rocks, 2—Si-undersaturated evolved magmatic rocks, 3—metasomatic rocks (fenite), 4—granite, 5—mafic and intermediate igneous rocks.

5.3.Ages and Emplacement Depths of Sub-Volcanic Reservoirs The U-Pb-(Th) ages of zircon and monazite in syenite xenoliths ejected in the Lučenec basin correspond to 5.3–5.9 Ma [1,18], whereas zircons recovered from volcanic structures in Cerová Upland returned U-Pb ages from 6.5 to 1.6 Ma [1,7,20,43]. These dates constrain

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Similarly, FeO, ΣYREE2O3, and CaO + SrO + MgO + Al2O3 contents in chevkinite-Ce from Pinciná projects along the boundary between Si-undersaturated and Si-oversaturated evolved igneous rocks (Figure 14). The chevkinite-Ce composition from Pinciná is similar Figure 13. REE–(Th + U)–(Nb + Ta) diagram [28] with projection points of zirconolites from Hajnáčka to that described from Middle-Miocene alkalic syenite from Ashizuri Peninsula, Japan [54]; (blue circles) and compositional fields for zirconolites from metasomatic rocks, skarns, syenites, Palaeocene granite of St. Kilda Complex, Scotland [55]; alkalic syenite complex of Puttetti, mafic and ultramafic rocks, and carbonatites compiled from literature. Violet squares are projection Indonesiapoints of laachite, [56,57]; stefanweissite, Archean peralkaline and nöggerathit granitee from complex an alkaline of Keivy, syenite-carbonatite Kola Peninsula; intrusive and Vishnevyecomplex subjacent Gory, Uralto the Mountain Laacher See Chain volcan ofo, Russia Eifel volcanic [58]. region, Germany [33–35].

FigureFigure 14.14. Discrimination diagram diagram [27] [27] correlating correlating chevkinite/perrierite chevkinite/perrierite compositions compositions with with their their hosthost rocks.rocks. Black squares are projection points of chevkinite-Ce from Pi Pincinnciná.á. 1—Si-oversaturated 1—Si-oversaturated evolvedevolved magmatic rocks, rocks, 2—Si-undersaturated 2—Si-undersaturated evolved evolved magmatic magmatic rocks, rocks, 3—metasomatic 3—metasomatic rocks rocks (fenite), 4—granite, 5—mafic and intermediate igneous rocks. (fenite), 4—granite, 5—maﬁc and intermediate igneous rocks.

5.4.5.3.Ages Ages and and Emplacement Emplacement Depths of Sub-Volcanic Reservoirs TheThe U-Pb-(Th)U-Pb-(Th) agesages ofof zirconzircon andand monazitemonazite inin syenitesyenite xenolithsxenoliths ejectedejected inin thethe LuˇcenecLučenec basinbasin correspondcorrespond toto 5.3–5.95.3–5.9 MaMa [[1,18],1,18], whereaswhereas zirconszircons recoveredrecovered fromfrom volcanicvolcanic structuresstructures inin CerovCerováá UplandUpland returnedreturned U-PbU-Pb agesages fromfrom6.5 6.5to to 1.61.6 MaMa [[1,7,20,43].1,7,20,43]. TheseThese datesdates constrainconstrain the lifespan of two separated magmatic reservoirs occurring in both areas. Both reservoirs produced ferroan, potassic, dominantly peraluminous syenites, but the southern, more alkalic reservoir beneath the Hajnáˇckadiatreme was recharged with carbonatite magma. Skarnoid xenoliths in this area also bear a piece of indirect evidence for crustal limestones interacting with or assimilated in the magmatic reservoir. The southern reservoir possibly extends from the area between Fil’akovo town and the Hajnáˇckavillage northwards (Hode- jov maar), eastwards (Gemerské Dechtáre maar), and north-eastwards (Tachty diatreme), covering an area of about 200 km2. The areal extent of the northern reservoir in the Luˇcenec basin is unknown, as the xenolith and xenocryst data are only restricted to the Pinciná maar. Both magmatic reservoirs occur in highly conductive zones detected by magnetotelluric sounding [59,60]. The conductive zone in the Luˇcenecbasin is traced down to the depth of 15 km and probably more. The conductive zone beneath Fil’akovo maar and Hajnáˇckadiatreme is underlain in the depth of ~5 km by moderately resistive domain interpreted as a low-conductivity (possibly Cadomian or Proterozoic) crystalline funda- ment. The high-conductivity zones are recently interpreted as water-saturated Neogene sediments in shallow parts and ﬂuid-saturated pathways with ore accumulations in deeper parts, aligned along regional fault zones. Stratiﬁed magmatic reservoirs beneath intra-plate volcanoes are located in relatively shallow depths, for example, 5–6 km beneath the Laacher See volcano [41], ~12 km beneath Minerals 2021, 11, 369 17 of 20

the Tenerife island [53], and ~10 km beneath the Coli Albani maar [48]. The emplacement depth of the reservoir sampled by the Pinciná maar in the Luˇcenecbasin has been estimated from the density of primary CO2-rich inclusions, and the approximate temperature of 800–900 ◦C inferred from zircon morphology. The inferred pressures—from 2.5 to 2.8 kbar for oxidized syenites, and 4.3–5.2 kbar for reduced syenites, including PI-12 and HP-3— testify to the magma underplating, stagnation, and differentiation in at least two separated horizons in the depth interval from 10 to 20 km [6]. However, these depth estimates can be biased by the loss of some fluid content from fluid inclusions through decrepitation cracks, and by the lacking independent thermobarometric control from mafic mineral assemblages. Emplacement depth of the carbonatite-syenite intrusives beneath the Hajnáˇckadia- treme in the Cerová Upland can only be roughly estimated from the titanite-ilmenite-rutile ◦ equilibrium, which has a triple point at ~13 kbar and 780 C in the tholeiite basalt-H2O system. The magmatic rutile is stable above 13–14 kbar at temperatures between 700 and 900 ◦C[61]. The lower pressure titanite-ilmenite assemblage is dominant in Ca-poor systems below 9 kbar at 700 ◦C, but may be stable at higher pressures in Ca-richer rocks [62]. The ilmenite-rutile transformation (Figure4b) accompanied by titanite resorption in the interstitial silicate melt (Figure4a) and especially aragonite crystallization in intergranular carbonate ocelli (Figure4d) are indicative of crystallization pressures above the titanite-ilmenite stability limit. Assuming the minimum temperature of 700 ◦C, a pressure of >15 kbar corresponding to depths >45 km would be needed to crystallize the magmatic aragonite and to allow the ilmenite-rutile transformation. The crystallization of primary magmatic aragonite is provisionally explained as a record of advective supra-lithostatic overpressure caused by expanding gas bubbles in a quasi-incompressible silicate melt [24]. If existing, this effect would make reliable pressure determinations from mineral assemblages problematic. The magmatic aragonite has never been detected in igneous xenoliths from the Luˇcenecbasin, although sporadic calcite inclusions in kaersutite and clinopyroxene megacrysts, as well as carbonatite and carbonated pyroxenite xenoliths, do occur in the basaltic lava flow near Mašková village [63], located northwest of Pinciná maar. It seems therefore likely that the aragonite crystallization from syenite and carbonatite in the Cerová Upland has a special, yet poorly understood, causal relationship with the pressure regime of the southern magmatic reservoir. It looks quite reasonable that the overpressures are due to extensive CO2-devolatilization resulting from the assimilation of limestones or the mingling of alkalic silicate and carbonatite magmas. The recently active, shallow magmatic reservoirs in the Luˇcenecbasin and the Cerova Upland are unlikely despite the occurrence of relatively young, 1.6 Ma old magmatic zircons recovered from maar sediments [43] and even younger, Early Pleistocene (1.2–1.5 Ma) basalts NE of Fil’akovo [16]. The modern regional heat flow in this area, ~90 mWm−2, indicates partially molten masses ~80–90 km deep [64,65].

6. Conclusions • Pliocene basalts in Carpathian back-arc basin ejected fragments of stratified subvolcanic magmatic reservoirs or intrusive complexes. • Zircon crystals and fragments in pyroclastic deposits represent relics of disintegrated syenite xenoliths. • Accessory mineral assemblages (AMA) in xenoliths and pyroclastic deposits reflect several types of parental melts: - Zirconolite and baddeleyite indicate carbonatite-alkalic syenite intrusives; - Yttrialite, monazite, chevkinite, and britholite are diagnostic of moderately alkalic to alkali-calcic syenite intrusives; - Pyrochore, columbite-Mn and Ca-niobates crystallized from anhydrous calc- alkalic granites with strong peralkaline affinity. • The stratified subvolcanic reservoir beneath Luˇcenec basin is emplaced 10–20 km deep. • The reservoir beneath Cerová Upland, probably ~200 km2 in area, is emplaced more than 45 km deep. Minerals 2021, 11, 369 18 of 20

• Zirconolite, yttrialite, and britholite-Y are new minerals described in the Western Carpathian realm.

Supplementary Materials: The following data are available online at https://www.mdpi.com/ article/10.3390/min11040369/s1, Table S1: EPMA analytical conditions, Table S2: Major and trace element analyses of xenoliths, Table S3: EPMA of monazite, Table S4: EPMA of fergusonite, Table S5: EPMA of silicate glass, Table S6: EPMA of ilmenite and rutile, Table S7: EPMA of apatite, Table S8: EPMA of titanite, Table S9: EPMA of zircon and baddeleyite, Table S10: EPMA of chevkinite, Table S11: EPMA of high-Th yttrium silicates, Table S12: EPMA of low-Th yttrium silicates, Table S13: EPMA of britholite, Table S14: EPMA of rhabdophane, Table S15: EPMA of zirconolite. Author Contributions: Conceptualization, M.H. and V.H.; methodology and software, P.K.; writing— original draft preparation, review, and editing, V.H.; project administration, M.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the VEGA grant agency, grant number 1/0143/18. Data Availability Statement: The data presented in this study are available in this article and the attached Electronic Supplementary Material. Acknowledgments: The original draft greatly benefited from constructive comments from three anonymous reviewers. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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