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

: 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 Pincin á 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 Hajn á ˇcka maars 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, britholite-Y, britholite-Ce, chevkinite-Ce, monazite-Ce, and rhabdophane(?). Baddeleyite and REE-zirconolite are typical of alkalic syenite associated with carbonatite. Pyrochlore, columbite-Mn, and Ca-niobates occur in calc-alkalic granites with strong peralkalic afﬁnity. Nb-rutile, niobian ilmenite, and fergusonite-Y are crystallized from mildly alkalic syenite and calc-alkalic granite. Zircons with increased Hf/Zr and Th/U ratios occur in all felsic-to-intermediate rock-types. If rock fragments are absent in the volcanic ejecta, the composition of the sub-volcanic reservoir can be reconstructed from the speciﬁc AMA and zircon xenocrysts–xenolith relics disintegrated during the basaltic magma fragmentation and explosion.


Introduction
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 fragments scavenged from the walls of volcanic conduits is resorbed in the hot basalt, whereas the larger xenoliths are disintegrated and ground during the magma fragmentation and phreatomagmatic eruption triggered by the magma-water interaction and explosive release 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 necessary to decipher the information on the composition of rocks sampled by the ascending magma [1,2].

Methods
Chemical compositions of minerals and silicate glass were determined using a wavelength-dispersive (WDS) mode of CAMECA SX-100 electron probe micro-analyzer maintained 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, calibrants, 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.

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% Fe 3+ 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 FeO tot 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.
Syenite xenolith HA-8 recovered from the Hajnáčka diatreme is composed of Kfeldspar, 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 ( Figure 4). 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 (Figure 4a).   The silicate glass domains that appear dark in BSE contrast represent molten feldspars, as is documented by the almost identical chemical compositions of both, and negligible amounts of Fe,Ti,Mg-oxides. Brighter domains with increased Fe, Ti, and Mg and lower Si contents correspond to potassic trachyte (Table S5).
The carbonate ocelli show polygonal fibrous texture in transmitted light, and fine domain zoning in back-scattered electron images (Figure 4a,c,d). Some ocelli contain arag- The silicate glass domains that appear dark in BSE contrast represent molten feldspars, as is documented by the almost identical chemical compositions of both, and negligible amounts of Fe,Ti,Mg-oxides. Brighter domains with increased Fe, Ti, and Mg and lower Si contents correspond to potassic trachyte (Table S5). onite crystals growing from the silicate glass-carbonate and feldspar-carbonate boundaries. Small aragonite crystals appear freely suspended in the ocelli without any preferred orientation (Figure 4d). X-ray and Raman mapping [24] revealed an incremental oscillatory growth zoning of aragonite, contrasting with an irregular patchy domain microtexture of the associated Mg-calcite.  The carbonate ocelli show polygonal fibrous texture in transmitted light, and fine domain zoning in back-scattered electron images (Figure 4a,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 ( Figure 4d). X-ray and Raman mapping [24] revealed an incremental oscillatory growth zoning of aragonite, contrasting with an irregular patchy domain microtexture of the associated Mg-calcite.
Euhedral fluorapatite is enclosed in feldspars. Resorbed apatites occur in the interstitial 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).
Titanite creates rounded resorbed grains in the silicate-carbonate glass matrix. BSE 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 gradually increasing (Nb,Ta) 2 O 5 and ZrO 2 contents, reaching the maximum concentrations of up to 6.9 and 1.0 wt.%, respectively, in the marginal zone. The total YREE oxide content attains 4.5 wt. % (Table S8) (Table S9). The zirconolites enclosed in feldspars are closely spatially associated with zircon with an HfO 2 content up to 1.6 wt.% (Table S9).
Euhedral fluorapatite is enclosed in feldspars. Resorbed apatites occur in the interstitial 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).
Titanite creates rounded resorbed grains in the silicate-carbonate glass matrix. BSE 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 gradually increasing (Nb,Ta)2O5 and ZrO2 contents, reaching the maximum concentrations of up to 6.9 and 1.0 wt.%, respectively, in the marginal zone. The total YREE oxide content attains 4.5 wt. % (Table S8) (Table S9). The zirconolites enclosed in feldspars are closely spatially associated with zircon with an HfO2 content up to 1.6 wt.% (Table S9).

Britholite-Ce
Representative analyses of silicates dominated by light rare earth elements associated with chevkinite-Ce are listed in Table S13. Atomic proportions recalculated from formulae based on eight cations plot along the britholite-apatite exchange vector (Ca + P) = ~7.86 − (Re + Si) [29]. The Re + Si values between 6.

Britholite-Ce
Representative analyses of silicates dominated by light rare earth elements associated with chevkinite-Ce are listed in Table S13. Atomic proportions recalculated from formulae based on eight cations plot along the britholite-apatite exchange vector (Ca + P) =~7.86 − (Re + Si) [29]. The Re + Si values between 6.40 and 6. complex REE-silicate-phosphate are also noteworthy. A more precise analysis of the mixing trends is precluded by the insufficient number of analyses caused by the mineral scarcity and small dimensions of mineral grains, making the EPMA analyses tricky, as the position of electron beam changes with switching between two different sample currents.

Ca-Ce Phosphate
Representative analyses of hydrous Ca-Ce phosphates associated with britholite-Ce and chevkinite-Ce are listed in Table S14. A crystalchemical formula based on four oxygen atoms returned approximately two cations, thus resembling a rhabdophane-like phase. However, low analytical totals (81-88 wt.%) in the analyzed phosphates indicate 2-3 water molecules compared to ideal rhabdophane CePO4.H2O with 7.12 wt.% H2O. The high CaO contents (5.3-7.2 wt.%, 0.23-0.37 apfu) are also unusual, possibly indicating a greyite component (Ca,Ce,Th)PO4.H2O. However, the charge of divalent cations is not counterbalanced by a sufficient amount of large tetravalent cations (Th and U). In addition, up to 1 wt.% halogens may be associated with structurally bound OH. Hence, identification of the hydrous Ca-Ce phosphate remains only provisional, demanding support from additional spectroscopic methods.

Zirconolite
The zirconolites from Hajnáčka diatreme (Table S15) are characterized by high Y + REE contents, 0.20-0.41 apfu, accommodated in the eight-fold-coordinated A-site by the incorporation of charge-balancing divalent (mostly Fe 2+ ) and pentavalent (Nb + Ta) cations in the five-or six-fold-coordinated C-site. Up to 0.13 apfu of actinides in the A-site are accommodated by substituting charge-balancing ferrous iron together with Ti 4+ in the Csite.
The chemical substitutions in zirconolites are characterized by the following exchange vectors, reflecting heterovalent substitutions in several sites ( Figure 10

Ca-Ce Phosphate
Representative analyses of hydrous Ca-Ce phosphates associated with britholite-Ce and chevkinite-Ce are listed in Table S14

Zirconolite
The zirconolites from Hajnáčka diatreme (Table S15) are characterized by high Y + REE contents, 0.20-0.41 apfu, accommodated in the eight-fold-coordinated A-site by the incorporation of charge-balancing divalent (mostly Fe 2+ ) and pentavalent (Nb + Ta) cations in the five-or six-fold-coordinated C-site. Up to 0.13 apfu of actinides in the A-site are accommodated by substituting charge-balancing ferrous iron together with Ti 4+ in the C-site.
The following end-members ordered by the decreasing abundance occur in Hajnáčka zirconolites: 1. The zirconolite compositions from Hajnáčka diatreme project within the triangle area bounded by REE-rich members of the zirconolite group-that is, steffanweissite, laachite, and nöggerathite (Figure 11c). One outlier falls outside 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,  Numbers on tie-lines refer to the substitutions documented in Figure 10. Boundaries are constructed using the dominant valency rule [32]. (c) Classification diagram based on atomic proportions of eightfold-coordinated cations, with projection points of zirconolites from Hajnáčka (black squares), steffanweissite, laachite, and nöggerathite holotypes from the Tertiary Laacher See volcano of the Eifel region, Germany [33][34][35]. Boundaries are constructed using the dominant component rule [32].

Chemical Composition of Silicate Glass
Zircon-hosted glass inclusions large enough to be analyzed by the defocused electron beam yielded a peraluminous alkalic trachyte composition, similar to that of interstitial glass ( Figure 12, Table S5). The silicate glass associated with yttrialite and britholite-Y was a metaluminous alkali-calcic trachyte. The interstitial glass associated with zirconolite corresponded to strongly alkalic metaluminous trachyte.
In summary, silicate glasses from Pinciná and Hajnáčka are dominantly potassic and ferroan trachytes with variable alkalinity and aluminosity. Rather peculiar are the presence of quartz in the strongly alkaline syenite xenolith from Hajnáčka and the absence of quartz in the quartz-normative syenite xenolith from Pinciná. Compositions of silicate melt inclusions indicate trachytic parental melts, which are more alkaline in Hajnáčka compared to Pinciná.  Figure 10. Boundaries are constructed using the dominant valency rule [32]. (c) Classification diagram based on atomic proportions of eightfold-coordinated cations, with projection points of zirconolites from Hajnáčka (black squares), steffanweissite, laachite, and nöggerathite holotypes from the Tertiary Laacher See volcano of the Eifel region, Germany [33][34][35]. Boundaries are constructed using the dominant component rule [32].

Chemical Composition of Silicate Glass
Zircon-hosted glass inclusions large enough to be analyzed by the defocused electron beam yielded a peraluminous alkalic trachyte composition, similar to that of interstitial glass ( Figure 12, Table S5). The silicate glass associated with yttrialite and britholite-Y was a metaluminous alkali-calcic trachyte. The interstitial glass associated with zirconolite corresponded to strongly alkalic metaluminous trachyte.
In summary, silicate glasses from Pinciná and Hajnáčka are dominantly potassic and ferroan trachytes with variable alkalinity and aluminosity. Rather peculiar are the presence of quartz in the strongly alkaline syenite xenolith from Hajnáčka and the absence of quartz in the quartz-normative syenite xenolith from Pinciná. Compositions of silicate melt inclusions indicate trachytic parental melts, which are more alkaline in Hajnáčka compared to Pinciná.

Genetic Constraints from Silicate Glass and Bulk Rock Compositions
Productive syenites enriched in REE-bearing minerals correspond to ferroan, potassic, and locally sodic types, typically representing flotation cumulates from fractionated alkali basalt. Variations in the parental basalt composition resulted in the variable alkalinity of the residual melts, ranging from strongly alkaline-to-peralkaline (HA-8), through medium alkaline (PI-12), to alkali-calcic compositions (HP-3, P-4).
The productive granites enriched in HFSE-and REE-minerals are ferroan and calcalkalic, whereas barren orthopyroxene granite (pincinite), practically devoid of accessory minerals except for zircon and monazite, is magnesian and calcic [9]. All granite xenoliths are peraluminous.
The melt aluminosity does not correlate well with the alkalinity, which is rather heterogeneous within a single xenolith. For instance, while the interstitial glass in the syenite HP-3 is metaluminous, silicate glass inclusions in zircon and the bulk xenolith composition are both peraluminous. The peraluminous melts are traditionally interpreted as either anatectic crustal melts or those resulting from extensive crustal assimilation of mantlederived melts. Despite the strong peraluminosity of the productive syenite and granite xenoliths, superchondritic εHf(t) values in accessory zircons from both rock types testify to mantle-derived parental magmas unaffected by the crustal assimilation [7].
The increased Ca content in the productive calc-alkalic granite from Čamovce [7,9] with strong peralkaline affinity indicated by AMA could result from the assimilation of Figure 12. Classification diagrams of magmatic rocks based on major elements [36][37][38], with projection points of silicate glass from Pinciná (PI, HP, P) and Hajnáčka (HA, MKD) (Table S5)

Genetic Constraints from Silicate Glass and Bulk Rock Compositions
Productive syenites enriched in REE-bearing minerals correspond to ferroan, potassic, and locally sodic types, typically representing flotation cumulates from fractionated alkali basalt. Variations in the parental basalt composition resulted in the variable alkalinity of the residual melts, ranging from strongly alkaline-to-peralkaline (HA-8), through medium alkaline (PI-12), to alkali-calcic compositions (HP-3, P-4).
The productive granites enriched in HFSE-and REE-minerals are ferroan and calcalkalic, whereas barren orthopyroxene granite (pincinite), practically devoid of accessory minerals except for zircon and monazite, is magnesian and calcic [9]. All granite xenoliths are peraluminous.
The melt aluminosity does not correlate well with the alkalinity, which is rather heterogeneous within a single xenolith. For instance, while the interstitial glass in the syenite HP-3 is metaluminous, silicate glass inclusions in zircon and the bulk xenolith composition are both peraluminous. The peraluminous melts are traditionally interpreted as either anatectic crustal melts or those resulting from extensive crustal assimilation of mantle-derived melts. Despite the strong peraluminosity of the productive syenite and granite xenoliths, superchondritic εHf(t) values in accessory zircons from both rock types testify to mantle-derived parental magmas unaffected by the crustal assimilation [7].
The increased Ca content in the productive calc-alkalic granite fromČamovce [7,9] with strong peralkaline affinity indicated by AMA could result from the assimilation of crustal limestones. This hypothesis is indirectly supported by the abundant anorthite-Ca-Tschermak clinopyroxene-forsterite-carbonate skarnoid xenoliths in the same lava flow.
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].

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 (Table 1). 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][41][42] similar to those detected in silicate glass inclusions ( Figure 12).
The baddeleyite-titanite-zirconolite assemblage from Hajnáčka diatreme 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 Hajnáčka-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 ThO 2 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 Ce 2 O 3 (up to 4 wt.%) and Nd 2 O 3 (up to 5.2 wt.%) 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 normally 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][52][53].

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 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 Hajnáčka diatreme 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 Hajnáčka village northwards (Hodejov maar), eastwards (Gemerské Dechtáre maar), and north-eastwards (Tachty diatreme), covering an area of about 200 km 2 . The areal extent of the northern reservoir in the Lučenec 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 Lučenec basin is traced down to the depth of 15 km and probably more. The conductive zone beneath Fil'akovo maar and Hajnáčka diatreme is underlain in the depth of~5 km by moderately resistive domain interpreted as a low-conductivity (possibly Cadomian or Proterozoic) crystalline fundament. 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 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 Lučenec basin has been estimated from the density of primary CO 2 -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-3testify 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 Hajnáčka diatreme 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-H 2 O 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 (Figure 4b) accompanied by titanite resorption in the interstitial silicate melt (Figure 4a) and especially aragonite crystallization in intergranular carbonate ocelli (Figure 4d) 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 Lučenec basin, 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 CO 2 -devolatilization resulting from the assimilation of limestones or the mingling of alkalic silicate and carbonatite magmas.

•
The reservoir beneath Cerová Upland, probably~200 km 2 in area, is emplaced more than 45 km deep.
• Zirconolite, yttrialite, and britholite-Y are new minerals described in the Western Carpathian realm.