Th-Rich Zircon from a Pegmatite Vein Hosted in the Wiborg Rapakivi Granite Massif

: Zircon from a pegmatite vein hosted in the rapakivi granites occurring in the Gubanov Intrusion of the Wiborg Massif was shown by the EPMA method to contain an anomalously high Th of 18.3 wt% ThO 2 (about 161,000 ppm Th). The SIMS method obtained a close value for thorium content—146,000 ppm. U increases to 6800 ppm. Zircon contains high values of other non-formula elements: Y (up to 51,500 ppm), Ti (up to 12,500 ppm), P (up to 9000 ppm), Nb (up to 3500 ppm), Ca (up to 2800 ppm) and Ba (up to 600 ppm). The average Hf value is 10,900 ppm. Total REE ranges from 25,500 to 49,400 ppm, and the average REE is 33,300 ppm. The REE pa�erns show poorly HREE fractionation over the LREE (Lu N /La N ratio average is 6.34) with well-marked negative Eu anomalies (Eu/Eu* averages 0.10) and slight positive Ce anomalies (Ce/Ce* averages 1.16). The formation of Th-rich zircon indicates that residual ﬂuid-saturated pegmatite melt is saturated with Th and other trace elements (REE, Y


Introduction
Zircon is a geochronometer mineral most commonly used for approaching various geological problems (e.g., [1][2][3]).The study of zircon trace elements, which has become possible with the arrival of high-precision microanalytical methods (SIMS, LA-ICP-MS, etc.), provides more information on petrogenesis [4][5][6].Therefore, it is important to obtain more evidence for zircon's isomorphic capacity for trace elements and zircon crystallization conditions when its impurity element concentrations are extremely high.The possible range and average concentrations of impurity elements in zircon have been assessed recently, based on strong evidence (e.g., [7][8][9]).However, detailed mineralogical and geochemical studies that are currently in progress provide more information on the anomalously high concentrations of some elements in zircon.Hf-rich zircon in rare-metal pegmatites from the Vasin-Mylk deposit (northeastern Kola Peninsula) was shown to contain zones consisting of hafnon.It was the fourth discovery of hafnon in the world [10].In this study, we report the occurrence of anomalous Th-rich zircon in a pegmatite vein emplaced in porphyritic rapakivi granite from the Wiborg Massif.We present EPMA and SIMS data for it, which provides a be er understanding of zircon's isomorphic capacity.
exposed over an area of more than 19,000 km 2 and formed during the consecutive intrusion of four portions of magma (Figure 1).Petrochemical, mineralogical and geochemical analyses of various granite phases are indicative of melt differentiation, which took place in the primary deep magmatic chamber during the crystallization of separate magmatic phases [18].Available isotopic data indicate that the massif was forming for at least 20 Ma over the time span of 1642-1622 Ma [19].From a practical point of view, the mineralogical and geochemical study of Wiborg rapakivi granites is essential for the assessment of the composition and quality of the region's underground waters (e.g., [20,21]).
Zircon is one of the most common accessory minerals present in all types of rapakivi granites.The prismatic faces of its crystals are be er defined than dipyramidal faces.The aim of the present project was to study a zircon sample with an anomalously high Th concentration (sample 110) obtained by Prof. V.V. Gavrilenko from a pegmatite vein in the rapakivi granites of the Gubanov Intrusion quarried in Vozrozhdenie quarry located in the Wiborg District of the Leningrad region (Figure 2a).The Gubanov Intrusion is a granitic intrusive body covering an area of about 20 km 2 within the Wiborg Massif [18].Vozrozhdenie facing stone deposits is its biggest exposure.Most researchers consider the intrusion as a body formed during a third phase of intrusion (after pyroxene-hornblende granites of phase I and ovoid biotite-hornblende wiborgites of phase II).Wiborgite is a regional term adopted by geologists in Finland, Sweden and Russia as a shorter synonym for the coarse-grained, ovoid-rich granites that are most common in rapakivi massifs.The Gubanov Intrusion consists of porphyritic biotite granites.Of note, the K-feldspar megacrysts (up to 30-40 mm) in the biotitic granites are idiomorphic with tabular habits rather than ovoids.These granites often display a trachytoid texture, especially near contact with host wiborgites.Feldspar ovoids, tentatively understood as xenocrysts trapped from wiborgites upon the intrusion of phase III, occur there as well.This conclusion is supported by the fact that the amount of ovoids in biotite granites decreases rapidly away from the contact.
Pegmatite veins are located inside the granites of phase III, near the contact with earlier granites of phase II.This suggests that pegmatites crystallized from the residual melt of granites of phase III, enriched with volatile and incompatible components.According to textural features, zircon crystallized simultaneously with the minerals of the central (giant-grained) part of the pegmatite vein-biotite and K-feldspar.

Methods
Anomalously high Th concentration in zircon from the above pegmatite vein (sample 110) was tentatively detected using the SEM-EDS method at the Institute of Precambrian Geology and Geochronology of the Russian Academy of Sciences on a JEOL JSM-6510LA scanning electron microscope with a JED-2200 energy dispersion X-ray spectrometer (JEOL Ltd., Akishima, Japan).A transparently polished, thin section was carbon-coated.
Measurements of zircon trace element composition were performed using the SIMS (secondary ion mass spectrometry) method using an IMS-4f (Cameca, Gennevilliers, France) ion microprobe at the Yaroslavl' branch of the Institute of Physics and Technology of the Russian Academy of Sciences.We mainly followed the analytical procedure described in [23,24].Concentrations of trace elements were calculated from the normalized to 30 Si + secondary ion intensities using calibration curves based on a set of reference glasses.NIST-610 reference glass was used as a daily monitor for trace element analyses.The primary O2 -ion beam spot size was ~20 µm.Each analysis was averaged over 3 measurement cycles.The accuracy of trace element measurements was up to 10% and 20% for concentrations of more than 1 ppm and between 0.1 and 1 ppm, respectively.The detection limit was 5-10 ppb.To construct REE distribution spectra, the composition of zircon was normalized to that of chondrite CI [25].

EPMA Analytical Results
The zircon grain studied is in the marginal zone of a coarse (about 1.5 cm across) quar crystal.It displays a well-defined xenomorphic habit (Figure 3a).The grain, 200 × 400 µm across, has an elongated shape.About half of the grain is highly brecciated and is healed with colloform aggregate streaks, consisting of Fe oxides (hydroxides?) and smaller amounts of Al and Si.The streaks vary in thickness from 10 to 30 µm.The zircon zones surrounded by streaks display zonal structure: zircon is darker in BSE in the centre of the zones and lighter towards the streaks.Relatively brighter areas have a higher mean atomic number than the darker areas, and so they are constituted by relatively heavier chemical elements on average.Micron-sized inclusions of other mineral phases of trace and minor elements in zircon, including Th (e.g., thorite or thorianite), do not occur.1 and 2.
Zircon contains a relatively high FeO concentration, varying from 2.16 to 10.3 wt% and averaging 6.13 wt%.The FeO value depends inversely on ThO2 (r = −0.50).In the ThO2richest lighter zones, FeO drops to 2 wt%.In the darker zones, which contain much less ThO2, the FeO value is the highest.
It should be noted that the elevated Ca value in zircon has a fluid effect on the mineral [28].CaO varies from 0.40 to 0.70 wt% (on average, 0.55 wt%), showing a positive correlation with ThO2 (r = 0.39).The CaO value shows either a negative or no correlation with other trace and minor elements.
The formula element for zircon is Hf, the only one that demonstrates a positive relationship between the HfO2 and ZrO2 (r = 0.95, Figure 4e) contents.A negative correlation was established for HfO2 with ThO2 (r = −0.86, Figure 4f).
Ti in the zircon analysed is anomalously high, varying from 693 to 12,565 ppm (on average, 2681 ppm).Hence, it cannot be used as a marker for zircon crystallization temperature [29].Ti is poorly correlated with other trace elements in zircon, showing a positive correlation only with Li (r = 0.80) and Nb (r = 0.71).
Hf varies over a wide range from 5863 to 17,364 ppm, averaging 10,910 ppm.The highest Hf value was obtained for the darker zircon zones (points 7 and 9) that typically contain the lowest Th.Th is negatively correlated with Hf (r = −0.41, Figure 5d).Two nonformula elements, Ba (r = 0.59) and Sr (r = 0.46), are positively correlated with Hf.Ba and Sr are negatively correlated with Th (for Ba-Th r = −0.60, Figure 5e).Ba (321 to 673 ppm, averaging 427 ppm) is higher by more than one order of magnitude than Sr (21.0 to 69.3 ppm, averaging 33.3 ppm).Such a correlation is not typical of zircons enriched in trace elements.It has been shown earlier that Ba and Sr values in anomalous zircon from Belokurikha leucogranites are correlated both for unaltered low-Ba-and low-Sr zircons and for zircons enriched in incompatible elements [30].
Ca varies from 1691 to 2796 ppm, averaging 2262 ppm.The Nb value is the same as Ca, varying from 1497 to 3510 ppm (on average, 2267 ppm).It has been noted above that Ca and Nb are positively correlated with Th.
The Y value is as high as 24,927 to 51,524 ppm, averaging 33,901 ppm.Y and P are positively correlated (r = 0.80, Figure 5f), as are Y and Hf (r = 0.63).P varies from 4685 to 9047 ppm (on average, 6673 ppm).REE varies from 25,532 to 49,374 ppm, averaging 33,256 ppm.The REE distribution spectra are poorly fractionated, so LREE is more abundant than HREE (Figure 6).The LuN/LaN ratio varies from 3.52 to 11.5, averaging 6.34.Th is negatively correlated with the LuN/LaN ratio (r = −0.68, Figure 7a) and separately with the SmN/LaN and LuN/GdN ratios.In other words, Th-rich zircon zones display fla er REE distribution spectra.Figure 6.Zircon REE distribution pa erns normalized to CI chondrite [25].The grey pa erns show the zircons from the rapakivi granites of the Suomenniemi Massif, located at the northeastern flank of the Wiborg Massif, Finland [31].
The spectra for light REE are almost flat (the SmN/LaN ratio varies from 0.86 to 2.06, averaging 1.30).All the zircons show negative Eu-anomaly (Eu/Eu* varies slightly from 0.09 to 0.11, averaging 0.10).Positive Ce-anomaly, characteristic of magmatic zircon [26], is largely reduced (Ce/Ce* varies from 0.84 to 1.55, averaging 1.16).2) has shown that only Li and Hf values are not anomalously high.Variations in Li from 0.67 to 11.6 ppm (on average, 3.32 ppm) are consistent with the Li value in zircon from the continental crust [32].Hf varies from 5863 to 17,364 ppm, averaging 10,910 ppm, which is even less than the average Hf value in zircon from granitoids (12,800 ppm) [7].

SIMS analysis (Table
Trace element values in zircon are high because the radioactive decay of Th and U with α-particle emission damages its crystalline structure.As a result, zircon becomes metamict, and non-formula trace elements and water easily enter the structure ( [33] and references therein).
The positive correlation of Y, REE and P and their negative correlation with ZrO2 and SiO2, confirmed by both analytical methods (EPMA and SIMS), supports a xenotime scheme of coupled substitution (Y,REE) 3+ + P 5+ → Zr 4+ + Si 4+ .On the other hand, highly inconsistent (by about one order of magnitude) Y, REE (total on average, 67,157 ppm) and P (on average, 6673 ppm) values, shown repeatedly for Y-rich zircons [30], suggest a different (not xenotime) scheme of coupled substitution.
The above correlation analysis of trace element values in Th-rich zircon has revealed two element associations, which show no positive correlation between the members of various groups.One consists of U, Y, P, REE and the associated Hf, Ba and Sr.All or some of these elements in zircon from rare-metal pegmatites and rare-metal granites are often abundant (e.g., [10,30]) because parent granitic melts are enriched in incompatible elements (LILE and HFSE) and because the melt is highly saturated with fluids [34].In the Th-rich zircon studied, Ba concentration dominates strongly over Sr concentration, demonstrating melt composition and emphasizing its specific feature.Ba is a large-ion lithophile (LIL) element compatible with K-feldspar and biotite, which are widespread in both rapakivi granites and pegmatite veins.It seems that the superabundance of Ba, which has not become part of K-feldspar or biotite, has become part of zircon at the final stage of pegmatite formation.
The other element associations in the zircon studied consist of Th, Ca, Nb and Ti.Its formation is hard to interpret.It is an association dominated by HFS elements, which are commonly immobile in geologic processes.However, fluorine fluids contribute to their mobility.The formation of such fluids upon rapakivi granite crystallization is observed due to an abundance (a few percent by volume) of fluorite in the rocks.F-enriched topazbearing granite stocks, recognized as phase IV of rapakivi granite intrusion, are known in SE Finland [35].Relatively high-to-moderate mobile element (U, Th, K and Ba) concentrations were reported for Dongargarh A-type rapakivi granites from India [36].
We compared REE distribution spectra for Th-rich zircon with those obtained by LA-ICP-MS for zircon obtained from the rapakivi granites of the Suomenniemi Massif, located at the northeastern flank of the Wiborg Massif, Finland [31].The total REE value in Suomenniemi zircon is much lower than in Wiborg Th-rich zircon, varying from 475 to 5019 ppm (on average, 1903 ppm).Suomenniemi zircon clearly displays negative Eu-anomaly (Eu/Eu* on average, 0.07) similar in magnitude to Eu-anomaly in Wiborg granites.The pa ern of the spectra for HREE is generally consistent with that for zircon from both massifs, except for a low value obtained for zircon from Suomenniemi rapakivi granites (Figure 6).In the LREE range, Suomenniemi zircon shows a wide range of values exceeding chondrite values by two orders of magnitude.Several spectra display a flat LREE distribution spectrum similar to those for Th-rich zircon.Other spectra for Suomenniemi zircon with lower LREE show a fractionated pa ern in which LREE becomes more abundant than HREE's.Th and U (40-250 ppm for both elements) in zircon from Suomenniemi rapakivi granites are common for granites [36].Its Th/U ratio is much lower than that for Th-rich zircon, varying from 0.7 to 1.4 (on average, 1.1).The Y value in Suomenniemi zircon is 2900 ppm, which is lower by at least one order of magnitude than that in Wiborg zircon (on average, 33,901 ppm, Table 2).
Modern estimates of Th and U in zircon from various rock types (over 2000 analyses) show that Th in zircon for granitoids, as well as mafic and intermediate rocks, varies from a few ppm to 9690 ppm [9].U in zircon from granitoids reaches a maximum of 10,800 ppm.Earlier researchers [7] also referred to 12,090 ppm as the highest Th value in zircon from granitoids.The highest U value in zircon from granitoids reported in the contribution cited is also about the same (12,380 ppm).The comparability of possible anomalously high Th and U values in zircon is deduced from the same thoritic-coffinitic scheme of coupled substitution, the electronegativity of these elements, and the similarity of their ionic radii [26].The dissolution limit of Th in the structure was determined experimentally as ThO2 = 5.5 ± 2.5 wt% [37].
Li-F granites from Podlesí, Czech Republic, were found to contain single zircon grains with up to 6.64 wt% ThO2 (58,350 ppm Th) [40].The same rocks were shown to carry zircon with about 14.75 wt% UO2.

Figure 2 .
Figure 2. (a) View of the quarry at the Vozrozhdenie deposit.The arrow shows the location of the pegmatite vein.(b) The site at which sample 110 was taken.A symmetrically zonal-structured granitic pegmatite vein in porphyry-like biotite granites.The length of the hammer from the head to the bill is 18 cm.

Figure 3 .
Figure 3. BSE image of zircon with analytical points: (a) EPMA method (off scale) and (b) SIMS method (crater is about 20 µm in diameter).Points are numbered as in Tables1 and 2.

Figure 4 .
Figure 4. (a-f) Co-variation diagrams showing the compositional variations of the studied zircon (EPMA method).

Figure 5 .
Figure 5. (a-f) Co-variation diagrams showing the compositional variations of trace elements of the studied zircon (SIMS method).

Figure 7 .
Figure 7. (a-f) Co-variation diagrams showing the compositional variations of REE of the studied zircon (SIMS method).

Table 1 .
EPMA analysis (wt%) of zircon from a pegmatite vein hosted in the rapakivi granites of the Wiborg Massif.

Table 2 .
Trace element concentrations (ppm) in zircon from a pegmatite vein hosted in the rapakivi granites of the Wiborg Massif.
ZrO2 is also lower than a stoichiometric value of 67.2 wt%.It varies from 30.03 to 42.11 wt%, averaging 34.38 wt%.SiO2 and ZrO2 show a positive correlation (r = 0.89, Figure