REE and Y Mineralogy of the Krudum Granite Body (Saxothuringian Zone)

Miloš René Institute of Rock Structure and Mechanics, v.v.i., Academy of Sciences of the Czech Republic, V Holešovičkách 41, Prague 8, 182 09, Czech Republic Correspondence: rene@irsm.cas.cz; Tel.: +420-266-009-228 Abstract: The Krudum granite body comprises highly fractionated granitic rocks ranging from medium-F biotite granites to high-F, high-P2O5 Li-mica granites. This unique assemblage is an ideal site to continue recent efforts in petrology to characterize the role of zircon, monazite and xenotime as hosts to REEs. The granitic rocks of the Krudum body analysed in this study were found to contain variable concentrations of monazite and zircon, while xenotime was only found in the high-F, high-P2O5 Li-mica granites and in the alkali-feldspar syenites of the Vysoký Kámen stock. For analysed monazites of all magmatic suites cheralite substitution was significant. The highest concentration of cheralite was found in monazites from the high-F, Li-mica granites and from the alkali-feldspar syenites. The proportion of YPO4 in all analysed xenotimes ranges from 71 to 84 mol. %. Some xenotimes were found to be hydrated and the observed water content estimated from analytical data ranged from 5 to 11 wt. % H2O. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 10 April 2018 doi:10.20944/preprints201804.0117.v1


Introduction
In the last ten years, several studies have emphasized the role of monazite, xenotime, and zircon as major hosts for rare earth elements (REE) and Y in granites [1][2][3][4][5][6][7][8][9].However, several factors controlling the composition of the above mentioned accessory minerals remain unclear.To expand on this knowledge, unique assemblages of variable fractionated granites in the Karlovy Vary pluton were selected for further analyses on these minerals.
The presented study concentrates on petrological and geochemical observations connected to the occurrence of monazite, xenotime, and zircon in compositionally different granitic rock types of the Krudum granite body.This granite body is a subsidiary intrusion of the Karlovy Vary pluton in the Slavkovský Les Mts.

Geological Setting
The Karlovy Vary pluton forms the southern edge of the Western Erzgebirge pluton that is part of the Variscan Krušné Hory/Erzgebirge batholith in the western part of the Bohemian Massif [10][11][12].This batholith consists of three individual plutons: Western, Middle and Eastern, each representing an assembly of shallowly emplaced granite units about 6-10 km paleodepth, with a maximum preserved vertical thickness of the pluton 10-13 km below the present surface level [12,13].The batholith belongs to one coherent and cogenetic, ca.400 km long plutonic megastructure of the Saxo-Danubian Granite Belt [14].
Geochemically, five groups of granites were previously distinguished in the Krušné Hory/Erzgebirge batholith: (i) low-F biotite granites, (ii) low-F two-mica granites, (iii) high-F, high P 2 O 5 Li-mica granites, (iv) high-F, low-P 2 O 5 Li-mica granites, and (v) medium-F biotite granites [10,11].However, the above type (iii) has been divided in the current study as high-F, high-P 2 O 5 Li-mica granites (iii), and muscovite-biotite high-F, high-P 2 O 5 granites (vi).Finally, a new discovery in this study was a quartz-free alkali-feldspar syenite (vii), which forms a distinct separate part of the Vysoký Kámen stock.The Western Krušné Hory/Erzgebirge pluton is interpreted as a sequence of separately emplaced magma batches or an assemblage of several magmatic pulses, emplaced more or less contemporaneously [10,11,15].The outcrops of this pluton could be divided into the Nejdek-Eibenstock and Karlovy Vary plutons [12,16].
All granitic rocks of the Karlovy Vary pluton were interpreted according their structural setting with respect to the Variscan collision as post-collisional intrusions [10].Various geochronology methods imply that all these granitic rocks formed between 327 and 318 Ma [10,11,15].
The Krudum granite body (KGB, ca.50 km 2 ) on the southwestern margin of the Karlovy Vary pluton (KVP) shows a concentric structure (Figure 1).

Geological Setting
The Karlovy Vary pluton forms the southern edge of the Western Erzgebirge pluton that is part of the Variscan Krušné Hory/Erzgebirge batholith in the western part of the Bohemian Massif [10][11][12].This batholith consists of three individual plutons: Western, Middle and Eastern, each representing an assembly of shallowly emplaced granite units about 6-10 km paleodepth, with a maximum preserved vertical thickness of the pluton 10-13 km below the present surface level [12,13].The batholith belongs to one coherent and cogenetic, ca.400 km long plutonic megastructure of the Saxo-Danubian Granite Belt [14].
Geochemically, five groups of granites were previously distinguished in the Krušné Hory/Erzgebirge batholith: (i) low-F biotite granites, (ii) low-F two-mica granites, (iii) high-F, high P2O5 Li-mica granites, (iv) high-F, low-P2O5 Li-mica granites, and (v) medium-F biotite granites [10,11].However, the above type (iii) has been divided in the current study as high-F, high-P2O5 Li-mica granites (iii), and muscovite-biotite high-F, high-P2O5 granites (vi).Finally, a new discovery in this study was a quartz-free alkali-feldspar syenite (vii), which forms a distinct separate part of the Vysoký Kámen stock.The Western Krušné Hory/Erzgebirge pluton is interpreted as a sequence of separately emplaced magma batches or an assemblage of several magmatic pulses, emplaced more or less contemporaneously [10,11,15].The outcrops of this pluton could be divided into the Nejdek-Eibenstock and Karlovy Vary plutons [12,16].
All granitic rocks of the Karlovy Vary pluton were interpreted according their structural setting with respect to the Variscan collision as post-collisional intrusions [10].Various geochronology methods imply that all these granitic rocks formed between 327 and 318 Ma [10,11,15].
The Krudum granite body (KGB, ca.50 km 2 ) on the southwestern margin of the Karlovy Vary pluton (KVP) shows a concentric structure (Figure 1).Porphyritic medium-F biotite granites, surrounded to the NW by younger, high-F, high-P2O5 topaz-bearing muscovite-biotite granites, form its center.The youngest, high-F, high-P2O5 Li-mica granite forms the outermost shell [17].The inner structure of the south-eastern edge of the KGB, partly overlain by metamorphic rocks of the Slavkov crystalline unit, is well stratified, comprising variable greisenised high-F, high-P2O5 Li-mica granites occurring also in the Hub, and Schnöd greisen Porphyritic medium-F biotite granites, surrounded to the NW by younger, high-F, high-P 2 O 5 topaz-bearing muscovite-biotite granites, form its center.The youngest, high-F, high-P 2 O 5 Li-mica granite forms the outermost shell [17].The inner structure of the south-eastern edge of the KGB, partly overlain by metamorphic rocks of the Slavkov crystalline unit, is well stratified, comprising variable greisenised high-F, high-P 2 O 5 Li-mica granites occurring also in the Hub, and Schnöd greisen stocks hosting the world-famous Sn-W-Nb-Ta-Li mineralization of the Horní Slavkov-Krásno ore district [18][19][20][21][22] (Figure 2).

Analytical Methods
The whole rock composition of selected granitoids was analyzed in a total of 66 samples.Rock samples of 2-5 kg weight were crushed in a jaw crusher and a representative split of this material was ground to fine powder in an agate ball mill, before being pressed into XRF-tabs.Major elements were determined using a Pananalytical Axios Advanced fluorescence (XRF) (PANalytical, Almelo, The Netherlands) spectrometer at Activation Laboratories Ltd., Ancaster, ON, Canada.The content of FeO was determined by titration, H2O + and H2O − were analyzed gravimetrically and F was analyzed using the ion selective electrode (ISE) (Krytur, Turnov, Czech Republic).Trace elements were quantified by inductively coupled plasma mass spectrometry (ICP MS) (Thermo Fisher Scientific, Waltham, MA, USA) techniques, also at Activation Laboratories Ltd., Ancaster, Canada, using a Perkin Elmer Sciex ELAN 6100 ICP mass spectrometer (PerkinElmer, Waltham, MA, USA), following standard lithium metaborate/tetraborate fusion and acid decomposition sample preparation procedures.All analyses were calibrated against international reference materials.

Analytical Methods
The whole rock composition of selected granitoids was analyzed in a total of 66 samples.Rock samples of 2-5 kg weight were crushed in a jaw crusher and a representative split of this material was ground to fine powder in an agate ball mill, before being pressed into XRF-tabs.Major elements were determined using a Pananalytical Axios Advanced fluorescence (XRF) (PANalytical, Almelo, The Netherlands) spectrometer at Activation Laboratories Ltd., Ancaster, ON, Canada.The content of FeO was determined by titration, H 2 O + and H 2 O − were analyzed gravimetrically and F was analyzed using the ion selective electrode (ISE) (Krytur, Turnov, Czech Republic).Trace elements were quantified by inductively coupled plasma mass spectrometry (ICP MS) (Thermo Fisher Scientific, Waltham, MA, USA) techniques, also at Activation Laboratories Ltd., Ancaster, Canada, using a Perkin Elmer Sciex ELAN 6100 ICP mass spectrometer (PerkinElmer, Waltham, MA, USA), following standard lithium metaborate/tetraborate fusion and acid decomposition sample preparation procedures.All analyses were calibrated against international reference materials.
The high-F, high-P 2 O 5 Li-mica granite (iii) is represented by more petrographic varieties, which could be classified as partly greisenised medium grained, equigranular granites, porphyritic, fine-grained granites and leucocratic granites that occur mainly in the Vysoký Kámen stock.The main granite variety is represented by a medium-grained, equigranular rock, consisting of quartz, albite (An 0-2 ), potassium feldspar, lithium mica, and topaz.Fluorapatite, zircon, Nb-Ta-Ti oxides, xenotime-(Y), and monazite-(Ce) are common accessory minerals.Cassiterite, uraninite, and coffinite occur usually as very rare accessory minerals.Porphyritic, weakly greisenised granites occur as relatively small lenses or layers in the main granite body of equigranular Li-mica granites.Their groundmass is fine-grained with phenocrysts of potassium feldspar.Granites contain quartz, albite (An 0-5 ), potassium feldspar, Li-mica and topaz.Apatite, zircon, Nb-Ta-Ti oxides, xenotime-(Y) and monazite-(Ce) are common accessories.The second sub-type of this granite, the leucocratic granite occurring in the Vysoký Kámen stock, is mostly composed of albite (An 0-2 ), potassium feldspar, quartz and subordinate amounts of lithium mica and topaz.Fluorapatite, Nb-Ta-Ti oxides, fluorite and rare beryl occur as accessory minerals.Quartz-free alkali-feldspar syenite (vii), composed exclusively of albite and potassium feldspar, also forms subhorizontal layers and lenses ranging from several decimetres to tens of meters in thickness in the Vysoký Kámen stock.Its contacts with leucocratic granite are typically diffuse.The alkali-feldspar syenite consists of albite (An 0-2 ), potassium feldspar, accessory lithium mica and topaz.Fluorapatite, triplite, Nb-Ta-Ti oxides, zircon, xenotime-(Y), monazite-(Ce), and very rare Nb-bearing wolframite are accessories.The alkali-feldspar syenite is described as feldspathite in some papers (e.g., [16,17]), but this name does not agree with the magmatic nature of this rock, which is in some places underlined by its striking magmatic layering (Figure 3).

Geochemistry
The medium-F biotite granite (v) is a weakly peraluminous Ca-poor granite with aluminum saturation index (ASI) ranging from 1.1 to 1.2.In comparison with common Ca-poor granites [25] it is enriched in P (0.16-0.1).

Accessory Minerals Textures
In the granite suites of the KGB rare earth element-(REE) and Y-bearing accessory minerals are represented by monazite, xenotime, and zircon.Monazite and zircon occur in all magmatic suites, whereas xenotime was found only in the high-F, high-P2O5 Li-mica granites (iii) and in the alkali-feldspar syenites (vii).Monazite, together with zircon and fertile apatite is usually enclosed in biotite and lithium mica flakes.Monazite occurs as small subhedral to anhedral grains (10-30 µm), often grows together with zircon in complex aggregates together with coffinite and xenotime (Figure 6).Monazite grains are not zoned, but zircon may have oscillatory zoning (Figure 7).Cores of possible inherited zircon grains were overgrown by younger zircon and xenotime (Figure 7D).The xenotime occurs as grains along the zircon rims (Figures 6 and 7B).

Accessory Minerals Textures
In the granite suites of the KGB rare earth element-(REE) and Y-bearing accessory minerals are represented by monazite, xenotime, and zircon.Monazite and zircon occur in all magmatic suites, whereas xenotime was found only in the high-F, high-P 2 O 5 Li-mica granites (iii) and in the alkali-feldspar syenites (vii).Monazite, together with zircon and fertile apatite is usually enclosed in biotite and lithium mica flakes.Monazite occurs as small subhedral to anhedral grains (10-30 µm), often grows together with zircon in complex aggregates together with coffinite and xenotime (Figure 6).Monazite grains are not zoned, but zircon may have oscillatory zoning (Figure 7).Cores of possible inherited zircon grains were overgrown by younger zircon and xenotime (Figure 7D).The xenotime occurs as grains along the zircon rims (Figures 6 and 7B).
Two main coupled substitution mechanisms have been proposed for monazite [2,27,28], namely the cheralite and huttonite substitutions.The analyzed monazite grains from all of the KGB magmatic suites plot between the cheralite and huttonite substitution vectors in the (Th + U + Si) vs. the (P + Y + REE) diagram.The huttonite substitution is characteristic for monazite from the low-F biotite granites (Figure 8).The highest fractions of the cheralite component were found in monazite from the alkali-feldspar syenites (vii) (up to 69.3 mol %).Similarly, the concentration of the xenotime (YPO4) component is relatively high in monazite from the high-F, high-P2O5 granites (iii, iv) (up to 9.1 mol %) and from the alkali-feldspar syenites (vii) (up to 9.4 mol %).

Xenotime Composition
The proportion of YPO4, the main component in xenotime, ranges from 70.66 to 83.75 mol % (Table 3).Some microprobe analyses from the high-F, high-P2O5 Li-mica granites reveal low totals, suggesting probably hydration of xenotime during its postmagmatic alteration.In slightly greisenised high-F, high-P2O5 Li-mica granites from the Hub stock of the F content reaches up to 1.12 wt %.Analyzed xenotime grains are commonly enriched in HREE (9.3-19.5 wt %HREE2O3), U and Th.The concentrations of Dy and Yb range from 3.05 to 7.67 wt % Dy2O3 (0.04-0.08 apfu Dy) and 2.24 to 8.04 wt % Yb2O3 (0.03-0.09 apfu Yb); the concentrations of U and Th range from 0.45 to 5.55 wt %UO2 (0.00-0.04 apfu U) and 0.04-1.73wt % ThO2 (0.00-0.01 apfu Th).Two charge balancing coupled substitutions involving Si (thorite-coffinite exchange, 0.0-4.5 mol % thorite component) and/or Ca (cheralite exchange, 1.2-19.8mol % cheralite component) for the replacement of Y and REE by U and Th are observed in xenotime.Both substitution mechanisms are found in the high-F, high-P2O5 Li-mica granites (Figure 9).In some cases, the xenotime grains are enriched in Sc (up to 2.03 wt % Sc2O3; 0.25 apfu Sc), Zr (up to 1.62 wt % ZrO2; 0.03 apfu Zr) and Bi (up to 0.07 wt % Bi2O3; 0.002 apfu Bi).The Sc and Bi contents show a lack of correlation with other cations in the octahedral position.The Zr content shows a negative correlation with Y content.The highest fractions of the cheralite component were found in monazite from the alkali-feldspar syenites (vii) (up to 69.3 mol %).Similarly, the concentration of the xenotime (YPO 4 ) component is relatively high in monazite from the high-F, high-P 2 O 5 granites (iii, iv) (up to 9.1 mol %) and from the alkali-feldspar syenites (vii) (up to 9.4 mol %).
The Sc and Bi contents show a lack of correlation with other cations in the octahedral position.The Zr content shows a negative correlation with Y content.

Substitution in Monazite
Two main coupled substitutions mechanisms have been proposed for monazite, the cheralite substitution (Th,U) 4+ + Ca 2+ = 2REE 3+ and the huttonite substitution (Th,U) 4+ + Si 4+ = REE 3+ + P 5+ [27][28][29][30][31].The cheralite substitution is dominant in the analyzed monazite from all the analyzed suites of the KGB.The predominance of the cheralite substitution over the huttonite substitution was also found in highly fractionated high-F, Li-mica granites from other parts of the Krušné Hory/Erzgebirge batholith and the Fichtelgebirge granites in NE Bavaria, Germany [3,9,27].High contents of the cheralite component (>20-30 mol %) were also found in highly fractionated S-type granites from the West Carpathian belt [2] and in similar S-type granites from the Belvís de Monroy pluton in the Iberian Variscan belt [7].The high-Th monazite was found also in the high-F, high-P 2 O 5 Li-mica granites from the German part of the Western Erzgebirge pluton (up to 51.7 wt % ThO 2 ) [27].Some monazite grains from similar high-F granites described in the Fichtelgebirge pluton [3] could be designed also as high-Th monazite.

Substitution in Xenotime
Like for monazite, two main mechanisms exist for the replacement of Y by REE, U and Th in xenotime: charge balancing coupled substitutions involving Si and Ca (thorite-coffinite-type and cheralite-type substitutions respectively) [2,5,31,32].In the analyzed xenotime from high-F, high P 2 O 5 granites (iii, iv) both substitutions mechanisms exist.Both mechanisms were also found in the S-type, high-F, Li-mica granites from the German part of Krušné Hory/Erzgebirge area [32].However, according to Pérez-Soba et al. [7], unlike zircon, xenotime from highly fractionated peraluminous granites from the Belvís de Monroy pluton in the Iberian Variscan belt showed predominance of one substitution, the cheralite substitution, over the thorite-coffinite substitution.
Enrichment of non-formula elements such as P, Al, Ca, Y and REE in P-rich zircon was found also in some other high-P peraluminous granites worldwide [7,[34][35][36].The entry of Y + HREE and P into the zircon structure is usually explained via xenotime substitution, whereas zircon and xenotime are isostructural [37][38][39].The apparent surplus on the A-site could be explained by the entrance of substantial amounts of interstitial cations (e.g., Fe, Ca, Al, As, Bi, Sc) [40,41].Sc enrichment in zircon is usually explained via the pretulite (ScPO 4 ) exchange [42].However, rational evaluation of non-formula elements enrichment in zircon is always matter of discussion.The most extended explanation is usually coupled with its metamictization and later postmagmatic alteration [43][44][45].Higher concentrations of P, Y and REE occur in metamictized zircon grains from the high-F, high-P 2 O 5 Li-mica granites (Figures 6 and 7C).Description of postmagmatic zircon alterations from these granites was presented in detail by René [46].On the other hand, the berlinite substitution found in zircon grains from the Podlesí granite stock and Belvís de Monroy pluton (Iberian Variscan belt) is believed to by primary magmatic [7,33].Similarly, the highest concentration of P (8.29 wt % P 2 O 5 ; 0.24 apfu P) was found in primary magmatic zircon grain from the examined alkali feldspar syenite.
Moderate to strong deviation of altered zircon from stoichiometry was also found in other occurrences of high-F, Li-mica granites in the Krušné Hory/Erzgebirge area (e.g., Cínovec, Podlesí, Altenberg, Seifen [33,36,47,48] as well as in altered zircon worldwide [49][50][51][52][53][54].Some xenotime analyses reveal low totals, suggesting probably their hydrothermal alteration.(d) Zircon grains from these granites are sometimes metamictized and fluorized.Zircon from the high-F, high-P 2 O 5 Li-mica granites and alkali-feldspar syenites is usually enriched in P (up to 8.29 wt % P 2 O 5 ; 0.24 apfu P).The higher concentrations of P, Y, REE and Sc returned by zircon grains from the high-F, high-P 2 O 5 Li-mica granites could be explained by their metamictization.However, the highest concentration of P (8.29 wt % P 2 O 5 ; 0.24 apfu P) that was found in zircon grains from alkali-feldspar syenite without visible metamictization could be believed as the result of a primary magmatic enrichment produced by fractionation.

Figure 4 .
Figure 4. (A) Binary plot of ASI vs. SiO2 for granitic rocks of the Krudum granite body, (B) Binary plot of Al2O3 vs. SiO2 for granitic rocks of the Krudum granite body, (C) Binary plot of K2O vs. SiO2 for granitic rocks of the Krudum granite body, (D) Binary plot of P2O5 vs. SiO2 for granitic rocks of the Krudum granite body (E) Binary plot of Rb vs. SiO2 for granitic rocks of the Krudum granite body, (F) Binary plot of Y vs. SiO2 for granitic rocks of the Krudum granite body.

Figure 4 .
Figure 4. (A) Binary plot of ASI vs. SiO 2 for granitic rocks of the Krudum granite body, (B) Binary plot of Al 2 O 3 vs.SiO 2 for granitic rocks of the Krudum granite body, (C) Binary plot of K 2 O vs. SiO 2 for granitic rocks of the Krudum granite body, (D) Binary plot of P 2 O 5 vs. SiO 2 for granitic rocks of the Krudum granite body (E) Binary plot of Rb vs. SiO 2 for granitic rocks of the Krudum granite body, (F) Binary plot of Y vs. SiO 2 for granitic rocks of the Krudum granite body.

Figure 7 .
Figure 7. High-contrast BSE images of zircon (Zrn) and xenotime (Xtm) from granites of the Krudum granite body.(A) Oscillatory zoning of zircon from the medium-F biotite granite, (B) Intergrowth of xenotime with zircon from the high-F, high-P2O5 Li-mica granite, (C) Oscillatory zoning of altered zircon in the high-F, high-P2O5 Li mica granite, (D) Inherited zircon grains overgrown by younger zircon and xenotime from the high-F, high-P2O5 Li-mica granite.

Figure 7 .
Figure 7. High-contrast BSE images of zircon (Zrn) and xenotime (Xtm) from granites of the Krudum granite body.(A) Oscillatory zoning of zircon from the medium-F biotite granite, (B) Intergrowth of xenotime with zircon from the high-F, high-P2O5 Li-mica granite, (C) Oscillatory zoning of altered zircon in the high-F, high-P2O5 Li mica granite, (D) Inherited zircon grains overgrown by younger zircon and xenotime from the high-F, high-P2O5 Li-mica granite.

Figure 7 .
Figure 7. High-contrast BSE images of zircon (Zrn) and xenotime (Xtm) from granites of the Krudum granite body.(A) Oscillatory zoning of zircon from the medium-F biotite granite, (B) Intergrowth of xenotime with zircon from the high-F, high-P 2 O 5 Li-mica granite, (C) Oscillatory zoning of altered zircon in the high-F, high-P 2 O 5 Li mica granite, (D) Inherited zircon grains overgrown by younger zircon and xenotime from the high-F, high-P 2 O 5 Li-mica granite.

Figure 8 .
Figure 8. Monazite composition vectors of monazite from the Krudum granite body.

Figure 10 .
Figure 10.Chemical composition of zircon from granites of the Krudum granite body.

Figure 11 .
Figure 11.Chemical compositions of zircon from the high-F, high-P2O5 Li-mica granites and alkali-feldspar syenites.(A) Distribution Al vs. P.The arrow represent vector of ideal berlinite type (P:Al = 1:1) substitution, (B) Distribution Ca vs. P.The arrow represent vector of ideal brabantite-type (P:Ca = 2:1) substitution, (C) Distribution of Y and REE vs. P.The arrow represent vector of ideal xenotime-type

Figure 10 . 20 Figure 10 .
Figure 10.Chemical composition of zircon from granites of the Krudum granite body.
(a) The Krudum granite body has a sequence of highly fractionated granitic rocks from the medium-F biotite granites to the high-F, high-P 2 O 5 Li-mica granites and alkali-feldspar syenites.(b) Analyzed monazite grains from the Krudum granite body display strong preference of cheralite substitution over the huttonite substitution with up to 69.3 mol % cheralite component in the alkali-feldspar syenites.Some monazite grains from the alkali-feldspar syenites are enriched in ThO 2 (up to 35.4 wt % ThO 2 ).(c) The coupled thorite-coffinite and cheralite substitutions are dominant in the analyzed xenotime.

Table 1 .
Whole-rock chemical analyses of granitic rocks of the Krudum granite body.

Variety wt % Medium-F Granite Medium-F Granite High-F Two-Mica Granite High-F Two-Mica Granite High-F Li-Mica Granite High-F Li-Mica Granite Alkali- Feldspar Syenite Alkali- Feldspar Syenite
b.d.l., below detection limit.