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GEOLOGICA CARPATHICA, AUGUST 2006, 57, 4, 227—242

www.geologicacarpathica.sk

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Electron microprobe dating of monazite from the Nízke

Tatry Mountains orthogneisses (Western Carpathians,

Slovakia)

IGOR PETRÍK

1

, PATRIK KONEČNÝ

2

, MARTIN KOVÁČIK

and IVAN HOLICKÝ

2

1

Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, P.O. Box 106, 840 05 Bratislava, Slovak Republic;

geolpetr@savba.sk

2

Geological Survey of Slovak Republic, Mlynská dolina 1, 817 04 Bratislava, Slovak Republic

(Manuscript received September 12, 2005; accepted in revised form March 16, 2006)

Abstract: The age and composition of monazite from ortho- and paragneisses of the Nízke Tatry Mts (Low Tatra Mts)
was studied by microprobe chemical method. The monazite shows relatively uniform composition characterized by
substitutions to xenotime (7—10 mol. %) and brabantite (6—11 mol. %), much less to huttonite (0—2 mol. %). Biotite-
hosted monazite is commonly overgrown by thin REE-epidote rims, in a less foliated sample it is apparently unstable and
breaks down to apatite, REE-epidote and a Th-silicate (huttonite?). Empirical thermometers based on Y in monazite give
600—650 

ºC for xenotime-bearing samples. Extensive dating by Cameca SX-100 microprobe yielded a range of ages: the

majority of data cluster around 340—360 Ma indicating either a Variscan origin of orthogneisses or strong reworking of
an older granitic protolith. Because monazite recrystallized at subsolidus temperatures it seems to have failed to record a
magmatic age. However, an Ordovician core (ca. 475 Ma) was found in one monazite grain from mylonitic Bystrá
augengneiss. This partially melted orthogneiss also contains younger monazites (320—330 Ma) which probably recrys-
tallized during this Variscan event. Decompression-related muscovite melting was probably the reason for the partial
melt formation. Metamorphic monazites from medium-grade biotite gneisses also record several ages: old inherited cores
(450 Ma) and metamorphic events occurring probably at ca. 390, 350 and 330 Ma.

Key words: Western Carpathians, Nízke Tatry, age, microprobe dating, gneiss, orthogneiss, monazite.

Introduction

Orthogneiss, an important rock type occurring in the
Western Carpathian basement, can be found in all main
tectonic units but the most extensive occurrence is in the
Nízke Tatry Mts (Low Tatra Mts). Both banded (stroma-
tite) and augengneiss (ophthalmite) varieties occur togeth-
er with paragneisses and amphibolites. While earlier
authors treated orthogneisses as migmatites (e.g. Miko &
Lukáčik 1983), the interpretations published in recent
years prefer their granitic protolith and most authors con-
sider them ductilely deformed syn-tectonic granitoids
(Adamija et al. 1992; Janák 1994; Petrík et al. 1998; Pol-
ler et al. 2000, 2001; Putiš et al. 2003). The first trace ele-
ment data on orthogneisses were brought by (Méres &
Hovorka 1992), who compared them with post-tectonic
Variscan granitoids. Petrík et al. (1998) studied their min-
eral composition, especially K-feldspars. The summary of
the existing data was recently presented by Kohút (2004).
Paragneisses were studied as part of the Nízke Tatry meta-
morphic complex by Spišiak & Pitoňák (1990), who inter-
preted them as a product of metamorphism of a mixed
protolith with variable extent of terrigeneous and volcan-
ogeneous material.

The presence of monazite, a main light rare earth ele-

ment (REE) mineral of orthogneisses enables their dating
by a chemical method (Montel et al. 1996). Monazite con-
tains enough Th and U to produce radiogenic Pb in

amounts measurable by microprobe, while containing no
common Pb (Parrish 1990). The electron microprobe dat-
ing method has been successfully applied to various gran-
itoids (Finger & Broska 1999; Finger & Faryad 1999;
Finger et al. 2003) or metapelites (Konečný et al. 2004) in
the Western Carpathians.

In this paper we present results of monazite geochrono-

logical study showing the main features of compositional
variations and geochronological history as recorded by
various monazite domains and yielding several Variscan
and pre-Variscan ages.

Monazite stability

Monazite is able to record a complex polymetamorphic

history and is commonly used for isotopic dating of meta-
morphic and magmatic events. Diffusion of radiogenic
lead is negligible even at high temperatures above 900 ºC
(Braun et al. 1998; Cherniak et al. 2004), so individual
metamorphic growth zones can be dated (Crowley & Ghent
1999). Monazites moreover commonly contain identifi-
able old inherited cores. New monazite may grow due to
the change of fluid regime mainly from apatite and allan-
ite, a source of LREE (Simpson et al. 2000; Foster et al.
2002). This process is opposite to the breakdown of mona-
zite observed in various metagranites (Finger et al. 1998;
Broska & Siman 1998). Monazite can form new domains
overgrowing older cores or it may recrystallize after previ-

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ous partial dissolution (Crowley & Ghent 1999). Dissolu-
tion and reprecipitation is an important growth mecha-
nism because (1) it does not require a net monazite
growth, and (2) new monazite may date the process with-
out complete resetting by volume diffusion of old lead,
which is according to most authors negligible. Harlov et
al. (2005) studied this process in experimentally metasom-
atized apatite. They found that replacement occurs after a
moving reaction front along which new monazite grows.
Micro- and nanoporosity allows fluids to permeate
through metasomatized areas as fluid-aided diffusion.
Monazites commonly show internal zonation or inhomo-
geneities visible in BSE images. These are usually caused
by increased Th (bright domains) or Y (dark domains). The
increase of Y, heavy REE and U results mainly from the
escape of light REE  and resulting decrease of crystallo-
graphical sites (Poitrasson et al. 1996). At 350 ºC in acid
and oxidation environment monazite may break down to
apatite and allanite (Poitrasson et al. 2000). This alteration
was observed in the Carpathians and Alps at higher P-T
conditions (Finger et al. 1998; Broska & Siman 1998).

Vavra & Schaltegger (1999) demonstrated Permian and

Triassic rejuvenation of monazite by lead loss, which
however was not caused by diffusion but by the activity of
fluids. The resetting of monazite by recrystallization may
occur below its closing temperature, even at 350—400 ºC
(Townsend et al. 2000).

Geological setting

Straddling the main Alpine superunits of the Western

Carpathians – the Tatric and Veporic Units, the mega-horst
of the Nízke Tatry Mountains is one of the largest mountain
ranges of the Western Carpathians (Fig. 1). The studied
rocks come from the pre-Mesozoic basement of its Tatric
part. The basement core consists of granitoids, high-grade
orthogneisses, paragneisses, amphibolites and ultramafic
rocks. The post-tectonic tonalites and granodiorites (Ďum-
bier and Prašivá type) forming the northern belt of the
mountains are separated from the gneisses and amphibolites
of the southern belt by a nebulite zone (Bezák & Klinec

Fig. 1. Sketch geological map of the Nízke Tatry Mts (adapted according to Bezák & Biely 1998). Sample localities and positions of some
valleys (e.g. Gelfúsová) are also shown.

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1983). The whole complex forms the upper Variscan tecton-
ic unit with amphibolites occurring at its sole. Madarás et
al. (1999) recognized in orthogneisses two deformation
events. The first event produced the foliation S

0

 (magmatic)

and  S

1

 (metamorphic) related to syn-metamorphic intrusion

into a shear zone. Kinematic indicators show prevailing
top-to-SE thrusting, the same as was found in the Západné
Tatry Mts (Western Tatra Mts) (Fritz et al. 1992). S

2

 folia-

tions are mylonitic and deformations indicate a temperature
of more than 450 ºC. They are attributed to the second de-
formation phase related to the collapse of thickened
Variscan crust (Madarás et al. 1999; Putiš et al. 2003),
which was accompanied by partial anatexis and retrogres-
sion.

The P-T conditions of metamorphism in the Nízke Tatry

are not yet precisely established. They were estimated for
massive garnet-clinopyroxene metabasites at 790—830 ºC
and 1300—1500 MPa (Janák et al. 2000b). The boudins of
these rocks occurring in banded amphibolites were iden-
tified as retrograde eclogites in the adjacent Western Tatra
Mts (Janák et al. 1996). Few existing data from metapelites
yield conditions of 620—660 ºC and 320—360 MPa (Janák
et al. 2000a). However, high-grade metamorphism and
melting can be inferred from the presence of sillimanite
and sporadic kyanite in paragneisses.

Orthogneisses were dated by conventional zircon dat-

ing, which gave 381 ± 6 Ma (LI, UI 1232 ± 31 Ma, Putiš et
al. 2003). 

40

Ar/

39

Ar dating of orthogneiss muscovites

yielded 331 ± 2 Ma (Dallmayer et al. 1993).

Methods

The new data presented in this work were obtained us-

ing the microprobe Cameca SX-100 in the laboratory of
Geological Survey of the Slovak Republic in Bratislava.
To achieve a sufficient number of counts with the lowest
rate of surface destruction we used the following condi-
tions (Konečný et al. 2004): 15 kV accelerating voltage,
80—150 nA sample current, 75—130 s counting time and
beam diameter about 5  m. The PAP correction procedure
was used for the conversion of counts to wt. %. Most ele-
ments were measured using the large crystals (LPET,
LTAP) which are about 8 times more sensitive than con-
ventional ones (Table 1). To avoid or minimize the effect
of interferences the lines of higher order were chosen.

Interferences PbM

1

 with Y

1

 and UM

1

 with ThM 1

were corrected by K

ovl

 factor obtained as the average

from repeatedly measured YPO

4

 and ThO

2

 standards (at

least 5—10 times). The readiness of the microprobe for
dating was proved by dating of five monazite standards
of various ages from 1840 to 77 Ma. The acceptable de-
viation from a true age is ± 5  Myr for monazite standards
younger than 500 Ma, and  ± 20 Myr for the oldest mona-
zite standard (1840 Ma).

The histogram base of monogenetic age monazite stan-

dards as revealed by multiple measurements is about
60 Ma for an average standard deviation  ± 15 Myr (1 ).
Therefore, the only events recorded by a polygenetic mon-

Table 1:  List of standards, element lines and crystals used for
monazite measurement. All lines are of the first order (according
to Konečný et al. 2004).

azite population differing by  > 30 Myr may be successful-
ly distinguished (Konečný et al. 2004). Weighed averages
were calculated using Isoplot/Ex v. 2.49 (Ludwig 2001).

Rock characterization

Orthogneisses are medium- to coarse-grained rocks with

stromatitic or ophthalmitic character depending on the pres-
ence of K-feldspar “augen”. In contrast to post-kinematic
Variscan granitoids occuring to the north (Ďumbier type)
they have banded appearance due to 1—2 mm thick biotite
bands alternating with quartz—feldspar bands. The K-feld-
spar occurs as porphyroblasts (originally phenocrysts) rang-
ing in size from several millimeters to 6 cm. Due to shearing
they commonly form more or less overgrown augen. In
some coarse-grained varieties the banding is discontinuous,
and the rock acquires a more granitic appearance. The or-
thogneiss complex contains abundant migmatitic varieties
and paragneiss xenoliths forming various inhomogeneities
and schlieren. The xenoliths are represented by a massive,
fine-grained biotite-garnet gneiss (e.g. in Kyslá area). Local-
ly, medium-grained metapelites were found containing gar-
net and sillimanite (to the E of Krpáčovo), Fig. 1.

The chemical composition of orthogneisses is monoto-

nous which is true also for orthogneisses from other
mountain ranges (Kohút 2004). Relatively acid and pera-
luminous composition, the presence of two micas and a
moderate to low content of accessories suggest a metasedi-
mentary source rock of granitoid protolith. The only spe-
cial feature of Nízke Tatry orthogneisses are increased
concentrations of W, which are 3—5 times higher than in
Ďumbier granitoids. Iron-rich biotite, the lack of magne-
tite and presence of pyrite indicate a reduced character of
magma. The rare earth elements (REE) are controlled by
monazite, apatite and xenotime. Whole rocks have dis-

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tinctly elevated contents of heavy REE (Gd-Lu), showing
flat patterns (La/Yb)

N

= 5—10. The trace element composi-

tion of paragneisses is not known.

Petrography and localization of samples

Orthogneisses are represented by samples NT-12, NT-27,

NT-754-2, NT-753, paragneisses by samples NT-757,
NT-764c.

Sample NT-12: Foliated biotite orthogneiss. SE slope of

Struhár Mts elevation  1220 m a.s.l. Fine discontinuous
bands ( < 1 mm) are defined by biotite. The rock has the
composition of tonalite with sub- to anhedral texture and
indistinctly foliated fabric. The biotite is fresh, fine, sub-

hedral, dark-red to pale-yellow. The plagioclase is anhe-
dral, zoned with basicity An

30—34

 in cores and An

20

 at

rims. The quartz is anhedral, shows undulous extinction,
intergrown with plagioclase. Only rare interstitial K-feld-
spar is present, while muscovite is absent in the rock.
Abundant accessories are represented by apatite, monazite
and zircon, mostly enclosed in biotite, where they form
distinct pleochroic halos. Biotite-hosted monazite is com-
monly surrounded by REE-epidote coronas (Fig. 3D). The
rock is only weakly mylonitic.

Sample NT-27: Coarsely banded mylonitic orthogneiss

with sporadic K-feldspar augen 1—3 cm in size. Bystrá do-
lina Valley, large outcrop on the W side of the road Tále—
Srdiečko, elevation 960 m a.s.l. The rock has the composition

Fig. 2. BSE images of monazite from orthogneisses NT-754-2, 27 and muscovite gneiss NT-764c. Points refer to Table 2. Sketch of the
old core in monazite m7 is shown in (D) with point ages. (A—B) show the breakdown of monazite to Th-apatite, rimmed by a Th mineral
(huttonite?) and REE-epidote.

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of muscovite-biotite granodiorite. It is distinctly foliated
and mylonitic. Two varieties of K-feldspar (30 %) can be
recognized: 1 – Large perthitic phenocrysts, which en-
close small euhedral plagioclases. The big augen consist
of several individuals separated by quartz septa. 2 – In-
terstitial, non-perthitic K-feldspar associated with quartz
drops in anhedral pools probably resulting from partial
melting. The plagioclase is sub- to anhedral, unzoned in-
tergrown with quartz. Basicity is An

30—26

. Quartz shows

undulouse extinction, in places dynamically recrystal-
lized. The biotite (10—15 %) is subhedral, dark red-brown
to straw-yellow, relatively fresh. The muscovite (3—4 %) is
usually intergrown with biotite showing exsoluted rutile,
occasionally is cross-oriented. The accessories are repre-
sented by apatite, zircon and monazite. The monazite is

hosted in biotite, quartz, plagioclase and muscovite
(Fig. 2C—E). The biotite-hosted monazite has thin REE-
epidote rims. A small amount of an ore mineral (pyrite?) is
also present.

Sample NT-753: Biotite schlier from orthogneiss. A cliff

5 m above road at the beginning of Kulichova dolina Val-
ley to Vajskovská  dolina Valley, 758 m a.s.l. The rock is
composed mainly of biotite (90 vol. %), quartz (5—10 %)
and plagioclase (2—3 %). The biotite is dark red-brown
to pale yellow, fresh but strongly deformed. Along cleav-
age it contains small exsolved titanites. The quartz shows
undulous extinction, and the plagioclase is strongly sericit-
ized. The accessories especially big columnar apatite and
zircon are abundant in biotite. Monazite is, however, not
common.

Fig. 3. BSE images of selected studied monazites (A,B,D—F)  and xenotime (C) from orthogneisses NT-761, 12 and paragneiss NT-757.
The numbers of analysed points correspond to Table 2.

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Sample NT-754-2: Coarse-grained granite-looking ortho-

gneiss with aligned biotite and subhedral texture. The same
locality as NT-753. The plagioclase is subhedral, entirely
sericitized containing small secondary phengitic mica.
K-feldspar (ca. 10 %) is mostly interstitial and finely perthit-
ic. The biotite (10 %) dark red-brown to straw yellow, forms
big flakes and encloses apatites, zircon and monazite. The
flakes are weakly aligned. It contains small excluded ores,
epidote—clinozoisite and secondary titanite. The muscovite
(6 %) intergrows with biotite and is commonly cross-orient-
ed. It has the composition of primary muscovite – relatively
pure with high TiO

2

 (max. 1.6 %). The Ti locally exsolves in

lamellae of secondary rutile. Small phengitic mica replaces
plagioclase cores. The accessories are represented by thick
columnar apatite, ores by goethitized pyrrhotite. The mona-
zite was found enclosed in biotite and quartz. This sample is
characterized by the monazite breakdown to REE-clinozoisite
(allanite), apatite and a Th phase (huttonite?) (Fig. 2A—B).

Sample NT-757: Massive, weakly banded biotite-garnet

gneiss. Outcrop on the forest road in the Gelfúsová  dolina

Table 2:  Representative analyses of studied monazites and xenotime. The crystalochemical formulae are based on 16 oxygens. Temperatures
(

°C) calculated according to Pyle et al. (2001) are given for samples NT-27 and 761. n.d. – element below detection limit, n.a. – element

not analysed.   Continued on the next page.

Valley, elevation 975 m a.s.l. The plagioclase is sub- to
anhedral, partly sericitized, bigger grains are slightly
zoned. Basicity is An

20—31

. Smaller grains are more basic.

The biotite is very abundant (45 %) fresh, dark red-brown
to straw yellow. It defines the banding of the rock. The
garnet (2 %) is a characteristic metamorphic mineral, ar-
ranged in biotite bands, where it forms large (0.2—0.5 mm)
anhedral, atol-like grains. In quartz-rich bands it is smaller
and euhedral. The garnet is not altered. Sillimanite com-
monly intergrows with biotite, in places it is overgrown by
muscovite. Myrmekite occurs occasionally. The accesso-
ries are abundant, plagioclase contains needle-like apatite,
zircon and monazite. Biotite-hosted monazite (Fig. 3E—F)
is surrounded by thin REE epidote rims. A xenotime was
identified by microprobe.

Sample NT-761: Finely-banded orthogneiss of tonalite-

granodiorite composition. The same locality as NT-27.
The rock consists of quartz, plagioclase, biotite, musco-
vite and K-feldspar. The bands are 1—2 mm thick defined
by micas. The dark red-brown biotite is abundant, fresh,

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233

ELECTRON MICROPROBE DATING OF MONAZITE (WESTERN CARPATHIANS, SLOVAKIA)

657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715

716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774

Table 2:  Continued.

commonly intergrown with muscovite the latter being also
cross-oriented. As in sample NT-27 from the same locality,
two varieties of K-feldspar (12 %) can be recognized: the
larger perthitic K-feldspar concentrated in stripes is
aligned along the foliation and may represent former au-
gen. The non-perthitic K-feldspar with undulous extinc-
tion forms small anhedral pools suggesting the presence of
partial melt. The sub- to anhedral sericitized plagioclase
has basicity of An

26

. The rock is rich in accessories, which

are hosted mostly by biotite, and represented by monazite,
zircon, xenotime and apatite (Fig. 3A—C). The monazite

and zircon form distinct halos. The biotite-hosted mona-
zite overgrows by thin REE-epidote rims.

Sample NT-764c: Biotite-muscovite-quartz-gneiss. Road

cuts on the road Tále—Krpáčovo, elevation  840 m a.s.l., ca.
2.5 km NW from Bystrá dolina Valley. The rock is leuco-
cratic composed mainly of quartz (50 %) and muscovite
(15 %) with subordinate amount of biotite (9 %). The mus-
covite forms large flakes intergrown with biotite or cross-
oriented. In places it forms fine symplectites with quartz.
The biotite is dark-brown, pale yellow, fresh. Uncommon
K-feldspar (3 %) occurs as subhedral perthitic crystals

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234

PETRÍK, KONEČNÝ, KOVÁČIK and HOLICKÝ

775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833

834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892

(1.2 mm) and small interstitial non-perthitic crystals. The
plagioclase is very rare (1 %), strongly retrogressed to
sericite. Measurable places have basicity of An

11

. The ac-

cessories are dominated by very common and large iso-
metric apatite grains (up to 0.5 mm in size) commonly
associated with muscovite. The monazite is small hosted
by the apatite or micas (Fig. 2F).

Monazite

Occurrence of monazite

Orthogneisses contain monazite 40—120  m in size,

enclosed by biotite, plagioclase and quartz. The mona-
zite in biotite seems to be least stable frequently showing
a loss of LREE, and replacement of them by Y + HREE. In

the sample NT-754-2 a more extensive replacement oc-
curs, whereby monazite breaks down to form a secondary
apatite enriched in Th and, along the biotite cleavage,
REE-enriched epidote (allanite). The secondary apatite
contains very small exsolutions of a Th mineral (hutton-
ite?), which follow original boundaries of monazite
(Fig. 2A,B). In orthogneisses NT-761, 754-2 xenotime
was also found. Samples NT-12, 27 have monazites over-
grown by REE-epidote rims, but monazite is still not re-
placed by apatite. The alteration only causes the
formation of irregular dark domains enriched in Y and
heavy REE. It is probably a variant of the monazite
breakdown described by Broska & Siman (1998) and
Finger et al. (1998). A difference is the formation of the
Th silicate rimming apatite.

The monazite from paragneiss NT-757 forms isometric

grains (30—50  m) enclosed in biotite, plagioclase or

Fig. 4. The correlation Ca + Si vs. Th + U + Pb  in  studied monazites. Numbers refer to mol. % of brabantite ( + huttonite).

Fig. 5. Compositional range of studied monazites expressed in terms of end-members.

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235

ELECTRON MICROPROBE DATING OF MONAZITE (WESTERN CARPATHIANS, SLOVAKIA)

893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951

952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010

quartz. At rims it is frequently overgrown by allanite (or
REE epidote), if located in biotite also by small secondary
titanite. The secondary epidote rim in paragneisses is
much thinner than in orthogneisses. Paragneiss NT-757
also contains xenotime and zircon.

Monazite chemical variability

The studied monazites are chemically characterized by

ca. 140 analyses for 15—18 oxides (except NT-764c, Ta-
ble 2). The chemical composition of monazite [LREE(PO

4

)]

is expressed in terms of solid solution end-members: hutto-
nite  Th(SiO

4

), brabantite CaTh(PO

4

)

2

 and xenotime

Y(PO

4

). Monoclinic monazite is isostructural with hutton-

ite and brabantite, and tetragonal xenotime is isostructural
with zircon. Monazite compositional variation can be ex-
pressed by the following substitutions:

2LREE

3+

=(Th, U)

4+

+Ca

2+

(Mnz Brb), LREE

3+

+P

5+

=(Th, U)

4+

+Si

4+

(Mnz

Hutt) and 2LREE

3+

=Y

3+

+H REE

3+

(Mnz Xno).

The first two substitutions correlate positively in coor-

dinates (Th + U + Pb) vs. (Si + Ca) (Zhu & O’Nions 1999).
From Fig. 4A,B it follows that these substitutions are re-
sponsible for the compositional variations of all mona-
zites. The analyses are shifted above the correlation 1 : 1,
probably due to slightly increased Ca. The largest composi-
tional range (up to 16 mol. % of Hutt + Brb) has NT-27 au-
gengneiss mainly due to high Th cores (see below). NT 754-2
and 761 are restricted to ca. 8 mol. % and monazites from
NT-12, 757 to about 5 mol. %. The biotite schlier NT-753
hosts the purest monazite. Monazite composition in terms
of (Mnz + Hutt)-Brb-Xno is shown in Fig. 5. Paragneiss
monazite has 84—90 mol. % monazite, 6—10 mol. % of bra-
bantite and 4—7 mol. % of xenotime. Orthogneiss mona-

Fig. 6. Histograms of monazite ages: (A) orthogneisses (NT-27, 754-2, 12, 761), (B) paragneisses (NT-764c, 757).

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236

PETRÍK, KONEČNÝ, KOVÁČIK and HOLICKÝ

1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069

1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129

zite is similar, showing a wider spread towards
brabantite end-member (ca. 6—19  mol. %) and
keeping 7—10 mol. % of xenotime.  A lower xe-
notime is characteristic of paragneiss monazites
which also show a low huttonite proportion (not
shown).

Monazite Th correlates positively with U with av-

erage Th/U ratio 9.3 ± 4.1. Individual samples show
similar values (NT-12 = 5.8 ± 1.7, NT-27 = 8.9 ± 1.7
and NT-761 = 7.7 ± 1). The exception is weakly
foliated sample NT-754-2 with higher Th/U ratio
12.3 ± 6.3 due to lower U.

Dating results

All the age data are listed in Table 3. The ages ob-

tained from the studied rocks are presented in histo-
grams (Fig. 6A,B) for individual samples. The total
number of analysed points is 196, all errors are 2  .

NT-12

The weighted average from 17 points mea-

sured on six monazites gives 349 ± 17 Ma
(MSWD = 0.77).  The monazites, mainly biotite-
hosted, are commonly surrounded by epidote co-
ronas. Th—U distribution is scattered monazites
having lower Th compared to other samples. The
Th-enriched corona epidotes (allanite) seem to act
as Th sinks in this tonalitic rock. Th escape may
be partly counterbalanced by of U influx, which is
documented by the good correlation of U with Y.
Y-enriched domains are darker and clearly indi-
cate a later overprint.

NT-754-2

The age based on all 25 points from 11 grains

gives a weighted average of 354 ± 15 Ma. The
MSWD = 1.6 suggests a heterogeneous data set.
Omitting seven outlying ages above 380 and below
310 Ma gives a new homogeneous set (n = 18) with
a weighted average of 345 ± 13 Ma (MSWD = 0.48).
The high ages (392—501 Ma) possibly indicate the
presence of older monazite cores although they do
not differ in composition from younger grains.
Fig. 7A provides an isochron representation with
the data divided into two groups. Both groups are
well distinguished: while the younger set has al-
most zero intercept, the isochron age of the older
set has a significant negative intercept and is
clearly overestimated. The monazites hosted by
biotite or muscovite are apparently not stable,
they break down into the Th-enriched apatite and
a corona of small grains of secondary huttonite(?).
This breakdown may contribute to younging of
the ages through opening of the system as sug-
gested by the scattered Th—U distribution. Mona-

Table 3:  Results of electron microprobe dating of monazite. m1-1 – mona-
zite grain-spot, an – analysis.   Continued on the next pages.

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237

ELECTRON MICROPROBE DATING OF MONAZITE (WESTERN CARPATHIANS, SLOVAKIA)

1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188

1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1338
1239
1240
1241
1242
1243
1244
1245
1246
1247

Table 3:   Continued.

zites enclosed in quartz are more stable giving the
age about 340 Ma.

NT-27

Thirteen grains from this sample were analy-

sed by 52 points giving total weighted average
383 ± 19 Ma and high MSWD = 8.7. The data set
splits into two groups in the histogram, the
younger  giving an average 340 ± 8 Ma, (n = 42,
MSWD = 0.95). It shows two peaks at 320—330 Ma
and 360—370 Ma. The older group (n = 10) gives
475 ± 19 Ma  (MSWD = 2.3), six of these points
come from one grain m-7 and show a high Th com-
position (7—9.5 % Th). The isochrons in Fig. 7B
confirm both weighted averages having zero inter-
cepts. Due to increased Ca the old core has braban-
tite content up to 15—19 mol. %. Monazites are
hosted by all minerals, mostly by quartz and pla-
gioclase, less by muscovite and biotite. The mona-
zite ages do not differ in various hosts: 345 ± 13 Ma
in biotite or muscovite, 341 ± 12 Ma in quartz and
plagioclase (except the old core data). Quartz-host-
ed monazites seem to preserve a slightly older age
(354 ± 13 Ma).

NT-761

Three grains were measured, one of them by a de-

tailed profile. Twenty nine point ages give the
weighted average 318 ± 11 Ma. The MSWD = 2.4
indicates a heterogeneous set. A detailed profile
across m-8 monazite (17 points, Fig. 3B) also yields
322 ± 10 Ma and MSWD = 0.68. This euhedral grain
is hosted by a K-feldspar pool, possibly indicating
partial melt. All measured points define a non-dis-
turbed correlation of Th vs. U (Th/U = 7.7 ± 1)
which supports their younger age. The profile age
is the same as the peak in the histogram, giving
the weighted average 322 ± 8 Ma (MSWD = 0.46).
The strongly foliated orthogneiss NT-761 thus
may date the last deformation – a melting event
with the lower age limit about 325 Ma.

NT-757, 764c

From the two metamorphic rocks sampled in re-

mote localities (Gelfúsová dolina Valley and
Tále—Krpáčovo, Fig. 1), we obtained 56 point data
which split into three groups in histograms
(Fig. 6B). The oldest group (n = 5) gives a weighed
average of 454 ± 38 Ma (MSWD = 0.043). The old
ages are found in both samples, always situated in
the centers of monazite grains, and therefore, are
considered inherited Ordovician cores. The main
group of ages (n = 48) gives a well defined average
of 355 ± 6 Ma (MSWD = 0.91). However, the base
of the peak is 110 Ma (NT-764c), twice a value
typical of monogenetic standard, which indicates

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238

PETRÍK, KONEČNÝ, KOVÁČIK and HOLICKÝ

1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306

1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365

that more than one age is recorded by the mona-
zites. The distinct peaks around 330, 350 and
390 Ma are tentatively ascribed to the same three
metamorphic events as recorded by metamorphic
monazites from the Tatra Mts (Konečný et al.
2004). The youngest seems to correspond to the de-
formation – melting event of orthogneisses and
migmatitization in the Vysoké Tatry Mts (Janák et
al. 1999; Poller & Todt 2000).

Discussion

Y in monazite thermometry

Heinrich et al. (1997) and Pyle et al. (2001)

found that the xenotime component in metamor-
phic monazite rises with temperature. Orthogneiss-
es NT-761, NT-754-2 and paragneiss NT-757
contain xenotime, which buffers Y and makes it
possible to use these empirical thermometers.
The logarithmical fit of Pyle et al. (2001) is pre-
ferred because empirical data are calibrated
against metamorphic temperatures and cover a
wider range of X

(Y + HREE)

. Compared to Heinrich

et al. (1997) data, it yields temperatures lower
by ca. 80 ºC. The fit is given by the equation:
T  (ºC) = 299.65*ln[X

(Y + HREE)

] + 1315.2. Only analy-

ses containing the same HREE (Gd, Dy Ho, Er and
Yb) as in the original calibration were used. The
NT-27 sample gives an average 613 ± 20 ºC (n = 11,
all errors due to range of Y + HREE). The profile in
the m-8 grain shows a peak in Y contents and corre-
sponding temperatures about 650 ºC in centre of
grain and ca. 600 ºC at rims. The ages in the center
vs. rim, however, do not differ significantly. The av-
erage for all 28 points is 625 ± 27 ºC. Other orthog-
neiss samples with no xenotime found in thin
sections, or incomplete HREE data give lower tem-

Fig. 7. Isochron diagrams of NT-27 and NT-754-2 orthogneisses. Bars indicate 1  error of radiogenic lead concentrations (B), the same
error is within the size of the symbols in (A). While zero intercepts in (A) confirm weighted averages, the negative intercept (—0.02)
in (B) indicates an overestimation of the older group age.

Table 3:   Continued from the page 237.

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239

ELECTRON MICROPROBE DATING OF MONAZITE (WESTERN CARPATHIANS, SLOVAKIA)

1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424

1425
1426
1427
1528
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483

Fig. 8. Correlation of temperature after Pyle et al. (2001) with the
age of monazite. Temperature was calculated only for points with
analysed Gd, Dy, Ho, Er, Yb and Y.

peratures about 550 ºC, which may be considered a low sta-
bility limit.

The paragneiss monazite has a significantly lower xeno-

time end-member: 4—8 mol. % (compared to 7—10 mol. %
in orthogneisses). The difference is explained by the pres-
ence of garnet in  biotite gneiss NT-757 where Y is parti-
tioned between monazite and garnet. The increased Y in
the rock is mineralogically documented by the occurrence
of xenotime. The presence of latter means that garnet
grows at buffered a

YPO4

. Pyle & Spear (2000) found that

the Y concentrations in monazite coexisting with xeno-
time increase with temperature while those in garnet de-
crease. Y in garnet is at the detection limit of the
microprobe in NT-757. The empirical equation of Pyle et
al. (2001) gives temperatures between 380—530 ºC with an
average of 440 ºC. They are considered to be the lower
limit because Dy and Ho were not analysed in this mona-
zite. The temperatures obtained in three orthogneiss sam-
ples do not correlate with the age (Fig. 8), which means
that we cannot attribute an age to a metamorphic event.

Ages 440—500 Ma

The age about 460—490 Ma was initially indicated by

two grains from sample NT-27. Repeated analyses situated
close to the former points confirmed the old core of mona-
zite m-7 (Fig. 2C,D). Moreover, the analyses showed that
composition of the core differs significantly from the bulk
of other monazites in higher Th/U ratio and increased bra-
bantite proportion.

The Ordovician age is a characteristic age of ortho-

gneisses from many Variscan domains. It is well docu-
mented from orthogneisses of the Moldanubian Zone of
the Bohemian Massif (Gföhl gneiss; Friedl et al. 2004),
southern Brittany (Brown & Dallmeyer 1996) or the Mas-
sif Central (Roger et al. 2004). Ordovician orthogneisses
also occur in the Variscan basement involved in the Al-
pine orogen (Schultz et al. 2004). In the Western Car-
pathians Janák et al. (2002) obtained the age of

468 ± 24 Ma (chemical monazite method) from a northern
Veporic metagranitoid which was metamorphosed under
high P-T conditions at ca. 342 ± 27 Ma. Another Carpathian
Ordovician rock is the Muráň gneiss from the southern Ve-
poric Unit dated at 464 ± 35 Ma by zircon vapour digestion
(UI, Gaab et al. 2005). A younger age of 405 ± 5 Ma was ob-
tained for Západné Tatry (Western Tatra) orthogneiss
(Poller et al. 2000). Putiš et al. (2001) dated zircons
from trondhjemite associated with banded amphibolite
(514 ± 24 Ma). The increasing recognition of pre-Variscan
rocks suggests involvement of the Carpathian basement in
the north Gondwana-derived Hun Superterran (von Raumer
et al. 2002).

It is, therefore, tempting to ascribe the monazite age

475 ± 19 Ma to the age of Nízke Tatry orthogneisses as well.
This, however, encounters problems: First, only one old core
was actually found among 13 measured monazite grains,
which otherwise give middle and late Variscan ages. Second,
the composition of the old core is quite unique among the
number of monazites analysed so far. Their high Th/U ratio
12—17 differs even more from the whole rock ratio (3—6), than
the bulk of Variscan monazites which typically have ratios of
8.5—10. This suggests a different source rock and thus a res-
tite rather than a magmatic origin of the old cores. A similar
old age (ca. 500 Ma, Gurk 1999) was obtained from clastic
metapelite zircons from the lower unit in the Západné Tatry
Mts. Old cores (540—550 Ma) revealed by chemical monazite
dating in Variscan (330 Ma) migmatites from the Cévennes
area of the Massif Central (Be Mezeme et al. 2006) were re-
cently interpreted as inherited.

The Ordovician age of granitic protolith of the orthog-

neiss would imply Early Paleozoic or Neo-Proterozoic
wall rocks as are demonstrated in the Armorican massif or
the Moldanubian Zone (Friedl et al. 2004). The old mona-
zite cores found in biotite gneisses (440—480 Ma), there-
fore, seem to have been inherited and this age indicates
rather, that the basin where the original protolith of para-
and orthogneisses was sedimented, drained an area of a
denudated Cadomian fundament.

Ages 340—350 Ma

The weighed averages of two samples (NT-12, 27, 754-2)

falling within the interval 340—350 Ma are based on the
largest number of measurements (60). This would mean that
the dominant mass of monazite formed and/or recrystal-
lized at this time implying the Variscan age of the parental
rock. The points forming a small peak at 380—390 Ma were
checked in view of the existing discordant zircon age
381 ± 6 Ma (Putiš et al. 2003). However, repeated measure-
ments located closest to original points failed to confirm
this age, producing younger data. The period 340—360 Ma
is the age of the main Variscan collision in the Western
Carpathian basement when, earlier in this interval, a medi-
um-P metamorphism took place followed by syncollision-
al S-type granites intrusions at ca. 350 Ma (Poller et al.
2000; Poller & Todt 2000; Finger et al. 2003).

On the other hand, syntectonic orthogneisses should

pre-date the post-tectonic undeformed Nízke Tatry to-

background image

240

PETRÍK, KONEČNÝ, KOVÁČIK and HOLICKÝ

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nalite (Ďumbier type) dated at 343 ± 3 Ma by Putiš et al.
l.c., and 330 ± 10 Ma by Poller et al. (2001) by common
lead. Resolution of monazite chemical dating with the
typical base of age peak corresponding to 60 Ma does not
allow us to establish a precise intrusion age. Since mona-
zites recrystallized at a lower temperature (ca. 600—650 °C)
compared to zircon, the obtained interval of 340—350 Ma
is considered a lower limit for orthogneiss formation.

Ages 320—330 Ma

Distinguishing between the monazite ages of about 350

and 330 is clearly at the edge of the chemical method pos-
sibilities, the distinct peaks in both ortho- and paragneiss-
es (Fig. 6) nevertheless suggest a thermal event ca.
330 Ma ago. The foliated orthogneiss NT-761 gives such
a younger age (325 ± 10 Ma) which may record the same
event as is documented by numerous zircon and monazite
data from the Western Carpathians (Poller & Todt 2000;
Poller et al. 2001; Finger et al. 2003). During this post-col-
lisional event many I-type granitoids were emplaced in
Tatric and Veporic Units. The thermal event which pro-
duced these granitoids was accompanied by rapid uplift
and decompression melting. Such melting was document-
ed in the Západné and  Vysoké Tatry (High Tatra) migma-
tite (orthogneiss?) by Janák et al. (1999) and dated at
332 ± 5 Ma by Poller & Todt (2000). An analogical situa-
tion is suggested by monazite data from the Nízke Tatry
orthogneisses. The sample NT-761 seems to have been re-
melted  via muscovite dehydration melting. It contains
new K-feldspar, apparently a product of the reaction:
Mu + Qtz = Kfs

melt

+ Sill (650—700 °C and 600—700 MPa,

Spear et al. 1999). The Y in monazite indicates an average
temperature of 601 ± 75 ºC (Pyle et al. 2005, error due to
range of Y), which means that it largely recrystallized lat-
er, in subsolidus conditions. The 330 Ma event, which is
also recorded by metamorphic monazites, appears impor-
tant in the Nízke Tatry Mts. The same age, 330 Ma, ob-
tained also by muscovite 

40

Ar/

39

Ar dating (Dallmeyer et

al. 1993) suggests a phase of rapid cooling following the
decompression (Brown & Dallmeyer 1996).

Conclusions

Monazite compositions from both the orthogneiss and

paragneiss of the Nízke Tatry Mts reflect their respective
meta(sedimentary) protoliths. The main substitutions in
both rock types are monazite—xenotime (max. 11.5 mol. %)
and monazite—brabantite (max. 11.8 mol. %, the old core in
NT-27 up to 19 mol. %), and to a lesser extent monazite—
huttonite (max. 3 mol. %). Monazites from all orthogneisses
appear unstable, showing local escape of the light REE,
which are captured in REE-epidote rims. In more advanced
degrees of the alteration monazite breaks down to Th-en-
riched apatite, REE-epidote and huttonite(?).

Extensive chemical dating of the monazite from Nízke

Tatry orthogneisses has shown that monazite can record
both old events in its cores and younger history in over-

grown and/or recrystallized rims. The overwhelming mass
of monazite gives Variscan ages between 340—350 Ma in-
dicating a strong Variscan reworking including partial
melting. The reason why monazite failed to record reliably
a primary age of the syn-tectonic intrusion probably re-
sults from its recrystallization, which seems to have oc-
curred at relatively low temperatures, typically between
600—660 °C. Although dissolution—precipitation process-
es culminated 350—340 Ma ago, the points giving range
between 380—440 may remember older processes. The
well defined old cores (ca. 480 Ma) found in one ortho-
gneiss sample are at present too scarce to give a solid basis
for a primary Ordovician age of the orthogneisses, even
though this possibility cannot be entirely dismissed.

Monazite from one strongly foliated sample also records

a younger event at ca. 330 Ma which is interpreted as the
end of the decompression phase (at 600 °C) and beginning
of the phase of rapid isobaric cooling. This is corroborated
by the coincidence of monazite and muscovite 

40

Ar/

39

Ar

ages. The muscovite is thought to have formed during ret-
rogression of the orthogneisses after the phase of partial
decompression melting. Such a P-T path is characteristic
of many Variscan plutons in the Western Carpathians
(Malá Fatra, Kohút et al. 1997; Tatra Mts, Janák et al.
1999) and other Variscan domains (southern Armorica,
Brown & Dallmeyer 1996).

Acknowledgment:

 This work was supported by the Project

GA-4096 (VEGA, Slovak Grant Agency) and Project 45s6
(Aktion Österreich—Slowakei) to I.P. All reviewers F. Fin-
ger, I. Broska and M. Janák are acknowledged for critical
reading of an earlier version of the manuscript. F. Finger is
thanked for his invaluable help with microprobe dating
work.

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