GEOLOGICA CARPATHICA, 51, 1, BRATISLAVA, FEBRUARY 2000
TRIASSIC AGE OF THE HRONÈOK PRE-OROGENIC A-TYPE
GRANITE RELATED TO CONTINENTAL RIFTING: A NEW RESULT
OF U-Pb ISOTOPE DATING (WESTERN CARPATHIANS)
, ALEXANDER B. KOTOV
, PAVEL UHER
EKATHERINA B. SALNIKOVA
and SERGEI P. KORIKOVSKY
Department of Mineralogy and Petrology, Faculty of Science, Comenius University, Mlynská dolina, 842 15 Bratislava,
Slovak Republic; firstname.lastname@example.org
Institute of Precambrian Geology and Geochronology, Russian Acad. of Sci., Makarova Embk. 2, 199034 St. Petersburg, Russia
Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 26 Bratislava, Slovak Republic
Institute of Geology and Ore Dep., Petrogr., Miner., Geoch., Russian Acad. of Sci., Staromonetny per 35, 109017 Moscow, Russia
(Manuscript received December 7, 1998; accepted in revised form September 28, 1999)
Abstract: The Hronèok leucocratic biotite granite represents a small intrusion located in the basement rocks of north-
ern part of the Veporic Superunit (Central Western Carpathians, the Vepor Mts., Slovakia). The rock geochemistry,
zircon typology and zircon Zr/Hf ratios reveal their A-type geochemical features and the higher degree of fraction-
ation. Isotope U-Pb data yielded a discordia line with a lower intercept age of 238.6 ± 1.4 Ma (MSWD=1.2), which is
interpreted as the crystallization age of the granite. This result shows a younger magmatic event than has often been
ascribed to the post-Variscan plutonic activity in the pre-Alpine basement of the Western Carpathians up to now.
However, it is synchronous and geochemically analogous with the acid volcanism, products of which occur in the
Veporic basement and its Permian-Lower Triassic cover rocks as well as within Triassic sequences of the Silica
Superunit. All previously given examples of magmatic/volcanic activity are considered to be indicators of early
Alpine continental rifting.
Key words: Triassic, Western Carpathians, Veporic Superunit, Hronèok A-type Granite, U-Pb isotope dating, zircon.
The Hronèok Granite focused the attention of West-Car-
pathian geologists for many years, in spite of its relatively
small outcropped area. The reason is based mainly on specif-
ic petrographic, geochemical and especially age data, as
compared to adjacent granitic rocks. The characteristic fea-
tures of the granite were described by Zoubek (1936), who
characterized the rock as Variscan orthogneiss due to its ex-
tensive metamorphic overprint. A distinctly porphyric struc-
ture and mylonitic nature of the Hronèok Granite have been
emphasized also by Andrusov (1958), Cambel et al. (1961),
Hók & Hrako (1990), Pitoòák & Spiiak (1994), Petrík et al.
(1995) and Puti et al. (1997). On the basis of geochemical
data, especially trace elements, as well as accessory zircon
typology and Zr/Hf ratio, the Hronèok Granite was interpret-
ed as a post-orogenic Variscan A-type granite (Petrík et al.
1995; Uher & Broska 1996; Petrík & Kohút 1997).
The first age isotope data of the Hronèok Granite were ob-
tained by the K-Ar method on biotite and K-feldspar which
gave Cretaceous model ages, 120 and 115 Ma, respectively
(Kantor 1959; recalculated by Cambel et al. 1990), later K-
Ar biotite data yielded an Alpine age again (96 Ma, Cambel
et al. 1979, 1990). The zircon U-Pb model age, 255 Ma
(Cambel et al. 1977) and whole rock Rb-Sr isochrone ages:
285 ± 5 Ma, 253 ± 2 Ma (Cambel et al. 1989), recalculated
later to 262 ± 29 Ma (Petrík et al. 1995), indicated a post-
Variscan, Permian age of magmatic crystallization of the
Hronèok Granite. Recent U-Pb zircon discordia ages of adja-
cent subvolcanic felsic dykes show a broad interval of Per-
mian to Triassic ages (278 ± 11 to 216 ± 5 Ma, Kotov et al.
1996). These relatively scattered results led us to analysing
the Hronèok Granite s.s. by U-Pb method again to obtain
new and more precise data. The group of A-type magmatic
rocks is moreover quite a reliable indicator of continental
rifting (Bonin 1987, 1990) geological setting, in this case re-
lated to the early Alpine time period.
Geological setting and rock description
The sample analysed (HRO-1) is a typical example of the
Hronèok Granite, as defined by Zoubek (1936) and Petrík et
al. (1995). The rock was collected from a large outcrop near
a forest road in the Kamenistá Valley, ca. 1 km NW of the
Èierny Potok gamekeepers lodge, and ca. 10 km SW of
Èierny Balog village (Fig. 1). The HRO-1 location is identi-
cal with the location of the ZK-26 sample of the Hronèok
Granite (Macek et al. 1982); thus, some zircon data were ob-
tained from this sample.
The Hronèok Granite forms an elongated body, ca. 7.5 km
by 1.52 km in size, in the NW part of the Veporic Superunit
of the Central Western Carpathians. Small isolated strongly
mylonitized outcrops of the Hronèok Granite also occur in
the northern surroundings of the main body or as dykes of
very fine-grained aplitic texture (Petrík et al. 1995; Petrík
60 PUTI et al.
1996). The host rocks are Early Paleozoic (?) medium to
high-grade metapelites-metapsammites (paragneisses to mig-
matites), locally also amphibolites and orthogneisses of the
Èierny Balog Complex (Krist et al. 1992; Puti et al. 1997).
The Pohorelá thrust fault represents the western tectonic
boundary of the Hronèok Granite: the granitic body (along
with Èierny Balog Complex) is thrust over low-grade Late
Carboniferous (?) metasediments of the Krak¾ová Formation
(Fig. 1). Miocene calc-alkaline andesites and pyroclastics
form the southern boundary of the Hronèok Granite body.
The Hronèok Granite is a medium-grained rock, locally
with porphyric pinkish or greyish K-feldspar phenocrysts,
0.5 to 2 cm in size. Oriented mylonitic fabric is widespread.
On the basis of modal composition, the rock shows leuco-
cratic biotite monzogranite composition (quartz 36.2,
plagioclase+muscovite 35.0, K-feldspar 24.4, biotite 3.9,
garnet 0.6 vol. %; Petrík et al. 1995). Quartz is anhedral and
strongly undulous, subhedral plagioclase is often replaced by
fine-grained muscovite (phengite variety), clinozoisite, and it
is locally albitized, K-feldspar locally shows Carlsbad twin-
ning. Biotite is subhedral to anhedral, yellow to dark brown,
often with oriented acicular rutile (sagenite variety) inter-
growths, locally replaced also by fine-grained muscovite
(phengite). Euhedral to subhedral garnet with a very peculiar
) is the most widespread accessory mineral. The
calcium-rich composition of the garnet and its occurrence to-
gether with phengite and albite in altered parts of plagioclase
indicates a post-magmatic character of the mineral caused by
low-grade Alpine metamorphic overprint, as it is a wide-
spread feature in the Veporic Superunit area (e.g. Vrána
1980; Puti 1991; Méres & Hovorka 1991; Puti et al. 1997;
Korikovsky et al. 1997). Zircon, apatite, monazite-(Ce), xe-
notime-(Y), allanite-(Ce), ilmenite, anatase, magnetite, and
pyrite are other accessory minerals found in the Hronèok
Granite (Petrík et al. 1995; Uher & Broska 1996).
On the basis of geochemical data (Table 1), the Hronèok
Granite is a leucocratic low-Ca and moderately Si- and Na-
Fig. 1. Schematic geological map of the Hronèok Granite body and the surrounding Tatro-Veporic complexes (after Puti 1995 and Kotov et
al. 1996). The position of the HRO-1 and ZK-26 samples in the Hronèok Granite is indicated. CB-1c metarhyodacite porphyry, GL-3 meta-
rhyodacite and GL-4 metagranite porphyry represent adjacent Permian-Triassic pre-orogenic acid rocks, dated by the U-Pb method (Kotov
et al. 1996). Explanations: 1 Undifferentiated Tertiary and Mesozoic rocks; 2 Tatric crystalline basement complexes; 3 Tatric (Per-
mian-Cretaceous) cover rocks; 4 Permian/Triassic low-grade volcano-sedimentary rocks (Bacúch Formation): subvolcanics (a), volcanics
and pyroclastics (b); 5 Carboniferous (?) metasediments (Krak¾ová Formation, Korikovsky & Miko 1992); 6 ¼ubietová medium/high
grade crystalline complex (Early Paleozoic?); 7 Hron medium-grade crystalline complex (Early Paleozoic?); 8 low-grade Permian-
Cretaceous cover rocks of the North-Veporic crystalline basement; 9 composite Èierny BalogVe¾ký Zelený Potok medium/high grade
crystalline complex; 10 Late Carboniferous granites to tonalites of the late Variscan Vepor pluton (ca. 303290 Ma, U-Pb, Bibikova et al.
1990) altered to Cretaceous low-grade granite-mylonites (Puti 1991; Dallmeyer et al. 1996); 11 Triassic Hronèok Granite and its Creta-
ceous low-grade granite-mylonites; 12 undivided the South-Veporic crystalline basement; 13 low-grade Permian-Triassic cover of the
Veporic crystalline basement; 14 main mid-Cretaceous thrust/transpression fault zones; 15 Krína Mesozoic Nappe; 16 Choè and
Muráò higher (unmetamorphosed) Mesozoic nappes; 17 internal thrust; 18 fault, or tectonic boundary; 19 geological contact.
TRIASSIC AGE OF THE HRONÈOK PRE-OROGENIC A-TYPE GRANITE 61
Table 1: Whole-rock compositions of the Hronèok Granite (main ele-
ments in wt. %, trace elements in ppm). HRO-1: orig. data, analytical
methods: XRF (main elem.), INAA (traces), lab.: IGEM, Moscow;
ZK-26: Cambel & Walzel (1982), Petrík et al. (1995), analytical
methods: wet (main elem.), OES (traces, lab.: Geol. Inst., SAS, Bra-
tislava) and INAA (traces, lab.: Strá pod Ralskem, Czech Rep.).
99.54 100.18 V
rich peraluminous granite. Relatively lower REE abundances
with strongly negative Eu-anomaly together with high I
0.710 documented a significant role of fractionation from a
continental crustal source material. Elevated contents of K, Y,
HREE, Ga, Nb, high Ga/Al, Fe/Mg ratios, very low P and V
contents, as well as Fe-rich biotite (close to annite, Petrík et al.
1995) and zircon typology (see the paragraph below) indicate
the A-type character of the Hronèok Granite (Petrík et al.
1995; Uher & Broska 1996; Petrík & Kohút 1997).
Zircon of the Hronèok Granite forms euhedral columnar
crystals, 40 to 250
m in length, translucent to transparent
Fig. 2. SEM microphotographs of the Hronèok Granite zircon (ZK-26 sample). A G
subtype, B S
subtype (after classification
of Pupin 1980). Scale bar: 100
with pinkish to yellow colour. The minerals have a length/
width ratio of 2.55.0. Zircon occurs as tiny inclusions in bio-
tite with characteristic pleochroic halos, or in plagioclase, lo-
cally overrimed by younger garnet. G
are the characteristic morphological types of the
Hronèok Granite zircon (sensu classification of Pupin 1980),
Figs. 2 and 3; they indicate a high alkalinity and moderate to
lower temperature of zircon origin (~ 700800 ± 50 °C). The
calculation of zircon saturation temperature (Watson & Har-
rison 1983) gave 780790 °C for the Hronèok Granite (Uher
& Broska 1996).
BSE microphotographs reveal strongly oscillatory zoning
of zircon crystals, locally with small inherited cores and gen-
erally Hf-enriched outer, and especially intermediate zones
(Fig. 4). Electron microprobe analyses documented a rapid
increase of HfO
content from 0.9 wt. % in the centre to 2.1
wt. % in intermediate parts of the crystals, together with no-
table P, U and Y increase; the narrow rim zones are often
poor in these elements (Table 2). This internal texture and
geochemical trend support a primary magmatic origin of zir-
con without extensive younger replacement phenomena and
metamictization, as well as only with small amount of old in-
herited zircon. Thus, such zircon from the investigated sam-
ple of the Hronèok Granite is suitable for further geochrono-
Zircon crystals were extracted from ca. 15 kg of the solid
granite rock by heavy liquid (bromophorm, methyleniodide)
and electromagnetic separation. Non-magnetic hand-picked
62 PUTI et al.
zircon from the sieve fraction >100
m and < 60
used for geochronology.
Geochronological studies were performed at the Institute of
Precambrian Geology and Geochronology, Russian Academy
of Sciences, St. Petersburg, on Finnigan MAT 261 8-collector
mass-spectrometer in static mode. Zircon was analysed fol-
lowing the method of Krogh (1973). The total blanks were
0.050.1 ng Pb and 0.005 ng U. The error of the U/Pb ratios is
0.6 %. An air-abrasion treatment of the zircon was performed
by the Krogh (1982) technique, modified by coating abrasive
walls with epoxy impregnated with diamond powder. Ages
were determined using the decay constants given by Steiger &
Jäger (1977). All errors are reported at the 2
tions for common Pb were made using the values of Stacey &
For the uncertainties and correlations of U/Pb, we used the
PbDat and ISOPLOT programs (Ludwig 1991a,b). In our
case we used the U/Pb uncertainties for individual data
points. These are virtually identical for both
U (0.310.34 %) due to low errors for measured
Pb ratios. For example, the
Pb ratio for zir-
con < 60
m (No. 1, Table 3) at 313.89 was measured with
an error of 0.086 %. This also resulted in a high error correla-
tion factor. We also calculated our laboratory uncertainties in
the U/Pb ratios that were calculated on the reproducibility of
the standard zircon sample which is 0.6 %. Lower intercept
age of discordia, recalculated with these uncertainties for our
data points yielded practically identical values with previous
calculations. Uncertainty in the common lead isotopic com-
positions also did not lead to an essentially different lower
intercept age. For example, we have recalculated data for zir-
con < 60
m (Table 3) using 400 Ma common Pb isotope
composition (in the manuscript we corrected for 238 Ma).
U isotopic ratio changed from 0.2817 to 0.2818
U ratio changed from 0.03920 to 0.03922.
Thus, all these uncertainties are within the given errors.
Two sieve fractions of euhedral and mostly transparent zir-
con (< 60
m and >100
m) and one abraded up to 40 % zir-
con fraction (>100
m) were analysed (Table 3). On a con-
cordia plot all three data points are discordant (Fig. 5). A
discordia line constructed for these points defines a lower in-
tercept age of 238.6 ± 1.4 Ma and an upper intercept age of
1096 ± 44 Ma (MSWD = 1.2).
Discussion and conclusions
The U-Pb dating results
The data points for the Hronèok Granite zircon cluster near
the lower intercept of discordia. However, the points for un-
abraded and abraded zircon from the fraction > 100
further from the lower intercept of the discordia and contain
an older inherited component of radiogenic Pb (Fig. 5). The
small inherited cores are also visible in some BSE micropho-
tographs (Fig. 4). Taking into account the igneous origin of
Fig. 3. Zircon typogram of the Hronèok Granite (ZK-26 sample).
Average I.A = 683, average I.T = 332, I.A index of alkalinity,
I.T index of temperature (after Pupin 1980).
Table 2: Representative electron-microprobe compositions of the
Hronèok Granite (ZK-26 sample), oxides in wt. %. Cameca SX-50
WDS microprobe (Univ. of Manitoba, Winnipeg, Canada), 15 kV,
20 and 30 nA, 40 s counting time, natural and synthetic standards.
1/C E N
2/C E N
Formulae based of 16 anions
> 20 %
TRIASSIC AGE OF THE HRONÈOK PRE-OROGENIC A-TYPE GRANITE 63
the studied zircon, the lower intercept age (238.6 ± 1.4 Ma)
is interpreted as the primary magmatic crystallization age of
the Hronèok Granite. As the closure temperature of zircon in
the U-Pb system is very high, ca. 7751000 °C (Mezger &
Krogstad 1997), the boundary low-/medium-temperature Pa-
leoalpine metamorphism of this part of the Veporic Superunit
(around 500 °C, Puti et al. 1997) could not disturb the U-Pb
isotope system of magmatic zircon.
However, several examples showing that the U-Pb zircon
age calculated for the lower discordia intersection for the
multigrain analyses of Phanerozoic zircons is younger than
the age of the same concordant zircon (Steiger et al. 1993;
Salnikova et al. 1998; our unpublished data). To this moment
we may interpret the obtained age of 238.6 ± 1.4 Ma as the
minimum age of the studied zircons. However, taking into
account the good quality of the studied zircons (transparent,
fractures- and visible core-free grains have been used), we
suggest that this age should not have a significant difference
from the maximum age.
On the other hand, previous biotite and K-feldspar K-Ar
dating of the Hronèok Granite which gave Cretaceous model
ages, 120 to 96 Ma (Kantor 1959; Cambel et al. 1979 re-
calculated by Cambel et al. 1990) apparently reflect a young-
er post-magmatic event, which could be interpreted as the
cooling during exhumation after Paleoalpine thrust deforma-
tion and metamorphism of the Hronèok Granite.
It is also noteworthy, that the Proterozoic age of the upper
intercept of the Hronèok Granite (1096 ± 44 Ma), is almost
identical with the upper intercept of the Triassic (216 ± 5
Ma) metagranite porphyry of the Krak¾ová Formation from
the neighbouring Kamenistá Valley: 1094 ± 71 Ma (Kotov et
al. 1996). Similar data was yielded by a plagiogranitic vein
located in this area and cutting the basement rocks (233.2±
3.6 Ma for the lower intercept age, and 1080 ± 40 Ma for the
upper intercept age, unpublished data of authors). This strik-
ing fact could indicate the same protolith for all these Trias-
sic acid magmatic rocks, containing zircon inherited from
unknown Proterozoic metaigneous rocks.
Table 3: U-Pb isotopic data of the Hronèok Granite zircon (HRO-1 sample). Notes:
uncertainties (95 % confi-
dence level) refer to the last digits of corresponding ratios;
correlation coefficients of
U ratios; Sample >100A
40 % of zircon removed during air-abrasion.
Isotopic ratios corrected for blank and common Pb
Fig. 5. U-Pb concordia diagram for three discordant zircon frac-
tions from the Hronèok Granite (HRO-1 sample). Error ellipses
are calculated according to Ludwig (1991b).
Fig. 4. BSE microphotographs of the Hronèok Granite zircon (ZK-26 sample).
64 PUTI et al.
Comparison to the other granites and acid volcanites
The zircon U-Pb isotope data of the Hronèok Granite (HRO-
1 sample) gave the lower intercept discordia age of 238.6 ± 1.4
Ma, which is Early Triassic. The age is younger than other ra-
diometric ages of late Variscan granites in the Western Car-
pathians and similar ages are also unknown in the broader
neighbouring region of the Alps, Carpathians and Pannonia
(ALCAPA) up to now. The Permian anorogenic A-type gran-
ites and S-type tin-bearing Spi-Gemer granites were consid-
ered to be the youngest pre-Alpine or post-orogenic Variscan
plutonic rocks in the Western Carpathians (~ 280250 Ma,
Uher & Pushkarev 1994; Uher & Broska 1996; Petrík &
On the other hand, Early to Middle Triassic age is suggested
in some rhyolites with acid tuffs in the West-Carpathian Silicic
Superunit (the Silica, Muráò and Drienok nappes located near
Ve¾ká Stoka, Telgárt/vermovo and Poniky). Those volcanics
occur as primary magmatic/pyroclastic intercalations within
Triassic carbonatic fossiliferous rocks (Zorkovský 1959a,b;
Slavkay 1965, 1971, 1981). It is remarkable that the geochem-
istry of the Hronèok Granite and the above mentioned Triassic
acid volcanics also show similar A-type geochemical features;
they are enriched in Si, K, Rb, Zr, Y, and depleted in Al, Mg,
Ca and Sr (unpubl. data of Uher, Puti and Ondrejka). More-
over, analogous A-type features are also found in post-orogen-
ic Permian leucogranites of the Western Carpathians and the
Pannonian area (Uher & Broska 1996) and a lot of of Permian
acid subvolcanic porphyries, rhyolites, trachyrhyolites and
rhyodacites determined in various Tatric, Veporic as well as
Gemeric domains (Broska et al. 1993; Korikovsky et al. 1995).
In contrast to the Triassic age of the Hronèok Granite, the ap-
parent Permian age of the most acid (meta)volcanites present
in the Northern Veporic area was confirmed by U-Pb and/or
Rb-Sr isotope dating (Kotov et al. 1996).
Consequently, our U-Pb data from the Hronèok Granite
support the idea of widespread acid plutonic and volcanic ac-
tivity during the Permian to Triassic in the West-Car-
pathian orogenic belt. Similar post-Variscan to early-Alpine
felsic alkaline plutonic and volcanic activity until the Trias-
sic (exceptionally to the Jurassic) is widely known in various
regions in Europe, especially in the Western-Mediterranean
alkaline province which also includes the Central and South-
ern Alps (De Vecchi & De Zanche 1982; Bonin 1987, 1990).
However, this Early Alpine A-type magmatic activity does
not appear to be a continuation of Variscan S- and I-type
calc-alkaline granitic magmatism and it rather reflects an in-
dependent Early Alpine long-lasting extensional tectonic re-
gime controlling the evolution mainly of southern domains
of the Central Western Carpathians, comprising the Veporic
and Gemeric (mid-Cretaceous) tectonic zones.
The geological interpretation of the A-type Hronèok Granite
can thus be related to pre-orogenic alkaline volcanics and sub-
volcanics, which have been dated in the area of consideration
in the time interval ca. 280215 Ma (Kotov et al. 1996). Some
of the volcanic rocks are directly exposed within the Lower
Triassic sedimentary cover sequence (enriched in tourmaline)
of the Veporic basement (Burda pass quarry, E of Fabova Ho¾a
Hill, Puti 1994). On the basis of the previously outlined data,
we regard this volcanic-plutonic complex, including the
Hronèok A-type Granite, as an indicator of Early Alpine conti-
nental rifting of the (Austroalpine-) Central Carpathian base-
ment during the early Tethys evolution. At that time large
master detachment faults could have been developed. During
its early evolution the Pohorelá thrust fault acted as a normal
fault/shear zone enhancing magmatic emplacement of the
Hronèok Granite. This extensional event within a stretched
continental margin crust culminated in the opening of the
Meliata (-Hallstatt) oceanic basin further to the south in the
Middle Triassic. On the other hand a Middle Triassic carbon-
ate platform developed towards the north in the Tatric zone
of the Central Western Carpathians, outside the area of dis-
tinct Early Alpine (Cimmerian) reactivation.
Acknowledgements: We thank S.Z. Yakovleva, V.P. Kovach
and N.G. Bereznaya (Inst. of Precambrian Geol. and Geochro-
nology, Russian Acad. of Sci., St. Petersburg) for help during
preparation and isotope measurements of zircon, I. Holický
(Geol. Inst., Slovak Acad. of Sci., Bratislava) and J. Stankoviè
(Faculty of Natural Sciences CLEOM, Comenius Univ.
Bratislava) for SEI and BEI microphotographs. The manu-
script was considerably improved as a result of reviews by U.
Schaltegger (ETH Zürich), I. Petrík (Geol. Inst., Slovak Acad.
of Sci., Bratislava) and J. Krá¾ (Geol. Survey of the Slovak
Rep., Bratislava). The study was financed by scientific Grant
#98-05-64058 from the Russian Foundation for Basic Re-
searche (S.P. Korikovsky), VEGA grants of the Slovak Repub-
lic #5228 (M. Puti) and #4078 (I. Petrík) and NSERC Grant
#1727-17 (P. Èerný, Winnipeg, Canada). We also thank
M. Miík and D. Plaienka for constructive criticism.
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