GEOLOGICA CARPATHICA, DECEMBER 2005, 56, 6, 463—472
www.geologicacarpathica.sk
40
Ar/
39
Ar dating of detrital mica from the Upper Paleozoic
sandstones in the Western Carpathians (Slovakia)
ANNA VOZÁROVÁ
1
, WOLFGANG FRANK
2,4
, JÁN KRÁ
3
and JOZEF VOZÁR
4
1
Department of Mineralogy and Petrology, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava,
Slovak Republic; vozarova@fns.uniba.sk
2
Department of
Geological Sciences, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
3
Geological Survey of Slovak Republic, Mlynská dolina 1, 817 04 Bratislava, Slovak Republic
4
Geological Institute of Slovak Academy of Sciences, Dúbravská cesta 9, P.O. BOX 106, 840 05 Bratislava 45, Slovak Republic;
geoljovo@savba.sk
(Manuscript received June 24, 2005; accepted in revised form October 6, 2005)
Abstract: Dating of detrital white mica taken from the Carboniferous-Permian sediments of the Western Carpathians
provided powerful results for determination of source areas. Two groups of samples were studied, first from the Carbonif-
erous formations of the Northern Gemeric Unit and second from the Pennsylvanian-Permian sequence of the Hronic Unit.
Both units are extremely important for the geodynamic reconstruction of the Western Carpathian Variscan Orogen, as the
Northern Gemeric Late Paleozoic reflects the development of a Variscan collision suture and the sedimentary history of
Hronicum formations allows reconstruction of the timing of rifting processes in the hinterland. Rift-related sediments of the
Hronic Unit show a trend of increasingly older detrital mica in an upward direction. Three samples from the Nižná Boca
Formation, belonging to Kasimovian—Gzhelian (Stephanian B-C), were investigated from the Hronic Unit. The
stratigraphically lowest sample yielded an age of 309 ± 3 Ma. The samples from the middle and upper portion of the Nižná
Boca Formation delivered successively older ages of 318 ± 2 Ma and 329 ± 2 Ma respectively. The time gap between the
average cooling in the source region and the age of sedimentation in the basal part of the Nižná Boca Formation does not
exceed 5 Myr, whereas at the top of the same formation this time difference reaches about 20 Myr. The trend to older ages
in higher stratigraphic levels in this tectonic unit is further corroborated by a sample derived from the Permian Malužiná
Formation, 2
nd
megacycle, which yielded a still older age of 342 ± 3 Ma. These cooling ages of the source area reflect
development of the Hronicum terrestrial rift and heterogeneity of the source area, with gradual rifting and cooling of the
lithosphere. Four samples from the Northern Gemeric Unit were also investigated. A multi-grain sample from the Mississip-
pian Hrádok Formation yielded a 465 ± 2.5 Ma cooling age of the source area. This proves the existence of Ordovician
crustal fragments in the Variscan collision suture. Newly-formed fine-grained muscovite gave a distinct staircase pattern
from 87 Ma at the rim to 142 Ma in the core. It reflects composite Alpine overprint of these metasediments. Multi-grain
analyses of detrital mica from the Moscovian (Westphalian) Rudňany Formation sandstone and from an orthogneiss pebble
from the associated conglomerate yielded ages of 385 ± 3 Ma and 372 ± 2.5 Ma, which reflect a uniform Variscan source.
Key words: Western Carpathians, Upper Paleozoic sandstones,
40
Ar /
39
Ar dating, source area, detrital mica.
Introduction
The present Alpine chain is built on continental crust con-
solidated at the end of the Variscan orogenic cycle. The
pre-Mesozoic Western Carpathian basement registered all
the main orogenic periods during its long evolution since
the Precambrian, which led to stepwise converging of
Gondwana-type pieces of crust (detached from Gondwana)
and to their final continent—continent collision in the
Variscan orogenic belt from the Late Devonian to the Car-
boniferous time. Due to development of syn- and post-oro-
genic Carboniferous and Permian sedimentary basins,
relics of which are preserved in all units of the Inner West-
ern Carpathians (Fig. 1), the detrital material derived from
the newly formed orogenic belt was preserved.
The
40
Ar /
3 9
Ar age of detrital mica grains in Upper Pale-
ozoic sandstones provides direct evidence for the ages of
source areas from which the sand detritus was derived. The
interpretation of provenance has to consider the post-sedi-
mentary evolution of the studied rocks, especially the Al-
pine low-grade metamorphic overprint in some tectonic
units. Samples rich in detrital micas were collected from
Carboniferous metasandstones and sandy metapelites from
the Northern Gemeric Unit and Pennsylvanian and Permi-
an sandstones from the Hronic Unit (Fig. 1). Both units are
extremely important for the geodynamic reconstruction of
the Western Carpathian Variscan Orogen, as the Northern
Gemeric Late Paleozoic reflects the evolution of a Variscan
collision suture. Additionally, the basal clastics of the
Hronicum can yield information on the former basement
of this unit and throw new light on the continental rifting
process responsible for the formation of these deposits.
Geological setting
Like most other collision fold belts, the Western Car-
pathians have been traditionally divided into external and
464
VOZÁROVÁ, FRANK, KRÁ and VOZÁR
internal structural zones, which are divided by the Pieniny
Klippen Belt (for summary see Biely et al. 1996a,b; Plašien-
ka 1995, 2003). The main differences between these struc-
tural zones are the age of the main Alpine events and the
intensity of their deformational and metamorphic effects.
These are: (1) external zone, the Late Cretaceous / Early
Paleocene to Oligocene / Early Miocene subduction / ac-
cretion and collision events; (2) internal zone, the Late
Jurassic HP / LT subduction event and Early / middle Creta-
ceous collision, associated with nappe stacking.
The internal part of the Western Carpathian orogenic
system is divided into the pre-Gosau nappe system and
overlying post-nappe Upper Cretaceous to Neogene sedi-
mentary and volcanic formations. The pre-Gosau nappe
system originated from sedimentary basins deformed dur-
ing Kimmerian to Mediterranean phases. Each such unit
consists of a crystalline basement and a characteristic Up-
per Paleozoic / Mesozoic envelope sequence, which either
rests on its original basement (Tatricum, Northern and
Southern Veporicum, Zemplinicum, Northern and South-
ern Gemericum) or was detached and displaced as a nappe
onto a different basement (Hronicum, Meliaticum, Turnai-
cum and Silicicum). Continental Upper Paleozoic sedi-
ments, unconformably overlain by Lower Triassic quartzose
clastic deposits and Middle Triassic shallow-water carbon-
ate are characteristic for Tatricum, Veporicum, Zemplini-
cum and Hronicum. The Carpathian Keuper Formation is
typical of the Late Triassic of the Tatricum and Northern
Veporicum, contrasting with dolomites and limestones
in the Hronicum.
The fragments of the Upper Paleozoic sedimentary basin
fillings, as well as their substrate are preserved only within
the internal Western Carpathians, as part of the principal
Alpine crustal-scale units (Tatricum, Northern and South-
ern Veporicum, Zemplinicum, Northern and Southern Ge-
mericum), and several cover nappe systems (Hronicum,
Meliaticum, Turnaicum and Silicicum). As mentioned
above, the investigation is focused on
40
Ar /
39
Ar multi-
grain dating of detrital white mica from the Carboniferous
rocks of the Northern Gemeric Unit and from the Pennsyl-
vanian and Permian rocks of the Hronic Unit (Fig. 1).
The Hronicum is a rootless Alpine unit, partly divided
into two subunits – Šturec and Choč Nappes (Andrusov
et al. 1973). Both subunits contain Late Paleozoic volca-
no-sedimentary formations, variably preserved as a conse-
quence of tectonic reduction during nappe thrusting.
Remains of these sedimentary complexes are known in
many mountain ranges in the Western Carpathians; the best
preserved segments being situated on the northern slopes of
the Nízke Tatry Mts. There are no remnants of the underly-
ing pre-Kasimovian—Gzhelian (pre-Stephanian) basement.
Tectonic slices of granitoid blastomylonites found in the
Fig. 1. Map showing the main tectonic units of the Slovak part of the Western Carpathians, studied area and localization of samples
(modified after Biely et al. 1996).
465
40
Ar /
39
Ar DATING OF DETRITAL MICA FROM THE UPPER PALEOZOIC SANDSTONES (SLOVAKIA)
basal part of the Šturec Nappe may be indicative of its com-
position (Andrusov 1936; Vozárová & Vozár 1979). The
Nižná Boca Formation, Stephanian according to macroflora
(Sitár & Vozár 1973) and microflora (Planderová 1979), is a
generally clastic regressive sequence with distinct tendency
of coarsening upward. Abundant massive and graded-bed-
ded grey or dark-grey sandstones with minor mudstone in-
tercalations, as well as layers rich in coalified plant detritus
indicate a fluvial-lacustrine delta association. Syngenetic
volcanism is represented by dacitic volcaniclastic detritus
mixed with non-volcanic material, less frequently by thin
dacite tuff layers and small lava flows. The Permian sedi-
mentary rock complexes (the Malužiná Formation) are de-
veloped gradually from the underlying Stephanian. They
comprise a thick succession of red-beds which developed in
three regional megacycles arranged above each other. An
integral part of this formation is the polyphase rift-related
andesite-basalt volcanism with a continental tholeiitic mag-
matic trend (Vozár 1977, 1997; Dostal et al. 2003). The mi-
croflora proved the Early and Late Permian age of the
Malužiná Formation (Planderová 1973; Planderová &
Vozárová 1982). This assumption is supported by
206
Pb /
238
U
and
207
Pb /
235
U dating of 263± 11 Ma copper sulphides from
uranium-bearing layers of the upper part of the second mega-
cycle (Legierski in Rojkovič et al. 1992).
The Northern Gemeric Unit comprises relics of Bashkiri-
an-Visean flysch (the Hrádok and Črme Formations in the
Ochtiná Group; Vozárová 1996) and fragments of two
Variscan terranes (the Klátov and Rakovec Terranes;
Vozárová & Vozár 1996), which differ from each other in
tectono-metamorphic development and probably also in
protolith age. Their gradual amalgamation took place in the
Mississippian. This can be concluded from the uppermost
Visean-Serpukhovian shallow-water carbonate-clastic de-
velopment (the Lubeník Formation; Vozárová 1996) which
after stratigraphic hiatus in the Bashkirian (Namurian B—C),
is unconformably overlapped by Moscovian marine post-
orogenic sediments. This overstepping marine “molasse”
also rests on Mississippian flysch (in the eastern part of the
Northern Gemeric Zone) and therefore seals a late Variscan
thrust belt. Detrital micas were also dated from the Missis-
sippian metasandstones of the Hrádok Formation and from
metasandstones of the Pennsylvanian Rudňany Formation.
The Hrádok Formation consists of a dark-grey and
black clastic flysch sequence – metaparaconglomerates,
metasandstones, metapelites, interlayered with metabasalts,
metamicrodolerites and basic metavolcaniclastics. Very rare
sedimentary rocks, found only in thin layers, are lydites and
siliceous metapelites. Slabs of ultramafic rocks (oceanic
crust fragments) represented by antigoritic serpentinites
and tremolite-talc schists are integral part of this deep-
water turbidite slope / rise sequence. A monotonous com-
plex of dark-grey metapelites, overlying the relatively
coarse-grained basal part of the Hrádok Formation, yield-
ed microflora indicating the Late Tournaisian—Visean
age (Planderová 1982; Bajaník & Planderová 1985).
Moscovian (Westphalian) formations (Dobšiná Group
consisting of Rudňany, Zlatník and Hámor Formations;
Vozárová 1996) are preserved within the Northern Ge-
meric Unit as tectonically reduced fragments. Basal con-
glomerates of the Rudňany Formation unconformably
overlap the Klátov Terrane gneiss-amphibolite complex,
metasediments and metabasalts of the Rakovec Terrane
as well as the Mississippian flysch sequence of the Črmel
Formation in the eastern part of Northern Gemeric Unit.
Coarse-grained delta-fan sediments contain detritus de-
rived from these underlying pre-Pennsylvanian rock com-
plexes. Black shales and micaceous metasandstones are a
normal member of the fining upward Rudňany Forma-
tion. The floral finds (Němejc 1947) confirm the West-
phalian A—B age.
Sample description
Hronic Unit: Three samples (WCA-4 / 01; WCA-7 / 01;
WCA-8 / 01) were selected for age determinations from the
Nižná Boca Formation sandstones. The dominant components
are quartz grains (from 44 to 55 %), lithic fragments (from
13 to 19 %), feldspars (from 11 to 14 %) and clastic mica
(from 2 to 7 %), with variable contents of primary matrix
(from 14 to 25 %). Fragments of dacitic volcanites and their
volcaniclastics, as well as different types of phyllites and
metaquarzites were recognized among the lithic clasts (ratio
Lv / L = 0.77). Plagioclase prevails among the feldspar
clasts (plagioclase / alkaline feldspar ratio is 0.6). The ma-
trix is slightly recrystallized, with newly-formed illite,
chlorite and calcite.
Clastic mica of the Malužiná Formation sandstone
(WCA-3 / 01) was separated from the upper part of the 2
nd
megacycle. Compared with the Nižná Boca Formation
sandstones this sediment is rich in feldspar. Generally, sand-
stones from the 2
nd
megacycle of the Malužiná Formation
contain: 49—58 % quartz grains, 10—12 % alkali feldspars,
8—9 % plagioclase, 6—7 % clastic mica, 1—3 % volcanic
clasts, 0.5—1 % other lithic clasts (mostly different type of
phyllites) and 15—22 % matrix, rich in hematite pigment.
Generally, the grade of metamorphism did not ex-
ceed the diagenesis / anchizone boundary (according to
pumpellyite + prehnite assemblages in diorite dykes: Vrána
& Vozár 1969; illite crystallinity indices from pelites:
Plašienka et al. 1989; Šucha 1989).
Northern Gemeric Unit:
Two groups of detrital mica were
analysed from the Rudňany Formation. The first of these was
derived from a white orthogneiss pebble of the Rudňany For-
mation boulder conglomerates (WCA-9 / 01) and the sec-
ond from the sandstones associated with the plant-bearing
horizon in the upper part of the Rudňany Formation
(WCA-9 / 98). Components of the Rudňany conglomerates
are different gneiss types, amphibolites, metabasalts and
their volcaniclastics and metasediments (more detail see
Vozárová 1973; Vozárová & Vozár 1988).
The white gneiss pebble is represented by medium-
grained, medium-banded orthogneiss showing structural
features of foliated granitic-gneiss. The porphyroblasts
of alkali feldspars (Or
95
Ab
5
) and plagioclases (Ab
88—74
An
9—25
Or
0.3—1.7
) are associated with quartz occurring in
the bands of mosaic type crystals, muscovite (content of TiO
2
466
VOZÁROVÁ, FRANK, KRÁ and VOZÁR
is ranging from 0.23 to 1.22 wt. %; Na / Na + K = 0.04—0.06;
Fe / Mg= 0.46—2.45) and strongly altered biotite (transformed
into iron-rich chlorite). Zircon and garnet (Alm
68—70
Pyr
3—8
Grs
2—20
Sps
5—22
) are the principal accessory minerals.
The mineral composition of associated sandstones reflects
the same source area as the Rudňany conglomerates, namely
a newly formed Variscan mobile zone. The sandstones are
rich in lithic fragments (51 %), with less quartz (36 %) and
feldspars grains (13 %) and detrital mica (2 %) (average of
8 analyses). The primary matrix is recrystallized consisting of
newly-formed chlorite + muscovite + quartz. The grade of Al-
pine metamorphism corresponds to the lower greenschist
facies (IC = 0.21—0.23 [
∆ °2Θ], Ir=1.0—1.08; Vozárová &
Šucha, unpublished). Coarse- to medium-grained metasand-
stones of the Mississippian Hrádok Formation are generally
relatively rich in quartz (in most samples more than 80 %).
Subordinate components are plagioclase, detrital mica and
lithic fragments. Primary coarse-grained detrital mica was
separated from sample WCA-1 / 00. The strong Alpine over-
printing is documented by fine-grained metamorphic mica,
which was separated from sample WCA-3 / 00.
Generally, the rocks of the Ochtiná Group were meta-
morphosed in LP greenschist facies (Sassi & Vozárová
1987; geothermal gradient approx. 40 °C / km, estimated
T = 350—370 °C at a low pressure). The illite crystallinity
indices correspond to these estimates (in the range of
0.17—0.23 [
∆ °2Θ]; Ir index=1; Vozárová & Šucha, unpub-
lished). These assessments are supported by the assemblage
Act + Chl + Alb+ Ep in the associated metabasic rocks. A
Variscan age of metamorphism is suggested by the presence of
pebbles derived from the lithologically and stratigraphically
corresponding rock complex (Črme Formation) within the
Westphalian conglomerates in the eastern part of Northern
Gemericum. A distinct Alpine overprinting is assumed.
Analytical procedure
The geochronological investigations were performed in
the Ar laboratory of the Institute for Geological Sciences,
Vienna University in 2002. The analytical procedure is
the same as described in Frimmel & Frank (1996).
Interpretation of the ages
General remarks:
The carefully separated and cleaned
samples of detrital muscovites were measured in the form
of multigrain samples of a few mg, which means that the
spectra represent an average age of several thousands of
individual grains. One can assume that during the separa-
tion procedure mainly the rims of the grains have been re-
moved and therefore the obtained age spectra mainly
reflect information from the inner portions of the detrital
grains. The disturbances in the low temperature steps do
not exceed a few percent. Apart from the reason just men-
tioned, this is well in line with the absent or negligible
thermal overprint of the samples and the well preserved
state of the detrital micas.
In addition to the now available age spectra of multi-
grain samples, histograms of single grain ages are needed
to understand the age variation among the detrital micas
in a single sample. Such investigations are planned in a
more detailed future study. However, we feel that the
rather well defined plateau-type age patterns of the dia-
grams indicates that mixing of micas from source regions
with largely different ages or thermal histories with dis-
cordant age patterns was limited or missing. Apart from
one sample of the Hrádok Formation the results reflect a
provenance from various domains of the evolving
Variscan Orogen.
Samples from the Hronic tectonic unit:
Three samples
from the Nižná Boca Formation, belonging to Stephanian
B-C, have been investigated from this unit. The strati-
graphically lowest sample (WCA-8 / 01) yielded an age
of 309± 3 Ma (Fig. 2, Table 1). The samples (WCA-7 / 01;
WCA-4 / 01) from the middle and upper portion of the
Nižná Boca Formation delivered successively older ages
of 318± 2 Ma and 329± 2 Ma respectively (Figs. 3, 4; Ta-
bles 2, 3).
The time gap between the average cooling in the source
region and the age of sedimentation in the basal part of
the Nižná Boca Formation does not exceed 5 Myr, where-
as on top of the same formation this time difference reach-
es about 20 Myr. If we assume that the result from these
Fig. 2.
40
Ar/
39
Ar apparent age diagram of detrital white mica from
the basal part of the Nižná Boca Formation, sample WCA-8 / 01.
Table 1: Analytic data of detrital white mica from the basal part of
the Nižná Boca Formation, sample WCA-8 / 01.
3068 WCA-8/01
Step T
(
o
C) %
39
Ar
%
40
Ar*
40
Ar*/
39
Ar App.
age
1
620
3.4
94.5
8.19 ± 1.7
244.8 ± 4.3
2
680
6.3
96.4
9.85 ± 0.7
290.5 ± 2.0
3
720
10.5
98.0
10.32 ± 0.2
303.3 ± 0.6
4
770
26.5
99.2
10.54 ± 0.1
309.3 ± 0.4
5
820
17.1
99.3
10.65 ± 0.3
312.3 ± 0.9
6
860
6.8
98.6
10.52 ± 0.5
308.8 ± 1.5
7
925
5.6
97.4
10.33 ± 0.4
303.6 ± 1.2
8
980
5.7
97.4
10.19 ± 0.9
300.0 ± 2.8
9
1060
13.7
98.4
10.66 ± 0.5
312.6 ± 1.6
10
1250
4.4
95.2
10.48 ± 1.4
307.8 ± 4.4
J = 0.017745 ± 0.4 %
Total gas age: 305.3 ± 2.8 Ma
80 % plateau age: 309.3 ± 2.7 Ma
467
40
Ar/
39
Ar DATING OF DETRITAL MICA FROM THE UPPER PALEOZOIC SANDSTONES (SLOVAKIA)
Fig. 3.
40
Ar/
39
Ar apparent age diagram of detrital white mica from
the middle part of the Niná Boca Formation, sample WCA-7/01.
Fig. 4.
40
Ar/
39
Ar apparent age diagram of detrital white mica
from the upper part of the Niná Boca Formation, sample WCA-4/01.
Table 3: Analytic data of detrital white mica from the upper part
of the Niná Boca Formation, sample WCA-4/01.
3048 WCA-4/01
Step T (
o
C) %
39
Ar %
40
Ar*
40
Ar*/
39
Ar
App. age
1
620
3.1
96.6
29.47 ± 0.4
250.8 ± 0.8
2
680
4.1
97.1
34.33 ± 0.4
289.1 ± 1.0
3
720
4.5
98.0
36.90 ± 0.7
308.9 ± 1.9
4
770
10.9
99.1
38.25 ± 0.6
319.3 ± 1.7
5
820
18.5
99.4
39.19 ± 0.1
326.5 ± 0.4
6
860
13.3
99.4
39.58 ± 0.2
329.5 ± 0.6
7
925
9.7
99.2
39.89 ± 0.3
331.8 ± 1.0
8
980
6.7
98.4
39.34 ± 0.3
327.6 ± 0.9
9
1060
16.5
98.8
39.53 ± 0.2
329.1 ± 0.5
10
1250
12.7
97.8
39.98 ± 0.2
332.5 ± 0.6
J = 0.005062 ± 0.4 %
Total gas age: 323.2 ± 2.4 Ma
77 % plateau age: 329.3 ± 2.1 Ma
three samples reflects a typical feature in these clastic de-
posits which has to be further tested this indicates
the evolution of the erosional source region from more lo-
cal and rapidly uplifted areas to a wider region in which
older portions are incorporated. In the simple case that
only deeper levels in the same basement source area were
eroded one should expect gradually younger ages in higher
stratigraphic levels. Of course, the age distribution in the
source area may have been complex considering also the
age variation among the various Variscan magmatic and
metamorphic crystalline rock complexes which contributed
a substantial portion of the rock fragments and the detrital
micas of the clastic deposits (broad spectrum of Variscan
muscovite K/Ar data: Cambel et al. (1990);
40
Ar/
39
Ar cool-
ing ages data from Tatricum and Veporicum: Maluski et al.
(1993), Dallmayer et al. (1996), Janák (1994); U-Pb zircon
data: Poller et al. (2000), Poller & Todt (2000)).
The trend to older ages in higher stratigraphic levels in
this tectonic unit is further corroborated by sample
WCA-3/01 which is derived from the Permian Maluiná
Formation, 2
nd
megacycle. It yielded a still older age of
342±3 Ma with negligible saddle shaped rejuvenation at
medium temperatures (Fig. 5, Table 4). Such features are
often observed when samples have suffered some over-
printing. As overprinting in sediments can be excluded,
overprinting in the source region could be a possible rea-
son for this feature.
Of course, the ages obtained cannot be interpreted as uni-
form ages in the source regions. As they are derived from
multigrain samples a mixture from micas with somewhat
different ages can be expected. Most probably the variation
did not exceed the time span of the Variscan cycle.
However, the limited data from Carboniferous/Permian
sediments from the Hronic tectonic unit are compatible
with the formation of a tectonic graben in a recently up-
lifted basement and later successive widening of the ero-
sional source area including older regions from the
Variscan belt.
Muscovite K/Ar (these data published between
19571988 were reviewed by Cambel et al. 1990) and
40
Ar/
39
Ar data from Tatric and Veporic basement units
generally give a broad age spectrum although Hercynian
cooling is completely preserved in many cases (Maluski
et al. 1993; Dallmayer et al. 1996; Krá¾ et al. 1997; Ko-
hút et al. 1998; Hók et al. 2000). Very typical
40
Ar/
39
Ar
plateau ages lie between 340320 Ma.
40
Ar/
39
Ar musco-
vite data often show rather discordant spectra, which are
interpreted as a result of Alpine rejuvenation.
Samples from the Carboniferous of the Northern Ge-
meric Unit: Four samples from this unit were investigat-
ed. The most interesting result was found from sample
WCA-1/00 of the Mississippian Hrádok Formation near
Ochtiná village. Here a well defined plateau age of
465±2.5 Ma was measured (Fig. 6, Table 5). The low tem-
Table 2: Analytic data of detrital white mica from the middle part
of the Niná Boca Formation, sample WCA-7/01.
3065 WCA-7/01
Step T (
o
C) %
39
Ar %
40
Ar*
40
Ar*/
39
Ar
App. age
1
620
2.8
94.2
9.49 ± 1.5
280.9 ± 4.2
2
680
5.2
97.2
10.56 ± 0.4
309.9 ± 1.3
3
720
7.0
98.6
10.71 ± 0.6
313.9 ± 1.9
4
770
19.5
99.2
10.81 ± 0.2
316.7 ± 0.6
5
820
20.4
99.4
10.90 ± 0.3
318.9 ± 0.9
6
860
8.7
99.5
10.80 ± 0.4
316.3 ± 1.4
7
925
6.7
99.0
10.65 ± 0.3
312.2 ± 0.9
8
980
6.1
98.8
10.63 ± 0.4
311.7 ± 1.4
9
1080
18.4
99.3
10.98 ± 0.1
321.3 ± 0.4
10
1250
5.1
99.0
11.01 ± 0.4
321.9 ± 1.3
J = 0.017745 ± 0.4 %
Total gas age: 316.0 ± 2.4 Ma
91 % plateau age: 317.8 ± 2.2 Ma
468
VOZÁROVÁ, FRANK, KRÁ¼ and VOZÁR
Table 5: Analytic data of detrital white mica from the Hrádok For-
mation metasandstone, sample WCA-1/00.
Fig. 6.
40
Ar/
39
Ar apparent age diagram of detrital white mica
from the Hrádok Formation metasandstone, sample WCA-1/00.
Fig. 7.
40
Ar/
39
Ar apparent age diagram of newly-formed fine-
grained white mica from the Hrádok Formation metasandstone,
sample WCA-3/00.
Table 6: Analytic data of newly-formed white mica from the Hrá-
dok Formation metasandstone, sample WCA-3/00.
Table 4: Analytic data of detrital white mica from the Maluiná
Formation, 2
nd
megacycle, sample WCA-3/01.
Fig. 5.
40
Ar/
39
Ar apparent age diagram of detrital white mica
from the Maluiná Formation, 2
nd
megacycle, sample WCA-3/01.
3064 G-1/00
Step T (
o
C) %
39
Ar %
40
Ar*
40
Ar*/
39
Ar
App. age
1
620
1.3
92.6
17.28 ± 1.2
482.4 ± 4.9
2
680
1.9
93.8
17.13 ± 0.7
478.9 ± 2.9
3
720
2.3
96.0
17.08 ± 1.0
477.5 ± 4.2
4
770
7.4
98.1
16.73 ± 0.2
469.0 ± 0.8
5
820
17.7
99.5
16.62 ± 0.1
466.2 ± 0.5
6
860
18.1
99.7
16.57 ± 0.1
465.0 ± 0.3
7
925
19.8
99.6
16.55 ± 0.1
464.6 ± 0.3
8
980
9.6
99.5
16.57 ± 0.1
465.0 ± 0.6
9
1060
20.0
99.7
16.58 ± 0.1
465.2 ± 0.4
10
1250
2.0
98.6
16.53 ± 0.6
464.0 ± 2.3
J = 0.017745 ± 0.4 %
Total gas age: 466.2 ± 2.7 Ma
87 % plateau age: 465.2 ± 2.4 Ma
2990 G-3/00
Step T (
o
C) %
39
Ar (rel.) %
40
Ar*
40
Ar*/
39
Ar
App. age
1
680
10.7
93.2
2.70 ± 0.6 87.4 ± 0.5
2
720
10.3
95.6
2.79 ± 0.6 90.4 ± 0.5
3
770
14.6
96.3
2.90 ± 0.5 93.8 ± 0.5
4
820
16.7
97.8
3.34 ± 0.5 107.6 ± 0.5
5
860
12.6
96.5
3.09 ± 0.4 101.2 ± 0.5
6
925
9.3
95.1
3.09 ± 0.4
99.9 ± 0.4
7
980
5.8
91.8
3.13 ± 0.6 100.9 ± 0.6
8
1060
15.4
97.2
3.52 ± 0.3 113.3 ± 0.4
9
1250
4.5
89.0
4.46 ± 1.2 142.2 ± 1.7
J = 0.018400 ± 0.4 %
Total gas age: 102.2 ± 1.2 Ma
80 % plateau age: 97.8 ± 1.2 Ma
perature steps of this diagram exhibit some higher ages,
which is unusual for undisturbed spectra of detrital mus-
covites. A
39
Ar recoil effect (
39
Ar loss from altered miner-
al lattice during irradiation) resulting from strongly
weathered rims of the detrital grains seems to be a possi-
ble explanation for this feature.
We think that this well defined concordant age spec-
trum reflects the cooling in the source region and not
mixing of widely differing ages. In the Alps such Early
Paleozoic ages have been often measured with Rb/Sr
whole rock dating on orthogneisses, but are only seldom
preserved in the minerals. During sedimentation of the
Hrádok Formation such a crustal element obviously de-
livered the dominating clastic contribution. The ques-
tion arises if such ages are typical of the Hrádok
Formation or restricted to local occurrences therein.
Metasandstone sample WCA-3/00 is also derived from
the Hrádok Formation. Here a metamorphic overprint in
the matrix is visible. Fine-grained phengite from the ma-
trix could be separated from this location and yielded a
distinct staircase pattern from 87 Ma at the rim to
142 Ma in the core (Fig. 7, Table 6). Alpine metamorphic
3067 WCA-3/01
Step T (
o
C) %
39
Ar
%
40
Ar*
40
Ar*/
39
Ar
App. age
1
620
1.1
95.3
10.55 ± 1.9
317.6 ± 6.2
2
680
4.4
97.4
11.28 ± 0.9
337.6 ± 3.2
3
720
6.8
99.3
11.54 ± 0.5
344.7 ± 1.7
4
770
22.4
99.5
11.49 ± 0.2
343.4 ± 0.7
5
820
14.6
99.5
11.45 ± 0.2
342.3 ± 0.8
6
860
6.4
99.1
11.30 ± 0.8
338.4 ± 2.7
7
925
7.1
98.6
11.20 ± 0.6
335.6 ± 1.9
8
980
7.9
98.4
11.32 ± 0.6
338.7 ± 1.9
9
1060
21.1
99.2
11.48 ± 0.2
343.2 ± 0.5
10
1250
8.3
98.3
11.52 ± 0.3
344.2 ± 1.1
J = 0.018250 ± 0.4 %
Total gas age: 341.6 ± 2.8 Ma
100 % plateau age: 341.6 ± 2.8 Ma
469
40
Ar/
39
Ar DATING OF DETRITAL MICA FROM THE UPPER PALEOZOIC SANDSTONES (SLOVAKIA)
Fig. 8.
40
Ar/
39
Ar apparent age diagram of gneiss pebble muscovite
from the Rudòany Formation conglomerate, sample WCA-9/01.
Table 7: Analytic data of gneiss pebble muscovite from the
Rudòany Formation conglomerate, sample WCA-9/01.
Table 8: Analytic data of detrital white mica from the Rudòany
Formation sandstone, sample WCA-9/98.
Fig. 9.
40
Ar/
39
Ar apparent age diagram of detrital white mica
from the Rudòany Formation sandstone, sample WCA-9/98.
3058 WCA-9/01
Step T (
o
C) %
39
Ar %
40
Ar*
40
Ar*/
39
Ar
App. age
1
620
1.9
94.4
13.10 ± 0.7
377.0 ± 2.3
2
680
4.2
97.2
13.19 ± 0.4
379.4 ± 1.5
3
720
5.5
98.1
13.06 ± 0.3
376.1 ± 0.9
4
770
19.0
98.7
12.92 ± 0.2
372.4 ± 0.7
5
820
22.9
99.4
12.95 ± 0.4
373.3 ± 1.4
6
860
9.3
99.5
12.93 ± 0.2
372.6 ± 0.6
7
925
7.1
98.9
12.90 ± 0.3
371.8 ± 1.0
8
980
8.2
99.0
12.92 ± 0.3
372.4 ± 1.0
9
1060
17.2
99.3
12.89 ± 0.1
371.7 ± 0.4
10
1250
4.7
98.9
12.89 ± 0.5
371.7 ± 1.6
J = 0.017745 ± 0.4 %
Total gas age: 373.0 ± 2.7 Ma
88 % plateau age: 372.4 ± 2.6 Ma
3051 G-9/98
Step T (
o
C) %
39
Ar %
40
Ar*
40
Ar*/
39
Ar
App. age
1
620
1.3
93.4
43.86 ± 1.6
361.7 ± 5.4
2
680
3.2
97.8
45.71 ± 0.5
375.5 ± 1.5
3
720
4.5
98.5
45.78 ± 0.9
376.0 ± 3.0
4
770
13.4
99.1
47.01 ± 0.1
385.1 ± 0.5
5
820
29.2
99.7
46.83 ± 0.4
383.8 ± 1.3
6
860
9.2
99.1
47.33 ± 0.8
387.5 ± 2.9
7
925
8.7
99.7
47.09 ± 0.3
385.7 ± 0.9
8
980
5.3
99.2
47.59 ± 0.9
389.3 ± 2.4
9
1060
17.6
99.6
47.22 ± 0.5
386.6 ± 1.6
10
1250
7.6
98.4
47.53 ± 0.4
388.9 ± 1.2
J = 0.005062 ± 0.4 %
Total gas age: 384.7 ± 3.4 Ma
90 % plateau age: 385.8 ± 3.2 Ma
evolution, in the Northern Gemeric/Veporic contact
zone, was related to the growth of the Western Car-
pathian orogenic wedge mainly during the Cretaceous
time. It was a consequence of crustal thickening and
post-burial thermal relaxation of geotherms during the
initial stage of exhumation, connected with local intru-
sions of the Cretaceous granitoids (Vozárová 1990; Janák
et al. 2001).
One can expect that small, still older age domains exist
in the tiny phengites. Because stepwise heating of multi-
grain samples cannot produce gas from a single domain,
it is always a mixture from different domains. We there-
fore interpret this result as a typical formation age over a
prolonged time of crystallization with some strongly re-
juvenated clastic remnants.
From the Westphalian Rudòany Formation we investi-
gated two samples. Sample WCA-9/01 (quarry near Zá-
vadka village) is a muscovite from a large orthogneiss
pebble, it yields an exceptionally well defined plateau
age of 372±3 Ma, reflecting the age of cooling of the or-
thogneiss in the source area (Fig. 8, Table 7).
A slightly older age (Fig. 9, Table 8) 386±3 Ma was
measured on detrital micas from sandstones near Rudòa-
ny village (sample WCA-9/98). Such Late Devonian
ages are obviously typical in the Westphalian clastic
sediments. They indicate a time difference between cool-
ing and sedimentation of approximately 5060 Ma.
Discussion and interpretation
The fundamental fact that the substantial part of the late
Variscan crust was remobilized and shortened during the
Alpine compression events, leads to the greatest difficulty
in the reconstruction of the Variscan Orogeny in the
Western Carpathians. The Western Carpathian Upper Pa-
leozoic sedimentary basins originated in consequence of
Variscan continent/continent collision of two micro-
plates. Relics of these colliding microplates are pre-
served as fragments of crystalline basement rocks within
the main crustal-scale units of the Alpine Western Car-
pathians internides (the Tatricum, Northern Veporicum,
Southern Veporicum, Hronicum, Zemplinicum and North-
ern Gemericum; Fig. 1). The three types of Variscan crys-
talline fragments in the Alpine Western Carpathians were
distinguished as: the Central Western Carpathian Crystal-
line Zone (CWCZ), the Northern Gemeric Zone (NGZ) and
Inner Western Carpathian Zone (IWCZ). The main differ-
ences between these zones are in their Variscan geody-
namic evolution as well as in the chronological and
spatial evolution of Upper Paleozoic basins (Vozárová
1998). These crustal fragments correspond to two collid-
ing microplates, one represented by medium- to high-
graded crystalline complexes and magmatites of the
CWCZ (crystalline rock complexes and derived Upper Pa-
leozoic post-orogenic sequence preserved in the Tatricum,
470
VOZÁROVÁ, FRANK, KRÁ and VOZÁR
Veporicum, Zemplinicum and Hronicum) and the second
represented by the IWCZ low-grade crystalline (Lower Pale-
ozoic complexes of the Southern Gemeric Unit with its
Permian cover). The CWCZ were separated from the IWCZ
by the NGZ (preserved in the Northern Gemeric Unit), with
the Mississippian deep water turbidites (Noetsch—Veitsch—
Ochtiná zone after Flügel 1990; North Gemeric—Veitsch
zone after Neubauer & Vozárová 1990).
The Variscan convergence led to closing and suturing
of the Rakovec oceanic domain, easternmost remnants of
the Rheic Ocean. The main collision stage occurred dur-
ing the Late Devonian—Mississippian. This was associated
with the origin of the intrasuture deep to shallow water em-
bayment during the Mississippian. Representative are sedi-
ments of the Ochtiná Group, primarily the turbidites of the
Hrádok Formation. The relatively old 465 Ma
40
Ar /
39
Ar
age of detrital mica (Fig. 6, Table 5), from the metasand-
stones of the Hrádok Formation, reflects the existence of
crustal fragments with the imprint of intra-Ordovician
events in the supposed collision suture zone. (The term
“Caledonian” is used for magmatic-metamorphic process-
es as a certain time interval.) Similar Caledonian age data
are preserved within the Central and Southern Alps, in ar-
eas of a lower-grade Alpine overprint. Orthogneisses and
paragneisses from different complexes were dated at about
440—470 Ma, based on Rb / Sr whole-rock analyses and U-Pb
zircon data (Grauert 1966; Köppel et al. 1980 and others).
Eclogite facies metamorphism at ca. 470 Ma is indicated by
Sm-Nd analyses on whole rock and garnet in the Gotthard
Massif (Gebauer et al. 1988; Gebauer 1990). There is a clear
proof for compressional events starting with the formation
of eclogites, probably in a subduction zone setting, at ca.
470 Ma (Gebauer et al. 1988; Gebauer 1990).
Cambrian-Ordovician magmatic and metamorphic activi-
ty is also documented by 514± 24 Ma U-Pb zircon data
(Putiš et al. 2001) and 468± 24 Ma EMPA monazite ages
(Janák et al. 2002) from the Tatricum and Northern Vepori-
cum metamorphic complexes in the Western Carpathians.
The
40
Ar /
39
Ar age of detrital mica from the Mississippian
of the Northern Gemeric Unit is the first evidence of Ordovi-
cian crustal fragments within the supposed Variscan collision
suture in the Western Carpathians. The sedimentary filling of
this intrasutural remnant basin was the only possibility for
the preservation of such old relics, because Variscan conver-
gence completely overprinted and reworked these Ordovi-
cian crustal fragments. The climax of the Variscan collision is
dated by the
40
Ar /
39
Ar ages of detrital mica as well as or-
thogneiss pebble muscovite from the Westphalian Rudňany
Formation, which gave cooling ages of 385.8± 3.2 Ma and
372± 3 Ma, respectively (Fig. 8, Table 7).
The dynamics of convergence were modified by the dex-
tral translation between Europe and Africa (Ziegler
1988; 1990) and the formation of the Pennsylvanian conti-
nental basins in the CWCZ. The
40
Ar /
39
Ar ages from detrital
mica of the Pennsylvanian and Permian sediments of the
Hronicum document clearly that increasingly older pieces of
the continental crust were involved in erosion due to wrench
faulting and rifting behind the Variscan suture zone
(Figs. 2, 3, 4, 5; Tables 1, 2, 3, 4). The relatively broad time
span in
40
Ar /
39
Ar ages of detrital micas is related to
a temporal progression of the Hronicum rift system. The tec-
tonic development of this terrestrial rift, interpreted from the
distribution of lithofacies, the composition of detrital materi-
al and the character of synsedimentary volcanism, shows that
the upper-crustal extension was spatially variable. According
to a numerical model of Buck (1991) the mode of extension
is controlled by crustal thickness and thermal conditions at
the time of rifting. In a core-complex mode the upper-crustal
extension is concentrated over a narrow region, while the
lower crust is thinned over a broad area. This occurs when the
crust, due to crustal thickening in the compressional stage, is
hot and thick. The younger
40
Ar /
39
Ar ages of detrital mica
from the Nižná Boca Formation, could reflect this initial
stage of rifting. When the lower crust flows slowly, then the
mode of extension depends on the strength of the lithos-
phere: a) wide-rift mode – hot and weak lithospere; b) nar-
row-rift mode – cold and strong lithosphere (Buck 1991).
As indicated by the chemistry of synsedimentary vol-
canism, and differences between the 1
st
and 2
nd
eruption
phase (Vozár 1997), the Upper Paleozoic Hronic sedi-
mentary basin may be an example of a region that has
passed through all three stages. Dominant silicic volcan-
ism during sedimentation of the Nižná Boca Formation
may have accompanied core-complex mode extension
and may have resulted from decompression melting. In
contrast, dominant andesite-basalt tholeiites as synsedi-
mentary volcanics during sedimentation of the Malužiná
Formation (Vozár 1997; Dostal et al. 2003) indicate rath-
er colder and stronger lithosphere, with mantle-derived
magma ascent along rift-related faults. The sedimentary
architecture was controlled by episodicity of footwall-
generated sediment discharge into depocenters leading
to a heterogeneity of provenance. The
40
Ar/
39
Ar ages of
detrital micas from Hronicum sediments support this
model.
Conclusions
Dating of detrital white mica taken from the Carbonifer-
ous-Permian sediments of the Western Carpathians provid-
ed important results for the determination of source areas.
Two groups of studied samples, from the Carboniferous
formations of the Northern Gemeric Unit and from the
Pennsylvanian-Permian sequence of the Hronic Unit
yielded the following ages:
1 – Northern Gemericum: A multi-grain sample from the
Pennsylvanian Hrádok Formation yielded a 465± 2.5 Ma
cooling age of the source area. This age proves the exist-
ence of Ordovician crustal pieces in the initial stage of
Variscan collision suture. Newly-formed fine-grained mus-
covite gave a distinct staircase pattern from 87 Ma at the
rim to 142 Ma in the core. It reflects a composite Alpine
overprint of these metasediments.
Multi-grain analyses of detrital mica and orthogneiss
pebble muscovite from the Westphalian Rudňany Forma-
tion yielded 385± 3 Ma and 372± 2.5, reflecting a uniform
Variscan source.
471
40
Ar /
39
Ar DATING OF DETRITAL MICA FROM THE UPPER PALEOZOIC SANDSTONES (SLOVAKIA)
2 – Hronicum: Rift-related sediments of the Hronic Unit
showed a trend of increasingly older detrital mica upward.
Cooling ages in the source area may reflect the development of
Hronicum terrestrial rift and the heterogeneity of the source
area, with gradual rifting and cooling of the lithosphere.
Acknowledgments: The authors are grateful to N.
Froitzheim, F. Ebner and I. Petrík for their helpful sug-
gestions and critical review of the manuscript. This work
was supported by Grant No. 1 / 1036 / 04 from Slovak
Grant Agency VEGA and Project No. APVT-51-002804.
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