GeolCarp_Vol63_No3_191

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA

, JUNE 2012, 63, 3, 191—200 doi: 10.2478/v10096-012-0016-4

Permian volcanics in the Northern Gemericum and Bôrka

Nappe system: U-Pb zircon dating and the implications for

geodynamic evolution (Western Carpathians, Slovakia)

ANNA VOZÁROVÁ

, MILOŠ ŠMELKO

, ILYA PADERIN

and ALEXANDER LARIONOV

Comenius University in Bratislava, Faculty of Natural Sciences, Department of Mineralogy and Petrology, Mlynská dolina, Pav. G,

842 15 Bratislava, Slovak Republic; vozarova@fns.uniba.sk

All-Russian Geological Research Institute (VSEGEI), Sredny prospect 74, 199 106 St.- Petersburg, Russia

(Manuscript received August 24, 2011; accepted in revised form September 30, 2011)

Abstract: U-Pb dating (SHRIMP) of magmatic zircon ages from the Northern Gemericum Permian volcanics (Petrová
Hora Formation) yielded the Concordia age of 272.4 ± 7.3 Ma for basaltic andesite, as well as the Concordia age of
275.2 ± 4 Ma for rhyodacites. Both zircon ages correspond to the Cisuralian Epoch in the time span of the Kungurian
Stage. Acquired

206

Pb/

238

U zircon age data support the nearly contemporaneous origin of the acid and basic volcanogenic

members in the Northern Gemericum Permian strata. The bimodal volcanic suite proves the transtension/extension
tectonic regime in the North Gemeric sedimentary basin during the Late Cisuralian. The magmatic zircon ages of
rhyodacites, occurring in the lower thrust sheet of the Bôrka Nappe (Jasov Formation), gave a younger Concordia age of
266 ± 1.8 Ma proving the Guadalupian Epoch, in the time span of the Wordian/Capitanian. In comparison to the North-
ern Gemericum realm, this age refers to the relatively younger stage of rift-related extensional movements. In the wide
Alpine-Dinarides realm the Middle Permian (Guadalupian) movements are related to the beginning of the Alpine sedi-
mentary cycle. Thus, the Middle Permian rifting expresses the beginning of the formation of the future Meliata oceanic
trough.

Key words: Permian volcanism, Western Carpathians, geodynamic evolution, zircon ages.

Introduction

Permian coarse-grained sediments of the Northern Gemericum
Unit unconformably overlap erosive relics of the Pennsylvan-
ian and Mississippian sedimentary sequences, as well as both
pre-Carboniferous crystalline complexes, Klátov and Rakovec.
As  the  deposits  of  the  continental  arid  to  semiarid  climate,
they  lack  any  relevant  faunal  and  floral  age  evidence.  The
only  possibility  to  prove  their  stratigraphic  position  is  radio-
metric  dating  of  the  accompanied  synsedimentary  volcanic/
volcaniclastic members. The first U-Pb dating from the urani-
um-bearing  horizon  in  the  Novoveská  Huta  ore  deposits
gave  240 ± 30 Ma,  and  was  interpreted  as  the  age  of  strati-
form mineralization (Arapov et al. 1984). The newest mona-
zite ages in the rhyolite tuff from the same locality confirm
the  age  of  278 ± 10 Ma  (Rojkovič  &  Konečný  2005).  The
Permian sedimentary sequence includes several volcanogenic
horizons, with acid and intermediate/basic members, thus in
situ  U-Pb  SHRIMP  zircon  dating  (Laboratory  of  VSEGEI,
Sankt Petersburg) has been applied, with the main objective of
proving the age of magmatism and to specify the stratigraphy
of the associated deposits.

The separate tectonic unit termed the Bôrka Nappe, consid-

ered as a high-pressure part of the Meliaticum Unit, contains
in its basal part the coarse-grained metasedimentary complex
with synsedimentary rhyodacites. No fossils were found due
to the strong metamorphic and structural reworking of this se-
quence. Its Permian age was presupposed based on lithofacial
similarities with the lower part of the Southern Gemeric Perm-

ian sequence (the Rožňava Formation). Therefore, our investi-
gation is focused on determination the age of rhyodacites and
the confirmation of stratigraphic specification and position of
the subdivided lithostratigraphic unit, compared to analogous
sequences in the Western Carpathians.

In this study we follow the time-scale calibration of Gradstein

et al. (2004) in order to compare geochronological data from
volcanic rocks with fossil bearing, sedimentary units.

Geological setting

The Western Carpathians orogenic belt is divided into an

Outer belt that is made up of neo-Alpine nappes (Outer West-
ern  Carpathians),  and  an  Inner  belt  (Inner  Western  Car-
pathians),  with  the  pre-Gosau  nappe  system  overlain  by  the
Late Cretaceous to Tertiary volcanic-sedimentary formations.
The  Inner  Western  Carpathians  consists  of  principal  thick-
skinned  crustal-scale  superunits  made  up  of  the  pre-Alpine
crystalline  basement  and  its  Late  Paleozoic/Mesozoic  enve-
lope and several cover and rootless nappe systems. The North-
ern Gemericum belongs to the pre-Gosau nappe system of the
Inner  Western  Carpathians  (Biely  et  al.  1996  and  references
therein).  The  innermost  part  of  the  Western  Carpathians  is
characterized by the extreme Early Cretaceous shortening due
to  the  paleo-Alpine  nappe  stacking.  Besides  the  basic  supra-
crustal units, formed by the Northern and Southern Gemericum
basement  and  their  dominant  Late  Paleozoic  envelope,  sev-
eral  rootless  nappe  systems  are  present.  The  lowermost  is

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GEOLOGICA CARPATHICA, 2012, 63, 3, 191—200

the  Meliaticum  Unit  that  was  over-
thrusted  by  the  Turnaicum  and  Sili-
cicum thrust nappe system. The Bôrka
Nappe,  with  relics  of  glaucophanites
that  immediately  overthrusts  the
Gemericum,  displays  lithological  and
tectonic  affinity  to  the  Meliaticum
Unit (Mello et al. 1998).

The Northern Gemericum: Bimodal,

rhyolite-andesite/basalt  volcanism  is  a
characteristic  member  of  the  Northern
Gemeric  Permian  sequences  (Fig. 1);
wide-spread  within  the  Petrová  Hora
Formation  (Fig. 2).  Pulses  of  volcanic
activity  correspond  to  large  regional
sedimentary cycles triggered by an ex-
tensional regime. It is the most charac-
teristic  feature  of  the  Petrová  Hora
Formation.  Bimodal  volcanism  was
dominated by rhyolite-dacite members,
accompanied  by  subordinate  andesites
and  basaltic  andesites  (Ivanov  1953;
Rojkovič  &  Vozár  1972;  Václav  &
Vozárová  1978).  Sediments  of  the
Petrova  Hora  Formation  are  character-
ized  by  the  low  degree  of  mineral  and

Fig. 1. Schematic geological map of the Inner Western Carpathians with indication of sample localities (modified after Bezák et al. 2004). Ex-
planations: Northern Gemericum (1—5): 1 – metapelites, metabasalts and their metavolcaniclastics of the Rakovec Complex; 2 – amphibo-
lites and gneisses of the Klátov Complex; 3 – Mississippian formations; 4 – Pennsylvanian formations; 5 – Permian formations; Southern
Gemericum

(6—9): 6 – turbidite metasediments and metavolcanites/metavolcaniclastics of the Gelnica Group, 7 – turbidite metasediments

of the Štós Formation, 8 – Permian overstep sequence, 9 – Permian apical granites; 10 – Meliaticum Unit including the Bôrka Nappe se-
quence; 11 – Turnaicum Unit – Mesozoic and Late Paleozoic sequences; 12 – Silicicum Unit – Mesozoic sequences; 13 – Veporicum
basement and its envelope sequence; 14 – Central Paleogene sediments; 15 – Neogene sediments; 16 – Main faults; 17 – Thrust fault,
thrust plane; 18 – Selected localities for the magmatic zircon dating presented in this paper.

Fig. 2. North Gemeric Permian lithostratigraphic scheme with indication of magmatic
zircon samples (modified after Vozárová 1996).

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U-Pb ZIRCON DATING OF THE PERMIAN VOLCANICS (WESTERN CARPATHIANS, SLOVAKIA)

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GEOLOGICA CARPATHICA, 2012, 63, 3, 191—200

structural maturity. The compositional mixing of the synge-
netic  volcanic  and  extraformational  non-volcanic  detritus  is
very common. Among the most striking features are the fin-
ing-upward  alluvial  cycles,  with  channel  lag,  point-bar  and
flood-plain lithofacies, alternating with playa and ephemeral
lake  sub-environments  at  the  topmost  part  of  the  large  re-
gional cycles.

Permian sequences of the Northern Gemericum are slightly

deformed and recrystallized, under the metamorphic P-T con-
ditions attaining P-T conditions from anchizone to the low-
temperature part of greenschist facies (Šucha & Eberl 1992).
The newly formed metamorphic mineral assemblage is repre-
sented by the fine-grained aggregate of quartz + illite + chlorite
± albite and/or microcline.

The Permian volcanites of the Northern Gemericum vary

from peraluminous acid to the metaluminous intermediate/ba-
sic volcanic rock suites, with Shand’s Index diagram in the
range of values A/CNK = 0.92—3.48 and A/NK = 1.29—3.51.
As the studied volcanites manifest strong secondary alteration,
in conformity with variability of Shand’s index, their classifi-
cation was based on incompatible elements Zr/TiO

vs. Nb/Y

(Winchester & Floyd 1977). They compile a continuous vol-
canic suite from rhyolites and dacites to andesite and basaltic
andesite  (Fig. 3a).  Selected  trace  elements  (Nb,  Ta,  Y,  and
Yb) suggest these acid volcanites were formed in a post-colli-
sional  tectonic  setting  (Fig. 3b),  but  with  distinct  affinity  to
the  syn-collisional  magmatites  in  chemical  composition.  Ba-
saltic andesite, based on Y : La : Nb ratio point to a continental
calc-alkaline basalt suite (Fig. 3c). Chondrite-normalized REE
abundances  (Fig. 3d)  in  the  acid  volcanites  are  enriched  on
light  REE  and  have  relatively  fractionated  heavy  REE  with
(La/Yb)

between 10 and 12 and (La/Sm)

between 2.1 and

3.2. Compared to this, chondrite-normalized REE abundances
in the basaltic andesite have relatively unfractionated heavy
REE with (LA/Yb)

= 5 and (La/Sm)

= 1.6 (Fig. 3d).

The Bôrka Nappe: The Bôrka Nappe is composed of a

changeable, incoherent and tectonically intensively segmented
package  of  the  Alpine  medium-  to  high-pressure  metamor-
phosed volcano-sedimentary complex of Permian-Jurassic age
(Fig. 4).  It  comprises  an  accretionary  prism  rock  complex
formed by the Late Triassic-Jurassic subduction of the oceanic
crust  and  adjacent  continental  margin  of  the  Meliata  Ocean
(Mello et al. 1998). The tectonic individualization of this ac-

Fig. 3. Northern Gemeric and Bôrka Nappe dated volcanite character-
istics based on their chemical composition. a – Zr/TiO

(wt. %) vs.

Nb/Y (ppm) classification diagram after Winchester & Floyd (1977).
b – Variations Nb (ppm) vs. Y (ppm) in the acid volcanites with in-
dication of tectonic setting after Pearce et al. (1984); Abbreviations:
WPG – within-plate granite, ORG – orogenic granite, VAG +
syn-COLG – volcanic arc and syn-collisional granite. c – Position
of basaltic andesite (sample no. 16/SM) in the Y/15 : La/10 : Nb/8 dis-
crimination diagram after (Cabanis & Leccole 1989); Abbreviations:
1A – calc-alkaline basalts, 1B – calc-alkaline basalts and island-
arc tholeiites, 1C – island arc tholeiites, 2A – continental basalts,
2B – back-arc basin basalts, 3A – alkali basalts of within-continen-
tal rift, 3B – enriched E-MORB, 3C – slightly enriched E-MORB,
3D – N-MORB. d – Chondrite normalized REE patterns. Normal-
izing values are after Taylor & McLennan (1985).

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GEOLOGICA CARPATHICA, 2012, 63, 3, 191—200

cretionary  prism  and  its  transport  into  the  present  structural
position  was  attained  during  the  nappe  stacking  stage  in  the
Early Cretaceous. On the basis of the lithology and mutual re-
lationships of the individual lithological complexes, the Bôrka
Nappe  is  subdivided  into  several  lithostratigraphic  units,  in-
cludig the Jasov Formation of the basal thrust sheet (Mello et
al. 1997, 1998).

The Jasov Formation consists of a monotonous sequence of

siliciclastic metasediments, mostly metapsamites interlayered
with  metaconglomerates  in  its  lower  part  and  with  metasilt-
stones and metapelites in its upper part. The metarhyolites and
their volcaniclastics comprise smaller bodies and lense-shaped
layers  in  the  lower  part  of  the  Jasov  Formation  (Fig. 4).  A
Permian age was presupposed on the basis of lithological sim-
ilarities  with  the  Southern  Gemericum  Permian  sediments  of
the  Gočaltovo  Group  (Mello  et  al.  1998).  The  Jasov  Forma-
tion,  inspite  of  the  lithological  similarities,  significantly  dif-
fers from the Southern Gemericum Permian by the character
of  metamorphic  and  structural  alteration.  The  metamorphic
mineral  associations  (Cld + Chl + Ab + Phn ± Pg)  and  b

331,060

spacing of potash white mica, proved temperatures around
350—400 °C at middle-high pressure regime during climax of
the Alpine metamorphism (Mazzoli et al. 1992).

The whole rock chemical analyses of volcanites indicate

rhyolite-dacite composition. This is also confirmed by trace
elements – Nb/Y vs. Zr/TiO

(Winchester & Floyd 1997)

classification, based on the ratios of Nb/Y vs. Zr/TiO

(Fig. 3a).  According  to  Shand’s  Index,  with  values  of
A/CNK = 1.59—2.64 and A/NK = 1.61—2.77 the Jasov Forma-
tion  acid  volcanites  correspond  to  the  peraluminous  suite.
The  Nb : Y  ratio  indicates  a  within-plate  origin  (Fig. 3b).
Chondrite-normalized REE pattern in the Bôrka Nappe rhyo-

dacite  is  enriched  in  light  REE  and
have  relatively  unfractionated  heavy
REE  (Fig. 3d),  with  (La/Yb)

ratio of

3.2 and (La/Sm)

ratio of 2.7. The dis-

tinct negative Eu anomaly is indicative
of  extensive  fractional  crystallization
involving  plagioclase.  According  to
the  selected  trace  elements,  mainly
Nb—Y—Ce, the rocks fit into the A

-type

post-orogenic  magmatic  suite  (in  the
sense  of  Eby  et  al.  1992),  with  ratios
Y/Nb >1.2.  The  increased  content  of
Rb  (70—262 ppm),  Zr  (254—600 ppm),
Y (28—72 ppm) and rare earth elements
excluding  Eu  (0.66—1.24 ppm)  along
with low content of Sr (6—21 ppm) and
V  (8—74 ppm)  is  their  characteristic
feature.

Analytical method

Zircons were separated from rocks by

standard  grinding,  heavy  liquid  and
magnetic  separation  analytical  proce-
dures.  The  internal  zoning  structures
and  shapes  of  the  half-sectioned  zircon

Fig. 4. Bôrka Nappe lithostratigraphic scheme with indication of magmatic zircon samples
(modified after Mello et al. 1997, 1998).

crystals  mounted  in  epoxy  resin  puck  with  chips  of  the
TEMORA (Middledale Gabbroic Diorite, New South Wales,
Australia  Black  et  al.  2003)  and  91500  (Geostandard  zircon,
Wiedenbeck  et  al.  1995)  reference  zircons,  were  imaged  by
optical microscopy, BSE and CL, in order to guide analytical
spots positioning. In situ U-Pb analyses were performed on a
SHRIMP-II  in  the  Center  of  Isotopic  Research  (CIR)  at
VSEGEI in St.-Petersburg, Russia.

Each analysis consisted of 5 scans through the 196—254

AMU mass range; primary beam diameter was about 25 µm,
with intensity of ca. 6 nA. The data were reduced in a manner
similar  to  that  described  by  Williams  (1998,  and  references
therein),  using  the  SQUID  Excel  Macro  of  Ludwig  (2000).
The Pb/U ratios were normalized relative to a value of 0.0668
for  the

206

Pb/

238

U ratio of the TEMORA reference zircons,

equivalent to an age of 416.75 Ma (Black et al. 2003); com-
mon lead was corrected using measured

204

Pb/

206

Pb (Stacey &

Kramers 1975). Age calculations and plotting was done with
ISOPLOT/EX  (Ludwig  1999).  Uncertainties  given  for  indi-
vidual analyses (ratios and ages) are at the one   level; howev-
er the uncertainties in calculated concordia ages are reported at
two   levels.

All dated samples were analysed for major and trace ele-

ments content including REE. Their chemical composition
was determined by ICP/ICP-MS in ACME Laboratories Ltd.,
Vancouver, Canada.

Results

Northern Gemericum – Petrova Hora Formation

: Two

samples from the Petrova Hora Formation were investigated:

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i) Sample no. 38/SM from the vicinity of Jaklovce vil-

lage (N 48°52

’160”, E 20°58’723”, 356 m above see

level) have the rhyodacite composition. The fine-grained
microporphyric  texture  is  formed  by  the  small  phenoc-
rysts  of  -quartz,  perthitic  K-feldspars  and  rare  Na-Ca
feldspars.  Mafic  minerals  are  represented  by  scarce,
strongly altered biotite. Besides the small grains of quartz
the blastofelzitic matrix contains relic thin crystallites of
feldspars. Zircon, monazite, xenotime, apatite, rutile and
Fe-Ti oxides are present as the accessory minerals. Major
and trace element analyses are presented in Table 1.

The U-Pb dating has been carried out on ten zircon

grains. Seven of them have euhedral shapes, and are 40—
70 250—350 µm in size. Th/U ratios range from 0.10 to
0.63 (Table 2). These seven results are similar in

206

Pb/

238

ratios and are considered to be a single population. The
Concordia age of 275.2 ± 4 Ma (Fig. 5) is interpreted as
the igneous crystallization age of the rhyodacite.

The other four analyses, three from the single grains

and  one  from  the  inherited  core  within  the  volcanic  zir-
con crystal (spot 6.1 in the Table 2), yielded Neoprotero-
zoic ages (Ediacaran) spanning from 602 to 637 Ma with
Th/U ratios ranging from 0.16 to 0.94. Their subhedral or
rounded  morphologies  and  significantly  older  apparent
ages suggest their xenocrystic nature: possibly they were
entrained  from  sedimentary  wall-rock  or  the  volcanics
protolith.

ii) The basaltic andesite sample no. 16/SM has been

collected  S  of  Krompachy  town  (N 48°55´074",
E 20°53´18", 445 m above see level). The sample is dark
rock,  and  has  aphanitic  and  fine-grained  texture,  with
rare preserved phenocrysts (max. 3 %). Plagioclase phe-
nocrysts  are  the  most  abundant  phase.  Fe-Ti  oxides  are
present  ubiquitously  both  as  microphenocrysts  and,  in
higher amounts, as a groundmass phase. The whole rock
is  strongly  altered.  The  main  secondary  minerals  are
chlorite, calcite and sericite. The bulk chemical composi-
tion as well as the trace elements composition including
REE concentration is given in Table 1.

The only two zircons from the analysed set gave Permian

ages  270.7 ± 5.3 Ma  and  273.6 ± 5.3 Ma.  Both  zircons
show  features  typical  of  magmatic  zircon  from  basic
rocks:  high  Th/U  ratios  (1.2—2.6),  with  relatively  high
contents  of  U  (829  and  1257 ppm)  and  Th  (1020  and
3238 ppm) (Table 2) and poorly pronounced growth zon-

Table 1: Northern Gemericum and Bôrka Nappe Permian volcanites
rock chemical analyses.

Meliaticum

Northern Gemericum

Locality

Bôrka

Nappe

Krompachy

Group

Krompachy

Group

Krompachy

Group

Sample

22/SM

16/SM

38/SM

38/SM-B

wt. (%)

SiO

78.25

55.94

82.50

80.95

11.57

16.07

8.75

11.02

2.72

9.02

2.25

2.22

MgO

0.40

1.85

1.71

0.30

CaO

0.04

3.58

0.04

0.07

2.57

4.91

0.06

0.18

2.74

1.75

2.52

3.22

TiO

0.13

1.19

0.12

0.03

0.60

0.05

0.07

MnO

0.02

0.09

0.01

0.03

0.01

0.00

0.01

29.00

20.00

4.00

15.00

3.00

4.00

LOI

1.50

5.00

1.90

1.70

Total

99.92

100.03

99.87

99.90

ppm

8.20

7.60

2.50

3.10

17.10

14.10

8.60

10.40

69.80

59.80

91.30

150.40

5.00

3.00

5.00

14.40

88.10

21.80

38.00

1.30

0.90

0.60

0.90

19.60

9.70

12.20

9.70

4.50

2.40

0.90

2.40

24.00

57.00

11.00

17.00

3.90

0.90

1.30

0.90

270.70

279.00

73.50

79.80

37.60

46.40

16.80

17.00

58.10

35.80

19.40

27.00

122.90

79.50

42.20

63.10

15.32

10.55

4.60

7.15

61.00

44.00

16.90

26.90

11.35

9.00

4.18

5.65

0.73

1.98

0.58

0.70

9.10

8.87

3.90

4.30

1.16

1.42

0.65

0.63

7.75

8.20

3.34

3.31

1.52

1.67

0.61

0.58

4.19

4.64

1.67

1.48

0.67

0.73

0.23

0.22

4.21

4.45

1.37

1.28

0.62

0.70

0.19

0.17

/Sm

3.22

2.50

2.92

3.01

/Yb

9.33

5.44

9.57

14.25

Eu/Eu*

0.22

0.68

0.44

0.43

ing. These youngest

206

Pb/

238

U ages are interpreted as the

best estimate of the basaltic andesite extrusion time.

Seven of eleven analysed zircons from basaltic andesite

16/SM cluster between 526 and 476 (Fig. 6) with weighted
average

206

Pb/

238

U of 504 ± 16 Ma. Elevated MSWD = 3.1

along  with  widely  variable  Th/U = 0.05—1.18  assume  this  is
heterogeneous  group.  Selection  of  zircons  with  Th/U < 0.2
(see Table 2) gives weighted average of 497 ± 18 (MSWD= 2.4).
Of four ca. 520—660 Ma old zircons three have Th/U > 1 typi-
cal  of  zircon  from  mafic  or  alkalic  rocks.  This  feature  is
shared  with  the  first  two  zircon  grains  (although  their  CL
structure is different) with pooled Concordia Permian age of
272.4 ± 7.3 Ma (Fig. 6). All but the first two zircons display
fine to coarse oscillatory zoning patterns, with growth zones

locally  interrupted  by  dissolution  surfaces.  These  structures
indicate zircon’s magmatic origin, which is supported by the
Th/U ratios (0.11—1.18). The two grains, with very low Th/U
(0.05—0.08)  may  either  be  interpreted  as  metamorphic  or
postdating  the  Th-absorbing  mineral  phase  in  their  parental
rock; the latter might find succour in their CL-structure quite
resembling  magmatic.  The  Neoproterozoic  ages  were  indi-
cated within the two other magmatic zircon grains, with ages
of 59311 Ma and 659 ± 12 Ma (Table 2).

All the Lower Paleozoic/Neoproterozoic zircon grains are

interpreted as material recycled and reworked from the base-
ment rocks possibly assuming prominence of ca. 500 Ma old
rocks in the volcanites source.

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Table 2:

Northern

Gemericum

and

Bôrka

Nappe

Permian

volcanites

ion

micro

probe

zircon

data.

Errors

are

1-sigma;

and

indicate

the

common

and

radiogenic

portion,

respectively.

(1)

Common

corrected

using

measured

204

Pb.

(1)

)

±%

)

±%

(1)

±%

)

±%

392

264

299

±4

0523

327

270

276

±5

0518

101

273

269

±1

–2

0516

153

278

280

±1

0519

280

254

±2

–9

0513

280

270

±2

–4

0516

366

283

254

±5

–1

0513

533

602

550

±2

–9

0586

301

609

± 1

551

±3

–1

0586

146

633

± 1

582

±6

–8

0594

569

637

± 1

532

±3

–1

0581

829

271

±3

0517

274

283

±2

0519

484

476

484

±3

0568

255

492

504

±4

0573

432

499

486

±2

–3

0569

297

506

527

±3

0579

332

515

544

±7

0584

365

518

± 1

520

±3

0578

526

± 1

517

±3

–2

0577

890

595

532

±1

–1

0581

659

± 1

565

±2

–1

0589

Errors

-s

d P

cat

e t

d r

c p

tio

. r

pec

dar

d c

alib

tio

(

b co

d u

eas

(1)

)

±%

)

±%

(1)

±%

)

±%

419

5.1

264

±3

270

±11

0517

449

6.1

264

±3

133

–

101

0487

865

1.1

264

±3

239

–

0510

970

4.9

265

±3

293

0522

3.6

265

±3

312

0526

503

8.2

265

±3

273

0517

587

1.2

266

±4

300

0523

557

0.2

266

±3

304

0524

652

3.8

269

±3

303

0524

647

3.8

270

±3

301

0523

Errors

-s

d P

cat

e t

d r

c p

tio

. r

pec

dar

d c

alib

tio

(

b co

d u

eas

197

U-Pb ZIRCON DATING OF THE PERMIAN VOLCANICS (WESTERN CARPATHIANS, SLOVAKIA)

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 3, 191—200

Fig. 5. Concordia diagrams of magmatic zircon ages from the
Northern Gemericum rhyodacite (sample no. 38/SM).

Fig. 6. Concordia diagrams of zircon age data from the Northern
Gemeric basaltic andesite (sample no. 16/SM).

Bôrka Nappe – Jasov Formation

206

Pb/

238

U zircon ages

were obtained from the metarhyodacite sample 22/SM, col-
lected in the abandoned quarry, situated west of Jasov vil-
lage (N 48°40´626", E 20°56´745", 281 m above see level).

The Jasov Formation metavolcanites and metavolcaniclas-

tics have dominant blastofelsitic and blastovitroclastic texture.
Related metavolcanics contain a few deformed phenocrysts of
quartz  and  abundant  recrystallized  volcanic  glass  in  the
groundmass.  The  newly  formed  metamorphic  mineral  as-
semblage  is  represented  by  the  fine-grained  aggregate  of
quartz + muscovite + phengite ± chlorite ± albite.  Zircon,  mon-
azite, xenotime, apatite, and Fe-Ti oxides are present as ac-
cessory  minerals.  The  representative  chemical  composition
of sample 22/SM is given in Table 1.

In the zircons analysed the Th/U ranges 0.44—1.37 are typi-

cal for that from igneous rocks. The zircon crystals have euhe-
dral shapes, characterized by the aspect ratios of ca. 1 : 2. Ten
zircon analyses from the sample 22/SM form a very coherent
cluster with

206

Pb/

238

U 266 ± 1.8 Ma Concordia age (Fig. 7).

This average

206

Pb/

238

U age is interpreted as the magmatic

crystallization age of the Jasov Formation metarhyodacite.

Fig. 7. Concordia diagram of zircon age data from the Bôrka Nappe
rhyodacite (sample no. 22/SM).

198

VOZÁROVÁ, ŠMELKO, PADERIN and LARIONOV

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 3, 191—200

Discussion

During the post-Variscan period an intense crustal re-equili-

bration  and  reorganization  took  place  under  alternating  tran-
stensional and transpressional tectonic regime. Following the
main  phases  of  Variscan  compression,  thermal  relaxation  of
the  crust  occurred  in  Early  Permian  times,  creating  the  rifts
and  grabens  that  allowed  accumulation  of  the  first  phase  of
continental sediments (Ziegler 1990).

U-Pb (SHRIMP) magmatic zircon ages obtained from the

Northern  Gemericum  Permian  sequence  proved  275 Ma
(Artinskian/Kungurian) as the date of the main phase of rift-
ing  and  volcanic  activity.  In  contrast,  the  most  voluminous
and evolved magmatism occurred in the North German Basin
(Benek  et  al.  1996)  and  is  dated  297—302 Ma  (Breitkreuz  &
Kennedy  1999).  The  Oslo  Rift  contains  the  most  extensive
and  best  preserved  sequences  of  basaltic  lavas  associated
also with this event (Naumann et al. 1992). The palynomor-
ph  assemblages  from  intravolcanic  sediments  from  the  Bol-
sano  volcanic  complex  in  the  Southern  Alps  document  its
Artinskian-Kungurian age (Hartkopf-Fröder & Krainer 1990).

The obtained

206

Pb/

238

U zircon age data from the Northern

Gemericum Permian volcanites support the nearly contempo-
raneous  origin  of  the  acid  and  basic  volcanogenic  members.
This bimodal volcanic suite proves the transtension/extension
tectonic regime in the North Gemeric sedimentary basin dur-
ing  the  upper  Cisuralian,  reflected  by  the  bimodal  magmatic
activity. The crustal shortening within the Western Carpathian
Variscan  fold-and-thrust  collisional  belt  terminated  at  the
end  of  the  Moscovian.  Following  the  main  phase  of  com-
pression,  the  Cisuralian  thermal  relaxation  of  the  newly
formed crust, results in extension and associated block fault-
ing,  accompanied  by  corresponding  magmatic  activity
(Saalian  movements).  A  feeder  rift  system  was  formed  for
sediment transport from the Variscan belt as well as from the
syngenetic volcanism to the irregularly subsiding basin.

The Carboniferous-Permian change from convergence to

extension has been attributed by Stampfli et al. (2002) to colli-
sion of the Paleo-Tethys mid-ocean ridge with the trench bor-
dering Laurussia. This tectonic setting produced pull-apart rift
basins. The irregularity of the continental basin margin gener-
ated local zones of extension and compression. Although the
North Gemeric volcanic rocks are bimodal, mafic types indi-
cate affinity to a within-plate setting, derived from the subcon-
tinental mantle during rifting. The analogous tectonic setting
was supposed by Dostal et al. (2003) for the Hronicum sedi-
mentary realm in the Central Western Carpathians.

The Neoproterozoic—Cambrian zircon ages from the North

Gemeric volcanites indicate recycling processes and rework-
ing from the older basement crust.

The

206

Pb/

238

U zircon data from the Permian volcanites of

the Bôrka Nappe yield essential younger ages of 266 ± 2 Ma.
In  comparison  to  the  Northern  Gemericum  realm,  this
Guadalupian age (Wordian/Capitanian) refers to the relatively
younger  stage  of  rift-related  extensional  movement  and  de-
rived  volcanism.  Equally,  in  the  lithologically  closest  South-
ern Gemeric domain, the zircon population from the Rožňava
Formation  acid  volcanites  gave  an  older  age  of  275 Ma
(Vozárová  et  al.  2009a).  In  the  wide  Alpine-Dinarides  realm

the Middle Permian (Guadalupian) movements are related to
the beginning of the Alpine sedimentary cycle (Krainer 1993;
Filipovič  et  al.  2003;  Vozárová  et  al.  2009b).  Generally,  the
thermal signature of the Permian rifting was a significant con-
trol of the subsequent Mesozoic and Cenozoic evolution of the
European  lithosphere.  In  the  Meliaticum  domain  this  conti-
nental  Permian  rifting  was  prolonged  intensively  in  the
Lopingian and Triassic and in their final stage led to the origin
of the Meliata oceanic basin (Kozur 1991; Mello et al. 1998).

Conclusions

i) The

206

Pb/

238

U zircon age data from the Northern

Gemericum  Permian  volcanites  confirm  the  Artinskian-
Kungurian  age  (Cisuralian).  The  obtained  data  show  that
both,  acid  and  basic  members  of  the  volcanogenic  sequence
are  coeval.  The  Cisuralian  thermal  relaxation  of  the  newly
formed  Variscan  crust,  resulted  in  extension  and  associated
block faulting, accompanied by corresponding bimodal mag-
matic activity. A feeder rift system was formed for sediments
transport from the Variscan belt as well as from the syngenetic
volcanism to the irregularly subsiding basin.

ii) The

206

Pb/

238

U zircon age data from the Bôrka Nappe

(Meliaticum  tectonic  unit)  acid  volcanites  indicate  the
Guadalupian  volcanism.  This  information  indicates  the
propagation  of  the  continental  rifting  further  into  the  fore-
land of the Variscan belt, closer to the northern margin of the
Paleo-Tethys  domain.  Thus,  the  Middle  Permian  rifting
expresses  the  beginning  of  the  origin  of  the  future  Meliata
oceanic trough.

Acknowledgment: The financial supports of the Slovak Re-
search and Development Support Agency (Project ID:
APVV-0438-06), as well as the Comenius University Bratis-
lava Grant Agency (Project UK 527/2010) are gratefully ac-
knowledged. We also thank Professor Fritz Ebner and dr.
Pavol Siman for their useful comments and suggestions.

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200

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Selected CL zircon images from the Permian volcanites from the Northern Gemeric Unit (samples 38/SM and 16/SM) and from the Bôrka
Nappe (sample 22/SM) and age data based on

206

Pb/

238

U ratios with indication of measurement points.

Appendix