GeolCarp_Vol53_No1_27

GEOLOGICA CARPATHICA, 53, 1, BRATISLAVA, FEBRUARY 2002

27 — 36

LOWER TRIASSIC POTASSIUM-RICH RHYOLITES

OF THE SILICIC UNIT, WESTERN CARPATHIANS, SLOVAKIA:

GEOCHEMISTRY, MINERALOGY AND GENETIC ASPECTS

PAVEL UHER

*, MARTIN ONDREJKA

, JÁN SPIŠIAK

, IGOR BROSKA

and MARIÁN PUTIŠ

Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 26 Bratislava, Slovak Republic

Department of Mineralogy and Petrology, Faculty of Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic

Geological Institute, Slovak Academy of Sciences, Severná 5, 975 01 Banská Bystrica, Slovak Republic

(Manuscript received October 13, 2000; accepted in revised form October 4, 2001)

Abstract: Detailed study of Lower Triassic high-K rhyolites of the Drienok and Muráň Nappes, the Silicic Unit, Inner
Western Carpathians, reveals their alkaline and anorogenic nature. The rhyolites occur within shallow-marine to conti-
nental shale, sandstone and limestone platform sequences. The rocks are unusually K-rich (4.9—8.7 wt.% K

O), enriched

in Si, Rb, REE‘s, Y and Zr and depleted in Al, Ca, Na, P and Sr. Rhyolite phenocrysts are represented by

β-quartz and

alkali feldspars. Zircon typology (mainly P

and D types) indicates a hot and dry magma environment which corresponds

to a high zircon saturation temperature: 820—895 °C. Accessory monazite-(Ce), xenotime-(Y), rutile, ilmenite, magne-
tite, hematite and barite were identified. The results indicate an origin of the high-K rhyolites in an extensional tectonic
regime during the early-Alpine rifting stage. The exceptionally high K in contrast to very low Ca and Na contents in the
rhyolites is probably a result of late-magmatic and/or post-magmatic (hydrothermal) overprint by K-rich fluids. This K-
enrichment of Early to Middle Triassic volcanic rocks is a characteristic feature of the South Alpine—Inner Carpathian
domaine during their Early Alpine continental rifting stage.

Key words: Western Carpathians, Silicic Unit, rhyolites, geochemistry, mineralogy, alkali feldspars, zircon.

Introduction

Occurrences  of  acid  volcanic  rocks  within  Lower  Triassic
clastic to carbonate sequences of the Silicic Unit, in the Muráň
and  Drienok  nappes  were  known  for  many  years  (e.g.  Štúr
1868; Oppenheimer 1931; Grenar & Kotásek 1956; Zorkovský
1959a,b;  Losert  1963;  Slavkay  1965,  1981;  Zuberec  1968;
Klinec  1976;  Hovorka  &  Spišiak  1988).  The  authors  men-
tioned  mainly  their  petrographic  features,  locally  with  bulk-
rock compositions (Zorkovský 1959a,b). A sole petrographical
and geochemical study is available for the Lower Triassic acid
volcanites  of  the  Drienok  Nappe  near  the  Poniky  village
(Slavkay 1965, 1981); the author characterized (trachy)andes-
ite—trachyte—rhyolite  lava  and  pyroclastic  sequences  as  be-
longing to the “K-alkalic association” (after de La Roche clas-
sification).  The  sequence  was  named    the  Skálie  Formation
and  correlated  with  the  Lower  Triassic  volcanic  suite  of  the
Bükk  Unit,  Hungary  (Hovorka  &  Spišiak  1988).  However,
trace-element  and  accessory  mineral  data  from  the  Triassic
acid volcanites in the Silicic Unit are still missing.

The aim of this study is to present new geochemical and

mineralogical data on the Lower Triassic rhyolites of the
Drienok and Muráň nappes of the Silicic Unit in comparison
with older results as well as analogous occurrences in the Al-
pine-Carpathian belt.

Regional geology

All the studied acid volcanites form lava flows or ignimbrite

layers in Lower Triassic siliciclastic and carbonate sequences
of the Muráň and Drienok nappes of the Silicic Unit.

The Silicic Unit represents the tectonically uppermost Al-

pine nappe structures in the Western Carpathians. It overlies
the Veporic, Hronic and Gemeric units. The Silicic Unit in-
volves Upper Permian to Upper Jurassic (Malmian) mainly

Fig. 1. Schematic map of Slovakia with the Silicic Unit (grey) and
localities of Lower Triassic volcanites: 1 – Telgárt, 2 – Ve ká
Stožka, 3 – Poniky.

Present address: Department of Economic Geology, Faculty of Sciences, Comenius Univerity, Mlynská dolina, 842 15 Bratislava,

Slovak Republic; puher@fns.uniba.sk

28 UHER et al.

sedimentary sequences which are facially analogous with the
Schneeberg  and  Mürzalpe  nappes  of  the  Juvavic  Unit  of  the
Northern Calcareous Alps (Mello Ed. 1997). A lower part of
the Silicic Unit consists of a 1200—1600 m thick sequence of
Lower Triassic continental to shallow marine sediments: vari-
coloured,  mainly  violet,  reddish-brown  and  greenish-grey
shales to sandstones, locally shales and limestones with silici-
clastic admixture (Bystrický 1959, 1964; Slavkay 1965; Biely
Ed.  1997;  Mello  Ed.  1997).  The  volcanic  rocks  occur  in  the
upper  part  of  the  Lower  Triassic  (Scythian)  members  of  the
Werfen Formation, which belongs to the Szin Member of Up-
per Nammalian to Middle Spathian age (according to Kovács
et  al.  1989;  cf.  Mello  Ed.  1997),  formerly  designated  as
“Campilian  Beds”  (e.g.  Bystrický  1959,  1964).  The  total
thickness of the Szin Member is 300—400 m (Mello Ed. 1997);
ca. 200 m of the thickness of this member was revealed by the
PO-1  borehole  near  Poniky  village  in  the  Drienok  Nappe
(Slavkay 1965).

Volcanic rocks of the Silicic Unit occur in several localities

(Figs. 1—3):

(1) The Gregová hill near Telgárt (formerly Švermovo) vil-

lage  represents  the  largest  continuous  volcanic  body  (3.3  by
1.8 km, Fig. 2) probably of a laccolith shape (Pouba 1953), ac-
companied  by  smaller  volcanic  outcrops.  The  rhyolite  body
lies  amidst  Griensbachian  to  Spathian  shales  to  fine-grained

Fig. 2. Schematic map of the Telgárt and Ve ká Stožka rhyolite bodies.

sandstones and limestones of the Muráň Nappe (Biely Ed.
1992, 1997).

(2) The Ve ká Stožka hill locality, ca. 9 km SSE of Závadka-

nad-Hronom  village  represents  a  smaller  rhyolite  body  (ca.
500 m in size) concordant with Lower Triassic violet and grey-
ish-green shales, locally sandy or clayish shales of the Werfen
Formation  (Zorkovský  1959a)  in  the  Muráň  Nappe  (Fig.  2).
A geological  map  1:50,000  by  Klinec  (1976)  does  not  show
this occurrence, but another rhyolite oucrop, 2 km S of the Po-
horelská Maša village (ca. 300 m in size) is depicted here.

(3) The largest area of Lower Triassic acid volcanites in the

Silicic Unit is situated in the vicinity of Poniky village. Volca-
nic and volcanoclastic rocks form a non-continuous belt, ca. 7
km long and up to 300 m wide plus a continuous 1200 by 300
m outcrop S of the Žiarec hill (Fig. 3).

According to the Poniky borehole material, the greyish-

green and violet-brown ”Werfenian” shales with intercalations
of  marly  limestones  and  calciferous  shales  containing
”Campilian” fauna are superposed by ca. 30 m thick volcanic
layer  of  subaqueous  andesite  tuff  and  (trachy)andesite
(Slavkay  1965).  This  volcanic  layer  is  superposed  ca.  5  m
thick bed of pyroclastic material with calcareous claystone to
marly limestone and two rhyolitic layers, 5 and 38.5 m thick
with 1 m thick intercalation of calcareous claystone to marly
limestone (Slavkay 1965).

LOWER TRIASSIC POTASSIUM-RICH RHYOLITES 29

Fig. 3. Schematic map of the Poniky rhyolites.

Analytical methods

Chemical analyses of the volcanic rocks were carried out by

X-ray fluorescence method (XRF: main oxides), optical emis-
sion spectrography (OES: Be, Sr, Ba, B, Ga, Sn, Pb, Zr, V, Cr,
Mo, Co, Ni, Cu, Sc and Y), atomic absorption spectroscopy
(AAS: Rb), instrumental neutron activaction analysis and in-
ductively coupled plasma (INAA, ICP: REE’s). Measurements
using XRF, OES and AAS methods were performed at the
Geological Institute, Slovak Academy of Sciences, Bratislava;
INAA at IGEM, Russian Academy of Sciences, Moscow and
ICP at the Geological Survey of the Slovak Republic, Spišská
Nová Ves (Slovakia).

Feldspars and zircon were analysed by electron microprobe

analysis (EMPA). JEOL Superprobe 733 apparatus was used in
wave-length dispersion mode and following analytical condi-
tions: accelerating voltage of 15 kV (feldspars) and 20 kV (zir-
con), respectively, beam current of 20 nA, beam diameter: 3—5
µm. Standards used: wollastonite (Si Kα, Ca Kα), Al

(Al

α), chromite (Fe Kα), albite (Na Kα) and orthoclase (K Kα)

for feldspars; zircon (Si K

α, Zr Lα, Hf Mα) and YAG (Y Lα)

for zircon. EMPA compositions were done at the Geological
Survey of the Slovak Republic, Bratislava.

Zircon was separated by conventional heavy-mineral separa-

tion technique (rock crushing, sieving, heavy liquid and elec-
tromagnetic separation) from large samples (ca. 5—7 kg in
weight).

Zircon morphology was studied under the Tesla BS-300

scanning electron microscope (SEM) at the Geological Insti-
tute, Slovak Academy of Sciences, Bratislava, internal zoning
of zircon and other minerals was investigated under the JEOL
JSM-825 scanning electron microscope in back-scattered
electrons mode (BSE) at the Geological Survey of the Slovak
Republic, Bratislava.

Results

Petrography and mineral composition

The studied rhyolitic rocks reveal common features of

acid volcanics. The texture is porphyric with granolepido-
blastic, locally microfelsic groundmass. In some cases, also
fluidal texture occurs. Phenocrysts, 2—4 mm in size, are rep-
resented by corroded bipyramidal

β-quartz and euhedral al-

kali feldspars. The feldspars are commonly replaced by
chessboard albite, late K-feldspar or fine-grained white mica
aggregates.

The devitrificated groudmass consists of very fine-grained

(~10

µm) aggregate of quartz, feldspar, white mica, hematite

pigment, rarely biotite, chlorite and accessory zircon, EDS
analyses reveal also rare monazite-(Ce), xenotime-(Y), rutile,
ilmenite, magnetite and barite. Microscopic hydrothermal
veinlets of quartz are locally common.

30 UHER et al.

Fig. 7. REE/chondrite normalized diagram of the rhyolites. REE
chondrite values after Taylor & Mc Lennan (1985). Sample loca-
tion: TR – Telgárt; PO, MPV – Poniky.

1000

2000

1000

2000

3000

4000

5000

Telgárt
Telgárt
Veľká Stožka
Poniky
Poniky

100

200

300

3600

3800

4000

4200

4400

Telgárt
Telgárt
Veľká Stožka

Poniky
Poniky

100

1000

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

REE/chondrite

TR-45
TR-52
PO-3/97
PO-4/97
MVP-10
MVP-15

Fig. 4. Total alkali-silica

diagram of the rhyolites (Le Bas et al.

1986).  Full  symbols:  our  analyses;  Open  and  grey  symbols:  older
analyses  (Zorkovský  1959a,b;  Slavkay  1965).  Poniky  TA  –  more
basic  members  of  the  Poniky  volcanics  (“trachyte-andesite”  mem-
bers according to Slavkay 1965).

Fig. 5. A/NK vs. A/CNK diagram of the rhyolites.

0.8

1.2

1.4

1.6

1.8

2.2

0.8

1.2

1.4

1.6

1.8

A/CNK

A/NK

Telgárt
Telgárt
V.Stozka
Poniky
Poniky

peraluminous

metaluminous

peralkaline

SiO

O + K

Telgárt

Veľká
Stožka

Poniky

Poniky TA

TRACHYTE

TRACHY-
DACITE

RHYOLITE

TRACHY-
ANDESITE

BASALTIC

ANDESIT

ANDE-
SITE

DACITE

BASALT

The studied rocks are volcanic rocks, however, volcanoclas-

tic rocks are also mentioned in the Poniky area (Slavkay
1965).

Chemical composition

The results of the new and published bulk-rock chemical

analyses of volcanics are presented in Tables 1 and 2. General-
ly, the investigated volcanic rocks are enriched in Si (72.8—
77.2 wt.% SiO

) and especially in K (4.9—8.7 wt.% K

O) and

depleted in Ti (0.08—0.30 wt.% TiO

), Mg (0.09—1.0 wt.%

MgO), Ca (0.03—1.1 wt.% CaO), Na (0.19—2.8 wt.% Na

and P (0.01—0.11 wt.% P

) – Table 1. TAS diagram (Le

Bas et al. 1986) discriminates the studied rocks into rhyolite
field, the more basic members of the Poniky area (trachytes,
trachyandesites to andesites after Slavkay 1965) fall into dac-
ite,  andesite  and  on  the  boundary  between  the  trachydacite
(trachyte) and rhyolite fields (Fig. 4). Remarkably high potas-
sium  and  low  calcium  and  sodium  contents  resulted  in  their
designation as potassium-rich rhyolites. Despite the relatively
low Al contents, due to depletion in Ca and Na, the rhyolites
are peraluminous with A/CNK = 1.15 to 1.7 (Fig. 5). High Si
contents connected with low Mg and Ca caused anomalously
high R1 parameter and very low R2 parameter in R1-R2 multi-
cationic  diagram  (Batchelor  &  Bowden  1985)  with  a trend
concordant with anorogenic magmatic suites (Fig. 6).

Trace-element geochemistry of the rhyolites (Table 2)

shows a slight enrichment in Rb, Zr, Y and REE, depletion in
Sr,  Ba  and  V,  as  well  as  elevated  Rb/Sr  and  Ga/Al  ratios,
which are typical for alkaline-rich (A-type) post-orogenic and
anorogenic Si-rich magmatic suites (cf. Whalen et al. 1987).
The A-type tendency is also evident in chondrite-normalized
REE distribution; the chondrite-normalized curves show char-
acteristic pronounced negative Eu-anomaly and slightly LREE
enrichment  (Fig.  7).  Conspicuously  high  Ba  content  (1200
ppm), in PO-2/97 sample in comparison to the others (Table 2)
is  caused  by  a  presence  of  hydrothermal  barite  which  was
identified in Poniky as well as the Telgárt rhyolites.

Fig. 6. R1-R2 diagram (Batchelor & Bowden 1985) of the rhyolites.
Analyses are compared to the fields of major granite associations: 1
– mantle fractionates, 2 – pre-plate collision, 3 – post-collisional
uplift, 4 – late-orogenic, 5 – anorogenic, 6 – syn-collision, 7 –
post-orogenic.

LOWER TRIASSIC POTASSIUM-RICH RHYOLITES 31

Unit

MURÁŇ NAPPE

DRIENOK NAPPE

Location

Telgárt, Gregová: rhyolites

V. Stožka

Poniky area: rhyolites

Reference

Sample

TR-45

TR-52

PV-1/90

Tel-1

VS-1

PO-1/97

PO-2/97

PO-3/97

PO-4/97

PO-5/97

PO-6/99

MVP-10

SiO

74.28

74.25

75.60

72.80

75.98

75.29

74.89

73.65

75.26

75.11

76.25

75.01

TiO

0.08

0.14

0.22

0.30

0.13

0.21

0.14

0.12

11.70

12.46

12.09

13.13

12.51

12.65

11.94

12.19

12.33

12.14

11.82

12.20

3total

2.48

1.69

2.02

3.10

2.72

1.38

2.28

2.13

1.77

1.50

1.89

1.74

MnO

0.01

0.03

0.01

trace

0.02

0.03

0.01

0.02

0.01

MgO

0.12

1.03

0.31

0.50

0.56

0.26

0.84

0.16

0.09

0.18

0.78

0.23

CaO

0.03

0.05

0.08

0.62

1.06

0.09

0.07

0.11

0.07

0.11

0.07

0.09

0.74

0.43

0.19

0.75

0.34

0.17

0.19

1.98

0.37

1.59

0.92

2.15

8.58

6.04

7.85

7.00

5.23

7.83

6.77

7.03

7.48

7.03

6.17

6.23

0.11

0.07

n.a.

0.01

trace

0.02

0.03

0.04

0.03

0.01

0.02

LOI

1.53

3.01

1.10

1.17

1.27

1.06

1.54

1.15

1.32

1.07

1.17

1.62

0.34

0.79

0.20

0.18

0.05

0.44

0.51

0.50

0.45

0.38

0.11

0.10

Total

100.00

99.97

99.61

99.49

100.02

99.34

99.21

99.15

99.33

99.29

99.33

99.52

Unit

DRIENOK NAPPE

Location

Poniky area: rhyolites

Poniky area: (trachy)dacites, andesites

Reference

Sample

MVP-13

MVP-15

KP-12

KP-14

KP-15

KP-18

KP-25

KP-26

KP-22

KP-16

KP-19

KP-21

SiO

74.82

74.04

75.33

76.67

75.65

77.23

73.44

73.14

67.67

63.89

64.66

59.60

TiO

0.12

0.14

0.13

0.14

0.16

0.14

0.17

0.55

0.96

0.85

0.83

12.80

12.01

12.35

11.53

12.18

12.02

13.08

13.18

14.87

15.62

15.35

15.16

3total

1.75

1.73

2.29

1.24

2.11

1.40

1.95

2.73

5.04

5.72

5.16

8.72

MnO

0.02

0.01

0.02

0.01

0.02

0.03

0.02

0.09

MgO

0.27

0.29

0.41

0.33

0.42

0.41

0.70

0.84

0.47

1.93

1.63

3.28

CaO

0.08

0.12

0.60

0.94

0.45

0.48

0.07

0.83

2.73

1.86

4.09

1.99

2.77

0.26

1.10

1.00

0.96

0.45

1.64

2.54

3.46

2.80

2.62

6.58

7.00

6.64

6.08

6.64

4.92

8.72

6.47

5.88

2.80

3.80

2.30

0.02

0.03

0.05

0.04

0.05

0.03

0.06

0.10

0.16

0.05

0.21

0.24

LOI

1.50

1.31

1.79

1.90

1.27

1.57

1.06

1.25

2.07

3.58

3.81

6.04

0.15

0.02

n.a.

Total

100.10

99.47

99.88

99.97

99.92

99.19

99.69

99.61

100.11

100.77

100.15

102.97

Table 1: Chemical analyses of the Lower Triassic volcanites from the Silicic Unit (wt. %). References: A – our results; B –Zorkovský
(1959b); C – Zorkovský (1959a); Slavkay (1965).

Unit

MURÁŇ NAPPE

DRIENOK NAPPE

Location

Telgárt, Gregová: rhyolites

Poniky area: rhyolites

Sample

TR-45

TR-52

PV-1/90

PO-1/97

PO-2/97

PO-3/97

PO-4/97

PO-5/97

PO-6/99

MVP-10

MVP-13

MVP-15

n.a.

219

228

251

222

263

230

222

230

255

241

n.a.

3.6

4.4

4.9

4.5

4.9

n.a.

14.5

n.a.

224

178

1200

295

234

257

141

330

250

360

n.a.

204

100

107

110

n.a.

4.7

3.3

4.7

5.1

4.2

n.a.

10.1

28.2

10.2

n.a.

360

162

190

234

170

240

204

145

155

150

8.3

n.a.

3.6

8.1

4.3

1.3

1.5

n.a.

1.4

n.a.

3.9

3.2

3.8

n.a.

1.3

1.1

0.9

1.3

3.6

3.2

3.4

n.a.

4.7

4.9

9.6

5.4

3.9

3.2

n.a.

15.1

10.2

5.2

4.8

6.4

4.3

4.2

4.6

4.8

6.2

n.a.

110

n.a.

116

n.a.

101

124

n.a.

7.7

n.a.

0.13

0.14

n.a.

0.7

0.2

n.a.

0.3

0.5

1.2

1.1

n.a.

1.3

n.a.

0.8

1.1

1.4

4.9

3.8

n.a.

4.9

4.3

n.a.

3.5

4.1

4.5

0.79

0.58

n.a.

0.56

0.51

n.a.

0.47

0.55

0.59

Table 2: Trace-element analyses of the Lower Triassic rhyolites from the Silicic Unit (ppm).

Feldspars composition

Electron microprobe analyses of alkali feldspar phenocrysts

from the Poniky rhyolites revealed three distinct phases: (1)

K>Na alkali feldspar (Or

55—65

35—45

~01

), (2) nearly pure

K-feldspar (Or

90—99

01—10

00—02

), and (3) albite (Ab

90—100

00—10

) replacing primary K-rich feldspars (Table 3, Fig.

8). The feldspar crystals are compositionaly homogeneous
without distinct changes from centre to rim of the crystals.

32 UHER et al.

Table 3: Representative microprobe compositions of alkali feldspars from the Poniky rhyolite (wt. %).

Fig. 9. Zircon BSE microphotographs, Poniky rhyolite. Size of
crystals: 110 (upper) and 170 (lower)

µm.

Zircon typology and composition

The zircon typology method (Pupin 1980) exhibits types

and subtypes typical for hot and dry alkali magmas: mainly
P

—P

and D (Figs. 9—10). Such zircon morphology indicates

a temperature of zircon crystallization of 800—900±50 °C (cf.
Pupin 1980). These results are in concordance with zircon sat-
uration temperatures (T

Zrn

) calculated from bulk-rock chemi-

cal composition (Watson & Harrison 1983): the T

Zrn

= 820—

845 °C for the Poniky rhyolites (6 samples) and 895 °C for the
Telgárt rhyolite.

BSE shows slightly oscillatory zoning of zircon, locally

with small inherited (?) oval core. EMPA reveals Hf contents
common for continental crustal granite zircon: 1.0—1.7 wt.%
HfO

(cf. Pupin 1992) – Table 4. The contents of Y are

slightly elevated: 0.4—1.0 wt.% Y

, and the concentration of

other elements (P, U, Th, REE, etc.) is below the EMPA detec-
tion limit (<0.1 wt.%). Profils across zircon crystals do not
show distinct variations of Zr, Hf and Y contents or systematic
Hf enrichment in the rims of zircon crystals.

Phase

K>Na feldspar

K-feldspar

Albite

Sample

PO-5

PO-6

PO-3

PO-6

PO-3

PO-5

Crystal/pos.

center

rim

center

rim

center

rim

center

rim

center

rim

center

rim

SiO

66.95

67.28

67.11

67.13

66.05

64.79

64.26

64.80

69.30

69.33

69.58

69.47

18.71

18.29

18.46

18.15

18.05

18.03

18.77

18.55

19.33

19.21

18.96

19.04

FeO

0.20

0.03

0.06

0.14

0.03

0.00

0.38

0.00

CaO

0.24

0.23

0.26

0.25

0.00

0.09

0.16

0.00

0.04

0.00

0.02

5.19

4.79

4.87

4.00

0.30

0.39

0.27

0.38

11.85

11.78

12.23

12.31

9.39

9.32

9.07

10.66

16.16

15.93

16.49

16.52

0.13

0.00

0.01

0.00

Total

100.68

99.94

99.83

100.33

100.59

99.23

99.95

100.25

100.65

100.74

100.78

100.84

Formulae based on 8 oxygen atoms

3.002

3.028

3.021

3.027

3.026

3.012

2.978

2.991

3.007

3.008

3.016

3.011

0.989

0.970

0.980

0.965

0.975

0.988

1.025

1.009

0.988

0.982

0.969

0.973

0.007

0.001

0.002

0.005

0.001

0.000

0.014

0.000

0.012

0.011

0.013

0.012

0.000

0.004

0.008

0.000

0.002

0.000

0.001

0.451

0.418

0.425

0.350

0.027

0.035

0.024

0.034

0.997

0.991

1.028

1.035

0.537

0.535

0.521

0.613

0.944

0.945

0.975

0.973

0.007

0.000

0.001

0.000

Total

4.998

4.963

4.962

4.972

4.973

4.984

5.010

5.007

5.001

4.997

5.014

5.020

FeO

= total Fe as FeO

Poniky (PO - 3)
Poniky (PO - 5)
Poniky (PO - 6)

Fig. 8. Feldspar ternary diagram of the rhyolites (atom. %).

LOWER TRIASSIC POTASSIUM-RICH RHYOLITES 33

Fig. 10. Zircon typograms of the rhyolites. Left – Telgárt rhyolite (PV-1); Right – Poniky rhyolite (PO-6).

Table 4: Representative microprobe compositions of zircon from the Poniky rhyolite (wt. %).

Sample#

PO-2

PO-3

PO-5

PO-6

Crystal/position

center

rim

center

rim

center

rim

center

rim

SiO

32.29

32.51

33.97

34.14

32.23

32.20

32.56

32.61

ZrO

65.35

65.15

64.98

64.31

64.52

64.97

64.94

65.66

HfO

1.45

1.35

1.21

1.23

1.73

1.71

1.57

1.69

Y2O

0.50

0.44

0.42

0.87

1.29

0.72

0.66

0.72

Total

99.59

99.45

100.58

100.55

99.77

99.6

99.73

100.68

Formulae based on 4 oxygen atoms

0.997

1.003

1.027

1.031

0.996

1.003

0.997

0.984

0.980

0.958

0.947

0.973

0.980

0.975

0.979

0.013

0.012

0.010

0.011

0.015

0.014

0.015

0.008

0.007

0.014

0.021

0.012

0.011

0.012

Total

2.002

2.003

2.005

2.003

100*Hf/(Hf+Zr)

1.304

1.210

1.033

1.148

1.518

1.508

1.416

1.509

Zr/Hf atomic

75.69

81.67

95.80

86.09

64.87

65.33

69.64

65.27

Discussion and conclusions

Paleotectonic implications

The Early Alpine evolution of the Inner Western Car-

pathians links with the Late Paleozoic evolution of the Intra-
Alpine terrain (Stampfli 1996). The Intra-Alpine terrain is
a Variscan continental fragment accreted to Eurasia due to clo-
sure of the Paleotethys ocean in the Late Carboniferous.
Northward subduction of the Paleotethys oceanic crust caused
the Late Carboniferous and Early Permian calc-alkaline mag-
matic activity in an active margin setting as being related to
subduction (Finger & Steyrer 1990). Some authors would re-
gard this late- to post-Variscan calc-alkaline magmatism as be-
ing related to post-orogenic processes rather than to arc mag-
matism (Bonin 1990, 1993).

According to our opinion this plutonic-magmatic process,

dated in the Western Carpathians Early Permian to Middle Tri-
assic (Kotov et al. 1996; Putiš et al. 2000, 2001) is related to

post-orogenic (post-Variscan) large-scale extension and hence
represents  a pre-orogenic  Early-Alpine  continental  rifting.
This  riftogenesis  is  finally  regarded  as  having  given  birth  to
the Meliata-Hallstatt ocean or marginal sea (Kozur 1991). One
of the best arguments for such an interpretation is the changed
geochemical  trend  of  anorogenic  magmatites;  instead  of  the
calc-alkaline, an alkaline trend is characteristic for the studied
Permian-Triassic  rhyolites  of  the  Drienok  Nappe  as  well  as
other magmatic occurrences regarded as related to this event
(Uher & Broska 1996; Kotov et al. 1996; Putiš et al. 2000).

We have proposed (Putiš in Kotov et al. 1996) to divide the

Permian  sedimentary-magmatic  complexes  of  the  Central
Western Carpathians into two different groups reflecting two
stages of continental riftogenesis: One group comes from deep
contemporary  active  transtensional  furrows  mainly  filled  by
sandy-shale  sediments.  Psephitic  intercalations  indicate  tec-
tonic erosion of the marginal parts of furrows. So called bimo-
dal volcanics (rhyolites, rhyodacites, andesites, basalts), with
alkaline  affinity,  are  characteristics  of  this  group.  They  are

34 UHER et al.

contemporaneous with emplacement of small granitic plutons
of the Hrončok type (A-type, Petrík et al. 1995; Uher & Bros-
ka 1996; Kotov et al. 1996; Putiš et al. 2000, 2001) in the up-
per crust. A thermal anomaly coupled with this stage of rifto-
genesis can also be determined by the numerous K-Ar and
Rb-Sr ages clustering around 250—230 Ma (Cambel et al.
1990; Krist et al. 1992). The second group of the mainly Veru-
cano-type Permian sediments reflects a dynamic setting con-
nected with erosion of uplifted blocks.

Lower to Middle Triassic acid to basic volcanic activity is

known from numerous localities of the internal and uppermost
nappe  units  in  the  broader  Alpine-Carpathian  orogenic  belt
(Dercourt et al. 1990). Triassic alkaline K-rich rhyolites analo-
gous to the Silicic rhyolites were described from the Southern
Alps (Vecchi & Zanche 1982). On the other hand, calc-alka-
line  mainly  andesite  and  younger  alkaline  basalt  formations
occur  in  Middle  Triassic  carbonate  sequences  of  the  Bükk
Unit (Szoldán 1990). Moreover, thin acid tuffitic layers of the
“Pietra Verde” type are known from the Middle Triassic Rei-
fling Limestone near Silická Brezová and Hucín villages in the
Silica Nappe of the Silicic Unit (Kuthan 1959; Mello 1997),
these tuffs and tuffites are common in the Upper Austroalpine
and Transdanubic Unit in Hungary (Ravasz 1973; Cros & Sz-
abó 1984). Recently, a small occurrence of green tufite layer

was also described in the Middle Triassic carbonates of the
Hronic Unit near Valaská Dubová village (Olšavský 1999).

We can find a lot of analogical settings along the northern

margin of Apulia, for example in the Southern and Eastern
Alps (Thöni & Jagoutz 1993; Pfeifer et al. 1993). Paleotecton-
ically, they indicate a rifted and unstable platform in the Per-
mian to Middle Triassic, leading at last to opening of the Mid-
dle Triassic—Early Jurassic Meliata-Hallstatt ocean. A similar,
but diachronous paleotectonic situation occurred in the north-
ern Tatric domain of the Central Western Carpathians, where
syn-rift mid-Jurassic limburgites and basanites are related to
the rifting stage of the Jurassic-Cretaceous Tethys ocean (Hov-
orka & Spišiak 1988).

Geochemical features

The above mentioned geochemical and mineralogical re-

sults of the investigated Lower Triassic volcanites of the Si-
licic  Unit  show  the  specific  and  similar  character  of  all  the
studied volcanic rocks. Our study described only rhyolites and
not  members  of  the  andesite—trachyte  or  trachyandesite—tra-
chyte  differentiation  sequences  in  the  Poniky  area  of  the
Drienok  Nappe.  However,  the  (trachy)andesite  and  trachyte
members occur in the older volcanic layer and they were rec-

Fig. 11. Distribution of alkali elements in the Silicic rhyolites in comparison to the “trachyte—andesite” members of the Poniky area (Poniky
TA; Slavkay 1965) and from the Middle Triassic high-K acid — intermediate volcanics and tuffs of the Southern Alps (Vicentinian and Tarvi-
sian Alps, Lugano area; Vecchi & Zanche 1982). Other symbols as in Figs. 4—6.

SiO

CaO

Telgárt
Telgárt
V. Stožka
Poniky
Poniky
Poniky TA
Vicen. Alps
Tarvisian A.
Lugano

SiO

Telgárt
Telgárt
V. Stožka
Poniky
Poniky
Poniky TA
Vicen. Alps
Tarvisian A.
Lugano

SiO

Telgárt
Telgárt
V. Stožka
Poniky
Poniky
Poniky TA
Vicen. Alps
Tarvisian A.
Lugano

LOWER TRIASSIC POTASSIUM-RICH RHYOLITES 35

ognized in the PO-1 borehole and also in some outcrops
(Slavkay 1965, 1981).

The most conspicuous feature of all the studied rhyolites of

the Silicic Unit is their unusually high potassium enrichment
(4.9—8.7 wt.% K

O, 6.8 wt.% K

O in average), together with

low Ca and Na contents (0.26 wt.% CaO and ~1 wt. % Na

in average). Such high K/Na ratio is unusual for common rhy-
olites of various geotectonic environments; rhyolites from var-
ious calc-alkaline and alkaline suites contain commonly ~1—6
wt.% K

O, 1—5.5 wt.% Na

O and 0.2—5.8 wt.% CaO (cf.

Yarmolyuk & Kovalenko Eds. 1987). However, Middle Trias-
sic rhyolites and dacites of the Southern Alps (Vicentinian and
Tarvisian Alps, Lugano area) also contain similar K-enrich-
ment and Na, Ca-depletion: 6.9—11.6 wt.% K

O and only

0.04—0.95 wt.% Na

O and 0.16—1.2 wt.% CaO; devitrification

of glassy groundmass was connected with late orthoclase crys-
tallization and the K-enrichment is explained mainly by mag-
matic gaseous transfer (Vecchi & Zanche 1982). The K-rich
and relatively Na, Ca-poor Sarmatian rhyolites (5.9—9.0 wt.%
K

O) of the Jastrabá Formation (central Slovakia) are also

connected with late magmatic to hydrothermal K-overprint
and origin of adularia, a secondary K-feldspar (Konečný et al.
1998). The alkalies vs. SiO

and Na

O vs. K

O distributions of

the Silicic rhyolites as well as the compared South-Alpine acid
volcanics show rather irregular patterns without apparent lin-
ear  trends,  whereas  the  (trachy)andesites  to  trachytes  of  the
Poniky area reveal a relatively regular linear pattern (Fig. 11).
Consequently, the geochemical data indicate some late-mag-
matic and/or post-magmatic (hydrothermal?) overprint of the
studied rhyolites by K-rich fluids. The origin of such fluids is
probably closely connected with the parental rhyolite magma
as a result of its fractionation, as the K-enrichment occurs only
in rhyolites and not in the neighbouring dacites, trachydacites
and  andesites.  On  the  other  hand,  the  post-magmatic  hydro-
thermal  overprint  is  evident  mainly  in  the  andesitic  and  tra-
chytic  members  of  the  Poniky  area,  as  manifested  by  their
strong chloritization and high Fe

(4.4—5.7 wt.%) and LOI

(3.5—6 wt.%) contents (Slavkay 1965).

Despite their unusual K-enrichment, the geochemistry of the

studied rhyolites indicates affinity to alkalic volcanic suite en-
riched also in Si, Rb, Zr, Y, REE and depleted in Mg, Ca, Na,
P, Sr, Ba and V, closely comparable to hot and dry post-oro-
genic  to  anorogenic  A-type  granitic  rocks  (cf.  Whalen  et  al.
1987). Zircon typology and saturation temperatures also sup-
port the high temperature and alkalic character of the studied
rhyolites. The Lower Triassic Drienok and Muráň nappe rhyo-
lites  also  show  geochemical  and  mineralogical  similarities
with  Permian  post-orogenic  A-type  rhyolites,  granites  and
granite  porphyries  of  the  Western  Carpathians  (Broska  et  al.
1993;  Uher  &  Broska  1996).  The  Lower  to  Middle  Triassic
Hrončok  Granite  also  reveals  A-type  features  (Petrík  et  al.
1995; Putiš et al. 2000).

Lower Triassic K-rich rhyolites of the Silicic Unit, together

with analogous volcanic occurrences in the Alpine-Carpathian
orogenic belt, represent the product of alkaline intra-plate vol-
canism connected with the initial Early-Alpine continental
rifting stage and opening of the Meliata-Hallstatt oceanic
trough in their vicinity.

Acknowledgments: The authors thank S. P. Korikovsky and

. Puškelová for providing the bulk-rock analyses, P. Siman

and D. Ozdín for electron-microprobe compositions and I.
Holický for SEM photographs. Helpful reviews from J. Ulrych
and J. Lexa are appreciated. The study was financed by VEGA
Grants #1143, 7091 (P.U.), 1/5228/98 and 1/8248/01 (M.P.) of
the Ministry of Education of Slovak Republic & Slovak Acad-
emy of Sciences, as well as Comenius University Grant 28/
2001/UK (M.O.).

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