GeolCarp_Vol59_No3_247

GEOLOGICA CARPATHICA, JUNE 2008, 59, 3, 247—260

www.geologicacarpathica.sk

K-Ar geochronology and petrography of the Miocene

Pohorje Mountains batholith (Slovenia)

MIRKA TRAJANOVA

, ZOLTÁN PÉCSKAY

and TETSUMARU ITAYA

Geological Survey of Slovenia, Dimičeva ulica 14, 1000 Ljubljana, Slovenia; mirka.trajanova@geo-zs.si

Institute of Nuclear Research of the Hungarian Academy of Sciences, Bem tér. 18c, 4001 Debrecen, Hungary; pecskay@namafia.atomki.hu

Okayama University of Science, Research Institute of Natural Sciences, I—I Ridaicho, Okayama, Japan; itaya@rins.ous.ac.jp

(Manuscript received March 26, 2007; accepted in revised form January 31, 2008)

Abstract: A series of K-Ar ages from the Alpine Pohorje Mountains igneous complex is presented. The granodiorite with
dacite was emplaced in a dynamic environment in the form of a single major intrusion, between 19—18 Ma (Ottnangian),
into metamorphic host rocks. The granodiorite includes an older mafic portion of transitional diorite to pyroxenite compo-
sition, cezlakite, yielding an age of ~ 20 Ma. Granodiorite magmatism was followed by major tectonic activity causing
uplift of the Pohorje Mountains complex, and the whole batholith cooled rapidly at rather shallow depths, yielding uniform
cooling ages of around 16.7 Ma, at the Karpatian/Badenian boundary. The process was accompanied by the intrusion of
minor rhyodacitic dykes in the north-western part of the Pohorje Mountains complex and of thin lamprophyre dykes
mostly into the metamorphic rocks on the western margin of the pluton. The pyroclastics within the Miocene sedimentary
rocks attest to the latter’s young age and subaerial emplacement conditions. In the final stage of the magmatism, aplite-
pegmatite melts intruded into the solidified granodiorite. The Pohorje Mountains batholith represents the westernmost
intrusion along the extensional structures of the Pannonian Basin. The main magmatic activity could be related to deep
transtensional fractures of the Labot fault system north of the Periadriatic zone. The tonalite and granodiorite from the
Pohorje Mountains are petrologically different and younger than the Oligocene tonalite from Železna Kapla (Eisenkappel),
as well as the tonalites further west, and the tonalites buried in the Zala Basin in Hungary (roughly between 40 to 30 Ma),
which belong to Paleogene Periadriatic intrusions.

Key words: Miocene, Pannonian Basin, Periadriatic zone, Pohorje, Labot fault, K-Ar dating, granodiorite batholith.

Introduction

Tertiary  calc-alkaline  intrusives  and  volcanics  are  wide-
spread  along  the  Periadriatic  zone  (e.g.  the  Bergell,  the
Rieserferner,  the  Adamello,  and  the  Železna  Kapla  (Eisen-
kappel,  Karawanke)  (Fig.

1). The Pohorje Mountains igne-

ous complex (usually abbreviated to PMIC) is situated at the
easternmost  end  of  the  Periadriatic  zone  that  separates  the
Eastern Alps from the Southern Alps and the north-western-
most Dinarides. It comprises a pluton and a volcanic stock.
They are lithologically subdivided into seven different rock
types:  tonalite,  granodiorite  transiting  to  porphyritic  grano-
diorite, a mafic portion of questionable diorite to pyroxenite
composition called cezlakite, dacite, rhyodacite and lampro-
phyre,  locally  called  malachite.  As  well  as  this,  aplite-peg-
matite veins intersect the plutonic rocks, most frequently in
the southern part of the PMIC. Tonalite represents the east-
ernmost  part  of  the  pluton  close  to  Slovenska  Bistrica,  and
has a an unclear transitional zone to granodiorite. In the area
between  Osankarica  and  Recenjak  the  latter  transits  to  por-
phyritic granodiorite that occupies the largest, central area of
the PMIC. Near the village of Cezlak a lens of cezlakite is in-
corporated into the granodiorite. The northeastern part of the
PMIC  consists  of  dacite.  Its  first  occurrences  can  be  found
on the traverse line from the area of Mala Kopa to Ribnica-
on-the-Pohorje.  In  the  same  area  infrequent  dykes  of  rhyo-
dacite and rare lamprophyre occur (Fig. 2).

During recent decades the PMIC has been the subject of

extensive research. This is because the area provides an ex-
cellent  opportunity  to  understand  the  geodynamic  signifi-
cance  of  this  magmatism.  Although  numerous  geological
studies  (including  works  which  deal  with  geochemistry,
structure,  paleomagnetism  and  stratigraphy)  have  been  per-
formed  by  many  researchers  (e.g.  Benesch  1918;  Dolar-
Mantuani  1935,  1938a,b;  Faninger  1970,  1973;  Mioč  &
Žnidarčič  1976,  1978;  Mioč  &  Žnidarčič  1983;  Dolenec  et
al. 1987; Činč 1992; Zupančič 1994a,b, 1994/95; Altherr et
al. 1995; Sachsenhofer et al. 1998, 2001; Pamić & Palinkaš
2000;  Trajanova  2002a;  Fodor  et  al.  2004;  Márton  et  al.
2006), chronological studies have been performed only spo-
radically, and in limited areas. Because of the overall petro-
logical  similarity  to  the  Periadriatic  intrusions,  the
geodynamic  framework  of  the  Pohorje  Mountains  magma-
tism  has  been  generally  attributed  to  the  Periadriatic  fault
system. This view is further supported by its spatial proximi-
ty to the Periadriatic line along which the Upper Oligocene
bodies are aligned further west (Fig. 1).

The age of the PMIC igneous rocks has been a subject of

vigorous debate over many years. The sequence of eruptions
cannot be established by mere observation since there is a gen-
eral lack of direct stratigraphic relations and intercalations. A
meaningful correlation within the relevant time span can,
however, usually be obtained by radiometric dating. Recently
numerous K-Ar age data have been accumulated for the igne-

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K-Ar GEOCHRONOLOGY AND PETROGRAPHY OF THE MIOCENE POHORJE MOUNTAINS (SLOVENIA)

ous and metamorphic rocks of this region, making it possible
to discuss the sequence of magmatism chronologically.

Here a description of the petrography and geochronology of

the studied area is given, as well as of the field relation of the
main PMIC rock types. The data thus obtained are then used
to propose a model of the volcano-plutonic complex forma-
tion, the reconstruction of the bulk shape of the body, and to
compare the structural controls on magmatism in the Periadri-
atic zone and in the Pannonian Basin.

Geological setting

The Pohorje Mountains complex represents the southeast-

ern part of the Eastern Alps. It extends toward the north into
the  Kobansko  region,  the  Saualpe  and  the  Koralpe  (Fig. 1).
The  Periadriatic  dextral  strike-slip  fault  zone  represents  its
southern  termination.  Toward  the  west  it  is  bounded  by  the
Karavanke  Mountains.  The  Pohorje  Mountains  complex  and
the Karavanke Mountains are separated by the NW-SE-trend-
ing Labot (Lavanttal) fault zone. The eastern prolongation of
the Pohorje Mountains complex is dismembered and covered
by Neogene sediments in the westernmost part of the Pannon-
ian Basin. Along the Labot fault the Pohorje Mountains block
was strongly tilted, that is downthrown at its western end and
uplifted at its eastern end (Trajanova 2002; Trajanova & Péc-
skay 2006).

Low to high pressure metamorphic rocks make up the ma-

jority of the Pohorje Mountains complex, and their character-
istics are described in several works by Hinterlechner-Ravnik
(e.g.  1971,  1982,  1988),  Hinterlechner-Ravnik  &  Moine
(1977), and Hinterlechner-Ravnik et al. (1991a,b). The deep-
est sequences are exposed in the southeastern part of the Po-
horje  Mountains.  On  the  basis  of  mineralogy  UHP
metamorphism was observed in eclogites and garnet peridot-
ites  near  Slovenska  Bistrica  (Jának  et  al.  2004,  2006).  HP
metamorphism  was  also  reported  by  Sassi  et  al.  (2004)  in
eclogites.

In the northwestern part of the Pohorje Mountains, the so-

called  Magdalensberg  thrust  sheet  rests  on  top  of  dia-
phthoresed  gneisses  and  micaschists.  It  consists  of  slightly
metamorphosed Silurian to Devonian pelagic sediments, with
intercalations  of  volcaniclastic  rocks,  diabase,  limestone  and
iron dolomite. This series is unconformably overlain by Per-
mian-Triassic  and  Miocene  sediments  (Mioč  &  Žnidarčič
1978 and Žnidarčič & Mioč 1989).

The PMIC intruded into already polymetamorphosed rocks.

Except in the case of the contact with the diaphthoresed
gneissic sequence, where andalusite schist and gneisses occur,
the pluton did not affect the metamorphic rocks significantly
(Hinterlechner-Ravnik 1971 and Mioč & Žnidarčič 1978).

The basic interpretation of the tectonic structure of the area

was established by Mioč & Žnidarčič (1978) and by Žnidarčič
&  Mioč  (1989).  The  main  tectonic  structures  are  the  follow-
ing:  Upper  Cretaceous  collisional  nappes  with  a  phyllonite
low angle shear zone, proposed by Trajanova (2002), the Pe-
riadriatic zone dislocated along the Labot fault and the Rib-
nica-Selnica  half-graben,  passing  to  a  graben  east  of  the
village of Selnica.

According to Fodor et al. (2002) the magmatism related to

the Neogene basin formation was practically coeval with the
cooling  of  metamorphic  rocks  in  northeast  Slovenia.  Subse-
quently it was followed by intensive brittle faulting, yielding
the Lovrenc and Selnica faults and local faults crosscutting the
Pohorje  block  mostly  in  the  northwest  to  southeast  direction
(Fig. 2). In accordance with these tectonic events, a Miocene
clockwise  rotation  and  a  subsequent  Pliocene  counterclock-
wise rotation occurred in the PMIC (Márton et al. 2006).

Mioč & Žnidarčič (1978) provided structural evidence for

the existence of Caledonian and Variscan metamorphism prior
to Alpine metamorphism, whereas Hinterlechner-Ravnik et al.
(1991b) speculated that metamorphic rocks on the eastern side
of the Pohorje Mountains could represent an older tectonic
melange, which was reworked during the Variscan and Alpine
histories.

Thöni (1999) determined a Sm-Nd age of 93—87 Ma on gar-

nets from metapelites in the southern Pohorje Mountains. Sm-
Nd and U-Pb ages of eclogites cluster around 90 Ma (Miller et
al.  2005).  The  conventional  K-Ar  and  zircon  and  apatite  fis-
sion-track  ages  of  metapelites  scatter  between  19  and  10 Ma
(Márton  et  al.  2002,  2004;  Fodor  et  al.  2007).  A  systematic
chronological study of the metamorphic rocks is in progress.
Although the polymetamorphic history of the rocks is unques-
tionable,  radiometric  dating  suggests  only  Alpine  metamor-
phic events up to now.

Petrography

Based on K

O versus SiO

plots, the main part of the PMIC

displays a clear medium to high-K calc-alkaline affinity (Pa-
mić & Palinkaš 2000), while the easternmost part is represent-
ed  by  tonalite,  which  was  also  confirmed  by  various
classification  criteria  (Faninger  1970).  This  rock  is  subordi-
nate and generally occurs as isolated outcrops, without a clear
relationship  with  the  granodiorite.  The  structure  is  massive
and  medium-  to  coarse-grained  (1  to  5 mm).  It  consists  pre-
dominantly  of  plagioclase,  biotite,  sparse  hornblende,  some
K-feldspar  and  quartz.  Accessory  minerals  are  allanite,  apa-
tite,  titanite,  zircon  and  opaque  minerals.  Traces  of  micro-
grains of garnet and pyroxene can be found.

The plagioclase is polysynthetically twinned, and shows nu-

merous deformation effects. In places it is overgrown, corrod-
ed and included in younger plagioclases, together with biotite
(Fig. 3) and quartz.

The mafic minerals are biotite and sparse hornblende (up to

1 %).  Biotite  frequently  shows  undulate  extinction,  kink
bands,  and  degradational  recrystallization,  and  is  extensively
corroded  by  plagioclases,  K-feldspar  and  quartz  (Fig. 4).
Sometimes it contains inclusions of accessory minerals (allan-
ite, apatite, zircon and opaque minerals). Opaque minerals and
sagenite were partly produced by secondary alteration. In such
cases  the  flakes  are  rimmed  or  completely  replaced  by  chlo-
rite.  Hornblende  grains  are  usually  fractured,  reaching  up  to
4 mm in size.

K-feldspar is sparse and can be included in the younger

minerals. Grains up to 10 mm in size belong to a subsolidus
metasomatic origin. They are often poikilitic with inclusions

250

TRAJANOVA, PÉCSKAY and ITAYA

of corroded plagioclases with myrmekitic reaction rims,
quartz and biotite (Fig. 4), and are restricted to the transitional
zone to granodiorite.

Quartz is squeezed between the grains of plagioclases and

of mafic minerals. It exhibits strained and broadly E-W
stretched grains with jagged grain boundaries, extremely un-

Fig. 3. Polysynthetically twinned plagioclase (pl1) overgrown and
corroded, together with biotite (bt), by younger plagioclase (pl2).
Tonalite/granodiorite transition. X nicols, Nadgrad.

Fig. 4.  Subsolidus  metasomatic  K-feldspar  grain  with  myrmekitic
reaction rim toward plagioclases, including plagioclase (pl), biotite
(bt),  hornblende  (hb),  apatite  (ap)  and  allanite  (al).  The  biotite  is
strongly  corroded  and  slightly  chloritized.  Tonalite/granodiorite
transition, X nicols, Cezlak.

dulate extinction and strong dynamic degradational recrys-
tallization.

The exact mineralogical composition of the tonalite, and in

particular of the quantity of the individual minerals, is greatly
obscured by subsequent alteration.

The main body of the PMIC belongs to a medium- to fine-

grained  granodiorite.  The  structure  is  massive  but  the  rock
displays an oriented fabric developed mostly as a result of ex-
ternal pressure during the magma emplacement, and especial-
ly  of  the  dynamic  environment  during  its  solidification.
Plagioclases, biotite, K-feldspar, and quartz are the main con-
stituents. Hornblende is rarely present. Allanite, some epidote,
opaque  minerals  (mostly  magnetite),  apatite,  zircon  and  rare
titanite occur as accessory minerals.

Plagioclase prevails among feldspars. The coarser grained,

polysynthetically  twinned  plagioclases  are  older  and  sparser,
and are restricted to the more eastern parts of the pluton (Nova
vas and the Nadgrad area), indicating mingling with tonalite.
They  sporadically  include  magmatically  corroded  homoge-
neous  plagioclase  grains  (noticed  also  by  Zupančič  1994/95)
that  could  belong  to  xenocrysts.  In  this  transitional  area  the
older  plagioclase  grains  are  overgrown  and  corroded  by
younger, zoned plagioclase (Fig. 5). The latter show interrupt-
ed oscillatory growth with partly resorbed zones, pointing to a
dynamic  environment  of  crystallization.  They  include  small
flakes  of  biotite  and  accessory  minerals.  In  places  the  outer
zone of the plagioclase includes micrograins of optically unaf-
fected quartz (Fig. 5), giving evidence of its epitactic growth
and rapid cooling. Their composition ranges from acid to in-
termediate, with an average An-content of 35 % (Dolar-Man-
tuani 1938; Faninger 1970 and Činč 1992).

Two generations of biotite occur squeezed between grains

of feldspars and of strained quartz. Biotite of the first genera-
tion shows larger grains, and is sometimes overgrown by fin-
er, younger biotite. The latter is characterized by minor
alteration and a preferred orientation.

According to Dolar-Mantuani (1938) and Faninger (1970),

around 5 %, and up to a maximum of 30 %, of the rock is rep-
resented  by  K-feldspar.  In  deeper  parts  of  the  pluton  in  the
area  of  transition  of  tonalite  to  granodiorite  two  generations
can be determined, hence the frequency is higher. The young-
er  K-feldspar  includes,  assimilates  and  corrodes  the  plagio-
clases,  biotite  and  quartz,  forming  myrmekitic  reaction  rims
towards the plagioclases (Fig. 4). Zupančič (1994b) proposed
that  this  K-feldspar  is  a  result  of  an  extensive  K-metasoma-
tism, which transformed tonalite to granodiorite. However, the
newly obtained data support this conclusion only for the tran-
sitional area of tonalite to granodiorite and not for the whole
granodiorite body.

Quartz represents late crystallization phase of the granodior-

ite, and is tectonically less affected. It may slightly corrode the
plagioclases and biotite.

Hornblende is present subordinately and restricted to the

eastern parts of the pluton, closer to the tonalite. Within the
rest of the body it is connected to the peripheral parts, and to
the more mafic enclaves. In the first case it has xenomorphic
fractured grains, similar to xenocrysts that are partly replaced
by biotite and corroded by plagioclases. In the peripheral areas
the hornblende is hypidiomorphic, and includes biotite flakes.

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K-Ar GEOCHRONOLOGY AND PETROGRAPHY OF THE MIOCENE POHORJE MOUNTAINS (SLOVENIA)

The granodiorite is crosscut by numerous aplite-pegmatite

veins.  They  have  caused  alteration  of  the  plagioclases  along
the fractures indicated by overgrowing sericite and some cal-
cite near the grain boundaries, as well as chloritization of the
biotite and crystallization of the K-feldspar micrograins inside
the  microfractures.  At  least  two  generations  of  the  veins  can
be  clearly  distinguished.  The  older  veins  usually  show  the
same deformation pattern as the surrounding granodiorite.

The composition of the porphyritic granodiorite is the same

as that of the granodiorite. It is characterized by phenocrysts
of plagioclase, which show complex twinning, zoning and
pronounced epitactic growth (Fig. 6). The biotite is fine-

Fig. 5.  Zoned  plagioclase  (pl2)  overgrowing  and  corroding  older
plagioclase (pl1). Younger plagioclase includes micrograins of op-
tically  unaffected  quartz  (q)  in  the  outer  epitaxic  rim  (er).  Grano-
diorite.  X nicols, Nadgrad.

Fig. 6. Porphyritic granodiorite with partly altered cores of zoned
plagioclase phenocrysts (pl) with epitaxic rims (er), biotite (bt) and
quartz (q). X nicols, Hudi Kot.

Fig. 7. Relics of pyroxene (px) in older hornblende (hb1). Euhedral
secondary hornblende (hb2) grows on older hornblende and pyrox-
ene. Quartz (q) replaces plagioclase (pl). X nicols, cezlakite from
the Cezlak II abandoned quarry.

grained, usually fresh and less corroded than in the granodior-
ite. The matrix is a fine to micro-grained mixture of plagio-
clase, K-feldspar and quartz. The latter can be enriched in
peripheral areas of the pluton.

Cezlakite, an uncommon transitional rock of diorite to py-

roxenite composition, is characterized by a medium to coarse-
grained ( ~ 4 mm) idiomorphic to xenomorphic texture and a
massive structure. Clinopyroxene, hornblende and plagioclase
are the main mineral constituents, whereas K-feldspar, biotite
and quartz are sparse and together with traces of muscovite are
secondary in origin. Titanite, opaque minerals and apatite rep-
resent accessory minerals. Chlorite, sparse epidote, calcite,
and traces of sericite are products of alteration.

Light-green augitic clinopyroxenes are prevailing constitu-

ents of the primary rock. Less altered grains contain numerous
patches  of  hornblende,  but  usually  just  represent  residual  in-
clusions  (Fig. 7).  In  the  peripheral  parts  pyroxenes  have  not
been preserved, and an oriented structure dominates due to the
lineated  amphibole  grains.  Hornblende  replaces  and  assimi-
lates  the  pyroxene,  and  includes  some  accessory  minerals.
Younger  amphibole  replaces  and  overgrows  the  hornblende
and  some  older  plagioclases  (Fig. 7),  and  includes  frequent
flakes  of  biotite.  The  amphiboles  show  the  characteristics  of
blastic growth, that is of metasomatism in a subsolidus state,
when  topometasomatic  processes  usually  play  a  major  role
(Augustithis  1973).  The  plagioclases  often  have  crossed
lamellas or are polysynthetically twinned. It has been estimat-
ed that there is less than about 10 % of them. They are exten-
sively  replaced  by  metasomatic  K-feldspar  and  quartz  that
occur in interstitial spaces, and usually have slightly undulate
extinction (Fig. 7).

Dykes

In the north-western part of the PMIC the groundmass of

the rock becomes microcrystalline and gradually transits to a
rock with porphyritic texture and holocrystalline ground-
mass. It has a clear transitional character to porphyritic gran-
odiorite, nevertheless we followed the original terminology

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TRAJANOVA, PÉCSKAY and ITAYA

keeping  the  traditional  name,  dacite.  Volumetrically,  dacite
represents only a small portion of the PMIC. The northeast-
ernmost dyke crops out near Vuzenica. It has a grey colour
with  a  greenish  tint.  Numerous  phenocrysts  of  plagioclase,
as  well  as  some  biotite,  hornblende  and  quartz,  are  embed-
ded  in  a  microcrystalline  groundmass.  In  some  localities
phenocrysts  of  biotite  and  plagioclases  are  slightly  altered,
sericitized  and  chloritized.  Biotite  is  usually  magmatically
corroded  and  deformed,  whereas  the  younger  generation
consists of finer flakes which are incorporated in the ground-
mass,  probably  demonstrating  rapid  cooling.  Small  sills
within  the  metamorphic  host  rock  show  oriented  structure
and  partly  resorbed  phenocrysts  of  plagioclase  and  biotite,
demonstrating  the  shear  stress  effect  (Fig. 8).  Bigger,  older
grains of xenomorphic hornblende can be overgrown by bi-
otite, and in some areas they are replaced by secondary min-
erals (mostly chlorite and calcite). It seems that they belong
to xenocrysts. Small idiomorphic phenocrysts of hornblende
are frequently skeletal, with salic inclusions in the core, indi-
cating  their  late  and  rapid  crystallization  (Fig. 9).  In  some
peripheral areas (e.g. near Stara Glažuta) and in sills the dac-
ite  is  significantly  enriched  with  hornblende,  yielding  lam-
prophyre-like rock.

Rhyodacite is sparser than dacite, and mostly forms thin

dykes. With respect to mineral composition, plagioclase phe-
nocrysts prevail over quartz and biotite, and sporadic K-feld-
spar  also  occurs.  Hornblende  is  present  rarely.  The
plagioclases  are  predominantly  zoned  and  have  numerous
glassy inclusions (Fig. 10) and/or dark rims. They sometimes
contain  inclusions  of  accessory  minerals  and  biotite.  Sparse
K-feldspar  displays  idiomorphic  phenocrysts  overgrowing
small  grains  of  biotite  and  plagioclase.  The  groundmass  is
nearly  glassy  or  sub-microscopically  crystallized.  The  shal-
lowest, vesicular dykes of rhyodacite occur at Trbonje. Beside
the  above-mentioned  constituents,  the  rhyodacite  frequently
contains  xenoliths  of  slates  and  sericite-bearing  quartz  sand-
stones. Compared to the dacite, hornblende is rarely present or

Fig. 8. A sill of dacite with an oriented structure showing the shear
stress  effect.  Brittle  deformed  phenocrysts  of  plagioclase,  slightly
chloritized  biotite  (bt1),  sparse  hornblende  (hb)  and  stretched
quartz  (q)  crystallized  prior  to  deformation,  while  younger  biotite
(bt2)  and  quartz  in  the  microcrystalline  groundmass  are  syndefor-
mational.  X nicols, E of Ribnica-on-the-Pohorje.

Fig. 9. A skeletal phenocryst of hornblende (hb) with salic inclu-
sions in the core, biotite (bt), plagioclase (pl); dacite. X nicols, area
of the peak Jesenko.

Fig. 10. Fresh, kinked biotite (bt) with an opaque grain (op) and al-
tered plagioclase phenocrysts (pl) including glassy material; rhyo-
dacite. X nicols, peak Mršak.

missing, K-feldspar and undeformed quartz phenocrysts are
more frequent and often magmatically resorbed (amoeboid,
Fig. 11), together outlining one of the most obvious composi-
tional differences.

Along the western margin of the pluton some small, mafic

dykes  of  lamprophyre,  variety  malchite,  occur.  The  dykes
crosscut the foliation planes of the metamorphic host rock at
low angles. They consist of phenocrysts of hornblende (often
with  salic  inclusions  in  the  core,  as  in  the  dacite)  and  zoned
plagioclase, rarely of biotite, and a micro- to cryptocrystalline
groundmass (Fig. 12). The presence of microxenoliths is char-
acteristic, as well as rounded, slightly altered plagioclase and
deformed hornblende phenocrysts, probably belonging to xe-
nocrysts.  In  the  core  of  some  xenoliths  colourless  fibres  of
older amphibole are found, probably belonging to tremolite or
anthophyllite, suggesting the metamorphic origin of the inclu-
sions. They are surrounded by and altered to chlorite and some
calcite. The transitional ductile to brittle character of the defor-
mation  indicates  syntectonic  emplacement  and  subsequent
post-cooling deformation of the lamprophyre.

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K-Ar GEOCHRONOLOGY AND PETROGRAPHY OF THE MIOCENE POHORJE MOUNTAINS (SLOVENIA)

Spatial relationships of the rocks

The broad spatial relationship of the rocks of the PMIC can

be seen in Fig. 2. The strongest influence of the post-cooling
brittle processes is expressed in the tonalite. The latter is inter-
sected  by  rare,  thin  veins  mostly  of  aplite  and  aplitic  grano-
diorite,  and  is  surrounded  by  high-grade  metamorphic  rocks.
The  relatively  wide  zone  of  interaction  with  the  granodiorite
can  be  based  on  petrographic  data,  reaching  the  area  of
Nadgrad and close to Osankarica and Cezlak.

No chilled margins were found along the contact of the gra-

nodiorite  body  towards  the  host  rocks.  This  could  be  ex-
plained  as  the  effect  of  a  turbulent  magma  flow  during
emplacement,  which  swept  away  crystallized  material  (Wil-
son  1989),  or  it  is  possible  that  the  surrounding  rocks  were
still hot enough to prevent chilling. The granodiorite fabric in-
dicates  rapid  crystallization  in  a  dynamic  environment.  The
marginal parts of the granodiorite body, especially towards the
south (e.g. in the Cezlak area), are criss-crossed by numerous
aplite-pegmatite veins (Fig. 13). In the area between Osankari-
ca and Recenjak increasing heterogeneity in the grain size of
the pluton can be observed, as well as a gradual transition to
porphyritic granodiorite. It is difficult to recognize a clear pat-
tern due to the later cleavage following intensive subhorizon-
tal  brittle  shearing  associated  with  normal  faulting,  and  poor
outcrop conditions.

In the southeast, in the area of Cezlak, the cezlakite lens is

incorporated in the granodiorite. It is intersected by granodior-
ite (Fig. 14), as well as by numerous aplite-pegmatite and rare
lamprophyre-like veins.

The transition of porphyritic granodiorite to dacite occurs

on the transverse line Mala Kopa—Hudi Kot. Dacite forms
dykes and smaller sills in the metamorphic host rocks. In the
marginal parts of the pluton of this area the dacite changes slow-
ly to more mafic, lamprophyre-like rock. No evidence has been
found to show that the dacite intruded into the granodiorite.

In the north-western part of the PMIC (e.g. in the area of

Mala  Kopa  and  Trbonje),  rhyodacite  has  intruded  mostly  as
thin,  subvertical,  undeformed,  light  grey  dykes,  with  a  pro-
nounced discordant relationship to the foliation of the neigh-
bouring  metamorphic  rocks.  Along  the  contact  zones  the
country  rocks  are  altered  into  epidote  hornfels  and  skarn.  In
the area of Mala Kopa minor rhyodacitic dykes have intruded
into  the  pluton,  and  at  one  place  seem  to  crosscut  the  small
dacitic  sill.  The  shallowest  rhyodacite  intrusion  was  found
near Trbonje.

The areas of the Stara Glažuta and Mislinja graben are char-

acterized  by  the  most  frequent  mafic  dykes,  lamprophyres,
with  thicknesses  mostly  of  less  than  1 m.  These  dykes  have
intruded into the surrounding metamorphic rocks at a low an-
gle to the foliation and sporadically shallowly into the pluton.
Within the medium-grade metamorphic rocks they are associ-
ated  with  amphibolite  schists  and  amphibolites  (e.g.  in  the
Mislinja graben). Along most of the contacts, extensional dis-
placements can be seen toward the southeast.

Very small aphanitic mafic dykes are included in and also

crosscut the aplite-pegmatite veins in the cezlakite, and appear
to be the youngest. Their relation to the lamprophyre has not
yet been established.

Fig. 11. Magmatically resorbed amoeboid quartz (q) phenocrysts in
rhyodacite. X nicols, Mala Kopa.

Fig. 12. A lamprophyre (LF) dyke intruding the margin of grano-
diorite (GD). Phenocrysts of hornblende (hb) often include salic in-
clusions in the core. The plagioclases (pl) are rounded and slightly
altered. Stara Glažuta.

254

TRAJANOVA, PÉCSKAY and ITAYA

Stratigraphy

Sporadic biostratigraphic data are available from the study

area. The bulk of the PMIC intrudes into Austroalpine base-
ment rocks. However, in the northwesternmost part, the roof
of  the  body  is  framed  by  the  nappe  of  the  low-grade  meta-
morphic  rocks  of  the  Magdalensberg  formation,  which  are
unconformably  overlain  by  Permotriassic  clastic  rocks  and
relics of Cretaceous and Tertiary sediments. In the same re-
gion  dacitic  tuffs  are  interbedded  with  Miocene  sediments.
According to Mioč & Žnidarčič (1978) they are mainly Mi-
ocene  (at  that  time  Helvetian).  Paleogene  sediments  are  re-
stricted  to  a  very  small  area  near  Zreče,  on  the  southern
margin  of  the  Pohorje  Mountains.  The  deposition  of  Neo-
gene  sediments  started  in  the  Early  Miocene.  By  means  of
recent investigations Jelen & Rifelj (2003) have determined
a  Karpatian  age  of  the  sediments  north  of  Maribor,  in  the
area  of  the  southernmost  Styrian  extensional  wedge  (Aus-
tria). Based on the lateral continuity, these authors have sug-
gested  the  same  age  for  the  Miocene  sedimentary  rocks  of
the  wider  Pohorje  area.  Numerous  dacite  and  frequent  to-
nalite pebbles occur in the Karpatian (?) unsorted conglomer-
ates.  Petrographic  and  chronological  studies  of  them  are  in
progress.

Earlier chronological data

A first estimate of the radiometric age of the Pohorje

Mountains igneous complex was proposed by Žurga in 1926.
Based  on  the  relationship  to  the  neighbouring  metamorphic
and sedimentary rocks, he proposed an Early Miocene age of
the pluton. Later, Germovšek (1954) placed the age between
the Late Cretaceous and Miocene. The first radiometric age
(an Rb-Sr model age of 19.5 ± 5 Ma), determined on tonalites
from  the  Pohorje  Mountains,  was  published  by  Deleon
(1969).  According  to  Faninger  (1970),  the  Austroalpine
crystalline rocks of the Pohorje massif were intruded by to-
nalites  during  Oligocene  times,  whereas  dacitic  volcanism
followed  in  the  Early  Miocene  (Faninger  1973).  Based  on
field relations, the Pohorje pluton was assigned the same age
as the Železna Kapla (Eisenkappel) intrusive, for which dif-
ferent authors have determined a Rb-Sr age of 29 to 28 Ma
(Mioč & Žnidarčič 1983).

Three K-Ar ages determined on cezlakite and granodiorite

(w.r. of cezlakite, 18.7 ± 0.7 Ma; biotite separated from cezlak-
ite, 16.9 ± 0.4 Ma; and biotite separated from granodiorite,
16.4 ± 0.4 Ma) indicate that the emplacement of the pluton oc-
curred in Neogene time (Dolenec 1994).

One dacitic dyke exposed at Vuzenica was dated by the

fission-track  method.  Based  on  the  apatite  FT  age
(14.6 ± 1.8 Ma)  Sachsenhofer  et  al.  (1998)  concluded  that  ei-
ther the depth of the emplacement was shallow or that exhu-
mation of the dated dyke took place soon after the magmatic
activity. However, they considered that the tonalite of the Po-
horje Mountains is an Oligocene Periadriatic intrusion. Pamić
&  Palinkaš  (2000)  assumed  that  the  Pohorje  Mountains  and
the Karavanke plutons are part of a series of mid-Tertiary in-
trusives which extend along the Periadriatic zone.

Fig. 13. Aplite-pegmatite veins (white) crosscutting granodiorite.
Active granodiorite quarry, Cezlak I.

Fig. 14. Granodiorite (light grey) crosscutting cezlakite (dark grey).
Abandoned cezlakite quarry, Cezlak II.

255

K-Ar GEOCHRONOLOGY AND PETROGRAPHY OF THE MIOCENE POHORJE MOUNTAINS (SLOVENIA)

Experimental methods

Sampling

Systematic sampling was performed in order to obtain in-

formation  about  the  3-dimensional  distribution  of  isotopic
ages in the PMIC. These samples cover an area of the pluton
about 10 km wide and 35 km long, with an elevation of nearly
1 km. The samples were collected at several localities along a
southeast-northwest  oriented  section  of  the  PMIC,  mostly
from quarries and natural outcrops: Nova vas in the Smrečno
area  (NV),  Nadgrad  (Ng),  quarries  Cezlak  I  and  Cezlak  II
(Cz),  Recenjak  (Rc),  Josipdol  (Jd),  Hudi  Kot  (HK),  Mislinja
graben (MG), Stara Glažuta (SG), Mala Kopa (MK), peak Mr-
šak  (pMs),  peak  Jesenko  (pJ),  Vuzenica  (Vu)  and  Trbonje
(Tb) (Fig. 2). For K-Ar dating 31 representative rock samples
were taken, and one sample was taken from Železna Kapla to-
nalite (Karavanke Mts). A piece with a weight of about 1 kg
was broken out of a larger block, free of weathering, xenoliths
and  joints.  The  samples  chosen  for  further  investigation
looked fresh and showed high resistance during the hammer-
ing procedure. Final selection of the specimens was performed
on  the  basis  of  thin  section  inspection.  After  this  they  were
crushed and sieved to 200—350

µm. The fine dust was elutriat-

ed with distilled water and dried at 110 °C for 24 h.

Based on the mineralogy and the texture of the rock sam-

ples, biotite, hornblende and feldspar were separated using con-
ventional techniques (heavy liquids, magnetic separator). The
purity of the monomineralic fractions was checked by means of
a binocular microscope and improved by hand picking.

Potassium determination

Approximately 0.05 g of each finely ground sample was di-

gested in acids and finally dissolved in 0.2 M HCl. Potassium
was determined by flame photometry with a Na buffer and a
Li internal standard. The inter-laboratory standards Asia 1/65,
LP-6, HD-B1, GL-O were used for checking the results of the
measurements.

Argon measurements

Argon was extracted from the samples by RF fusion in Mo

crucibles, in a previously baked stainless steel vacuum sys-
tem.

Ar spike was added from a gas pipette system and the

evolved gases were cleaned using Ti and SAES St707 getters
and liquid nitrogen traps, respectively. The purified Ar was
transported directly into the mass spectrometer and the Ar iso-
tope ratio was measured in the static mode, using a 15 cm ra-
dius magnetic sector type mass spectrometer built in
Debrecen.

Details of the instruments, the applied methods and the re-

sults of calibration have been described elsewhere (Odin
1982; Balogh 1985).

Age calculations

The atomic constants suggested by Steiger & Jäger (1977)

were used for calculating the ages of the samples. All analyti-

cal errors are given in terms of ± 1

σ (i.e. with a 68% analytical

confidence level). In order to check the reproducibility and ac-
curacy of the argon and potassium analysis, duplicate mea-
surements were performed on two samples (designated Nos.
5379 and 5272) at Okayama University and at ATOMKI, De-
brecen, respectively.

At Okayama University the K-Ar dating was performed us-

ing the methods described by Nagao et al. (1984) and Itaya et
al. (1991). The analytical errors in Okayama are given in
terms of ± 2

σ.

Results and discussion

The analytical results of the K-Ar dating are summarized in

Table 1. Forty-one K-Ar age determinations were carried out
on  different  mineral  separates  and  whole-rock  samples  from
the PMIC and on one biotite separated from the Železna Kapla
tonalite.  Except  for  the  biotite  separate  No. 5694  from  the
Železna Kapla tonalite that gave an age of 32.4 ± 1.2 Ma (Oli-
gocene), all the other ages from the PMIC range between 20.3
and 14.9 Ma (Miocene). No significant gaps were observed in
the  K-Ar  ages  of  the  different  rock  types  (Figs. 15  and  16).
Due to the analytical errors the ages generally overlap and re-
flect their transitional character. However, there is geological
proof that this magmatism was episodic.

Fig. 15. Distribution of the K-Ar ages for the rocks of the PMIC.
The abbreviations of the localities are given in the text, and their
succession is shown in Table 1.

257

K-Ar GEOCHRONOLOGY AND PETROGRAPHY OF THE MIOCENE POHORJE MOUNTAINS (SLOVENIA)

order  to  increase  the  reliability  of  the  radiometric  ages  ob-
tained from the granodiorite varieties, the K-Ar ages of a bi-
otite  fraction  with  the  same  grain  size  were  determined.  The
reason  for  this  is  that  the  closure  temperature,  and  thus  also
the time of closure of a mineral, depends on the grain size in
the  cooling  pluton  (Hess  et  al.  1993).  In  our  work,  only  the
0.200—0.350 mm fraction was used for dating. The ages range
between  16.9 Ma  and  15.7 Ma  (Table 1).  Highly  consistent
ages  were  obtained  on  biotite  separates  ( ~ 16.5 Ma)  because
all the ages were in good accordance within the analytical er-
ror. Due to the low K content (3.32 %) of the biotite separate
No. 6034,  a  duplicate  analysis  was  performed  in  order  to
check the reliability of the analytical age. Considering that the
ages  obtained  on  the  same  biotite  separate  were  within  the
limits of the analytical error, a mean age (16.9 ± 0.5 Ma) was
accepted. Furthermore, two representative samples (No. 5379
and No. 6034) were analysed at the Geochronological Labora-
tory of Okayama University: they yielded the same result. The
general agreement between the biotite and feldspar age (Sepa-
rate No. 5386, 15.7 ± 0.6 Ma) suggests that the measured ages
may refer to a rapid cooling of the granodiorite. Besides this,
the  mean  age  (16.5 Ma)  of  the  biotite  fractions  provides  the
best information regarding the cooling history of the pluton.

Porphyritic granodiorite

Samples were taken from two different localities, and nine

separates were dated. Slightly different ages were obtained on
the  biotite  (16.5 ± 0.7—17.5 ± 0.7 Ma)  and  the  feldspar  sepa-
rates  (18.0 ± 0.6 Ma).  The  only  exceptional  older  biotite  age
(No. 5382)  was  determined  on  a  porphyritic  granodiorite,
transiting to dacite. Taking into consideration that the biotite
ages are generally slightly younger than the feldspar ages, the
presence  of  excess  argon  in  feldspar  separates  is  probable.
However, the biotite and feldspar ages overlap, because of the
analytical error (Table 1). Therefore the biotite ages can be re-
garded as the cooling ages of the porphyritic granodiorite.

Cezlakite

Almost identical ages (19.5 ± 0.8 Ma and 20.3 ± 1.1 Ma)

were  obtained  for  the  amphibole  mineral  fractions  separated
from samples Nos. 5387/A and 5387/B. A significant decrease
in age (17.3 ± 0.7 Ma, No. 5387/A) was determined on the ma-
fic  mineral  fraction  enriched  in  biotite.  The  older  amphibole
ages  ( ~ 20 Ma)  suggest  that  this  gabbroic  body  was  already
formed,  when  the  granodiorite  intrusion  was  emplaced.  Yet,
based on the analytical data the presence of excess argon in the
amphiboles  cannot  be  excluded.  This  assumption  is  also  sup-
ported by the U-Pb zircon age (18.64 ± 0.11 Ma) of the pluton
(Fodor  et  al.  2007).  On  the  contrary,  the  younger  K-Ar  age  of
sample No. 5387/A may reflect the heat effect caused by the in-
trusion of the granodiorite. This interpretation is also supported
by the available geological (e.g. Fig. 14) and petrographical data.

Dacite

The dacitic dykes were sampled at four different localities,

and six samples were prepared. The biotite separates

(No. 5272  –  duplicate  analysis,  No. 5655  and  No. 6851)
gave  identical  ages  ( ~ 17 Ma)  except  for  one  separate
(No. 6851,  16.0 ± 0.6 Ma).  The  whole-rock  ages  (Nos. 6033
and  4996b,  ~16.7 Ma)  are  slightly  lower,  but  similar.  One
separate of mafic minerals (intergrowing of biotite and horn-
blende,  No. 6033)  yielded  the  oldest  age  (18.2 ± 0.7 Ma),
which  could  show  some  influence  of  the  excess  argon  and
mixed  age  of  the  two  minerals.  The  apparent  ages  obtained
on the whole-rock samples can be regarded as the minimum
age,  and  could  be  slightly  younger  than  the  real  geological
age. On the other hand the whole-rock ages support the reli-
ability  of  the  biotite  ages  as  determined  on  the  same  rock
sample.  According  to  the  petrography,  some  of  the  dacite
dykes  (e.g.  Vuzenica)  are  slightly  altered,  but  since  the  bi-
otite  is  mostly  fresh  it  is  not  supposed  that  a  significant  Ar
loss could have occurred from the biotite because of this al-
teration.

Dacite dykes of mafic character (transiting to lampro-

phyre) were sampled in the areas of the Mislinja graben, Sta-
ra  Glažuta  and  Mala  Kopa.  Because  of  their  fine-grained
porphyritic  structure  and  overgrowing,  only  whole  rock
(Nos. 6014,  6029,  6012,  and  6013)  and  one  mafic  separate
(No. 6014)  were  dated.  The  ages  range  between  18.5 ± 0.7
and  16.5 ± 0.6 Ma.  The  older  ages  are  interpreted  as  being
closer to the emplacement age of the dacite, although some
excess  Ar  could  be  present  due  to  the  xenocrysts.  The
younger ages probably reflect cooling of the dacite.

On the basis of the available radiometric data it is not possi-

ble to define a gap between the formation of the granodiorite
and the dacite.

Rhyodacite

Taking into account the texture and freshness of the rhyo-

dacite, whole-rock samples and biotite separates (5 measure-
ments)  were  obtained  at  three  exposures  (Trbonje,  peak
Mršak  and  Mala  Kopa).  The  whole-rock  ages  are  slightly
lower, but consistent with the biotite ages. They range from
16.4 ± 0.5 Ma  (No. 6854)  to  14.9 ± 0.6 Ma  (No. 4999).  Two
determinations on the biotite separates (Nos. 5385 and 6854)
yielded almost identical results (16.2 ± 0.7 and 16.4 ± 0.5 Ma).
The  results  obtained  from  whole-rock  fractions  (Nos. 4998,
4999  and  4997)  show  slight  variation,  from  16.1 ± 0.6  to
14.9 ± 0.6 Ma. The fresh biotite separate from the shallowest
outcrop at Trbonje gives a reliable age (16.4 ± 0.5 Ma).  The
shallow  rhyodacite  dykes  cooled  rapidly,  so  that  the  biotite
ages,  together  with  field  evidence,  could  reflect  the  age  of
their intrusion. The somewhat lower whole-rock ages could
be the consequence of the slightly altered glassy groundmass
of  the  rocks,  since  the  glass  retentivity  of  Ar  is  very  poor.
The results obtained for the rhyodacite can be used to make
comparisons of the isotopic ages with the stratigraphic data
of  the  surrounding  sediments.  Such  comparisons  indicate
rhyodacite  volcanism  at  the  Karpatian/Badenian  boundary.
These ages could be related to the main tectonic phase which
affected  the  PMIC  (Trajanova  &  Pécskay  2006).  This  as-
sumption  is  supported  by  the  available  paleomagnetic  data
(Márton et al. 2006).

258

TRAJANOVA, PÉCSKAY and ITAYA

Lamprophyre

Samples were taken from the contact between lamprophyre

and granodiorite at Stara Glažuta, and from one lamprophyre
dyke  within  metamorphic  rocks  at  the  Mislinja  graben.  Be-
cause  of  the  fine-grained  porphyritic  structure  with  phenoc-
rysts of hornblende only a whole-rock sample (No. 6030) and
an  amphibole  separate  (No. 5653/2)  were  dated.  Both  ages
(17.7 ± 0.7 and 18.2 ± 0.7 Ma) show higher values than can be
supported by the field evidence. It was assumed that one of the
reasons for this could be the presence of excess argon in am-
phiboles, in xenocrysts or in micro-xenoliths. However, the
older  amphibole  ages  can  be  the  consequence  of  its  higher
closure  temperature  (500—550

C K-Ar, 530 ± 40

C Ar-

Ar method, Harland et al. 1990).

Aplite-pegmatite

At the Cezlak granodiorite quarry a single sample from an

aplite-pegmatite  vein  was  dated.  The  K-feldspar  separate
(No. 6988) yielded an age of 16.1 ± 0.5 Ma. Aplite-pegmatite
intruded  into  already  brittle  deformed  rocks,  thus  providing
geological evidence that it represents the last phase of magma-
tism. The K-feldspar age can give the uppermost limit of the
magmatism  termination  on  the  PMIC,  and  strongly  supports
the  validity  of  the  cooling  ages  (around  16.7 Ma)  obtained
from different rocks.

Conclusions

The Pohorje Mountains igneous complex is composed pre-

dominantly of granodiorite and dacite. The easternmost part
of the pluton is composed of tonalite. Small-sized rhyodacite
and  lamprophyre  dykes  represent  minor  intrusions.  No  evi-
dence  has  been  found  to  show  that  the  dacite  intruded  into
the  granodiorite.  The  rocks  show  a  clear  gradual  transition
from plutonic to shallow intrusive rocks.

The results of this systematic geochronological study, as

well as the radiometric ages obtained on some of the metamor-
phic  rocks  (Fodor  et  al.  2004),  provide  strong  evidence  that
the PMIC was formed in the Miocene. The K-Ar ages range
between approximately 19.0 Ma and 16.0 Ma. The older ages
(19—18 Ma) are close to the emplacement age of the batholith,
which  is  also  confirmed  by  the  U-Pb  zircon  age
18.64 ± 0.11 Ma (Fodor et al. 2007). The age of the transitional
diorite  to  pyroxenite  rock  named  cezlakite  is  not  well  con-
strained,  but  the  radiometric  data  are  supported  by  field  evi-
dence, proving that this small body within the granodiorite is
the oldest.

No apparent younging direction has been noticed within

the studied area. The K-Ar ages of the granodiorite and dac-
ite  generally  overlap.  The  consistent  biotite  ages  indicate
that  synchronous  and  rapid  cooling  of  the  whole  batholith
most  probably  occurred  at  about  16.7 Ma  (Fig. 16),  on  the
Karpatian/Badenian  boundary.  The  pronounced  marginal
oriented  rock  structure  indicates  crystallization  in  an  exten-
sional  stress  field,  and  a  connection  between  magmatic  and
tectonic activity.

At the northwestern part of the PMIC, extensional processes

opened pathways for the emplacement of small-sized rhyodac-
ite  dykes.  The  dykes  cooled  rapidly,  so  that  the  biotite  ages
might  reflect  the  age  of  their  intrusion.  Synchronously,  thin
lamprophyre dykes intruded into the metamorphic rocks along
the marginal western part of the pluton, followed by the intru-
sion of residual aplitic-pegmatitic melts into the already frac-
tured pluton, mostly on its southern part. K-feldspar separated
from  the  aplite-pegmatite  is  the  most  suitable  tool  to  deter-
mine the age of the last magmatic event in the PMIC, which
occurred around 16 Ma.

Tectonic activity characterized by strong tilting of the entire

Pohorje Mountains massif continued and rapid unroofing oc-
curred. Extensional processes indicated by post-cooling low
angle shearing and brittle faulting is expressed on all rock
types. Broadly NW to SE directed thinning formed the thick
shear zones within the PMIC.

The magmatic activity in the PMIC is probably connected

to the deep transtensional rift zones related to the development
of the Labot fault system north of the Periadriatic zone. This
magmatism represents the westernmost intrusion along the ex-
tensional structures of the Pannonian Basin. In contrast, mag-
matism along the Periadriatic line was active in the Paleogene.
The  Pohorje  Mountains  granodiorite  differs  petrologically
from the Oligocene Železna Kapla tonalite (the Črna tonalite
has an age of approximately 32.4 Ma) and is much younger. It
is  also  different  from  the  tonalites  which  are  found  further
west  (marked  1—16  on  Fig. 1)  and  the  tonalites  buried  in  the
Zala Basin in Hungary (with ages of between 40 and 30 Ma),
which, together with the Železna Kapla tonalite, belong to the
Periadriatic intrusions.

Acknowledgments:  The  financial  support  for  this  research
work, which was provided by the Geological Survey of Slove-
nia  and  the  Institute  of  Nuclear  Research  of  the  Hungarian
Academy  of  Sciences,  is  gratefully  acknowledged.  Special
thanks  are  due  to  the  reviewers  N.  Thorsten,  J.  Lexa  and  V.
Cvetković, for their very constructive corrections to the manu-
script, as well as to M. Janák for useful suggestions, and to P.
Sheppard for final corrections of the English text. The authors
also express their thanks to E. Toth, M. Štumergar, S. Zakra-
jšek and S. Čertalič, for technical support.

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