GeolCarp_Vol61_No6_454

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA, DECEMBER 2010, 61, 6, 451—461 doi: 10.2478/v10096-010-0027-y

Tectonic evolution of the southeastern part of the Pohorje

Mountains (Eastern Alps, Slovenia)

FREDERIK KIRST

, SASCHA SANDMANN

, THORSTEN J. NAGEL

, NIKOLAUS FROITZHEIM

and MARIAN JANÁK

Steinmann-Institut, University of Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany; fredster@uni-bonn.de

Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovak Republic; marian.janak@savba.sk

(Manuscript received April 7, 2010; accepted in revised form June 10, 2010)

Abstract: Field relations and deformation structures in the southeastern part of the Pohorje Mountains constrain the
tectonic evolution of the Austroalpine high-pressure/ultrahigh pressure (HP/UHP) terrane. The Slovenska Bistrica
Ultramafic Complex (SBUC) forms a large (ca. 8 1 km size) body of serpentinized harzburgite and dunite including
minor garnet peridotite and is associated with partly amphibolitized eclogite bodies. The SBUC occurs in the core of an
isoclinal, recumbent, northward closing antiform and is mantled by metasedimentary rocks, mostly gneisses and a few
marbles, including isolated eclogite/amphibolite lenses. Before this folding, the SBUC formed the deepest part of the
exposed terrane. We interpret the SBUC to be derived from near-MOHO, uppermost mantle which was intruded by
gabbros in the subsurface of a Permian rift zone. During Cretaceous intracontinental subduction, the SBUC was most
likely part of the footwall plate which experienced HP to UHP metamorphism and was folded during exhumation. In the
Miocene, the Pohorje Pluton intruded and, subsequently, the metamorphic rocks together with the pluton were de-
formed probably due to east-west extension and contemporaneous north-south shortening, thus forming an antiformal
metamorphic core complex.

Key words: Eastern Alps, Pohorje, UHP metamorphism, eclogite, garnet peridotite.

Introduction

Eclogite-facies tectonic units in collisional orogens often con-
tain garnet-bearing peridotite lenses and boudins. This is typi-
cally,  but  not  exclusively,  the  case  for  tectonic  units  that
reached  ultrahigh-pressure  (UHP)  conditions  (Coleman  &
Wang  1995).  There  are  in  principle  two  different  ways  in
which  these  ultramafic  bodies  may  have  been  incorporated
into  the  surrounding  felsic  and  mafic  rocks.  One  is  that  they
originated from the mantle wedge above the subducting crust
(“mantle”  peridotites  sensu  Brueckner  &  Medaris  2000)  and
were incorporated into the downgoing plate either during sub-
duction or exhumation. Alternatively, ultramafic bodies were
transferred from the mantle into the crust prior to subduction
(“crustal” peridotites sensu Brueckner & Medaris 2000). Un-
derstanding  these  processes  yields  important  constraints  for
the  tectono-metamorphic  and  geochemical  processes  in  sub-
duction zones.

In this paper we show the field relations between ultramafic

mantle rocks, including garnet peridotite, and crustal gneisses
in  the  Pohorje  Mountains  of  the  Eastern  Alps  which  experi-
enced  HP  to  UHP  metamorphism  (Hinterlechner-Ravnik
1987;  Hinterlechner-Ravnik  et  al.  1991;  Janák  et  al.  2004,
2006, 2009; Sassi et al. 2004; Vrabec 2004, 2007; Miller et al.
2005;  De  Hoog  et  al.  2009).  Janák  et  al.  (2006)  documented
that  garnet  peridotites  in  Pohorje  reached  UHP  metamorphic
conditions of up to 900 °C and 4 GPa during Cretaceous intra-
continental subduction. They proposed that garnet peridotites
were  derived  from  depleted  mantle  rocks  which  were  subse-
quently  metasomatized  in  the  plagioclase-peridotite  or  the

spinel-peridotite stability field. Subduction of these peridotite
protoliths  resulted  in  the  development  of  garnet-bearing  as-
semblages at HP and UHP conditions of metamorphism. As a
possible tectonic scenario, Janák et al. (2006) and De Hoog et
al.  (2009)  suggested,  based  on  petrological  and  geochemical
data,  that  peridotites  were  incorporated  into  the  subducting
crust from the overlying mantle wedge. The field relations re-
ported here, however, show that garnet peridotites and the as-
sociated serpentinites and eclogites formed the deepest part of
the exposed HP/UHP metamorphic terrane, suggesting an al-
ternative scenario in which the peridotites may have been part
of the downgoing plate already from the beginning.

Regional geological setting

The Pohorje Mountains in NE Slovenia are located at the

southeastern margin of the Eastern Alps in the vicinity of the
Periadriatic  Line  (Fig. 1).  The  basement  of  the  area  largely
consists  of  Cretaceous-age  high-grade  metamorphic  rocks,
mainly  gneisses  and  micaschists,  with  bodies  and  lenses  of
metabasics  and  a  body  of  ultramafic  rocks,  the  Slovenska
Bistrica Ultramafic Complex (SBUC), at the southeastern end.
The  SBUC  extends  over  8 km  from  Modrič  in  the  west  to
Šentuvec in the east. It has been strongly serpentinized except
for a few garnet peridotite remnants in which UHP-assemblages
are  preserved.  The  entire  metamorphic  series  is  called  the
Pohorje  Nappe  after  Janák  et  al.  (2006)  and  belongs  to  the
Lower  Central  Austroalpine  after  Janák  et  al.  (2004)  or  the
Koralpe-Wölz nappe system after Schmid et al. (2004). In the

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KIRST, SANDMANN, NAGEL, FROITZHEIM and JANÁK

NW  part  of  the  Pohorje  Mts,  the  high-grade  metamorphic
rocks  are  overlain  by  low-grade  slates  and  phyllites  of  the
Upper Central Austroalpine (Drauzug-Gurktal nappe system)
along  a  top-to-the-E  low-angle  normal  fault  of  Late  Creta-
ceous age. The central part of the Pohorje Mts is built by the
Pohorje  Pluton,  a  granodiorite  to  tonalite  intrusion  with  an
Early Miocene crystallization age of ca. 18.6 Ma, followed by
rapid cooling (Fodor et al. 2008; Trajanova et al. 2008). The
plutonic  rocks  are  associated  with  subvolcanic  dacite  bodies
to the northwest, representing a shallower crustal level. In this
area,  the  pluton  just  reaches  into  the  basal  part  of  the  Upper
Central Austroalpine slates and phyllites. The whole magmat-
ic complex extends over ca. 35 km in a WNW-ESE direction.

Paleogeographically the Austroalpine realm represents the

northwestern continental margin of Apulia. From bottom to
top,  the  Austroalpine  nappe  stack  is  subdivided  into  the
thrust  sheets  of  the  Lower  Austroalpine,  the  Lower  Central
Austroalpine  and  the  Upper  Central  Austroalpine  (Janák  et
al. 2004). The Permian to Mesozoic sediment nappes of the
Northern Calcareous Alps represent the former sedimentary
cover of the Central Austroalpine. The sedimentary rocks are
still partly connected with the Upper, partly with the Lower
Central Austroalpine (Janák et al. 2004; Nagel 2006). In gen-
eral,  more  southeasterly  located  units  of  the  Austroalpine
were thrust over more northwesterly located ones during the
Eo-Alpine orogeny in the Cretaceous.

Together with Saualpe, Koralpe and some other areas, the

Pohorje  Mts  belong  to  the  Koralpe-Wölz  nappe  system
which is part of the Lower Central Austroalpine. Before their
HP-UHP  metamorphism,  these  units  experienced  a  HT-LP
event  during  Permian  rifting  (e.g.  Schuster  et  al.  2001;
Schuster & Stüwe 2008) causing metamorphism of Paleozoic

and older sediments of continental and oceanic affinity. Rift-
ing  also  led  to  underplating  and  emplacement  of  gabbroic
bodies into thinned continental crust (Thöni & Jagoutz 1993;
Thöni et al. 2008). During the Cretaceous, these mafic rocks
and  their  host  rocks,  mainly  metasediments  with  minor  or-
thogneisses,  were  overprinted  under  HP  to  UHP  conditions
(Janák et al. 2004, 2006, 2009; Sassi et al. 2004; Miller et al.
2005, 2007) in a south- to southeast-dipping, intracontinental
subduction zone (Janák et al. 2004). The direction of subduc-
tion can be inferred from a general pressure gradient along the
Koralpe/Pohorje traverse with lower pressures in the NW and
higher pressures in the SE (Tenczer & Stüwe 2003; Janák et
al. 2004; Bruand et al. 2010). Subduction and exhumation re-
sulted in strong deformation and nappe stacking of these units.
The peak of (U)HP metamorphism in Pohorje was reached at
ca. 91 Ma according to garnet Sm-Nd and zircon U-Pb dating
of  eclogites  and  gneisses  (Thöni  2002;  Miller  et  al.  2005;
Thöni et al. 2008; Janák et al. 2009).

Cretaceous subduction and collision were followed by an

extensional stage between  ~ 80 and 67 Ma within the Austro-
alpine units (e.g. Rantitsch et al. 2005). Extension resulted in
lithospheric  thinning  and  formation  of  half-grabens  in  the
upper crust and thus may have contributed to the exhumation
of the deeply buried units. The rifting was probably caused
by  westward  rollback  of  the  Penninic  subduction  zone
(Froitzheim  et  al.  1997).  During  the  Paleocene  and  Eocene
the  Austroalpine  nappe  stack  was  thrust  on  top  of  the  Pen-
ninic  and  Helvetic  units  due  to  the  collision  between  the
Apulian and European continents. In the Miocene, the East-
ern Alps underwent orogen-parallel extension and lateral ex-
trusion  of  crustal  blocks  to  the  east  (Ratschbacher  et  al.
1991) as a result of east-directed rollback in the Carpathians

Fig. 1. Geological sketch map of the Pohorje Mountains and adjacent areas with marked location of the map area; modified from Mioč &
Žnidarčič 1977.

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KIRST, SANDMANN, NAGEL, FROITZHEIM and JANÁK

and gravitational collapse. Lithospheric thinning also result-
ed in magmatism within the Pannonian Basin as well as the
adjacent Austroalpine units, including the Early Miocene
Pohorje Pluton (Trajanova et al. 2008; Fodor et al. 2008). In
the Pohorje Mts, paleostress analysis showed that Neogene
E-W extension was temporarily accompanied by N-S short-
ening (Fodor et al. 2008).

In order to clarify the structural relations between ultramafic

rocks, eclogites, and gneisses, we mapped an area of about
35 km

in the scale of 1 : 5,000 northwest of Slovenska Bistrica

(Figs. 2 and 3) including the eastern end of the Pohorje Pluton
with surrounding gneisses and micaschists as well as metaba-
sics and meta-ultrabasics occurring south of the intrusion.
This area includes all the formerly described UHP localities
(Hinterlechner-Ravnik 1987; Janák et al. 2004, 2006, 2009).

Field relations and petrography

The Lower Central Austroalpine basement rocks in the

study area mostly comprise strongly sheared gneisses and mi-
caschists which have been retrogressed to variable degrees un-
der amphibolite to upper greenschist facies conditions after
their Upper Cretaceous HP metamorphism. These rocks host
bodies and lenses of metabasics, an ultramafic complex, and
the southeastern end of the Miocene Pohorje Pluton.

Metamorphic rocks

Serpentinites vary in colour from black and dark green to

very light green. Bastite as a pseudomorph of lizardite after
enstatite or bronzite is quite common; its grain-size of up to
1 cm  suggests  a  coarse-grained  precursor  material.  Kernel
structures  and  other  mesh-like  textures  formed  during  ser-
pentinization. All of the serpentinites have a massive texture
without any macroscopically visible preferred orientation of
minerals  indicating  that  serpentinization  was  essentially
post-tectonic. According to geochemistry the protolith of the
serpentinite was harzburgite and dunite, highly depleted fol-
lowing  melting  within  the  spinel  stability  field  and  later
metasomatized by melts or fluids before serpentinization. It
probably originated in the oceanic mantle or in a continental
rift zone (De Hoog et al. 2009).

Garnet peridotites occur as small lenses, several meters to

tens of meters in size, or as loose boulders. These rocks con-
sist  of  garnet,  olivine,  orthopyroxene,  clinopyroxene  and
brown  spinel,  which  are  variably  replaced  by  amphibole,
green  spinel,  serpentine,  talc  and  chlorite  (see  Janák  et  al.
2006  for  more  details).  All  garnet  peridotite  localities  are
within the SBUC (Fig. 2) in contrast to the map of Mioč &
Žnidarčič (1977) in which one of the localities (119) is away
from this body (see also Janák et al. 2006). Two localities of
garnet peridotite (119 and VI01/04) show UHP metamorphic
conditions of up to 4 GPa and 900 °C (Janák et al. 2006).

Eclogites occur as lenses and bodies several meters to sev-

eral hundreds of meters in size south of the Pohorje Pluton.
They are often associated with or even surrounded by am-
phibolite but also occur as individual bodies within gneisses
and serpentinites. The HP metamorphic assemblage is gar-

Fig. 3.

NE-SW

trending

cross-sections.

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TECTONIC EVOLUTION OF THE SOUTHEASTERN PART OF THE POHORJE MOUNTAINS (SLOVENIA)

net,  omphacite,  phengite,  kyanite,  zoisite  and  quartz.  It  is
usually  retrogressed  to  some  extent  so  that  amphibole,  pla-
gioclase  and  diopsidic  clinopyroxene  occur  as  secondary
phases (see Janák et al. 2004 and Sassi et al. 2004 for more
details).  The  eclogites  differ  in  their  macroscopic  appear-
ance,  probably  reflecting  the  composition  and  texture  of
their  protoliths  as  well  as  strain  partitioning.  Some  of  them
show  a  distinct  foliation  (Fig. 4a)  with  alternating  garnet-
rich layers and layers rich in omphacite and zoisite, while in
others garnet is concentrated in patches a few cm in diame-
ter. Janák et al. (2004) reported two UHP localities of kyan-
ite  eclogites  (JV03,  PO6)  which  are  shown  on  the  map

(Fig. 2). Metamorphic conditions of these eclogites reached
3.0—3.1 GPa and 760—825 °C (Janák et al. 2004).

Amphibolites consist of hornblende and plagioclase and

they are partly foliated. The amphibolites form lenses and
bodies from several tens of meters to two kilometers in size.
There is a transition between eclogite and amphibolite; typi-
cally, the inner part of mafic lenses is preserved as eclogite
whereas the outer part is sheared and retrogressed to amphib-
olite (Fig. 4b).

Gneisses occur as several types. (a) Leucogneiss is domi-

nated by quartz, K-feldspar, and plagioclase, together with
white mica and minor biotite. (b) Biotite gneiss consists of

Fig. 4. a – Foliated eclogite. The outcrop surface is perpendicular to the foliation. Outcrop coordinates: 46°24

’35” N, 15°30’58” E. b – Mar-

ginal amphibolitization of an eclogite body. Amphibolitization was accompanied by the formation of a new, subhorizontal foliation and a
WNW-ESE-trending stretching lineation. The older eclogite-facies foliation in the eclogite is parallel to the surface of the outcrop. Outcrop
coordinates: 46°24

’59” N, 15°31’33” E.

Fig. 5. a – Impure marble with boudinaged layers of aplite and am-
phibolite. Outcrop coordinates: 46°25

’44” N, 15°32’14” E. b – Iso-

clinally folded pegmatite with S-dipping fold axis plane (180/57)
subparallel to the foliation of the surrounding amphibolite (186/33).
Outcrop coordinates: 46°24

’43” N, 15°31’20” E. c – Brittlely

sheared Miocene pegmatite in gneiss showing top-to-the-E shear
sense. Foliation of gneiss: 114/13. Outcrop coordinates:
46°25

’58” N, 15°33’17” E.

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KIRST, SANDMANN, NAGEL, FROITZHEIM and JANÁK

biotite, K-feldspar, quartz, and plagioclase with minor white
mica. (c) Augengneiss contains large K-feldspar porphyro-
blasts a few cm in diameter, with plagioclase, quartz, biotite
and white mica in the matrix. The texture of the augengneiss
is inhomogeneous and migmatitic.

Micaschists occur as layers and lenses of different sizes

within surrounding gneisses. They consist of variable modal
amounts of garnet, white mica and biotite as well as quartz,
plagioclase, K-feldspar, kyanite and staurolite. Their Al-rich
composition corresponds to metapelites (see Janák et al. 2009
and  Hurai  et  al.  2009  for  more  details).  Janák  et  al.  (2009)
determined P-T conditions of 2.2—2.7 GPa at 700—800 °C for
these metapelites (JV02/04, PO6MS and PO6OR, Fig. 2) but
assumed  that  they  probably  experienced  the  same  UHP
metamorphism at  ~ 3 GPa as the associated kyanite eclogites.
Zircons  from  the  metapelites  yielded  an  age  of  92 ± 0.5 Ma
using the SIMS ion microprobe (Janák et al. 2009) which is
interpreted  as  the  age  of  HP-UHP  metamorphism.  Some  of
the  zircon  cores  record  Permian  to  Triassic  events,  most
probably  reflecting  HT-LP  metamorphism  at  that  time.
Chemical Th-U-Pb (EMPA) dating of monazite (Krenn et al.
2009) yielded an age of 100 ± 6 Ma.

Marbles occur within a metamorphic lens surrounded by

granodiorite and in two elongate lenses northeast of the in-
trusion. Marbles are coarse-grained and foliated.

Leucogneisses and biotite gneisses are more abundant than

augengneisses and micaschists which form layers and lenses
within them. Textures are rather heterogeneous on the out-

crop scale as a result of tectonically transposed layering. As
an example, Fig. 5a shows an outcrop of impure marble with
boudinaged layers of aplite and amphibolite.

Pohorje Pluton

Granodiorites are the most abundant intrusives, composed

of  quartz,  K-feldspar,  plagioclase,  biotite  and  hornblende.
They are usually slightly foliated, especially at the rims of the
intrusion. According to geochemical data the granodiorite origi-
nated from a mantle source, was subsequently modified by as-
similation of crustal material, and differentiated (Dolenec et al.
1987;  Zupančič  et  al.  1994;  Altherr  et  al.  1995).  Fodor  et  al.
(2008) estimated a pressure of 0.6—0.7 GPa which corresponds
to magma crystallization at 23—26 km depth (assuming a crustal
density  of  2.7 g/cm

) using Al-in-hornblende barometry on a

granodiorite from the northwestern part of the map area, which
confirms the results of Altherr et al. (1995). Towards the north-
west, the pressure decreases to 0.3—0.4 GPa (11—15 km) near
the northwestern end of the pluton (Fodor et al. 2008).

Pegmatites and aplites occur within the granodiorite and

surrounding metamorphic rocks. They form veins and bodies
of several centimeters to tens of meters in length which intrud-
ed partly subparallel, partly discordant to the foliation of the
surrounding rocks. The pegmatites consist of quartz and feld-
spar only, the aplites also contain biotite. The pegmatites were
often isoclinally folded with axial planes parallel to the folia-
tion of the host rocks, or they were sheared under ductile and

Fig. 6. a – Photomicrograph of a quartz mylonite showing sinistral,
top-to-the-E  shear  sense.  Primary  fabric:  F(139/28)  L(96/15).  Dy-
namic  recrystallization  of  quartz  grains  by  combined  subgrain  rota-
tion  and  grain  boundary  migration  is  indicative  for  temperatures  of
ca. 400—500 °C.  Outcrop  coordinates:  46°23

’58” N, 15°28’34” E.

b—c – Photomicrographs of garnet micaschist with the assemblage
Grt + Ky + Ms + Bt + Qtz. F(137/25) L(88/14). Outcrop coordinates:
46°24

’53” N, 15°33’19” E. b – Shear bands (e.g. in the upper

right corner) and sheared micas indicate a dextral, top-to-the-W shear
sense. c – Mica fish indicating a dextral, top-to-the-W shear sense.

457

TECTONIC EVOLUTION OF THE SOUTHEASTERN PART OF THE POHORJE MOUNTAINS (SLOVENIA)

Fig. 7. Poles of foliations and stretching lineations of a – metamorphic rocks and b – granodiorite in the stereonet; equal area projection,
lower hemisphere.

brittle conditions. There are also pegmatites that appear as
strongly sheared lenses of some decimeters length within bio-
tite gneisses. The aplites are mostly undeformed.

Deformation structures

The metamorphic rocks generally display a strong and pen-

etrative foliation, with the exception of most of the eclogites,
some amphibolites, and the serpentinites. Microstructures re-
lated to the foliation record amphibolite- to upper-greenschist-
facies conditions, as indicated by grain-boundary migration
deformation in quartz layers (Fig. 6a) and dynamic recrystalli-
zation of plagioclase. In contrast, the foliation in the eclogites

formed  under  eclogite-facies  conditions  and  is  typically  dis-
cordant to the younger, amphibolite-facies foliation of the host
rocks (Fig. 4b). Apart from this, the foliation of the metamor-
phic  rocks  dips  at  a  shallow  to  moderate  angle  to  the  south-
southwest,  or less frequently to the southeast (Fig. 7a).

The magmatic rocks of the Pohorje Pluton are in many

places foliated as well. This foliation is generally weaker
than that in the metamorphic rocks. The orientation of the fo-
liation is similar to that in the metamorphic rocks but more
scattered (Fig. 7b). The foliation in the pluton formed under
greenschist-facies conditions (Fodor et al. 2008). In the Bistri-
ca dell in the area between cross-sections 1 and 2, foliations
in the granodiorite dip at angles of 12°—37° to the SW and S
whereas stretching lineations mostly dip SE at angles of

458

KIRST, SANDMANN, NAGEL, FROITZHEIM and JANÁK

10°—22°; the shear-sense criteria show a top-to-the-SE sense
of shearing.

The stretching lineation in the metamorphic rocks shows ir-

regular orientation within the foliation (Fig. 7). Samples with
E-W-striking lineation often show a top-to-the-E shear sense
(Fig. 6a) but the opposite is observed as well (Fig. 6b and c).
We were unable to assign a distinct direction of the lineation
to some particular stage of the tectonic evolution. This is prob-
ably due to a polyphase deformation history with several
shearing events and different shear directions which reorient-
ed older lineations. Miocene (post-intrusive) stretching linea-
tions in the magmatic rocks of the Pohorje Pluton are also
variably oriented but rather weakly developed.

Outcrop-scale folds occur only rarely. Some folded pegma-

tite dykes can be observed; their axial planes are parallel to the
foliation  of  the  metamorphic  rocks.  For  example,  Fig. 5b
shows  an  isoclinally  folded  pegmatite  with  S-dipping  axial
plane subparallel to the foliation of the surrounding amphibo-
lite.  Fig. 5c  shows  a  pegmatite  in  gneiss  which  was  sheared
under brittle conditions, displaying top-to-the-E sense of shear
as a result of Miocene extension. The presence of large-scale,
tight folds along the northern border of the SBUC is inferred
from structural field relations as desribed below.

Relations between the pluton and the metamorphic

rocks

The contacts between the pluton and its metamorphic coun-

try rocks are in general roughly parallel to the foliation of the
pluton  and  the  country  rocks.  Both  the  northeastern  and  the
southwestern contacts dip southwest to south at moderate an-
gles. Therefore, the granodiorite body in the investigated area
displays  a  tabular  shape  dipping  towards  the  southwest,
strongly thinning towards the east and finally wedging out. As
mentioned  above,  Al-in-hornblende  barometry  indicates  that
the depth of the granitoid intrusion increases from the north-
west to the southeast. Accordingly, the “tail” of the pluton is
the deepest exposed part of the intrusion. Since the foliation in
the pluton is much weaker than in the country rocks and the
contacts are sub-parallel to the foliation, it appears that in the

Fig. 8. Sketch of the structural relations in the Pohorje Mountains.

Further northwest the pluton becomes discordant, cutting the
foliation  of  the  metamorphic  rocks  (according  to  the  map  of
Mioč  &  Žnidarčič  1977)  and  also  the  Cretaceous  low-angle
normal fault at the base of the Upper Austroalpine. The Pohorje
Antiform may be interpreted as a large-scale antiformal corru-
gation  resulting  from  the  contemporaneous  E-W  extension
and N-S shortening (Fodor et al. 2003), like the ones observed
in  the  footwall  of  the  Simplon  Line  in  the  western  Central
Alps (Mancktelow & Pavlis 1994).

Relations between the SBUC and the surrounding

rocks

Although the main part of the SBUC is formed by ultramaf-

ic rocks, several lenses of eclogite occur within it. In addition
to these internal eclogite lenses, the three largest bodies of ma-
fic  rocks  are  in  direct  contact  with  the  serpentinite  (Fig. 2).
One of these is located at the western end of the SBUC associ-
ated with the garnet peridotites. The other two are located on
the  northern  boundary  of  the  SBUC  and  extend  northwards
into  the  gneisses.  The  amphibolitized  parts  of  eclogites  are
mostly  in  contact  with  the  gneisses  whereas  well  preserved
eclogites are mostly in contact with the serpentinite, within the
SBUC. Therefore we assume that amphibolitization of eclog-
ites was facilitated by deformation focused along the contact
between  eclogites  and  gneisses.  The  ultramafic  rocks  were
serpentinized  after  amphibolite-facies  overprint  and  after  re-
gional  ductile  deformation,  possibly  during  the  intrusion  of
the Pohorje Pluton.

Along the southwestern border of the SBUC, the gneisses

show  a  S-  to  SW-dipping  foliation  and  must  be  structurally
above  the  SBUC.  Along  the  northern  border  the  foliation  of
the gneisses is generally dipping to the south which indicates
that the gneisses are structurally below the SBUC. Therefore,
the SBUC occupies the core of a tight antiform with a south-
dipping  axial  plane,  which  is  termed  the  Slovenska  Bistrica
Antiform  in  the  following.  Locally,  however,  the  outcrops
along  the  northern  border  of  the  SBUC  show  that  eclogites
and amphibolites of the SBUC lie below the gneisses. There-
fore, we assume that the northern contact is folded and that the

study  area  the  rising  melt  roughly  followed
the pre-existing foliation of the metamorphic
rocks. According to the map of Mioč & Žni-
darčič  (1977),  north  of  the  investigated  area
the  gneissic  country  rocks  are  folded  into  a
large, approximately E-W trending open anti-
form, which we refer to as the Pohorje Anti-
form  in  the  following.  The  trace  of  this
antiform  is  in  the  gneisses  but  towards  the
west  it  enters  the  pluton.  West  of  this  point,
the pluton appears as the core of the antiform;
its northern contact is dipping to the north and
its southern contact to the south, in both cases
approximately  parallel  to  the  foliation  of  the
country rocks. This indicates that the Pohorje
Antiform was formed at least partly after the
intrusion of the pluton and therefore must be
of Neogene age (see also Fodor et al. 2003).

459

TECTONIC EVOLUTION OF THE SOUTHEASTERN PART OF THE POHORJE MOUNTAINS (SLOVENIA)

Fig. 9. Tectonic scenario for the evolution of the Austroalpine units. a – Late Permian: Development of a rift between the Lower Central
Austroalpine (LCA) and the Upper Central Austroalpine (UCA); HT/LP metamorphism of basement rocks; emplacement of gabbroic bod-
ies at the base of continental crust; refertilization of depleted mantle rocks (SBUC) by melt impregnation. b – Late Cretaceous: Intraconti-
nental subduction of LCA under UCA; (U)HP metamorphism of LCA units; extraction of the UCA lower crustal and mantle wedge; sketch
modified after Janák et al. (2006). c – Latest Late Cretaceous: Exhumation of HP-rocks to lower/mid-crustal levels as a result of slab ex-
traction and extension within the Austroalpine units due to rollback of the Penninic subduction zone. The island arc in the SE has been re-
placed by the Southern Alps due to sinistral strike-slip movement along the Paleo-Periadriatic Line.

“fingers” of mafic rocks which extend northwards into the
gneisses partly represent fold cores. The map-scale geometry
suggests that the fold axes trend east-west (Fig. 2).

The foliation in the gneisses at the western border of the

map  area  dips  uniformly  towards  the  south  to  southwest  at
moderate to low angles, although the axial trace of the Sloven-
ska  Bistrica  Antiform  projects  into  this  area.  From  this  we
conclude that the foliation of the gneisses is not deformed by
the  antiform  but  represents  its  axial  planar  foliation  (Fig. 8).
This foliation formed under amphibolite- to upper greenschist-
facies conditions, at least during the late stages of its develop-
ment which may have started under eclogite-facies conditions.
Consequently, the same is true for the Slovenska Bistrica An-
tiform. In contrast, the Pohorje Antiform is folding the folia-
tion of gneisses (Fig. 8) and therefore the Slovenska Bistrica
Antiform is older than the Pohorje Antiform.

Discussion

The field relations suggest that the SBUC forms the core of

the Slovenska Bistrica Antiform. This antiform is mantled by
the gneiss-dominated series. The highest pressures of the Cre-
taceous  metamorphism  in  eclogites  ( ~ 3.0 GPa  according  to
Janák  et  al.  2004)  were  determined  from  the  vicinity  of  the
SBUC. Even higher pressures of 4.0 GPa are recorded by the
garnet peridotites from the core of the Slovenska Bistrica An-
tiform. Therefore, we assume that the Slovenska Bistrica Anti-
form  developed  during  the  exhumation  of  the  deeply
subducted  rocks  and  accommodated  some  part  of  this  exhu-
mation,  possibly  in  the  way  of  an  upward-directed  channel
flow (see Janák et al. 2009; Fig. 13c) accompanying slab ex-
traction (Froitzheim et al. 2003; see also Roffeis et al. 2008 for
the Koralpe) as the major exhumation mechanism (Fig. 9b).

460

KIRST, SANDMANN, NAGEL, FROITZHEIM and JANÁK

Unfolding of the Slovenska Bistrica Antiform leads to the

following  tectonostratigraphy:  (1)  at  the  base,  the  ultramafic
rocks  of  the  SBUC,  with  internal  eclogite  lenses;  (2)  above,
relatively  large  eclogite  bodies  which  are  partly  connected
with  the  internal  eclogites  of  the  SBUC;  and  (3)  at  the  top,
mixed gneisses, schists and marble with smaller eclogite lens-
es. Such a tectonostratigraphy resembles the one observed in
former crust-mantle boundary complexes like the Ivrea Zone
in the Western Alps. There, peridotites of the uppermost man-
tle (e.g. at the localities Finero and Balmuccia) are closely as-
sociated  with  gabbroic  bodies  and  are  overlain  by  various
gneisses interlayered with marbles and metabasic rocks. All of
these  were  affected  by  Permian-age  HT-LP  metamorphism
(Handy  et  al.  1999).  As  mentioned  above,  HT-LP  metamor-
phism is indicated for the metapelites in the study area by the
presence  of  Permian  to  Triassic  metamorphic  zircon  cores
(Janák et al. 2009). In contrast to the Ivrea Zone, such a crust-
mantle boundary assemblage in Pohorje may have been sub-
ducted,  metamorphosed  under  (U)HP  conditions,  and
exhumed (Fig. 9). In this interpretation, the association of ul-
tramafic  (serpentinites,  garnet  peridotites)  and  mafic  (eclog-
ites,  amphibolites)  rocks  in  the  SBUC  would  be  a  primary
feature,  in  the  way  that  the  uppermost  mantle  was  invaded,
probably in the course of Permian rifting, by mafic melts gen-
erated  at  a  deeper  level  of  the  mantle.  Impregnation  by  such
melts  could  also  explain  the  refertilization  of  the  ultramafic
rocks, in particular the garnet peridotites, as indicated by their
major  and  trace  element  composition  (Janák  et  al.  2006;  De
Hoog et al. 2009; see also Müntener et al. 2004).

Although the above presented scenario for crustal emplace-

ment of the SBUC seems to be the most likely, there are also
some difficulties with this model. A Permian age of the gab-
broic  precursors  of  the  eclogites  has  not  been  determined  in
Pohorje, only the Cretaceous age of metamorphism (Miller et
al.  2005;  Thöni  et  al.  2008).  The  chemical  composition  of
eclogites  shows  an  oceanic  affinity  (Hinterlechner-Ravnik
1982) and N-MORB characteristics (Sassi et al. 2004) and ser-
pentinites show a high degree of melt depletion which is more
typical for an oceanic than a subcontinental lithospheric man-
tle origin (De Hoog et al. 2009). There is no evidence on the
P-T conditions of the Permian metamorphism and initial stage
of  Cretaceous  subduction  in  Pohorje;  these  are  obscured  by
the HP-UHP metamorphism. Janák et al. (2006) showed that
garnet  exsolution  from  clinopyroxene  in  peridotite  occurred
under P-T conditions of 2.5 GPa and 700—750 °C, before sub-
duction culminated at 4 GPa and 900 °C. They proposed that
the exsolution process may correspond to the incorporation of
a high-temperature peridotite into a subducting crust. Experi-
ments  show  that  garnet  exsolution  from  clinopyroxene  is  es-
sentially  a  consequence  of  cooling  (Harte  &  Gurney  1975)
and therefore unlikely to occur at rising temperature. Further
study is therefore needed to resolve these problems.

Conclusions

The extent of the SBUC has been revised clarifying that the

occurrence of garnet peridotites is restricted to localities with-
in the body of serpentinized ultramafics. The serpentinites are

closely associated with large eclogite bodies which we inter-
pret  as  former  metagabbros  underplated  during  the  Permian
rifting. This coherent ultramafic/mafic body is interpreted as
being  derived  from  near-MOHO  uppermost  mantle  and  sub-
ducted  with  continental  crust en  bloc  to  (U)HP  depth  during
the Cretaceous orogeny. Following the (U)HP metamorphism,
the SBUC was folded together with the overlying gneiss-dom-
inated series during exhumation and emplaced in the core of
the  tight  to  isoclinal,  northward  closing  Slovenska  Bistrica
Antiform.  Later  in  the  Miocene  the  Pohorje  Pluton  intruded
and  finally,  probably  in  the  Neogene,  the  gneissic  series  to-
gether with the pluton were deformed into the upright Pohorje
Antiform.  This  structure,  resembling  a  metamorphic  core
complex, resulted from E-W extension and contemporaneous
N-S shortening.

Acknowledgments: We would like to thank Mirka Trajanova
and Kurt Stüwe whose reviews helped to improve the manu-
script.  We  also  thank  Cees-Jan  De  Hoog  for  fruitful  discus-
sions and Mirijam Vrabec for her help with the maps and field
logistics.  Field  work  of  F.  Kirst  and  S.  Sandmann  was  sup-
ported by the DAAD. This work was also financially supported
by  the  Slovak  Research  and  Development  Agency  (Project
APVV-51-046105),  and  the  VEGA  Scientific  Grant  Agency
(Grant No. 2/0031/09).

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