GeolCarp_Vol65_No2_117

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, APRIL 2014, 65, 2, 117—130 doi: 10.2478/geoca-2014-0008

Introduction

One  of  the  major  conundrums  in  geodynamic  reconstruc-
tions of the Balkan Peninsula is the problem of the final clo-
sure  of  the  Tethys  Ocean.  The  most  recent  and  widely
accepted  interpretation  suggests  that  the  Sava-Vardar  Zone
(SVZ) is the relict of the youngest Tethyan realm. According
to this standpoint, most Balkan ophiolites, which occur along
three  sub-parallel  belts  roughly  extending  NNW—SSE,  were
emplaced  during  the  Late  Jurassic  (Robertson  &  Karamata
1994;  Bortolotti  et  al.  2002;  Schmid  et  al.  2008;  and  refer-
ences therein). These ophiolite belts have been given different
names, and those most often used are East and West Vardar
and  Dinaric  ophiolites  (see  Schmid  et  al.  2008).  Although
there are strong disagreements about how many oceans were
involved  in  Mesozoic  geodynamic  events,  most  authors
share  the  opinion  that  an  oceanic  realm  still  existed  during
Late Cretaceous. The term Sava-Vardar Zone was first sug-
gested by Pamić (1993, 2002) who regarded it as the contin-
uation  of  the  Periadriatic  Zone.  Accepting  his  view  that  it
was  the  latest  suture  in  this  region,  Schmid  et  al.  (2008)
slightly changed the name into Sava Zone in order to distin-
guish it from other Vardar ophiolitic units. In this study we
use the name Sava-Vardar Zone by defining this unit which
had survived the Late Jurassic ocean(s) closure and the em-
placement of the West and East Vardar ophiolites, and which
supposedly represents the last suture between the Tisza/Dacia
and Dinarides (Karamata 2006; Schmid et al. 2008; Robertson
et al. 2009). The age of the final closure of this last oceanic

The Upper Cretaceous ophiolite of North Kozara – remnants

of an anomalous mid-ocean ridge segment of the Neotethys?

VLADICA CVETKOVIĆ

, KRISTINA ŠARIĆ

, ALEKSANDAR GRUBIĆ

, RANKO CVIJIĆ

and

ALEKSEJ MILOŠEVIĆ

University of Belgrade, Faculty of Mining and Geology, Đušina 7, 11000 Belgrade, Serbia;

cvladica@rgf.bg.ac.rs; kristinas@rgf.bg.ac.rs

Institute of Mining, Prijedor, Bosnia and Herzegovina; aleksandar_grubic@yahoo.com

University of Banja Luka, Faculty of Mining, Save Kovačevića bb, 79101 Prijedor, Bosnia and Herzegovina; rip@teol.net

(Manuscript received September 20, 2013; accepted in revised form March 11, 2014)

Abstract: This study sheds new light on the origin and evolution of the north Kozara ophiolite, a part of the Sava-Vardar
Zone. The Sava-Vardar Zone is regarded as a relict of the youngest Tethyan realm in the present-day Balkan Peninsula.
The north Kozara ophiolite consists of a bimodal igneous association comprising isotropic to layered gabbros, diabase
dykes and basaltic pillow lavas (basic suite), as well as relicts of predominantly rhyodacite lava flows and analogous
shallow intrusions (acid suite). The rocks of the basic suite show relatively flat to moderately light-REE enriched patterns
with no or weak negative Eu-anomaly, whereas those of the acid suite exhibit steeper patterns and have distinctively more
pronounced Eu- and Sr- negative anomalies. Compared to the known intra-ophiolitic granitoids from the Eastern Vardar
Zone, the acid suite rocks are most similar to those considered to be oceanic plagiogranites. The new geochemical data
suggest that the basic suite rocks are similar to enriched mid-ocean ridge basalts. The geochemical characteristics of the
acid suite rocks indicate that their primary magmas most probably originated via partial melting of gabbros from the lower
oceanic crust. Our study confirms the oceanic nature of the north Kozara Mts rock assemblage, and suggests that it may
have formed within an anomalous ridge setting similar to present-day Iceland.

Key words: Balkan ophiolites, Sava-Vardar Zone, E-MORB, rhyodacite, acid magmatism.

realm is constrained by a regional metamorphic overprint at
~

65 Ma, which was recorded on the Maastrichtian siliciclastic

rocks belonging to the deepest parts of the SVZ accretionary
wedge (Ustaszewski et al. 2010).

One of the key SVZ localities is exposed on the northern

slopes  of  the  Kozara  Mts  (north  Bosnia  and  Herzegovina).
This is the place where it was first documented that, besides
the  generally  known  Upper  Jurassic  ophiolites,  remnants  of
Upper Cretaceous oceanic crust also exist (Jelaska & Pamić
1979;  Karamata  et  al.  2005;  Ustaszewski  et  al.  2009).
Ustaszewski et al. (2009) reported the first U/Pb radiometric
ages  and  provided  a  detailed  geological  and  petrological
study of the north Kozara ophiolite-related igneous rocks. In
addition to providing accurate age data, the authors conclude
that the north Kozara ophiolite represents a bimodal igneous
association. They suppose that the north Kozara ophiolite is
the  relict  of  an  oceanic  plateau,  leaving  a  possibility  that  it
formed in a back-arc setting still open. Their hypothesis about
an  intra-oceanic  geotectonic  setting  was  postulated  using  a
combination  of  geological  and  geochemical  arguments  and
the latter was mainly based on the observation that the north
Kozara  basic  rocks  are  geochemically  more  enriched  than
normal mid-ocean ridge basalts (NMORB).

In this study we report and discuss a new set of major ele-

ment  and  trace  element  data  of  igneous  rocks  of  the  north
Kozara  ophiolite  complex.  The  main  aim  of  this  study  was
twofold:  firstly,  to  try  to  further  constrain  the  geotectonic
setting  of  this  ophiolite  by  taking  a  closer  look  at  the
geochemistry of the basic igneous rocks, and secondly to ad-

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dress the petrogenesis of the acid rocks of this bimodal asso-
ciation, which was neglected in the previous research. Our
results confirm earlier views that there is a close petrogenetic
link between the basic and the acid rocks of this bimodal as-
sociation and that this has important implications for geody-
namic reconstructions in this area.

Geological setting

The SVZ trends N—S in central Serbia and, then, toward the

northwest,  it  bends  parallel  to  the  Sava  River  (see  inset  of
Fig. 1). In central Serbia, the SVZ suture assemblage is repre-
sented by Senonian flysches, whereas more to the north along
the  southern  margin  of  the  Pannonian  Basin  it  is  exposed  in
the  form  of  several  scattered  inselbergs  (Belak  et  al.  1998;
Pamić 2002; Slovenec et al. 2010). One of these is the north
Kozara body, the largest exposure of ophiolites in this area.

The north Kozara ophiolite was thrust onto the ophiolite

melange of south Kozara, which belongs to the Upper Jurassic
West  Vardar  ophiolite  belt  during  the  latest  Cretaceous  to
Early  Paleogene  (Schmid  et  al.  2008;  Ustaszewski  et  al.
2009).  In  map  view  (Fig. 1),  the  north  Kozara  complex  ap-
pears as a partly dismembered slice of ophiolite rocks, which
is  mostly  covered  by  Cenozoic  formations.  It  is  approxi-
mately  15 km  long  and  extends  ENE—WSW,  starting  from
Gornji  Podgradci  to  Maglajci  in  the  east  and  the  west,  re-
spectively (Fig. 1). In map view, the ophiolite slice consists
of  several  blocks  surrounded  by  unconformably  overlying
Maastrichtian  to  Paleocene  mostly  siliciclastic  sediments.
They  are  represented  by  arkoses,  lithic  sandstones,  conglo-
merates  and  shallow-water  limestones  (Ustaszewski  et  al.
2009). The ophiolite blocks show a regular WNW—ESE dis-
tribution, with pillowed basalts situated in the south, a series
of diabase dykes in the middle part and gabbros in the north.
A similar ophiolite slice is found approximately 20 km north-
west,  in  the  area  of  Kostajnica  (Fig. 1).  The  north  Kozara
ophiolites  are  separated  from  the  Upper  Jurassic  ophiolites
of south Kozara by a major N-dipping thrust (Ustaszewski et
al. 2009). In contrast to the south Kozara ophiolite complex,
the north Kozara ophiolite does not have exposed ultramafic
rocks  and  has  distinctively  larger  masses  of  acid  rocks.
Moreover, the north Kozara acid rocks are characterized by a
strong predominance of volcanic rocks.

Approximately 5 km north of the north Kozara ophiolite

there is the series of Prosara Mt (Fig. 1). It is another insel-
berg of the SVZ, which consists of two tectonic units sepa-
rated  by  a  northward  dipping  thrust.  This  structure  was
recently re-defined by Ustaszewski et al. (2010) as a low-an-
gle  detachment  recording  latest  Oligocene/Miocene  exten-
sional unroofing of the Prosara inselberg. The lower, southern,
unit  is  slightly  or  non-metamorphosed  whereas  the  upper,
northern, unit consists of various rocks metamorphosed under
up to greenschist facies conditions (Šparica & Buzeljko 1984;
Jovanović & Magaš 1986). Both units are cut by decametric
intrusions of alkali feldspar granites that were dated by U/Pb
zircon age to 82.68 ± 0.13 Ma (Ustaszewski et al. 2009). The
granites  show  the  same  schistosity  as  their  host  rocks  indi-
cating that deformation is post-Late Cretaceous.

Samples and analytical methods

A set of 23 samples of both groups of the north Kozara bi-

modal  magmatic  association  was  analysed  optically  and
chemically. For the basic rock group only diabases and fine-
grained isotropic gabbros were sampled, whereas the samples
of acid rocks cover all rock types found in the field, from al-
most aphyric silicic lava flows, through fine-grained rhyolite/
rhyodacite dykes to medium-grained granite. Sixteen samples
were  analysed  on  major  and  selected  trace  elements  in  the
Laboratory of the Department of Earth Sciences – University
of Perugia (Italy) using an XRF device. The XRF included an
X-ray  tube  with  Rn  and  W  anode,  with  acceleration  voltage
and electric current ranging from 40 kV to 45 kV to and from
30 mA  to  35 mA,  respectively.  An  LiF-200  crystal  analyser
was  used  for  radiation  separation  in  working  regime  without
vacuum.  In  addition,  eight  samples  were  analysed  for  major
and full range trace elements in the ACME Laboratories Ltd.
Vancouver  (Canada).  Major  element  oxides  were  deter-
mined  using  ICP  atomic  emission  spectrometry  (detection
limits  around  0.001—0.04 %).  Concentrations  of  trace  and
rare  earth  elements  were  measured  by  ICP-MS  (detection
limits  0.01—0.5 ppm).  STD  SO-17  was  certified  in-house
against 38 Certified Reference materials including CANMET
SY-4 and USGS AGV-1, G-2, GSP-2 and W-2. The accuracy
of the analyses is within limits of 2—5 % for major elements,
10—15 % for trace elements and 1—5 % for REEs.

Results

Rock classification and petrography

The accurate rock classification of the north Kozara igne-

ous rocks is difficult because some rocks are, at least to some
extent, affected by low temperature metamorphic processes.
In  general,  by  combining  fieldwork  data,  petrography  and
chemical characteristics, the studied rocks are distinguished
into:  (1)  the  basic  and  (2)  the  acid  suite.  The  basic  suite,
which may comprise some intermediate rocks as well, vastly
predominates, however, in spite of that, the relative propor-
tion  of  acid  rocks  is  very  high.  The  relative  abundance  of
acid rocks is remarkably higher than the relative proportion
of  acid  magmatic  rocks  in  other  ophiolites  of  the  Balkan
Peninsula (e.g. Šarić et al. 2009). Ustaszewski et al. (2009)
reported  a  detailed  petrographic  study  of  the  north  Kozara
basic  rocks,  including  field  images  and  photomicrographs.
Thus, in this study we provide only brief descriptions for the
basic suite, whereas a special emphasis is put on the lithology
and  petrography  of  the  acid  rocks,  which  was  mainly  ne-
glected by earlier researchers.

Basic suite

The basic suite is mostly represented by gabbros, diabases

and basalts. Gabbros compose the lower part of the north
Kozara ophiolitic sequence. They mainly occur in the north of
the complex, where they appear as km-sized irregular masses
(Fig. 1). The rocks are commonly hypidiomorphic, coarse- to

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medium-grained  textures  and  show  isotropic  fabric.  Occa-
sionally, coarser-grained varieties with observable cumulitic
textures  are  also  observed.  The  gabbros  consist  of  hypidio-
morphic  plagioclase  and  commonly  uralitized  clinopyro-
xene,  the  interstitial  space  is  filled  by  fine-grained  Fe-Ti
oxide  and  secondary  minerals,  mostly  albite,  chlorite,  epi-
dote and calcite. At some places, very coarse-grained gabbro
masses  and  veins  of  gabbro  pegmatite  are  found.  Diabase
mostly  occurs  within  typical  sheeted  dyke  complexes,  the
best exposed are found near Trnova (Fig. 1). More rarely, di-
abase appears as isolated dykes and irregularly shaped shal-
low  intrusions,  which  cut  the  gabbro  zone.  In  general,
diabase dykes vary in thickness from a few cm up to 0.5 m.
Diabase is composed of idiomorphic to hypidiomorphic pla-
gioclase  and  hypidiomorphic  clinopyroxene  as  primary
phases,  whereas  Fe-Ti  oxide,  sphene  and  apatite  appear  as
accessories. Like the gabbros, the diabases are also affected
by low temperature ocean floor metamorphism. The most al-
tered varieties usually contain albite, epidote, chlorite, preh-
nite,  uralite,  secondary  opaque  minerals  and  leucoxene.
Basalts appear as pillow lavas or coherent and volcaniclastic
effusions.  More  rarely,  they  form  feeding  dykes  cutting  the
extrusive facies. Most basalts contain scarce phenocrysts or
are almost aphyric with only rare elongated laths of plagio-
clases  enclosed  by  volcanic  glass  commonly  replaced  by
chlorite or clay minerals. The pillow lavas are found interca-
lated with ‘Scaglia Rossa’ red pelagic limestones. This field
observation was taken as first evidence that the north Kozara
basalts  are  late  Campanian  to  early  Maastrichtian  in  age
(Karamata et al. 2000, 2005; Grubić et al. 2009; Ustaszewski
et al. 2009; Vishnevskaya et al. 2009).

Acid suite

Acid rocks are generally distinguished into volcanic and

subvolcanic/intrusive  facies.  The  largest  outcrops  of  acid
volcanics are found south and south-west of Gornji Podgradci
(Fig. 1).  There  relicts  of  presumably  larger  piles  of  silicic
lava crop out. These lavas do not have direct magmatic con-
tacts with the adjacent masses of basalt and diabase, but they
commonly  display  a  similar  N-dipping  orientation  as  the
bulk  of  the  north  Kozara  ophiolite.  The  silicic  lavas  appear
as up to 15 m high roadcuts and cliffs (Fig. 2a). The lava is
commonly  platy  jointed  with  individual  plates  commonly
between 0.3 and 1 m in thickness. Because they can be vari-
able in colour, these rocks were sometimes mistaken for al-
tered  diabase,  and  this  is  probably  a  reason  why  their
abundance  (and  significance)  was  merely  underestimated.
They are predominantly aphyric or have few phenocrysts and
usually show banding or foliated fabric related to magmatic
flow.  Plagioclase  is  most  abundant  among  the  phenocrysts,
whereas quartz and K-feldspar are rare. The groundmass typ-
ically displays banding in combination with classical perlitic
texture (Fig. 2c). Most samples underwent high-temperature
devitrification  that  resulted  in  microspherulitic  and  mi-
cropoikilitic  textures  (Fig. 2e,d).  Devitrification  bands  are
sometimes folded (Fig. 2f) suggesting that devitrification oc-
curred  while  the  acid  magma  was  still  hot  and  deformable.
Plagioclase  is  idiomorphic,  sometimes  displaying  perfect

crystal  shapes  against  the  glassy  groundmass.  It  appears  as
individual  phenocrysts  and  only  some  samples  show  the
presence  of  irregular  mm-sized  glomeroporhyritic  nests.
Quartz  phenocrysts  are  usually  ~ 0 .3 mm  in  diameter.  They
are  commonly  rounded  and  partially  embayed.  Quartz  also
appears  as  a  product  of  secondary  recrystallization  filling
lens-like  voids  and  lithophysae  in  the  groundmass.  Alkali
feldspars as phenocrysts are very rare. More often, they ap-
pear  as  submilimetric  laths  in  the  groundmass,  mostly  as
product  of  devitrification.  The  subvolcanic/intrusive  acid
rocks  are  predominantly  represented  by  leucocratic  dykes
that  are  cutting  the  sheeted  diabase  complexes.  The  acid
dykes are up to 1—2 m in thickness and are often parallel to
adjacent diabase (Fig. 2b). They are plagioclase- rarely also
quartz-phyric rocks with a holocrystalline groundmass com-
posed of the same phases (Fig. 2g). Subordinate amounts of
devitrified volcanic glass can be observed extremely rarely.
Granitoid rocks are found only near Moštanica (Fig. 1). The
granite is leucocratic and has a hypidiomorphic granular tex-
ture.  It  is  predominantly  composed  of  variable  amounts  of
plagioclase, quartz and K-feldspar. Plagioclase is tabular and
hypidomorphic, whereas K-feldspar and quartz are anhedral
and usually fill the interstitial space, sometimes forming mi-
crographic  intergrowths  (Fig. 2h).  Primary  mafic  minerals
are replaced by fine-grained chlorite aggregates. The form of
these  aggregates  suggests  that  biotite  was  originally  present.
Opaque minerals, apatite and zircon are the main accessories.

Rock geochemistry

The results of chemical investigations are given in

Table 1a,b. In most diagrams data reported by Ustaszewski
et al. (2009) are also plotted. Most studied igneous rocks
show less than 53 wt. % and more than 65 wt. % SiO

, with

the exception of three samples having ~ 57, ~ 61 and
~

64 wt. % SiO

. Given that these three rock samples contain

magmatic quartz, they are plotted within the acid suite. Be-
cause of petrographic evidence of low-temperature alteration
processes,  and  relatively  high  loss  on  ignition  (LOI)  values
(mostly between 2 and 6 wt. %), the chemical classification
is based on so-called ‘immobile elements’. The Nb/Y vs Zr/Ti
diagram  (Winchester  &  Floyd  1977;  Fig. 3)  shows  that  the
basic suite samples predominantly plot within the corner of
the  subalkaline  basalt  field  and  toward  the  andesite/basalt
and  andesite  fields,  whereas  those  of  the  acid  suite  stretch
along the rhyolite/rhyodacite/dacite fields.

Most samples of the basic suite comprise a relatively nar-

row silica range of 45—50 wt. % SiO

. They are also charac-

terized by relatively high titanium contents, ranging from 1.2
to 2.5 wt. % TiO

. MgO and CaO contents vary 4—8 wt. %

and 4—9.5 wt. %, respectively. Mg#mol[MgO/(MgO + FeOt)]
values are mostly between 0.6 and 0.75. MgO and CaO show
negative correlations with silica contents, whereas other major
oxides display either a poor correlation or data scattering
(Fig. 4). The acid suite samples display a much wider silica
range of 57—75 wt. % SiO

They also show remarkable varia-

tions in the abundance of other major oxides, in particular
Al

(9—18 wt. %), Na

O (1—9 wt. %) and K

O ( < 1—7 wt. %).

Although some of these variations can be the result of alter-

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ation processes, these effects should not be very large be-
cause there is no correlation between LOI values and Na

O, Rb or Ba contents and simultaneously the concentra-

tions of these elements are very well correlated (R

K—Na

> 0.7,

Ba—Rb

> 0.9). It is generally observed that the samples of

acid lava flows are relatively homogeneous with high silica
( > 65 wt. % SiO

) and high potassium contents ( ~ 5 wt. %

O). These rocks are also slightly to moderately peralumi-

nous with Alumina Saturation Index [ASI = mol(Al

(CaO + Na

O + K

O))] ranging from 1 to 1.4. On the other

hand, four samples of leucocratic dykes have invariably low
potassium contents and approach trondhjemitic composition.
The variations between SiO

contents and CaO and MgO are

generally negatively correlated both for the acid and basic

rocks. However, the contents of some major oxides, in par-
ticular Al

, form almost parallel trends whereas others,

such as TiO

and P

display inflections (Fig. 4).

The basic suite samples show generally elevated concen-

trations of compatible trace elements, for instance Ni con-
tents are up to 100 ppm, Cr up to 350 ppm and Co up to
50 ppm (Table 1a). The contents of these trace elements are
strongly positively correlated with MgO contents and nega-
tively with SiO

contents (Fig. 5). Compatible trace elements

are present in low concentrations in the rocks of the acid
suite (Ni < 40 ppm, Cr mostly < 50 ppm). Sr also behaves
compatibly in both suites and is more enriched in the rocks
of the basic suite (Fig. 5). The concentrations of most incom-
patible trace elements are higher in the rocks of the acid suite

Explanation: Fe

– Total iron as Fe

. The analyses are ordered according to silica contents with ICP-MS analyses (Acme, Canada)

given in italic. D – Diabase, FG-G – Fine-grained gabbro, MG-G – Medium-grained gabbro, LOI – Loss on ignition.

Table 1a: Chemical composition of the rocks of the northern Kozara basic suite.

No.

KZ-53/2 MAG 11/2

KZ-44 KZ-46 KZ-54

MAG 11/4

KZ-40 TRNAV

1/2 TRNAV 1/1

KZ-26

Rock

FG-G

FG-G D D MG-G

MG-G

D D D D

SiO

46.80 47.64

47.79 48.14 48.60 48.70

48.79 48.89

50.02

50.96 53.50

TiO

1.68 1.43

1.33 1.48 1.86 1.54

1.84 1.33

2.42

2.25 1.87

14.15 15.67

14.55 13.16 12.90 15.69

12.17 16.67

14.81

12.24 12.45

11.30 11.55

11.26 12.30 10.75 11.63

14.21 10.29

13.28

13.18 10.65

MnO

0.16 0.20

0.16 0.18 0.15 0.19

0.19 0.15

0.22

0.23 0.17

MgO

7.21 7.69

7.91 7.98 7.38 7.12

6.86 7.25

4.75

6.20 3.65

CaO

9.20 6.95

9.61 8.75 6.21 7.23

7.31 9.50

7.20

6.05 4.28

2.88 2.97

2.64 3.01 3.88 4.30

3.16 2.61

4.20

3.80 5.03

0.35 0.68

0.23 0.18 0.75 0.34

0.30 0.28

0.84

0.50 0.45

0.22 0.25

0.20 0.21 0.19 0.25

0.27 0.17

0.37

0.28 0.28

LOI

5.16 5.89

3.95 3.88 6.83 3.75

3.91 3.38

2.79

3.79 6.05

Tot

99.11 100.92

99.64 99.27 99.50 100.74

99.02 100.52

100.90

99.48 98.38

63.5

141

147.5

108

188

45.3

36.3

30.7

0.53

0.62

4.98

17.8

15.9

18.5

3.3

5.3

6.2

7.1

8.7

20.5

11.7

183.5

203

193

174

224

163

140

169

259

64.7

0.4

0.5

0.91 10

1.08 9

3.92

0.27

0.49

1.56

271

250

225

271

287

218

323

233

312

287

229

110

134

118

134

120

127

175

248

236

205

29.4

32.6

7.4

8.8

16.9

18.6

21.2

2.87

3.28

5.41

13.8

15.5

23.5

3.93

4.71

6.07

1.47

1.66

1.62

4.78

5.45

6.77

0.95

1.07

1.21

5.58

5.98

1.23

1.35

1.49

3.33

3.8

4.23

0.5

0.57

0.63

3.11

3.44

3.84

0.46

0.53

0.6

260

114

301

345

240

113

272

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(e.g. Nb = 14—80 ppm, Zr = 300—800 ppm, Th = 13—36 ppm)
than in the rocks of the basic suite (e.g. Nb < 15 ppm,
Zr < 250 ppm, Th < 10 ppm).

Chondrite- and primitive mantle-normalized multi-element

diagrams for rare earth elements (REE) and incompatible trace
elements, respectively, are shown in Fig. 6a—f. Only samples
that were analysed by ICP-MS method are plotted and they are
compared  with  data  reported  by  Ustaszewski  et  al.  (2009).
The  patterns  of  the  basic  suite  are  compared  with  those
shown  by  basalts  from  known  geotectonic  settings  (normal
mid  ocean-ridge  basalts  –  NMORB;  enriched  mid  ocean-
ridge  basalts  –  EMORB  and  ocean  island  basalts  –  OIB)
(Hofmann 1997), whereas the rocks of the acid suite are com-
positionally  compared  with  intra-ophiolite  acid  rocks  of  the

Eastern  Vardar  Zone  (Šarić  et  al.  2009)  and  rhyolites  from
the Torfajökull and Ljósufjöll volcanic fields (South Iceland
Volcanic  Zone;  Martin  &  Sigmarsson  2007).  The  data  re-
ported in this study show very similar patterns to those of the
north Kozara samples reported by Ustaszewski et al. (2009).
The  basic  suite  rocks  display  relatively  flat  or  slightly
LREE- and LILE-enriched patterns with low to moderate Eu
and Sr negative anomalies (Fig. 6a,b). Their normalized pat-
terns REE and trace element patterns are most similar to the
pattern of EMORB. The samples of the acid suite are charac-
terized  by  LREE-  and  LILE-enriched  chondrite-  and  primi-
tive  mantle-normalized  patterns,  respectively,  and  exhibit
pronounced  Eu  and  Sr  negative  anomalies  (Fig. 6c—f).  Four
samples  of  the  north  Kozara  acid  rocks  were  analysed  by

Table 1b: Chemical composition of the rocks of the northern Kozara acid suite.

Explanation: Fe

– Total iron as Fe

. The analyses are ordered according to silica contents with ICP-MS analyses (Acme, Canada)

given in italic. G – Granite, QD – Quartzdiorite, RD – Rhyodacite, RH – rhyolite.

No.

KZ-21/1 KZ-21/1

MOST

13/1

MAG

11/3

KZ-32

TRN 5

KZ-27

KZ-23/2

TRN 3

CR 8

MOST

Rock

RD RD RD RH

RD RH RH RH

RH RH G

SiO

57.30 61.13 64.25 66.13 66.66 68.92 69.42 70.10 70.70 73.85 74.29 72.14

TiO

0.80 0.65 1.36 0.39 0.36 0.47 0.36 0.30 0.31 0.27 0.21 0.30

17.95 18.51 14.59 14.13 15.77 14.55 11.75 12.00 15.11 14.14 13.75 13.80

3.89 3.75 7.67 3.62 2.10 1.87 5.50 5.35 1.84 1.91 2.04 4.45

MnO

0.02 0.03 0.07 0.12 0.01 0.04 0.02 0.02 0.01 0.02 0.04 0.04

MgO

1.84 2.19 2.60 2.18 0.58 2.03 0.87 0.49 0.39 0.40 0.97 0.19

CaO

2.77 0.86 0.54 2.64 0.34 1.82 0.20 0.05 0.15 0.51 0.27 0.23

9.00 8.06 5.18 4.09 2.38 7.18 1.49 1.46 2.58 5.81 5.57 5.49

1.19 0.21 0.44 2.65 6.40 0.25 5.83 5.72 6.51 2.21 1.70 2.53

0.17 0.25 0.24 0.10 0.09 0.12 0.08 0.01 0.17 0.04 0.06 0.05

LOI

4.38 4.24 3.28 4.07 5.11 2.83 4.30 2.72 1.94 0.88 1.19 0.82

Tot

99.31 99.87 100.22 100.12 99.80 100.08 99.82 98.22 99.60 100.04 100.09 100.04

27.1

103

371

31.5

468

419

835

236

219

354

10.4

7.3

1.5

1.8

0.3

4.47

11.6

5.81

16.1

29.6

24.7

24.7

9.2

61.8

17.1

13.1

17.5

81.9

34.3

18.7

4.5

7.7

114

111.5

214

66.3

102

58.9

35.4

111

1.3

6.5

2.2

1.5

14.05

33.8

13.35 24

2.08

14.25

4.73 6.76

122

101

336

337

347

334

2290

345

764

710

453

315

289

809

32.6

116.5

73.8

116

33.3

72.7

49.7

48.6

73.8

181.5

99.4

82.9

151

8.94

21.3

13.6

11.9

78.8

52.4

41.7

6.87

18.75

12.45 8.49

1.34

2.23

1.3

0.86

7.01

8.8

1.13

4.17

2.29 1.55

6.21

25.5

12.85 9.16

1.36

5.65

2.86 2.09

4.04

17.25

8.53 6.15

0.61

2.79

1.34 0.96

4.19

18.85

8.31 6.01

0.64

3.01

1.31 0.9

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shows  that  although  Nb  concentrations  generally  increase
with  nickel  concentrations,  Zr/Nb  ratios  remain  unaffected.
Such  roughly  uniform  Zr/Nb  ratios  suggest  that  they  were
mostly  insensitive  to  partial  melting  and  fractionation  pro-
cesses,  and,  importantly,  that  this  ratio  can  be  used  to  eluci-
date  the  mantle  source  geochemistry.  Most  basic  rocks  of
north Kozara have Zr/Nb ratios varying  ~ 15 and that is close
to primordial mantle values (McDonough & Sun 1995). These
values  are  slightly  higher  than  the  average  EMORB  Zr/Nb
ratio of  ~ 10, but distinctively lower and higher from the av-
erage Zr/Nb ratios of NMORB ( ~ 30) and OIB ( ~ 6), respec-
tively. Three samples analysed by XRF show remarkably low
Nb  contents  (2—3 ppm)  and,  therefore,  they  have  very  high
Zr/Nb  ratios  ( ~ 60).  Apart  from  possible  analytical  problems
(i.e. higher detection limits of XRF analyses), potential expla-
nations  can  be  that  these  rocks  either  crystallized  from  mag-
mas originating from a strongly depleted upper mantle, or they
do not compositionally represent melts but crystal cumulates.

The previous discussion indicates, first, that the conclu-

sion of Ustaszewski et al. (2009) that the basic rocks of north
Kozara  did  not  originate  from  an  NMOR  environment  or
from volcanic arc settings is robust even taking into consid-
eration  possible  effects  of  fractionation  processes.  The  pre-
diction that this ophiolite segment formed in an OIB setting
should, at least, be reconsidered on the basis of our new data.
The  above  presented  values  of  the  Zr/Nb  ratio  indicate  that
the north Kozara primary magmas formed by partial melting
of an EMORB-like source or, more likely, of a mixture be-
tween EMORB and NMORB mantle sources. This conclusion
has  certain  geodynamic  significance  because  it  would  mean
that the ophiolitic segment of the north Kozara represents part
of  an  anomalous  mid-ocean  ridge  or,  alternatively,  an  island
plateau that was presumably situated not far from the ridge.

The origin of acid magmatism: A petrogenetic link to the
basic suite

In general, the acid magmatic rocks that are spatially related

to  ophiolites  are  most  frequently  represented  by  so-called
oceanic plagiogranites (Coleman & Peterman 1975; Pedersen
& Malpas 1984; Floyd et al. 1998). Alternatively, they corre-
spond  to  granitoids  and  their  volcanic  equivalents  that  result
from  pre-  or  post-collisional  geodynamic  events  (Brown  &
D’Lemos  1991;  Li  &  Li  2003;  Kamei  2004;  Karsli  et  al.
2007). However, the intra-ophiolitic acid rocks of north Kozara
show important differences with respect both to the plagiogran-
ites  and  the  pre-  to  post-collisional  granites.  First,  the  north
Kozara acid rocks have greater relative abundance in respect to
mafic products than the intra-ophiolitic acid rocks of any known
ophiolite  segment  of  the  Balkan  Peninsula,  and,  second,  they
have a much larger proportion of volcanic products.

Šarić et al. (2009) gave a detailed geochemical and Sr-Nd-Pb

overview of granitoids and related acid/intermediate rocks that
occur within the Upper Jurassic East Vardar Zone ophiolites.
Apart from oceanic plagiogranites, the authors identified two
additional types of intra-ophiolite granitoids and interpreted
them  as  originating  from  either  pre-collisional  subduction-
related  magmas  or  from  magmas  which  formed  due  to  ob-
duction-induced  melting  of  various  protoliths.  The  authors

suppose that the latter event was associated with the ophiolite
emplacement. As already demonstrated, the normalized trace
element patterns of the north Kozara acid rocks are generally
similar  to  those  shown  by  the  oceanic  plagiogranites  and
partly  to  granites  deriving  from  obduction-induced  melting
(Fig. 6). The geochemical similarity between the north Kozara
acid rocks and the plagiogranites occurring in Upper Jurassic
ophiolites of the Eastern Vardar Zone is present, although the
former do not have the trondhjemitic composition, typical for
oceanic plagiogranites (Coleman & Peterman 1975; Coleman
& Donato 1979). Plagiogranites are generally interpreted as
originating  from:  i)  extensive  fractionation  of  parental
tholeiitic magma (e.g. Montanini et al. 2006), ii) partial melt-
ing  of  basaltic  protolith  (e.g.  Pedersen  &  Malpas  1984),  or
iii)  liquid  immiscibility  processes  (e.g.  Dixon  &  Rutherford
1979).  However,  irrespectively  of  their  true  origin,  most
authors agree that there is a tight petrogenetic link between the
plagiogranites  and  the  host  basic  rocks  and  a  similar  hypo-
thesis can be postulated for the north Kozara acid suite (see
also Ustaszewski et al. 2009). The geochemical similarity of
the  north  Kozara  acid  rocks  and  the  obduction-related  East
Vardar granitoids may be used to support such a hypothesis,
because obduction of hot ophiolites is capable of producing
melts  of  various  underlying  protoliths  (Cox  et  al.  1999;
Whitehead et al. 2000). If the protoliths are basic rocks, than
the resulting partial melts can easily have geochemical char-
acteristics similar to oceanic plagiogranites and, therefore, to
the north Kozara acid suite rocks, as well.

The genetic relationship between the two suites of north

Kozara  is  also  inferred  by  comparing  the  north  Kozara  acid
rocks with the modern acid volcanic rocks of Iceland. As pre-
viously shown in Fig. 6, there is a general similarity between
the  normalized  REE  and  trace  element  patterns  between  the
north  Kozara  and  rocks  of  the  South  Iceland  Volcanic  Zone
(Martin & Sigmarsson 2007). Moreover, these two groups of
acid rocks are similar because: i) both are part of bimodal ig-
neous  associations  with  relative  abundance  of  acid  igneous
rocks  in  excess  of  1 %,  ii)  there  is  a  vast  predominance  of
volcanic  products  within  the  acid  suite,  and  iii)  in  both  re-
gions the basic rocks mostly have an EMORB ( ± NMORB)
geochemical  signature.  These  petrological  and  geochemical
similarities imply that both acid suites can have a similar ori-
gin.  In  general,  the  origin  of  the  Icelandic  acid  rocks  is  ex-
plained  by  extensive  fractionation  of  primary  basic  tholeiitic
melts (e.g. Prestvik et al. 2001) or by direct partial melting of
altered  gabbros  and  amphibolites  in  the  oceanic  crust  (e.g.
Sigmarsson et al. 1991; Martin & Sigmarsson 2007).

Diagrams Rb vs Ba*/Ba and Rb vs Sr*/Sr (Fig. 8) show

fields reflecting chemical compositions of acid rocks originat-
ing by crystal fractionation of basic magmas (grey field) and
those  formed  by  partial  melting  of  hydrated  basaltic  oceanic
crust (stippled field). The Ba*/Ba and Sr*/Sr ratios represent a
quantification of strontium and barium anomalies, respectively
(see  figure  caption  for  details).  Most  samples  of  silica  rich
lava of north Kozara plot within the field of melts formed by
1—10 %  partial  melting  of  altered  basaltic  crust.  This  field
overlaps with the field of melts produced by fractionation of
basaltic  magmas  but  the  required  amounts  of  fractionation
are  far  too  high  ( > 80 %)  to  be  considered  possible.  More-

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to  microcrystalline  mosaic  quartz  aggregates.  The  presence
of micropoikilitic or so-called snow-flake texture (Andersen
1969;  Lofgren  1971),  in  the  form  of  patchy  quartz  crystals
enclosing laths of alkali feldspar, strongly suggests that these
lavas  emplaced  at  high  temperatures  and  underwent  slow
cooling  devitrification  (Ryan  &  Sammis  1981;  Manley
1992; Orth & McPhie 2003). Such slow cooling is likely as-
sociated  with  subaerial  emplacement  and  that  is  also  sup-
ported by the lack of hyaloclastic deposits.

Concluding remarks

The discussion presented above allows us to derive four ma-

jor conclusions. First, the rocks of the basic suite of the north
Kozara  have  an  E-MORB  geochemical  signature.  Second,
there is a close petrogenetic link between the basic and acid/
intermediate  suite.  Third,  the  acid  magmas  most  probably
originated  through  partial  melting  of  hydrated  oceanic  crust,
similar  in  composition  to  the  rocks  of  the  basic  suite.  And
fourth,  the  most  voluminous  acid  magma  most  likely  em-
placed as subaerial high-temperature rhyodacite/rhyolite lava.

The most important geodynamic implication provided by

these conclusions is that the entire north Kozara ophiolite slice
could  represent  the  remnant  of  an  anomalous  ridge  segment
that  is  similar  to  present  day  Iceland.  The  above  recognition
that the north Kozara acid volcanic originated by partial melt-
ing of basic rocks is tightly related to the thermal state of the
oceanic  crust.  Partial  fusion  of  hydrated  basaltic  material  in
shallow  crust  is  only  possible  in  regions  with  elevated  geo-
thermal gradient (Sigmarsson et al. 1991, 1992). This is so be-
cause  a  cold  crust  generally  needs  a  larger  input  of  heat  to
reach its solidus, while a hotter crust is more readily melted.
The  high-temperature  emplacement  of  the  north  Kozara  acid
lavas further supports this opinion. It can imply that the rhyo-
dacitic  magma  had  travelled  through  hot  oceanic  crust  and
reached the surface relatively fast, because of a large density
contrast in combination with relatively low viscosity. The pre-
sented evidence that is provided by the study of acid rocks, is
in accordance with a typical E-MORB geochemical signature
of the host basic volcanic and shallow intrusive.

Acknowledgments: A part of the investigations of the geo-
logy of Kozara was financially supported by the Ministry of
Science  and  Technology  of  the  Republic  Srpska  (Contract
No. 06/0-020/961-102-08  from  12.  12.  2008),  Project  No.
OI176016  granted  by  the  Serbian  Ministry  of  Education,
Science  and  Technological  development  and  project  “Geo-
dynamics”  of  the  Serbian  Academy  of  Sciences  and  Arts.
The  authors  thank  Prof.  Dr.  Giampiero  Poli  (University  of
Perugia)  for  XRF  analyses.  Kamil  Ustaszewski  and  Volker
Hoeck are acknowledged for their constructive reviews.

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