www.geologicacarpathica.sk
GEOLOGICA CARPATHICA, AUGUST 2010, 61, 4, 273—292 doi: 10.2478/v10096-010-0016-1
Introduction
Mélanges are by consensus understood as chaotic tectono-sed-
imentary complexes that have been initially deposited by tec-
tonically induced sedimentary processes in an accretionary
wedge from a fore-arc region. Consequently, an ophiolite mé-
lange mostly incorporates mixed magmatic material derived
from both interfaced oceanic sides. Because of a long tectonic
history of obduction various lithoclasts representing diferent
tectonomagmatic stages of an oceanic basin may be identified
in the final product. Many mélanges are overlain by slices of
coherent fragments of genetically related oceanic lithosphere
(ophiolites) which may include lithologies not found in the as-
sociated mélange and vice versa. The comparative petrologi-
cal and geochemical investigations of magmatic inclusions
found in a mélange combined with analogue rocks of related
ophiolite sequences always provide important, although in-
complete constraints on the tectonomagmatic evolution of an
oceanic basin and its segments from continental rifting to
opening and formation of oceanic lithosphere through spread-
ing and convergence till the final ophiolite emplacement.
Geochemistry, petrology and tectonomagmatic significance
of basaltic rocks from the ophiolite mélange at the NW
External-Internal Dinarides junction (Croatia)
DAMIR SLOVENEC
1
, BOŠKO LUGOVIĆ
2
and IRENA VLAHOVIĆ
3
1
Croatian Geological Survey, Sachsova 2, HR-10000 Zagreb, Croatia; damir.slovenec@hgi-cgs.hr
2
Institute of Mineralogy, Petrology and Mineral Deposits, Faculty of Mining, Geology, and Petroleum Engineering, University of Zagreb,
Pierottijeva 6, HR-10000 Zagreb, Croatia; blugovic@rgn.hr
3
GEO-EKO d.o.o., Nikole Pavića 11, HR-10090 Zagreb, Croatia; irena.kloc@geo-eko.hr
(Manuscript received October 28, 2009; accepted in revised form March 11, 2010)
Abstract: At the NW inflexion of the Sava-Vardar Suture Zone ophiolite mélanges, known as the Kalnik Unit, form the
surface of the slopes of several Pannonian inselbergs in the SW Zagorje-Mid-Transdanubian Zone. The Mt Samoborska
Gora ophiolite mélange, thought to be a part of the Kalnik Unit, forms a separate sector obducted directly onto Dinaric
Triassic carbonate sediments. Basaltic rocks, the only magmatic rocks incorporated in the mélange, include Middle—
Triassic (Illyrian—Fassanian) alkali within-plate basalts and Middle Jurassic (uppermost Bathonian—Lower Callovian)
tholeiitic basalts. The latter sporadically constitute composite olistoliths, and are geochemically divided into N-MORB-
like (high-Ti basalts) and transitional MORB/IAT (medium-Ti basalts). These geochemically different rocks suggest crystal-
lization at various tectonomagmatic settings, which is also indicated by the rock paragenesis and host clinopyroxene compo-
sitions. Alkali basalts reflect melts derived from an OIB-type enriched mantle source [Ti/V= 62.2—82.4; (La/Lu)
cn
= 6.4—12.8]
with Nd-Sr isotope signatures close resembling the Bulk Earth [
ε
Nd(T= 235 Ma)
= + 1.6 to + 2.5]. They are recognized as pre-
ophiolite continental rift basin volcanic rocks that closely predate the opening of the Repno oceanic domain (ROD) of the
Meliata-Maliac ocean system. The high-Ti and medium-Ti basalts from composite blocks derived from a similar depleted
mantle source (
ε
Nd(T= 165 Ma)
= + 6.01 vs. + 6.35) succesively metasomatized by expulsion of fluids from a subducting
slab leading to a more pronounced subduction signature in the latter [Ti/V= 31.6—44.8 and (Nb/La)
n
= 0.67—0.90 vs. Ti/
V= 21.5—33.9 and (Nb/La)
n
= 0.32—0.49]. These composite blocks indicate crust formation in an extensional basin spread-
ing over the still active subducting ridge. The majority of high-Ti basalts may represent the fragments of older crust formed
at a spreading ridge and incorporated in the mélange of the accretionary wedge formed in the proto-arc—fore-arc region.
The Mt Samoborska Gora ophiolite mélange represents the trailing edge of the Kalnik Unit as a discrete sector that records
the shortest stage of tectonomagmatic evolution related to intraoceanic subduction in the ROD.
Key words: Triassic—Jurassic, Croatia, Samoborska Gora Mts, proto-arc extension, back-arc spreading, alkali within-
plate basalts, tholeiitic basalts, ophiolite mélange.
In the SW segment of the Zagorje-Mid-Transdanubian Zone
(ZMTDZ) of the Sava Suture Zone the tectono-sedimentary
ophiolite mélanges lacking overthrusted ophiolites are com-
monly found as individual sectors on the surface of slopes of
intra-Pannonian inselbergs of Kalnik, Ivanščica and Medved-
nica (Pamić & Tomljenović 1998, and references therein). On
account of similar textural features all these mélange sectors
are assumed to constitute a single unit (Babić et al. 2002) re-
ferred to as the Kalnik Unit (Hass et al. 2000), which brings
strong evidence for a discrete oceanic domain (Repno oceanic
domain = ROD) within the Meliata-Maliac ocean system
(Babić et al. 2002; Goričan et al. 2005; Slovenec & Lugović
2008, 2009). However, excluding the Mt Medvednica ophio-
lite mélange where magmatic megaclasts or olistoliths have
been geochemically studied in detail and stages of terminal
spreading and initial convergence were dated by isotopes
(Lugović et al. 2007; Slovenec & Lugović 2008, 2009) other
sectors still wait for such complementary data that would con-
firm other sectors as true integral parts of the Kalnik Unit.
The Samoborska Gora ophiolite mélange has a peculiar po-
sition as it was obducted directly onto Triassic sediments of
274
SLOVENEC, LUGOVIĆ and VLAHOVIĆ
the Adria amagmatic passive continental margin and traces
the headmost edge of a larger mélange unit. This mélange is
separated from the Medvednica ophiolite mélange by the re-
gional Sava fault and thus it is not clear whether it consti-
tutes a discrete ophiolite mélange unit, or represents a
detached segment of the Kalnik Unit. Present study revealed
that besides apparently similar blocks of magmatic rocks as
in the other mélange sectors of ZMTDZ, Mt Samoborska
Gora mélange includes blocks of within-plate alkali basalts
which were not up to now identified in other mélange sec-
tors. However, during the testing of correlation between
ophiolitic blocks from the Mt Samoborska Gora and those
from the Mt Medvednica, similar alkali basalts were also
found for the first time in the ophiolite mélange of the Mt
Medvednica. In this work we performed geochemical and
petrological characterization of almost all ophiolitic blocks
from the Mt Samoborska Gora with the purpose of testing
possible correlation with Mt Medvednica ophiolite rock
fragments. The age of massive lava blocks was obtained by
isotope age determination, and the age of pillow lavas was
determined from the fauna content in the intrapillow matrix
or from atop pillow lavas associated cherts. New data were
utilized to infer or improve the geodynamic evolution of the
oceanic segment(s) from which ophiolites of Mts Samobors-
ka Gora and Medvednica have been derived.
Geological outlines
Samoborska Gora is the cornerstone that links the External
Dinarides, Southern Alpine units, southwestern tip of the
Zagorje-Mid-Transdanubian Zone (ZMTDZ) as a part of the
Sava-Vardar Suture Zone (SVSZ) and the continental block of
the Tisia Unit (Fig. 1A). Its exact tectonic position in the
framework of Alpine-Dinaric-Pannonian triple junction zone
sensu Hass & Kovács (2001) is not clearly explained yet (e.g.
Pamić & Tomljenović 1998; Tari & Pamić 1998; Hass et al.
2000; Pamić 2002; Babić et al. 2002; Goričan et al. 2005;
Schmid et al. 2008; Robertson et al. 2009). However, a posi-
tion within the transitional zone between the External and In-
ternal Dinarides seems to be most acceptable (Placer 1999).
Mt Samoborska Gora consists of two tectonostratigraphic
units, the pre-Eocene Žumberak Autochthony overthrusted by
the south-west vergent Žumberak Nappe (Šikić et al. 1979)
(Fig. 1B). The Žumberak Autochthony shows a sedimentary
succession identical to the eastern border of Adriatic carbon-
ate platform (Tomljenović 2002). The oldest rocks of the
Žumberak Autochthony are Permian molasse-type clastic
rocks which are unconformably overlain by Lower Triassic
clastic rocks followed by Upper Triassic carbonate rocks with
minor cherts (Herak 1956; Šikić et al. 1979). Locally, the Late
Anisian sedimentary succession may be interstratified with
acidic pyroclastic rocks (Goričan et al. 2005). The ophiolite
mélange is thrusted onto Middle Triassic carbonates. The age
of the ophiolite mélange is supposed to be Middle Jurassic to
Hauterivian by analogy with similar mélanges from the
Kalnik Unit (Babić et al. 2002). Unlike in the Mt Medvednica,
where the Paleozoic-Triassic sedimentary succession was sub-
jected to an Early Cretaceous low grade metamorphic over-
print (Belak et al. 1995, with references) related to obduction
of an intraoceanic island arc (Lugović et al. 2006), the Mt
Samoborska Gora analogue succession lacks any metamor-
phism. The youngest rocks of the Žumberak Autochthony are
the Late Campanian—Paleocene alluvial and delta fan deposits
(Šikić et al. 1979; Fig. 1C) which in many lines of evidence
resemble post-ophiolitic Gossau- or Ugar-type sedimentary
sequence.
The Žumberak Autochthony is thrusted by the Žumberak
Nappe composed of a thick sedimentary succession showing
identical lithostratigraphic sequences as in the Žumberak Au-
tochthony (Fig. 1B). Top-WSW thrusting, internal imbrica-
tion and folding of the Žumberak Autochthony related to an
Eocene deformational event affected Mt Samoborska Gora
and Mt Medvednica (D3 event of Tomljenović 2002). The
Žumberak Nappe consists of three individual thrust sheets (Ja-
petić, Cirnik and Vrhovčak) which represent the uppermost
pre-Neogene structural units (Šikić et al. 1979). The best ex-
posure of the ophiolite mélange is under the Vrhovčak thrust
sheet where fragments of basaltic rocks were sampled in de-
tail. The Neogene and Quaternary strata mostly unconform-
ably overlay the pre-Eocene basement on the southeastern
slopes; on the northern slopes the basement rocks are trans-
gressively overlain by the younger strata (Fig. 1C).
The ophiolite mélange of the Mt Samoborska Gora shows
characteristics of a chaotic olistostrome complex mixed with
fault-bounded fragments of different lithologies varying in
size from a few centimeters to several tens of meters set in a
strongly sheared pelitic-siltous continent derived matrix.
Ophiolite mélange contains clasts of greywacke, minor shale,
reddish and greyish cherts, scarce limestones along with frag-
ments of ophiolitic basaltic rocks (Herak 1956; Brajdić &
Bukovec 1989; Kloc 2005). Other ophiolite rocks are notably
absent. Massive lavas predominate over pillow lavas. The
fragments are homogeneous in the term of texture and
geochemistry with the exception of a unique, texturally uni-
form hektometer sized block (Fig. 1C, location 2) composed
of tholeiitic high-Ti and medium-Ti basaltic rocks. K-Ar
measurement on plagioclase separate from the high-Ti seg-
ment of this composite block has yielded an uppermost Ba-
thonian—Early Callovian age of 165.4 ± 5.8 Ma (Balogh 2009;
unpublished). The observed age of sedimentary rock frag-
ments range from Middle Triassic for “exotic” limestones to
Middle Jurassic for greyish radiolarian cherts (Goričan 2008,
unpublished).
Fragments of alkali basalts are concentrated in the eastern-
most part of the Mt Samoborska Gora ophiolite mélange
(Fig. 1C; locations 10—13) where reddish radiolarian cherts
crop out above massive alkali basalts (Fig. 1; location 12).
The radiolarian assemblage of these cherts indicates Middle
Triassic age (Goričan 2008, unpublished). Assuming that
they are integrated in a coherent block, they would represent
the oldest fragments incorporated in the ophiolite mélange of
Mt Samoborska Gora.
In the westernmost part of the Mt Medvednica ophiolite mé-
lange near the village Gornja Bistra (Fig. 1B) crop out the
hectometer large block of similar massive alkali basalts and
their pillow lavas with interpillow pelagic limestones of Illyri-
an-Fassanian age of Middle Triassic (Halamić et al. 1998). For
275
BASALTIC ROCKS FROM THE OPHIOLITE MÉLANGE AT THE EXTERNAL -INTERNAL DINARIDES (CROATIA)
the purpose of correlation the data on these rocks were
integrated in the present work.
The Mt Samoborska Gora ophiolite mélange in many re-
spects shares the overall textural characteristics of the Mt
Medvednica ophiolite mélange (Babić et al. 2002; Slovenec
& Pamić 2002, and references therein) suggesting that both
mélanges may have formed by accretion along a discrete
sediment-starved trench and were tectonized during the on-
set of obduction. The pre-Neogene Mt Medvednica base-
ment experienced regional-scale tectonic transport from the
NW, and 130° clockwise rotation during the Oligocene—ear-
liest Miocene (Tomljenović et al. 2008), which caused its re-
cent perpendicular orientation to the overall NW-SE Dinaric
structural trend (Fig. 1A). Therefore, the ophiolite mélange
of Mt Samoborska Gora should be tested as a potential
prolongation of the Mt Medvednica ophiolite mélange, and
so of the Kalnik Unit.
Like elsewhere in the Dinarides the onset of tectonic re-
working of Mt Samoborska Gora ophiolitic mélange took
place successively during and after the first ophiolite emplace-
ment onto the Adria continental margin (Schmid et al. 2008;
Robertson et al. 2009, and references therein) and continued
during the Cretaceous until the Senonian as confirmed by
mantle peridotite clasts in the Campanian basal conglomerates
(Halamić 1998) and subaerial weathering of peridotites to the
Ni-lateritic crust (Palinkaš et al. 2006).
Fig. 1. A – Geotectonic sketch map of
the Alps, Dinarides and Hellenides show-
ing the position of the Periadriatic-Sava-
Vardar Suture Zone (after Pamić 2000).
Legend: 1 – External units (External
Dinarides and Alps); 2 – Internal units
[Passive continental margin, Central
Dinaride Ophiolite Belt (CDOB), Mirdita Zone]; 3 – Periadriatic-Sava-Vardar Zone; 4 – Serbo-Macedonian Massif; 5 – Pelagonides; 6 –
Golija Zone; 7 – Zagorje-Mid-Transdanubian Zone; 8 – Pannonian Basin. Faults: BL – Balaton; DF – Drava; PL – Periadriatic; SF – Sava;
SP – Scutari-Peć; SN – Sava Nappe; ZZ – Zagreb-Zemplín. Mountains: I – Ivanščica; K – Kalnik; Ko – Kopaonik; Md – Medved-
nica; SG – Samoborska Gora; SD – Szarvaskő-Darnó; Bü – Bükk; B – Bódva Valley; JK – Jaklovce. B – Interpretations of over-
thrust relations in the Mt Samoborska Gora and western part of the Mt Medvednica (after Šikić & Basch 1975 and Šikić et al. 1978, 1979;
taken from Tomljenović 2002). Legend: 1 – Žumberak Nappe; 2 – Žumberak Autochthony; 3 – Ophiolite mélange (Oph); 4 – Upper
Cretaceous-Paleocene sediments; 5 – Lower Cretaceous metamorphic complex; 6 – picture break. C – Simplified geological map of the
Mt Samoborska Gora (modified after Šikić et al. 1978 and Tomljenović 2002). Legend: 1 – Neogene and Quaternary sedimentary rocks;
2 – Upper Cretaceous-Paleocene flysch; 3 – ophiolite mélange with blocks of basalt (black fields) and Triassic-Jurassic radiolarites,
sandstones and shales (not separated on the map); 4 – Upper Triassic dolomites and limestones; 5 – Middle—Lower Triassic dolomites,
limestones, cherts and clastic rocks (shale, siltite and sandstone); 6 – Upper Permian clastic rocks (conglomerate, sandstone, siltite and
shale), limestones, dolomites and gypsum; 7 – reverse or thrust faults; 8 – normal faults; 9 – geological contact line; 10 – discordance
line, tectonic-erosion discordance; 11 – sample location (1 = js-73/1; 2 = jf-33/2, jf-34, t-23b/1, t-23b/2; 3 = sas-8; 4 = js-60; 5 = 92-18;
6 = sat-242; 7 = t-56/1; 8 = sas-202; 9 = sas-206; 10 = 92-20, 92-21, 92-22, 92-23, 92-25, 92-30; 11 = sas-203; 12 = sas-210; 13 = 92-16).
276
SLOVENEC, LUGOVIĆ and VLAHOVIĆ
Analytical techniques
Mineral analyses from eleven representative samples were
performed at the Mineralogisches Institut, Universität Heidel-
berg, using a CAMECA SX51 electron microprobe equipped
with five wavelength-dispersive spectrometers. The operating
parameters were 15 kV accelerating voltage, 20 nA beam cur-
rent, ~ 1 µm beam size (10 µm for plagioclase) and 10 s count-
ing time for all elements. Natural minerals, oxides
(corundum, spinels, hematite and rutile) and silicates (albite,
orthoclase, anorthite and wollastonite) were used for calibra-
tion. Raw data for all analyses were corrected for matrix ef-
fects with the PAP algorithm (Pouchou & Pichoir 1984,
1985) implemented by CAMECA. Formula calculations
were done using a software package authorized by Hans-Pe-
ter Meyer (Mineralogisches Institut, Universität Heidelberg).
The mineral composition of higly altered samples was analy-
sed by X-Ray diffraction on powdered samples (XRD).
Bulk-rock powders for chemical analyses of twenty five
samples were obtained from rock chips free of visible veins.
Six samples with amygdulas or calcite veins were dissolved
in Na-acetate (NaOAc) at pH around 6 controlled by acetic
acid (HAc).
Major elements were measured by ICP and all trace ele-
ments by ICP-MS at Activation Laboratories in Ancaster,
Canada. A series of international standards were used to con-
struct calibration curves. Referent samples W2 and WMG-1
were run as unknowns. Major element and trace element con-
centrations were measured with accuracy better than 1 % and
5 %, respectively.
Isotopic analyses were done in CRPG in Vandoeuvre,
France on Finnigan MAT 262 mass spectrometer following
the procedure described in Hart & Brooks (1977). Sr frac-
tions were deposited on single W filaments and TaF
5
—H
3
PO
4
was added as an activator. Double filaments (Ta for emis-
sion, Re for ionization) were used for Nd analyses. Nd frac-
tions were deposited on the Ta filaments, with H
3
PO
4
added
as an activator. All analyses were made in multi-dynamic
mode, using software developed by Spectromat. An expo-
nential law was used for fractionation correction. Normaliz-
ing ratios of
86
Sr/
88
Sr= 0.1194 and
146
Nd/
144
Nd= 0.7219 were
assumed. The
87
Sr/
86
Sr value for the NBS 987 Sr standard for
the period of measurement was 0.710254± 0.000028 (2
σ,
n= 92). The
143
Nd/
144
Nd value for the La Jolla Nd standard
was 0.511841± 0.000020 (2
σ, n=22). An in-house Nd standard
was also analysed during this period, yielding a
143
Nd/
144
Nd
ratio of 0.511110 ± 0.000020 (2
σ, n=100), consistent with
the value obtained for this standard over the past 15 years.
Total procedural blanks were ~ 700 pg and ~ 300 pg for Sr
and Nd, respectively.
Petrography and mineral chemistry
Extrusive rocks from the Mt Samoborska Gora ophiolite
mélange comprise pillow lavas and massive lavas of high-Ti
and medium-Ti tholeiitic and alkali basalt composition, re-
spectively. Pillows of amoeboidal shape with tortoise shell
joints were noticed occasionally (Fig. 1C, location 2). In spite
of polyphase alterations which may be severe, igneous tex-
tures are preserved in all samples.
Tholeiitic extrusives are mostly aphyric and composed of
plagioclase, clinopyroxene, Fe-Ti oxide, spinel and accessory
apatite (Fig. 2A,B). Plagioclase from the high-Ti group shows
normal zoning with labradorite core and andesine rim ranging
in overall composition from An
71—54
to An
48.6—31.5
, respective-
ly. The highest individual core to rim variation is An
71—40
(Ta-
ble 1). In the medium-Ti tholeiitic group, igneous plagioclase
is altered to albite (An
0.1—2.9
) or peristerite (An
~
7
) with minor
sericite, calcite, prehnite, analcime and pumpellyite, and
therefore the rocks may be classified as spilites. Clinopyrox-
ene shows two stages of alteration. Some clinopyroxenes from
the high-Ti samples show deuteric alterations to ferohorn-
blende-feroedenite and in both geochemical groups are partly
hydrothermally altered to chamosite-clinochlore, epidote and
pumpellyite (Fig. 2A,B; Table 1). In the high-Ti basalts Fe-Ti
oxide is ulvöspinel-magnetite (Usp
16.6—46.8
Mgt
50.2—79.0
Spl
0.9—7.5
)
coexisting with ilmenite (Ilm
92.8—96.2
Hem
3.8—7.2
) whilst in the
medium-Ti basalts low-Ti chromian spinel (Mg#= 70—77,
Cr#= 26—30, TiO
2
<0.38 wt. %) crystallized along with minor
Fe-Ti oxides (Table 1). The pillow lavas are aphyric to slight-
ly plagioclase-phyric or plagioclase-clinopyroxene-phyric.
The pillows show characteristic increasing crystallinity from
the outer chilled margin, showing plumose-variolitic to
hypocrystalline porphyric texture, to the hollocrystalline core
with aphyric ophitic to intergranular texture. Calcite and/or
chlorite filled amygdaloidal tholeiitic pillow lavas are rarely
observed. Massive lavas are mostly aphyric and show fine- to
coarse-grained ophitic to intergranular texture undistinguish-
able from pillow core. Petrographical evidence in the tholeiitic
basalts suggests the following order of crystallization: plagio-
clase
→ clinopyroxene+plagioclase+Fe-Ti oxides (Fig. 2A).
Additionally, in some medium-Ti pillow and massive basalts
spinel coexists with the early fractionated plagioclase whilst
rare Fe-Ti oxides are confined in matrix.
Alkali basalts from the Mts Samoborska Gora and Medved-
nica ophiolite mélanges show similar aphyric quenched tex-
tures suggesting a fast cooling rate at the time of effusion. In
thin sections they show spinnifex- to variolitic-like domains
(variolites) formed by sheaf- or plumose-textured pinkish cli-
nopyroxene intergrown with acicular plagioclase with intersti-
tial glass infillings (Fig. 2C). Primary spinel (Mg#=59—62,
Cr#=42—46; Table 1) and Ti-magnetite are accessory phases.
In all samples plagioclase is completely altered to albite, preh-
nite and occasionally to pumpellyite. Clinopyroxenes from the
Mt Medvednica alkali basalts are fresh whilst those from the
Mt Samoborska Gora are always altered to chlorite. Glass is
devitrified to palagonitic mesostasis and consists of chlorite,
calcite, hematite, prehnite, pumpellyite and titanite. The lavas
are amygdaloidal with up to 30 % amygdals filled by calcite
suggesting relatively shallow water effusion compared to
tholeiitic lavas.
Clinopyroxene chemistry
. Selected matrix clinopyroxene
compositions from the analysed tholeiitic and alkali rocks are
shown in Table 2 and all are plotted in the classification
diagram in Fig. 3.
Clinopyroxenes from tholeiitic host rocks show normal and
reverse zoning. Normally zoned grains have homogeneous
277
BASALTIC ROCKS FROM THE OPHIOLITE MÉLANGE AT THE EXTERNAL -INTERNAL DINARIDES (CROATIA)
cores and show decreasing Al
VI
/Al
IV
, Mg# and generally in-
creasing Ti to the grain periphery. In reverse zoned grains the
patterns show opposite compositional variations. In Fig. 3 cli-
nopyroxene reveals two compositional groups. The majority
of clinopyroxene from high-Ti host rocks stretch in the field of
augite (Wo
38.1—42.8
En
40.9—48.9
Fs
10.6—20.1
) and forms a composi-
tional trend concordant with analogue rocks from Mt Medved-
nica ophiolite mélange described by Slovenec & Lugović
(2009). On the contrary, the clinopyroxenes hosted in the larg-
est block from the Mt Samoborska Gora ophiolite mélange
(Fig. 1C; location 2), composed of high-Ti basalts (sample
jf-33/2) associated with medium-Ti basalts (sample jf-23/b2)
as well as clinopyroxenes from an individual medium-Ti
block (Fig. 1C; location 1 sample js-73/1) form a separate
compositional trend with a distinctly higher Wo- and Fs-
content. Clinopyroxenes hosted in the high-Ti rock from the
block shows transitional diopside-augite composition
(Wo
43.4—46.3
En
39.7—413
Fs
13.4—16.8
) whilst those from the medi-
um-Ti rock entity stretch along the line separating the heden-
bergite-augite compositional fields (Wo
43.7—46.1
En
21.9—26.6
Fs
28.4—34.3
).
The clinopyroxenes hosted in both analysed medium-Ti
rocks compared with clinopyroxene compositions from
high-Ti rock entity of the composite blok, contain more TiO
2
(1.58—3.05 wt. % vs. 1.25—1.69 wt. %), Na
2
O (0.45—0.59 wt. %
vs. 0.35—0.43 wt. %), and generally higher Al
2
O
3
(3.49—7.19 wt. %
vs. 3.38—5.51 wt. %) (Fig. 3A—C), and show significantly
Fe-enriched (Mg#= 43.2—74.2.0 vs. 74.6—80.2). This is not
expected for clinopyroxene hosted in the rocks derived from
a more depleted source.
Clinopyroxenes of alkali basalts from Mt Samoborska Gora
are completely altered and only the clinopyroxene composi-
tion from Mt Medvednica alkali basalts are shown in Table 2.
These clinopyroxenes are normally zoned and show diopsidic
composition (Wo
47.4—49.6
En
30.1—36.5
Fs
14.3—22.5
; Fig. 3). Their
high
content
of
other-than-quadrilateral
components
(TiO
2
= 2.99—4.17 wt. %; Al
2
O
3
= 5.89—7.40 wt. %; Fig. 4A—B)
clearly reflected the alkali non-orogenic nature of the host
rocks (Leterrier et al. 1982). High-Ti content of clinopyroxene
is favoured by cooling rate of crystallization, magma chemis-
try and cotectic opaque phase (Tracy & Robinson 1977). High
titanium content in the clinopyroxene is consistent with en-
riched composition of relatively primitive alkali basaltic mag-
ma, coexisting low-Ti spinel (TiO
2
>1.6 wt. %) and fast cooling
rate as suggested by quenching textures of host pillow basalts.
Bulk-rock chemistry
Chemical compositions of the analysed rocks are shown in
Table 3. As virtually shown by petrography and the high LOI
of the samples (up to 8.46 wt. %), and by experience from
similar rocks elsewhere (e.g. Pearce & Cann 1973; Thomson
1991) significant element mobility is expected to occur in
most of the samples. Here, potential element mobility was
tested by plotting their concentrations against Zr selected as
differentiation index (Fig. 5).
For the tholeiitic rock suite, except for TiO
2
and P
2
O
5
which
are positively correlated with Zr, the major elements do not
Fig. 2. Back-scattered electron image of (A) high-Ti tholeiitic mas-
sive basalt, sample 92-30, showing ophiolitic texture indicating or-
der of crystallization: plagioclase
→ clinopyroxene → Fe-Ti oxide.
Plagioclase is altered to albite, clinopyroxene is fresh or pseudo-
morphosed by ferroedenite/ferrohornblende. Ilmenite is an acces-
sory phase. (B) Medium-Ti massive basalt, sample js-73/1, show-
ing in part spinifex texture of skeletal clinopyroxene and albitized
plagioclase. Spinel (no. 12) surrounded by ferrite-chromite (no. 13)
coexists with accessory Ti-magnetite (no. 18). (C) Alkali pillow
basalt, sample sb-17, showing spinifex texture formed by subparal-
lel needle-like clinopyroxene and plagioclase altered to an aggre-
gate of albite and prehnite (no. 16). Primary spinel is accessory
phase. Legend: Ab – albite, Amp – amphibole, Chl – chlorite,
Ilm – ilmenite, Pl – plagioclase, Pmp – pumpellyite, Spl – spinel.
278
SLOVENEC,
LUGOVIĆ
and
VLAHOVIĆ
Table 1: Selected microprobe analyses and formulae of feldspars, amphibole, chlorite, spinel, ulvöspinel-magnetite and ilmenite from the tholeiitic and alkali volcanic rocks in the Mts Samo-
borska Gora and Medvednica ophiolite mélange.
Formulae calculated on the basis of 8 oxygens and total Fe as trivalent for feldspar; 23 oxygens and fixed number of 15 cations excluding Na and K for amphibole; 14 oxygens and total Fe as
divalent for chlorite; 4 oxygens and 3 cations for magnetite-ulvöspinel and chromian spinel; 3 oxygens and 2 cations for ilmenite. Fe
2
O
3
is calculated on the basis of fixed number of cations for
amphibole, spinels and ilmenite, H
2
O corresponds to 2 (OH) and 8 (OH) per formular unit in amphibole and chlorite, respectively. An = 100* Ca/(Ca+ Na+ K); Mg# = 100* Mg/(Mg+ Fe
2+
),
Cr# = 100* Cr/(Cr+ Al). c = core, r = rim; MB = massive basalt, PB = pillow basalt; Th = tholeiitic basalt, Alk = alkali basalt.
279
BASALTIC ROCKS FROM THE OPHIOLITE MÉLANGE AT THE EXTERNAL -INTERNAL DINARIDES (CROATIA)
show correlation suggesting significant mobilization during
alterations. Large ion lithophile elements (= LILE; Cs, Rb, K,
Ba and Sr) represented by Ba in the Ba—Zr plot (Fig. 5A)
show highly inconsistent variations which make them unreli-
able for petrogenetic and discriminatory constraints. High
field strength elements (= HFSE; Ti, Th, Hf, Nb, Ta, P, and Y)
shown by Ti (Fig. 5B) and rare earth elements (= REE, La-Lu)
displayed by La and Sm (Fig. 5C—D) showing good positive
correlation with fractionation index have obviously remained
immobile. Therefore the HFSE and REE concentrations of our
tholeiitic samples may be confidently used to characterize
geochemical and petrogenetic features of the rocks as was al-
ready successfully tested for similar mafic rocks from differ-
ent oceanic provenances (e.g. Pearce & Norry 1979; Shervais
1982; Beccaluva et al. 1983). Transitional metals (V, Cr, Mn,
Fe, Ni and Zn) represented by Ni and V (Fig. 5E—F) retain
magmatic correlation but are strongly related to the abundance
and type of the opaque phase hosted in a sample. Similar rela-
tions concerning element mobilization are observed in alkali
basalts (Fig. 5).
In the Zr/TiO
2
vs. Nb/Y diagram (Winchester & Floyd
1977) frequently used to classify altered and metamorphosed
extrusives, the analysed rocks are divided between the fields
of subalkali andesite/basalts and alkali basalts, (not shown).
Jurassic ophiolitic fragments show exclusively tholeiitic
chemistry whilst extrusives associated with Middle Triassic
pelagic sediments plot in the field of alkali basalts. Tholeiitic
rocks from ophiolite complexes and mélanges are best dis-
criminated by geochemical parameters which include Ti/Cr
ratio and Ni concentration (Beccaluva et al. 1983) or simply
TiO
2
content (Bortolotti et al. 2002). Following this scheme
(not shown) tholeiitic rocks from the Mt Samoborska Gora
ophiolite mélange are distinguished into the high-Ti group and
medium-Ti group (Table 3). High-Ti basalts are widely ac-
Fig. 3. Plot of clinopyroxene compositions in the En—Wo—Fs (Mg
2
Si
2
O
6
—Ca
2
Si
2
O
6
—
Fe
2
Si
2
O
6
) diagram with the nomenclature fields of Morimoto (1988) for tholeiitic volca-
nic rocks from the Mt Samoborska Gora and alkali volcanic rocks from the Mt Medvedni-
ca ophiolite mélange. Fields for clinopyroxene compositions from high-Ti, medium-Ti
and low-Ti tholeiitic basalts of the Mt Medvednica ophiolite mélange (Slovenec &
Lugović 2008 and 2009) plotted for correlation constraints.
cepted as representing crystallization in a
middle ocean ridge (= MOR) setting where-
as tholeiitic basalts with lower TiO
2
may
suggest formation in various suprasubduc-
tion zone (= SSZ) settings (Serri 1981; Bec-
caluva et al. 1983).
Tholeiitic rocks display Ti/V ratios rang-
ing from 21.5 to 44.8 and spread in the
field of recent MORB and BABB whereby
medium-Ti samples form a separate group
with lower V at given Ti (Fig. 6). Alkali
basalts from the Mt Samoborska Gora
show an increased concentration of Ti at
relatively low V (Ti/V= 62.2—82.4) and
suggest derivation from an enriched mantle
source. The referent alkali basalts from the
Mt Medvednica with Ti/V ratios of around
50 straggle the boundary line between
MORB/BABB and OIB/WPAB.
The element abundance patterns nor-
malized to N-MORB values for analysed
extrusive rocks are displayed as spider di-
agrams in Fig. 7A1 and 7A2. Tholeiitic
rocks show a wide range of LILE enrich-
ment consistent with the observed alter-
ations. The rock suite displays negative Nb—Ta anomaly rel-
ative to La which is typical of SSZ related magmas. The in-
tensity of the anomalies significantly increases from high-Ti
group [(Nb/La)
n
=0.67—0.90] to medium-Ti group [(Nb/
La)
n
= 0.32—0.49] suggesting a more subduction influenced na-
ture of the latter. They have nearly flat La—Lu profiles which
range from ~ 1 to ~ 3 times relative to N-MORB for the high-
Ti group and 0.7—1.0 times for the medium-Ti group. A strong
to significant positive Sr anomaly in the medium-Ti basalt
suggests fractionation of plagioclase. Alkali basalts in general
show a smooth pattern with typical continuous enrichment of
more incompatible elements in the profile from Th to Lu and
may show HFSE (P, Nb) positive anomalies. Strong negative
anomalies of Ba, K, and Sr are caused by their mobilization
due to devitrification and albitization. The alkali basalts from
Mt Samoborska Gora are more enriched [(Th/Lu)
n
= 70—85]
relative to the Mt Medvednica samples [(Th/Lu)
n
= 32—46]. All
samples have Lu
n
< 1 which may indicate residual garnet in the
source. In the spider diagram their profiles, excluding negative
anomalies for Ba, K and Sr which are related to alterations,
perfectly match the variation patterns of alkali basalts from
East African rift zone (Fig. 7A2).
Chondrite normalized REE patterns of analysed rocks are
displayed in Fig. 7B1 and 7B2. Tholeiitic rocks show various
intensities of LREE depletion and nearly flat HREE profile
[(Tb/Lu)
cn
= 0.96—1.29] at 12—20 times relative to chondrite for
the high-Ti group and 10—12 for the medium-Ti group. The
intensity of LREE depletion expressed by the ratio (La/Sm)
cn
increases from the high-Ti group (0.69—0.87) to medium-Ti
(0.51—0.62). Alkali basalts show strong enrichment of LREE
over HREE in the Mt Samoborska Gora samples [(La/
Lu)
cn
= 9.4—12.8] and relatively lower in the Mt Medvednica
samples [(La/Lu)
cn
= 6.4—7.6] concordant with relations in the
spider diagram (Fig. 7B2). Both groups show slight Eu anom-
280
SLOVENEC,
LUGOVIĆ
and
VLAHOVIĆ
Table 2: Selected microprobe analyses and formulae of clinopyroxene from the tholeiitic and alkali volcanic rocks in the Mt Samoborska Gora (SG) and Mt Medvednica (MD) ophiolite mélange.
SG Tholeiitic High-Ti basalts
SG Tholeiitic Medium-Ti basalts
MD Alkali basalts
Sample
jf-33/2 jf-33/2 92-20 92-20 92-21 92-21 92-22 92-22 92-30 92-30 js-73/1 js-73/1 js-73/1 js-73/1
jf-23/b2
jf-23/b2 sb-9 sb-9 sb-17 sb-17
Anal. nr.
3 4 30 3 1 2 20 21 16 18 2 16 22 25 13 14 3 4 1 2
Site
c r c r c r c r c c c c c c c r c r c r
Rock type
PB PB MB MB MB MB PB PB MB MB MB MB MB MB PB PB MB MB PB PB
SiO
2
48.79 49.10 51.65 50.54 50.91 51.93 52.33 50.51 51.60 52.28 46.85 48.34 47.43 48.68 44.69 45.79 45.65 44.39 47.09 45.65
TiO
2
1.46 1.69 0.55 1.10 0.96 0.65 0.62 0.97 0.70 0.54 2.75 1.78 1.63 1.58 3.05 2.49 3.18 4.17 2.93 3.50
Al
2
O
3
5.36 5.51 0.92 2.39 3.00 2.47 1.86 2.15 2.59 1.91 5.34 3.49 5.02 3.92 5.17 4.29 6.72 7.25 6.31 6.20
Cr
2
O
3
0.29 0.19 0.01 0.05 0.24 0.12 0.29 0.02 0.45 0.27 0.24 0.05 0.12 0.16 0.26 0.16 0.15 0.10 0.09 0.03
FeO
7.76 8.17 8.64 11.82 8.16 7.28 7.88 10.68 6.87 6.58 12.07 13.77 10.60 11.60 17.38 19.24 9.53 10.37 8.16 11.10
MnO
0.22 0.23 0.35 0.31 0.31 0.21 0.18 0.32 0.21 0.13 0.30 0.41 0.28 0.30 0.40 0.38 0.20 0.19 0.17 0.18
MgO
13.62 13.31 14.03 13.84 15.97 16.23 17.04 14.52 16.42 16.89 10.30 10.80 11.41 11.78 7.40 7.04 11.58 10.75 11.95 10.84
CaO
21.39 21.57 22.90 18.37 19.33 20.00 18.47 19.13 21.11 20.52 21.44 19.92 22.03 20.86 20.40 19.50 21.40 21.84 22.41 21.51
Na
2
O
0.39 0.39 0.17 0.39 0.33 0.29 0.22 0.35 0.28 0.30 0.50 0.52 0.49 0.47 0.53 0.55 0.43 0.45 0.41 0.47
Total
99.28
100.17 99.22 98.81 99.20 99.18 98.88 98.64
100.23 99.43 99.79 99.08 99.01 99.36 99.28 99.44 98.84 99.51 99.52 99.49
Si
1.822 1.823 1.941 1.917 1.892 1.925 1.946 1.909 1.892 1.927 1.784 1.857 1.801 1.847 1.751 1.801 1.735 1.687 1.777 1.736
Ti
0.041 0.047 0.015 0.031 0.027 0.018 0.017 0.027 0.019 0.015 0.079 0.051 0.047 0.045 0.090 0.074 0.091 0.119 0.099 0.100
Al
IV
0.178 0.181 0.041 0.083 0.107 0.074 0.054 0.091 0.108 0.073 0.216 0.143 0.199 0.153 0.237 0.198 0.265 0.313 0.223 0.264
Al
VI
0.058 0.059 0.000 0.024 0.024 0.033 0.027 0.005 0.004 0.010 0.024 0.015 0.026 0.023 0.002 0.001 0.036 0.011 0.057 0.014
Cr
0.009 0.006 0.000 0.002 0.007 0.004 0.008 0.000 0.013 0.008 0.007 0.002 0.004 0.005 0.008 0.005 0.005 0.003 0.003 0.001
Fe
3+
0.056 0.042 0.060 0.023 0.047 0.022 0.000 0.055 0.072 0.047 0.064 0.063 0.112 0.070 0.110 0.088 0.073 0.093 0.054 0.083
Fe
2+
0.187 0.211 0.212 0.352 0.206 0.203 0.245 0.282 0.138 0.156 0.321 0.379 0.225 0.299 0.459 0.544 0.229 0.237 0.203 0.270
Mn
0.007 0.007 0.011 0.010 0.010 0.007 0.006 0.010 0.007 0.004 0.010 0.013 0.009 0.010 0.013 0.013 0.006 0.006 0.005 0.006
Mg
0.758 0.737 0.786 0.783 0.885 0.897 0.944 0.818 0.892 0.920 0.585 0.618 0.646 0.666 0.432 0.413 0.656 0.609 0.672 0.615
Ca
0.859 0.858 0.922 0.747 0.770 0.794 0.736 0.775 0.829 0.810 0.875 0.820 0.896 0.848 0.857 0.822 0.891 0.889 0.876 0.877
Na
0.026 0.028 0.013 0.029 0.024 0.032 0.016 0.026 0.020 0.021 0.037 0.039 0.036 0.035 0.040 0.042 0.032 0.033 0.030 0.035
Mg#
80.2 77.7 78.8 68.9 80.9 81.5 79.5 74.4 86.68 85.61 64.57 61.99 74.17 69.16 48.49 43.16 74.1 72.0 76.8 69.5
Al
VI
/Al
IV
0.33 0.32 0.00 0.29 0.22 0.44 0.50 0.06 0.04 0.14 0.11 0.10 0.13 0.15 0.01 0.01 0.14 0.04 0.26 0.05
Formulae calculated on the basis of 4 cations and 6 oxygens. MB = massive basalt, PB = pillow basalt, c = core, r = rim. Mg# = 100* Mg/(Mg+ Fe
2+
).
281
BASALTIC ROCKS FROM THE OPHIOLITE MÉLANGE AT THE EXTERNAL -INTERNAL DINARIDES (CROATIA)
aly (Eu/Eu*= 1.16—0.93) typical for low accumulation or frac-
tionation of plagioclase. The REE paterns of analysed alkali
basalts are highly comparable with the profiles of alkali ba-
salts from the East African rift zone (Fig. 7B2).
The Nd and Sr isotopic compositions of two tholeiitic and
two alkali basalts are shown in Table 4. In tholeiitic samples
the
143
Nd/
144
Nd ratios are very consistent ranging from
0.512939 to 0.513002 and
87
Sr/
86
Sr ratios show a spread be-
tween 0.704353 and 0.704422. The initial
ε
Nd
and Sr initial
isotopic ratios were calculated for 165 Ma which is assumed
as the age of crystallization for the SSZ tholeiitic extrusives of
Mt Samoborska Gora ophiolites. The initial
ε
Nd
vary from
+ 6.01 to + 6.35 whilst the (
87
Sr/
86
Sr)
i
ratios vary from
0.703862 to 0.704001. The initial
ε
Nd
and (
87
Sr/
86
Sr)
i
ratios of
the tholeiitic Mt Samoborska Gora mafic extrusives plot in the
field of recent back-arc analogues (Fig. 8). In the alkali ba-
salts the range of
143
Nd/
144
Nd ratios narrows from 0.512602 to
0.512661. The
87
Sr/
86
Sr ratios show a spread between
0.705445 and 0.705851. The initial
ε
Nd
and Sr initial isotopic
ratios were calculated for 235 Ma (Illyrian—Fassanian) which is
assumed as the age of crystallization for alkali basalts of the Mt
Samoborska Gora and Mt Medvednica. The initial
ε
Nd
varies
between +1.58 to +2.54 whilst the (
87
Sr/
86
Sr)
i
ratios ranges from
0.705271 to 0.705442. The initial
ε
Nd
and (
87
Sr/
86
Sr)
i
ratios of
both analysed alkali basalts plot in the area of magmas generat-
ed from slightly enriched mantle sources bearing Nd-Sr isotopic
characteristics close to Bulk Silicate Earth (BSE; Fig. 8).
Discussion
In the Mt Samoborska Gora ophiolite mélange tholeiitic and
alkali basalts were geochemically identified. Tholeiitic lavas
with N-MORB-like geochemical signatures (high-Ti basalts)
and tholeiitic rocks with SSZ characteristics (medium-Ti ba-
salts) are the only ophiolitic lithologies archieved in this mé-
lange which occasionally constitute composite blocks
suggesting that diverse ophiolitic lithologies interfere in space
Fig. 4. Discriminant diagram: A – Ti—Al
IV
(simplified after Becca-
luva et al. 1989 and Komiya et al. 2004); B – MnO—Na
2
O—TiO
2
(simplifed after Nisbet & Pearce 1977) and C – SiO
2
/100—Na
2
O—
TiO
2
(simplifed after Beccaluva et al. 1989) for clinopyroxene
from tholeiitic volcanic rocks of the Mt Samoborska Gora and al-
kali volcanic rocks of the Mt Medvednica ophiolite mélange.
MORB – mid-ocean ridge basalts; BABB – back-arc basalts;
IAT – island-arc tholeiites; BON – boninite; OIB – ocean-is-
land basalts; WPAB – within plate alkali basalts. Fields for cli-
nopyroxene compositions from high-, medium- and low-Ti
tholeiitic basalts of the Mt Medvednica ophiolite mélange (Slovenec
& Lugović 2009) plotted for correlation constraints.
282
SLOVENEC,
LUGOVIĆ
and
VLAHOVIĆ
Table 3: Chemical analyses of tholeiitic and alkali volcanic rocks from the Mt Samoborska Gora (SG) and Mt Medvednica (MD) ophiolite mélange.
SG Tholeiitic High-Ti basalts
SG Tholeiitic
Medium -Ti basalts
SG
Alkali
basalts
MD
Alkali
basalts
Sample
jf-33/2 92-20 92-18 92-21 t-56 92-22 92-23 92-30
sas-206 jf-34 js-60
sas-202
t-23b/2 sas-8
t-23b/1
js-73/1
sat-242
sas-203
sas-210 92-16 92-25 sb-9 sb-17 sb-21 sb-30
Rock type
PB MB MB MB PB PB MB MB MB MB MB MB PB MB PB MB MB MB MB MB MB PB PB PB MB
SiO
2
48.57
48.66 49.36 48.96 46.55 49.54 49.97 49.28 46.54 48.59 50.83 49.18 46.98 47.58 46.95 48.01 51.52 54.12 52.32 46.94 55.76 46.66 42.37 47.22 48.63
TiO
2
1.41
1.76 1.79 1.79 1.83 1.88 1.93 1.95 1.96 2.01 2.20 2.31 0.93 0.96 0.97 1.07 1.19 2.57 2.62 2.74 2.70 2.08 1.99 2.08 2.10
Al
2
O
3
17.94
14.74 14.71 14.53 15.53 14.55 14.26 14.38 16.13 16.02 19.49 17.74 15.66 16.02 15.27 16.46 17.28 16.30 16.05 15.28 16.15 15.30 15.32 15.28 15.98
Fe
2
O
3
total
7.48
11.93 12.03 12.05 10.47 12.49 12.71 12.10 12.11 11.04 8.58 13.38 11.63 11.46 11.09 8.69 9.83 10.38 10.04 11.78 9.90 9.09 9.93 9.10 7.39
MnO
0.20
0.19 0.23 0.20 1.24 0.22 0.30 0.27 0.60 0.14 0.11 0.20 0.20 0.21 0.12 0.21 0.69 0.27 0.17 0.31 0.06 0.16 0.15 0.14 0.19
MgO
6.75
6.41 6.83 6.71 4.67 6.41 6.47 6.68 3.94 5.44 4.92 3.77 5.69 5.53 4.35 8.10 3.67 3.17 4.82 8.13 1.08 8.58 8.11 8.07 7.98
CaO
7.97
7.81 7.31 7.02 7.14 7.56 7.21 6.58 4.32 4.26 4.60 2.32 7.87 7.66 8.93 6.88 2.49 1.16 1.92 3.71 1.30 5.03
13.33 7.51 9.62
Na
2
O
3.05
4.26 3.85 4.12 4.18 3.98 3.87 4.39 5.88 4.67 4.62 4.51 4.45 4.47 4.59 4.68 6.94 7.03 6.35 5.00 6.41 2.84 1.48 3.21 3.05
K
2
O
0.43
0.45 0.50 0.41 0.48 0.37 0.48 0.45 0.32 0.63 0.30 0.57 0.24 0.27 0.21 0.10 0.15 0.68 0.52 0.19 0.68 1.40 0.14 0.38 0.29
P
2
O
5
0.15
0.18 0.18 0.17 0.21 0.19 0.19 0.21 0.21 023 0.23 0.24 0.07 0.07 0.09 0.09 0.11 0.82 0.94 0.97 0.93 0.35 0.33 0.39 0.44
LOI
5.69
2.80 3.27 2.93 7.73 2.57 2.93 3.25 7.60 7.12 4.30 5.48 6.16 5.73 7.36 5.58 5.88 3.59 3.56 4.42 4.94 8.46 7.04 6.60 4.26
Total
99.64
99.19
100.06 98.90
100.03 99.76
100.32 99.72 99.69
100.15
100.17 99.70 99.92
100.00 99.83 99.86 99.86
100.09 99.31 99.29 99.91 99.97 99.99 99.98 99.93
Mg#
66.72
54.18 55.54 55.90 48.04 53.04 52.83 55.00 41.25 51.86 55.60 35.89 50.35 49.63 46.27 67.34 44.98 38.56 48.83 60.30 19.39 65.65 64.37 64.25 70.33
Cs
0.5
5.3 2.3 4.7 0.1 1.3 0.8 0.6 0.1 0.4 0.2 0.4 0.3 0.2 0.5 0.1 0.1 0.3 0.2 0.1 0.4 2.0 1.0 1.4 1.6
Rb
13
12
14
12 5 9
13
11 3 9 3 4 7 5
16 2 2
25
21 3
39 6
10
43
41
Ba
268
1070
1270
737 81
631
273
258 77
240 47 80 62 69
233 78 85 83 71 74 60 79 30 74 49
Th
0.28
0.45 0.46 0.41 0.60 0.45 0.47 0.60 0.43 0.61 0.32 0.49 0.06 0.07 0.07 0.07 0.09 7.05 7.14 6.60 7.79 3.69 2.70 3.98 4.12
Ta
0.14
0.33 0.32 0.28 0.30 0.27 0.26 0.31 0.22 0.23 0.22 0.21 0.04 0.05 0.04 0.04 0.06 3.88 3.93 3.68 4.02 2.10 1.91 2.31 2.42
Nb
2.2
5.4 5.2 4.5 4.9 4.5 4.5 4.7 3.4 3.6 3.4 3.3 0.6 0.8 0.7 0.7 1.0 60.3 60.7 58.2 62.6 33.9 30.8 35.2 38.7
Sr
142
301 415 307 175 369 307 264 63 109 162 90 719 711 308
1407 98 85 72 167 58 139 70 92 103
Zr
89
116 122 122 115 134 141 132 141 140 135 151 47 49 50 69 59 245 251 255 274 152 147 168 173
Hf
2.4
3.5 3.6 3.5 3.5 3.9 4.1 3.6 3.7 3.9 4.0 4.0 1.5 1.4 1.6 1.8 1.7 6.1 6.2 6.3 6.8 3.5 3.5 3.8 3.9
Y
30
34 36 35 36 37 39 44 38 34 35 33 28 28 26 27 24 36 34 30 35 29 27 31 33
Sc
33
37 36 34 46 35 36 37 41 40 36 33 43 45 43 43 45 19 17 23 13 31 29 32 32
V
230
2.88 283 279 245 289 301 355 315 382 406 414 256 269 234 221 211 238 241 268 196 261 254 251 245
Cr
430
213 177 152 226 100 98 115 410 190 120 145 384 460 233 470 440 80 92 271 20 239 301 292 252
Co
46
- - - 33 - - 39 43 38 72 42 58 61 57 57 64 35 29 - 23 36 43 34 31
Ni
140
70 60 53 56 50 47 41 140 50 45 42 265 270 182 260 290 32 39 117 20 146 230 152 122
La
3.21
6.41 6.64 6.29 4.09 6.83 7.21 6.37 4.53 5.12 4.73 3.92 1.62 1.80 1.71 2.35 2.19
32.71
33.41
43.91
31.40
21.21
19.72
22.25
24.86
Ce
10.18
17.90 18.51 18.10 13.03 19.32 20.60 18.8 14.20 15.11 14.40 12.21 5.34 5.75 5.55 7.79 6.91 69.10 70.33 97.82 67.11 47.79 45.13 49.32 54.98
Pr
1.62
2.80 2.87 2.79 1.98 2.98 3.14 2.78 2.15 2.23 2.24 1.88 0.91 0.95 0.90 1.34 1.12 8.33 8.24
12.10 7.44 5.29 5.04 5.62 6.22
Nd
8.89
14.60 15.11 15.22 10.92 16.31 17.10 14.52 12.20 12.21 12.01 10.20 5.30 5.62 5.20 7.56 6.12 32.22 33.15 47.71 31.21 22.81 21.41 23.99 25.99
Sm
2.92
4.79 4.96 4.74 3.36 5.16 5.35 4.62 4.21 3.84 3.67 3.22 2.00 2.06 2.00 2.59 2.23 7.03 7.62 9.52 6.51 4.79 4.59 4.99 5.32
Eu
1.21
1.71 1.78 1.74 1.33 1.83 1.89 1.80 1.49 1.45 1.41 1.31 0.88 0.97 0.82 1.14 0.94 2.22 2.26 3.01 2.12 1.61 1.64 1.75 1.87
Gd
3.83
5.77 5.88 5.78 4.39 6.20 6.36 6.08 5.35 4.88 4.75 4.28 3.40 3.50 3.15 3.49 3.19 6.83 6.99 7.99 6.23 4.92 4.70 5.13 5.39
Tb
0.79
1.11 1.12 1.09 0.85 1.12 1.20 1.18 1.00 0.98 0.91 0.84 0.66 0.70 0.64 0.72 0.59 1.15 1.19 1.99 1.10 0.89 0.88 0.93 0.95
Dy
4.96
7.06 7.06 7.05 5.47 7.42 7.67 7.58 6.36 6.31 6.07 5.50 4.10 4.72 4.00 4.70 3.79 6.40 6.42 6.21 6.34 4.79 4.65 5.10 5.29
Ho
1.12
1.42 1.47 1.45 1.24 1.54 1.63 1.56 1.32 1.27 1.32 1.19 0.98 1.02 0.93 0.97 0.88 1.21 1.24 1.18 1.15 1.01 0.97 1.07 1.10
Er
3.22
4.53 4.57 4.49 3.55 4.75 5.03 4.72 3.99 3.75 4.18 3.68 2.97 3.15 2.71 2.94 2.59 3.22 3.27 3.27 3.13 2.76 2.64 2.84 2.88
Tm
0.493
0.661 0.668 0.667 0.541 0.703 0.741 0.699 0.592 0.562 0.636 0.554 0.441 0.472 0.413 0.426 0.399 0.437 0.439 0.442 0.426 0.382 0.371 0.390 0.393
Yb
3.13
3.97 3.98 4.09 3.50 4.23 4.46 4.39 3.88 3.59 4.10 3.42 2.95 2.90 2.55 2.75 2.50 2.58 2.59 2.56 2.53 2.46 2.35 2.49 2.52
Lu
0.482
0.558 0.567 0.572 0.482 0.594 0.634 0.666 0.606 0.538 0.615 0.504 0.452 0.461 0.401 0.416 0.383 0.359 0.361 0.355 0.347 0.331 0.321 0.338 0.342
Major elements in wt. %, trace elements in ppm. LOI = loss on ignition at 1100
o
C. PB = pillow basalt; MB = massive basalt. Mg# = 100 * molar (MgO/(MgO+FeO
total
)).
283
BASALTIC ROCKS FROM THE OPHIOLITE MÉLANGE AT THE EXTERNAL -INTERNAL DINARIDES (CROATIA)
and time. Thus the studied rocks from the Mt Samoborska
Gora ophiolite mélange contribute to the ophiolite controversy:
how rocks, which have been formed in different tectonomag-
matic settings (e.g. Western and Eastern Albanian ophiolites,
Dinaric and Vardar ophiolites, etc.), come together in a small
regional scale lacking any tectonic contact. In an ophiolite
Fig. 5. Variation diagrams for selected elements with Zr as index of fractionation for the tholeiitic and alkali volcanic rocks from the Mt
Samoborska Gora and alkali volcanic rocks from the Mt Medvednica ophiolite mélange.
Fig. 6. V—Ti/1000 discrimination diagram (Shervais 1982) for the
tholeiitic and alkali volcanic rocks from the Mt Samoborska Gora and
alkali volcanic rocks from the Mt Medvednica ophiolite mélange.
IAT – island-arc tholeiites, MORB – mid-ocean ridge basalts,
BABB – back-arc basin basalts, CAB – calc-alkaline basalts,
CFB – continental flood basalts, OIB – ocean-island basalts and
AB – alkali basalts.
mélange various sequences of the oceanic crust which are un-
related in the term of time and crystallization setting may be
found juxtaposed in the finally formed mélange (see detail
study of Saccani & Photiades (2005) for Albanian ophiolite
mélanges). Materials detached from the oceanic lithosphere,
which are incorporated in an ophiolite mélange, record
polyphase history of formation which includes a variety of
sedimentary and tectonic processes during deposition in an ac-
cretionary wedge and subsequent tectonic incorporation
through onset of thrusting and final emplacement onto a pas-
sive continental margin. The Mt Samoborska Gora ophiolite
mélange closely exposes Middle Triassic non-orogenic alkali
basalts and different tholeiitic extrusives, some of them show-
ing uppermost Bathonian—Early Callovian age, which may
facilitate the study of evolution of the oceanic domain where
these ophiolites were formed from the initial stage of opening
till the initiation of shortening of the oceanic domain.
Tectonomagmatic significance of tholeiitic basalts
The extrusive rocks archived in the Mt Samoborska Gora
ophiolite mélange show geochemical signatures that reflect
magmas derived from several parental mantle sources which
are obviously related to the different tectonomagmatic settings
(Pearce & Norry 1979; Pearce 1983). Composite blocks of
tholeiitic extrusives suggest that the high-Ti and medium-Ti
magmatism may be temporally and spatially closely interrelat-
ed. Similar overlaping of contrasting magma types seems to
be very common in Neotethyan ophiolites as exemplared by
the Mirdita ophiolites in Albania (Bébien et al. 2000; Hoeck et
al. 2002; Bortolotti et al. 2002; Dilek et al. 2007).
The high-Ti and medium-Ti basalts from the Mt Samobors-
ka Gora ophiolite mélange show N-MORB-like REE patterns
284
SLOVENEC, LUGOVIĆ and VLAHOVIĆ
Fig. 7. N-MORB-normalized (A) multielement and (B) REE patterns for tholeiitic and alkali volcanic rocks from the Mt Samoborska Gora
and alkali volcanic rocks from the Mt Medvednica ophiolite mélange. Field for alkali basalts from the East African Rift-Kenya Rift (Wil-
son 1989 and references therein; Spath et al. 2001) and Japan Rift (Okamura et al. 2005) are shown for comparision. Normalization values
are from Sun & McDonough (1989).
(Fig. 7B1) and at the same time show HFSE negative anoma-
lies (Fig. 7A1) which are unique characteristics of MORBs
with arc signatures (Shervais 2001). Surprisingly, although
the LREE depletion in the medium-Ti basalts is more pro-
nounced they display considerably higher relative depletion of
Ta—Nb thus confronting MORB and SSZ signatures in one
single geochemical group. In the diagram V-Ti/1000 all
tholeiitic basalts from the Mt Samoborska Gora plot in the
field of ocean ridge basalts (Fig. 6). However, they may plot
between the fields of MORB and IAT extrusives forming a
SSZ array similar to the recent back-arc basin basalts
(Fig. 9A—C). The Nd isotopic composition of the tholeiitic
rocks from the Mt Samoborska Gora ophiolite mélange ex-
pressed in the term of
ε
Nd(T =165 Ma)
shows vary small range
from + 6.01 in the high-Ti basalts to + 6.35 in medium-Ti ba-
salts and strongly suggests that medium-Ti basalts must have
derived from a similar but slightly more depleted mantle
source during the second stage of partial melting. Combined
with initial
87
Sr/
86
Sr ratios ranging from 0.703862 to
0.704001, respectively (Table 4), the tholeiitic rocks better
match BABB then N-MORB (Fig. 8).
Clinopyroxene chemistry is frequently used to discriminate
and characterize the tectonomagmatic setting of parental ba-
salts (Beccaluva et al. 1980; Pearce & Wanming 1988; Pearce
2003). Beccaluva et al. (1989) promote clinopyroxene compo-
sition as a robust discriminator for host basalts from different
ophiolitic types. The composition of clinopyroxenes from the
Mt Samoborska Gora ophiolite mélange clearly discriminate
tholeiitic basalt host rocks in the fields of MORB and BABB
(Fig. 4A—C). Clinopyroxenes hosted in the medium-Ti sam-
ples plot at high-Ti corner of the MORB/BABB field (Fig. 4A
and 4C), although following the petrochemical parameters of
this discriminatory concept, they should plot between the
MORB/BABB and IAT fields. The clinopyroxene from our
medium-Ti basalts exceptionally coexists with abundant
spinel and minor Ti-magnetite. The significantly higher parti-
tion of Ti in clinopyroxene compared with coexisting spinel in
a tholeiitic melt, cause clinopyroxene to become enriched in
Ti and also in Fe (Figs. 3 and 4), which is generally atypical
for clinopyroxenes hosted in ophiolitic medium-Ti and low-Ti
basalts (e.g. Bortolotti et al. 2002; Slovenec & Lugović 2009).
Due to lower partition of Ti in spinel relative to Fe-Ti
285
BASALTIC ROCKS FROM THE OPHIOLITE MÉLANGE AT THE EXTERNAL -INTERNAL DINARIDES (CROATIA)
Fig. 8. Initial
143
Nd/
144
Nd—
87
Sr/
86
Sr isotope ratios diagram for se-
lected high-, and medium-Ti tholeiitic rocks from the Mt Samobor-
ska Gora (time: 165 Ma) and alkali extrusive rocks from the Mts
Samoborska Gora and Medvednica (time: 235 Ma) ophiolite mé-
lange showing the main oceanic mantle reservoirs of Zindler & Hart
(1986). Fields for the Mt Medvednica tholeiitic basalts (Slovenec &
Lugović 2009) plotted for correlation constraints. DM – depleted
mantle, BSE – bulk silicate Earth, EMI – enriched mantle,
HIMU – mantle with high U/Pb ratio, PREMA – frequently ob-
served PREvalent MAntle composition. The mantle array is defined
by many oceanic basalts and a bulk Earth value for
87
Sr/
86
Sr can be
obtained from this trend. Data for back-arc basin basalts – BABB
(shaded field) compiled from Wilson (1989) and references therein,
Cousens et al. (1994) and references therein, Pearce et al. (1995),
Gribble et al. (1998) and Ewart et al. (1998). Data for mid-ocean
ridge basalts – MORB (solid line) compiled from Wilson (1989)
and references therein and Cousens et al. (1994), references therein
and Peate et al. (1997). Data for oceanic island arcs and active con-
tinental margins – IAB (broken line) compiled from Wilson
(1989) and references therein, Cousens et al. (1994) and references
therein, Pearce et al. (1995) and Peate et al. (1997).
oxide(s), caution is recommended concerning liability of cli-
nopyroxene in paragenesis with spinel as a tectonomagmatic
discriminatory tool.
Coexistence of the high-Ti and medium-Ti basalts in the
ophiolite mélange of the Mt Samoborska Gora suggests at
least two stages of partial melting and magma generation: an
older stage producing crust with N-MORB-like geochemical
signatures at an ocean spreading ridge closely followed by the
stage marked by formation of the younger crust which
involves subduction related melt. The high-Ti group from the
Mt Samoborska Gora ophiolite mélange derived from a man-
tle source and experienced low total partial melting ranging
from < 5 to 10 % (Fig. 10A), leaving residual fertile mantle
peridotites closely resembling the composition of the lherzo-
lites from the Mt Kalnik (Lugović et al. 2007) and from the
entire CDOB (Lugović et al. 1991; Bazylev et al. 2009). This
geochemical group is accepted as representing remnants of
the crust formed at the ocean ridge at any time in the ocean’s
spreading history.
On the contrary, the Upper Bathonian—Lower Callovian
high-Ti basalts, crystallized at around 165 Ma, that are associ-
ated with the medium-Ti basalts in the composite block, indi-
cate terminal formation of high-Ti crust which may be related
to subducting ridge and indicate incipient crust formation in
the hanging wall or, in other words, in the upper plate. These
composite blocks give strong evidence for the “stage of birth”
from the geodynamic model of SSZ ophiolites proposed by
Shervais (2001). Stern (2004) thought that such association of
extrusive rocks were formed by extensive magmatism in the
extensional proto-arc—fore-arc basin that spreads over the still
active subducting ridge. Extension in the proto-arc or infant-
arc basin lasts 5—10 Ma and is normally marked by almost si-
multaneous crystallization of high-Ti and medium-Ti to low-Ti
basalts as a consequence of melt derivation from successively
more depleted mantle which was progressively more metaso-
matized by expulsion of fluids from the subducting slab.
Since the high-Ti basalts from ocean spreading center and
from proto-arc—fore-arc extensional basin were derived from
essentially similar sources they are geochemically hardly dis-
tinguishable. Following the proposed geodynamic models, the
medium-Ti basalts from Mt Samoborska Gora ophiolite mé-
lange showing significantly higher HFSE depletion (Fig. 7A1)
and Nd isotopic composition (Table 4) clearly resemble mag-
matism of the second stage partial melting from an already
moderately depleted mantle source contaminated by an
amount of subduction related components. Here, the second
stage melting is somewhat atypical since it allows cotectic
crystallization of clinopyroxene and spinel± Ti-magnetite in-
stead of ordinary Fe-Ti oxide(s) as recorded in analogue rocks
from ophiolite mélanges included in the nearby Kalnik Unit.
This may suggest a different thermal regime governing incipi-
Table 4: Nd and Sr isotope data of tholeiitic and alkali volcanic rocks from the Mt Samoborska Gora and Mt Medvednica ophiolite mélange.
Location number corresponds to the locations in Fig. 1C for the samples from the Mt Samoborska Gora; MD = Mt Medvednica. Rock types:
Th = tholeiitic, Alk = alkali, H-Ti = high-Ti, M-Ti = medium-Ti, PB = pillow basalt, MB = massive basalt.
a
Errors in brackets for Nd and Sr
isotopic ratios are given at the 2
σ-level.
147
Sm/
144
Nd calculated from the ICP-MS concentrations of Sm and Nd following equation:
147
Sm/
144
Nd = (Sm/Nd)*[0.53151+0.14252*
143
Sm/
144
Nd].
b
Initial
ε
Nd(t)
calculated assuming I
o
CHUR
= 0.512638, (
147
Sm/
144
Nd)
o
CHUR
= 0.1966, and
λ
Sm
= 6.54*10
—12
a
—1
.
c
87
Sr/
86
Sr
(t)
calculated using ICP-MS Rb and Sr concentrations and assuming
λ
Rb
= 1.42*10
—11
a
—1
. * The initial
ε
Nd
and
initial isotopic ratios for Sr in analysed tholeiitic (Th) rocks are calculated for 165 Ma, and in alkali (Alk) rocks are calculated for 235 Ma.
286
SLOVENEC, LUGOVIĆ and VLAHOVIĆ
ent crust formation in the upper plate of the oceanic segment
represented by the Mt Samoborska Gora ophiolite mélange.
The medium-Ti basalt derived from a metasomatized mantle
region experienced 13—22 % total partial melting (Fig. 10A).
Transitional harzburgites, tectonically inserted into the Cam-
panian-Maastrichtian rudist limestones near the village of
Gornje Orešje in the Mt Medvednica (Lugović et al. 2007),
showing ~ 20 % partial melt extraction probably represent the
Fig. 9. Discrimination diagrams for the tholeiitic and alkali volcanic rocks from the Mt Samoborska Gora and alkali vocanic rocks from the
Mt Medvednica ophiolite mélange. A – Ta/Yb—Th/Yb diagram (Pearce 1983); S – subduction zone enrichment, C – crustal contamina-
tion, W – within-plate enrichment. N-MORB, E-MORB and OIB are from Sun & McDonough (1989). B – Th—Nb/16—Hf/3 diagram (Wood
1980); A – normal mid-ocean ridge basalts (N-MORB), B – enriched MORB (E-MORB) and within-plate tholeiites (WPT), C – alkaline
within-plate basalts (AWPB), D – calc-alkali basalts (CAB), E – island-arc tholeiites (IAT); 1 – crustal contamination, 2 – SSZ ophiolites
trend, 3 – MORB ophiolites trend. Data for back-arc basin basalts – BABB (shaded field) compiled from Saunders & Tarney (1979),
Weaver et al. (1979), Crawford & Keays (1987), Jahn (1986), Ikeda & Yuasa (1989), Ewart et al. (1994), Gribble et al. (1998), Leat et al.
(2000), Münker (2000). C – La/10—Nb/8—Y/15 diagram (after Cabanis & Lecolle 1989); 1A – calc-alkali basalts (CAB), 1B – area of over-
lap between 1A and 1C (CAB, IAT), 1C – island-arc tholeiites (IAT), 2A – within plate basalts (WPB), 2B – back-arc basin basalts
(BABB), 3A – intercontinental rift alkali basalts (ICRAB), 3B – enriched mid-ocean ridge basalts (E-MORB), 3C – weakly enriched mid-
ocean ridge basalts (E-MORB), 3D – normal mid-ocean ridge basalts (N-MORB). D – DF
1
—DF
2
diagram (Agrawal et al. 2008;
DF
1
= 0.5533log
e
(La/Th) + 0.2173log
e
(Sm/Th)—0.0969log
e
(Yb/Th) + 2.0454log
e
(Nb/Th)—5.6305; DF
2
= —2.4498log
e
(La/Th) + 4.8562log
e
(Sm/Th)—2.1240log
e
(Yb/Th)—0.1567log
e
(Nb/Th)+0.94). IAB–island-arc basalts, OIB–ocean-island basalts, CRB–continental rift basalts.
most depleted residual mantle from which the medium-Ti ba-
salts from Mt Samoborska Gora were derived.
Tectonomagmatic significance of alkali basalts
Alkali basalts are very commonly associated with tholeiitic
rocks in many Mesozoic ophiolite mélanges and were mostly
interpreted as remnants of intraoceanic islands (OIB) or sea-
287
BASALTIC ROCKS FROM THE OPHIOLITE MÉLANGE AT THE EXTERNAL -INTERNAL DINARIDES (CROATIA)
mounts (Saccani & Photiades 2005; Monjoie et al. 2008; Sayit
& Göncüoglu 2009). However, alkali basalts from continental
rifts (= CRB) closely preceding formation of early oceanic
crust as a rule have a similar composition (e.g. Fitton 2007)
which discriminates them as within-plate volcanic rocks
(Fig. 9B). We do not exclude either OIB or CRB origin of al-
kali basalt fragments incorporated in the Mts Samoborska
Gora and Medvednica ophiolite mélanges.
Although alkali basalts from the Mts Samoborska Gora and
Medvednica ophiolite mélanges in general share geochemical
characteristics of OIB and CRB, they show slightly pronounced
geochemical differences (Fig. 7A2, 7B2) suggesting a more en-
riched or less depleted source for the Mt Samoborska Gora al-
kali basalts (Figs. 6, 9A). Low relative abundance of HREE in
these alkali basalts suggests presence of garnet as a residual
phase in both sources (e.g. Wilson 1989; Spath et al. 1996).
Crustal contamination of alkali basalts increases the LILE
or LREE/HFSE ratios, with higher intensity in CRB than in
OIB. Uncontaminated CRB have La/Ta ratio < 22 (e.g. Fitton
et al. 1988; Hart et al. 1989) and the ratio of the analysed alka-
li basalts is even lower (7.8—11.9) suggesting insignificant
crustal contamination. Therefore the analysed alkali basalts
represent uncontaminated melts and plot in the field of mantle
array in the diagram Th/Yb—Ta/Yb wherein alkali basalts from
the Mt Samoborska Gora also show a more primitive nature
(Fig. 9A). Many attempts based on geochemical parameters
were done to discriminate between OIB and CRBs. Recent
multielement ratios discriminate function analysis resolve
CRB and OIB in two separate fields at 78% and 85% confi-
dence level, respectively (Agrawal et al. 2008). Alkali basalts
from the Mts Samoborska Gora and Medvednica mélanges
Fig. 10. Petrogenic model for: A – mafic tholeiitic volcanic rocks from the Mt Samoborska Gora ophiolite mélange. Partial melting lines:
DM – depleted mantle source, PM – primitive mantle source (Kostopoulos & James 1992). Model parameters = spinel-lherzolite source
(ol
57
—opx
25.5
—cpx
15
—sp
2.5
), melting proportion = ol
1.21
-opx
8.06
-cpx
76.37
-sp
14.36
, distribution coefficients are from Kostopoulos & James
(1992). Fractional crystallization lines: initial magma = 10% melting of DM and PM mantle source, respectively, fractionated mineral
assemblage = ol
30
—cpx
40
—pl
30
, distribution coefficients are from Chen et al. (1990). Data for N-MORB, E-MORB are from Sun & McDonough
(1989). Fields for high-, medium- and low-Ti tholeiitic basalts from the Mt Medvednica ophiolite mélange (Slovenec & Lugović 2009)
plotted for correlation constraints. B – mafic alkali volcanic rocks from the Mt Samoborska Gora and Mt Medvednica ophiolite mélange.
Partial melting lines: moderately enriched mantle source (OIB-like) (La = 1.5 ppm, Yb = 0.5 ppm). Model parameters = garnet-lherzolite
source (ol
60.1
—opx
18.9
—cpx
13.7
—gt
7.3
), melting proportion = ol
1.3
—opx
8.7
—cpx
36
—gt
54
, distribution coefficients are from Kostopoulos & James
(1992). Fractional crystallization lines: initial magma = 4% and 6% melting of the moderately enriched mantle source, fractionated mineral
assemblage = ol
30
—cpx
40
—pl
30
, distribution coefficients are from Chen et al. (1990).
show overall geochemical signatures comparable with CRB
(Fig. 9D). Thus, the alkali basalts are interpreted as the pre-
ophiolitic continental rift basin volcanic rocks, rather than as
the remnants of intraoceanic islands or seamounts.
Continental intraplate alkali basalts were interpreted as vol-
canic products of partial melting of the upper mantle related to
lithospheric extension causing upwelling of the asthenosphere
(Perry et al. 1990; Kent et al. 1992). Alternatively, alkali ba-
salts may erupt when the mantle plume impinges on the base
of the continental lithosphere causing partial melting and ini-
tial rifting (Morgan 1981; McKenzie & Bickle 1988). The
analysed alkali basalts reflect melt derived from an OIB-type
enriched mantle source (Fig. 9A), which experienced ~ 8% to-
tal partial melting for the Mt Medvednica and 5—7% for the Mt
Samoborska Gora (Fig. 10B). Assuming that both groups of
the alkali basalts were derived from the same or similar paren-
tal asthenospheric material, which resembles Nd-Sr isotopic
signatures of the bulk Earth at the time of crystallization
(Fig. 8), then the alkali basalts from the Mt Samoborska Gora
with more primitive geochemical significance, particularly
lower
ε
Nd(T= 235 Ma)
(+1.58 vs. + 2.54; Table 4) represents melts
of an older stage of partial melting. If this assesment is correct,
Illyrian-Fassanian age is promoted as the age of initial Neo-
tethyan opening in the ROD.
Geodynamic evolution of the Mt Samoborska Gora ophio-
lites in the context of the Repno oceanic domain
The ophiolite mélange of the Mt Samoborska Gora is tec-
tonically emplaced directly on the Adria amagmatic continen-
tal margin and represents the southwesternmost detached
288
SLOVENEC, LUGOVIĆ and VLAHOVIĆ
leading edge of a larger ophiolite mélange unit, most likely of
the Kalnik Unit (Haas et al. 2000) from the Zagorje-Mid-
Transdanubian Zone (Fig. 1A,B). The tectonomagmatic histo-
ry of the Kalnik Unit has been in part successfully inferred
from the remnants of the oceanic upper crustal rocks archived
in the Mt Medvednica ophiolite mélange (Slovenec &
Lugović 2008, 2009) thought to have been generated in a dis-
crete Neotethyan oceanic domain referred to as the Repno
oceanic domain (ROD) (Babić et al 2002; Slovenec &
Lugović 2008). The ROD should be included in easternmost
segment of the Tethys (Bortolotti & Principi 2005). Some au-
thors regard the ROD as a domain of Dinaric provenance (e.g.
Pamić 1997; Haas & Kovács 2001) whilst others relate it to
the Meliata-Maliac ocean system (e.g. Goričan et al. 2005;
Slovenec & Lugović 2008). These basins opened as back-arc
basins in response to the delayed subduction of the Paleo-
tethyan lithosphere beneath the European continental margin
(Stampfli & Borel 2002, 2004).
The geochemical affinities and age of crystallization of
analogous ophiolitic rocks from the Mt Samoborska Gora
ophiolite mélange and the Mt Medvednica segment of the
Kalnik Unit were used for correlation of these two ophiolitic
segments. Normalized multi-element concentration (Fig. 11A),
normalized REE concentrations (Fig. 11B) and Nd and Sr iso-
topic signatures (Fig. 8) of the analogue high-Ti N-MORB-
like extrusives from both ophiolitic segments, assumed to
represent the oceanic crust formed at a spreading or subduct-
ing ridge, show identical patterns suggesting a similar mantle
source. The high-Ti basalts from the composite blocks in Mts
Samoborska Gora and Medvednica ophiolite mélanges, which
indicate incipient crust formation in a converging upper plate,
show identical K-Ar ages of 165.4± 5.8 Ma and 165.1± 3.3 Ma,
respectively. The medium-Ti extrusives which represent rel-
atively younger incipient crust for Mt Samoborska Gora
show more pronounced subduction signatures (Fig. 11A)
and different crystallization regime as revealed by paragenesis
of clinopyroxene and spinel (Figs. 3 and 4A,C), and reveal a
higher intensity of partial melting (Fig. 10A). However, in-
spite of this, the Mt Samoborska Gora lacks the fragments of
typical IAT-like lithologies (low-Ti basalts) that are related to
the early stage of true subduction sensu Stern (2004). This
may suggest that the Mt Samoborska Gora ophiolites repre-
sent a discrete segment at the stage of incipient intraoceanic
convergence. The overlapping of geochemical signatures and
age of crystallization of analogue rocks from these two ophio-
lite mélange sectors allow them to be integrated into a single
ophiolite mélange unit, namely the Kalnik Unit. This finding
will serve to improve the geodynamic evolution of the ROD
that was already proposed by Slovenec & Lugović (2009). In
this respect, the newly discovered alkali basalts play a key role
for initiation of the ROD.
(1) Incipient opening of the ROD most likely started in the
Early Ladinian, namely in the Fassanian, soon after the erup-
tion of the alkali basalt during the Illyrian—Fassanian
(Fig. 12A). This stage is documented by the close association
of alkali basalts with Middle Triassic cherts in the Mt
Samoborska Gora and interpillow Illyrian—Fassanian pelagic
limestones in the Mt Medvednica, and also by the absence of
any crustal contamination of the alkali lavas, suggesting their
effusion in a highly evolved intracontinental rift basin. These
statements are also confirmed by similar geochemical patterns
with intracontinental alkali basalts from East African rift zone
(Fig. 7A2, 7B2).
(2) The onset of oceanic crust formation at ensialic BAB
spreading centre in the ROD continued through the Carnian
(Fig. 12B) as documented by radiolarians cherts above the pil-
low lavas (Halamić & Goričan 1995; Goričan et al. 2005). The
fragments of the early oceanic crust of the ROD were not en-
countered elsewhere in the Kalnik Unit. The oceanic crust for-
mation recorded in Eastern Mediterranean ophiolites, such as
Fig. 11. (A) N-MORB-normalized multielement patterns and (B) chondrite-normalized REE patterns for average high-, medium- and low-
Ti tholeiitic basalts from the Mt Medvednica ophiolite mélange. Fields for tholeiitic high- and medium-Ti basalts and alkali basalts from
the Mts Samoborska Gora and Medvednica ophiolite mélange are shown for comparision. Data for the Mt Medvednica tholeiitic high-, me-
dium- and low-Ti basalts from Slovenec & Lugović (2009). N-MORB normalization values are from Sun & McDonough (1989), and Chon-
drite normalization values are from Taylor & McLennan (1985).
289
BASALTIC ROCKS FROM THE OPHIOLITE MÉLANGE AT THE EXTERNAL -INTERNAL DINARIDES (CROATIA)
the Albanide-Hellenide ophiolite mélanges, commenced by E-
MORB (Saccani & Photiades 2005). The absence of E-MORB
extrusives in the Mts Samoborska Gora and Medvednica ophi-
olite mélanges may indicate initially fast-spreading ridge seg-
ment of the ROD wherein interaction between the uprising
asthenosphere and OIB-type enriched mantle source was sup-
pressed to produce E-MORBs. Continuation of sea floor
spreading during the Jurassic (Pliensbachian and Bajocian)
produced N-MORB-like crust (Pamić 1997) in the ROD as a
consequence of partial melting of pure suboceanic mantle
with SSZ signatures inherited from an earlier (Hercynian?)
subduction. The maximum evolved stage of spreading in the
ROD is reflected by the Bathonian typical N-MORB extru-
sives (Slovenec & Lugović 2009).
(3) Intraoceanic convergence in the ROD commenced in the
Late Bathonian—Early Callovian (Slovenec & Lugović 2009)
as indicated also in the Mt Samoborska Gora ophiolite mé-
lange, and may have continued until the Middle Callovian in
the Mt Medvednica (Babić et al. 2002). Initial convergence
led to formation of an extensional proto-arc basin in the lead-
ing edge of the oceanic lithophere overriding the still active
oceanic ridge (Fig. 12C). This stage of partial melting generat-
ed N-MORB-like magmatism, similar to the spreading-ridge
stage, and almost simultaneous medium-Ti magmatism in
both Mts Samoborska Gora and Medvednica. Subsequent IAT
magmas that reflect transition from incipient to true SSZ mag-
matism in the fore-arc basin are located only in the Mt
Medvednica (Slovenec & Lugović 2009). It is most likely that
the Mt Samoborska Gora ophiolite mélange represents a dis-
crete segment of the ROD which records the shortest subduc-
tion related evolution. The oceanic crust fragments in the
Kalnik Unit do not provide any evidence of formation of
island arc in the ROD.
(4) The age and processes that led to the closure of the
ROD are unclear and may be inferred only from the relevant
metamorphic rocks assumed to represent the ancient ROD
crust. The lower geenschist facies metamorphic complex of
the Mt Medvednica formed by obduction of an island arc
succession onto the Adria continental margin (Lugović et al.
2006) dated to 124—114 Ma ago (Belak et al. 1995) and a
metamorphic sole from the Mt Kalnik which has a BABB-
type crust protoliths (Ignjatić 2007), dated to 126—110 Ma,
Fig. 12. Schematic geodynamic model for interaction of back-arc rifting, active spreading and subduction-related processes at the infant intra-
oceanic arc setting for Mt Samoborska Gora and Medvednica Mt ophiolites in Repno oceanic domain as part of Meliata-Maliac-Vardar ocean
system. A – the intracontinental back-arc rifting stage, B – the spreading stage and formation ensialic back-arc basin, C – the subduction
stage of an active ocean ridge, D – the closure stage of the ROD. 1 – mantle diapires, 2 – oceanic crust with radiolarian cherts, 3 – raising
of the mantle diapir, 4 – melting zone; IA = island arc (infant proto-arc/island arc system), BAB = back-arc basin, AP = accretionary prism.
290
SLOVENEC, LUGOVIĆ and VLAHOVIĆ
suggest that the final closure most likely took place in
Barremian—Aptian (Fig. 12D).
Conclusions
Our findings provide evidence that the Mt Samoborska
Gora ophiolite mélange is the headmost edge of the Kalnik
Unit which is obducted onto the Adria passive margin. It
represents a discrete sector of the ROD which records a com-
plexity of tectonomagmatic and sedimentary processes from
the terminal intracontinental rifting in the Middle—Triassic
(Illyrian—Fassanian) through various stages of back-arc
spreading until the intraoceanic convergence in the Middle
Jurassic (uppermost Bathonian—Early Callovian). From the
incorporated clasts the stage(s) of spreading cannot be in-
ferred properly in details. Intraocenic convergence led to for-
mation of the crust in the upper plate by magmatism in an
extensional proto-arc—fore-arc basin that was spreading over
an active subducting ridge. This oceanic sector records the
shortest duration of the convergence related evolution in the
ROD which may have been suppressed before the true sub-
duction commenced. The incorporated fragments were intro-
duced into the mélange mostly by tectonically induced
sedimentary processes in the accretionary wedge in the front
of the proto-arc—fore-arc region, whilst older rocks, recog-
nized by alkali basalts and Triassic limestones, repeatedly re-
ferred to as “exotic” in ophiolite mélanges, actually
represent tectonic inclusions integrated in the trailing edge
of the mélange in the latest phase of obduction.
Acknowledgments: This work is a contribution to the scien-
tific projects Mesozoic magmatic, mantle and pyroclastic
rocks of northwestern Croatia (Grant No. 181-1951126-1141
to Da.S.), Geological map of Republic of Croatia 1 : 50,000
(Grant No. 181-1811096-1093) and Tectonomagmatic corre-
lation of fragmented oceanic lithosphere in the Dinarides
(Grant No. 195-1951126-3205 to B.L.) carried out under the
support of the Croatian Ministry of Science, Education and
Sport. We are greatful to Špela Goričan for radioalarian anal-
ysis and Kadosa Balogh for K-Ar age determinations. We
are also greatful to Tonći Grgasović and Radovan Filjak for
assistance with rock sampling. Our thanks go to H.-P. Meyer
and Rainer Altherr for microprobe facilities and Ilona Fin for
excellent polished thin sections. Critical comments by
Vesnica Garašić and Bruno Tomljenović improve an early
version of the manuscript. Constructive reviews by Ján
Spišiak and Dragan Milovanović greatly helped to achieve
the final version of this paper.
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