GEOLOGICA CARPATHICA, AUGUST 2008, 59, 4, 277—293
www.geologicacarpathica.sk
Introduction
An ophiolite mélange is a tectono-sedimentary unit initially
accreted in a deep ocean trench (accretionary wedge) over a
subducting plate in front of the leading edge of the overrid-
ing plate. The mélange is an archive of very heterogeneous
rock fragments, derived from both sides of the accretionary
wedge, representing remnants of different tectonostrati-
graphic units formed during the long lasting evolution of an
oceanic system. Systematic petrological and geochemical
analyses of these rocks fragments give an opportunity to
study the history of an oceanic system from the steady state
oceanic crust formation to the final closure. This method is
particularly efficient when regional field relations fail to re-
solve the geotectonic affiliation of poorly exposed ophiolitic
rocks as confirmed in the Panonnian Basin and Sava Zone.
Mt Medvednica is located on the southernmost segment of
the Zagorje-Mid-Transdanubian Shear Zone (ZMTDZ) and
exposes one of the largest ophiolitic mélanges in the entire
Pannonian Basin and NW Sava Zone. The Mt Medvednica
ophiolite mélange is peculiarly positioned between the Ma-
liak ophiolites to the NW and Dinaric/Vardar ophiolites to
the SE (Fig. 1A). The mélange consists of remnants of Me-
sozoic oceanic crust along with fragments of sedimentary
rocks derived from different geotectonic provenances (Slo-
venec 1998, 2003; Slovenec & Pamić 2002, and references
therein).
Amphibole gabbroic rocks from the Mt Medvednica ophiolite
mélange (NW Croatia): geochemistry and tectonic setting
DAMIR SLOVENEC
1
and BOŠKO LUGOVIĆ
2
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
(Manuscript received December 27, 2007; accepted in revised form March 31, 2008)
Abstract: Amphibole gabbroic rocks with heteradcumulate and isotropic fabric constitute centimeter to hectometer
large fragments in the Early Callovian to the Late Valanginian ophiolite mélange of the Mt Medvednica located at the
SW tips of the Zagorje-Mid-Transdanubian Shear Zone. Normalized multielement concentration patterns have strong
Ta-Nb anomaly [(Nb/La)
MORBN
= 0.28—0.72] for all rocks, while normalized REE concentration of isotropic gabbros
show patterns transitional between mid-ocean ridge and island arc magmatic rocks [(La/Lu)
MORBN
= 0.92—1.12]. Low
Ti, Cr and Na content of clinopyroxene from the isotropic gabbros ( < 0.98 wt. % TiO
2
, < 0.94 wt. % Cr
2
O
3
, < 0.39 wt. %
Na
2
O) combined with high Ca-plagioclase (up to An
93
) and crystallization of plagioclase after augite-diopside with
tschermakite-magnesiohornblende as intercumulus oikocrystal bring in the evidence of formation in a suprasubduction
setting. The rocks were severely albitized and uralitized in greenschist facies on the sea floor and altered in prehnite-
pumpellyite facies during emplacement. Due to the alterations the LILE may have been selectively enriched while
HFSE and REE retained magmatic ratios. Overall geochemical data and age determination of gabbropegmatite (161.1 Ma)
are liable to geotectonic constraints and advocate a proto-arc—immature intra-oceanic island arc source of the Mt Medvednica
gabbroic fragments. It was suggested that the Mt Medvednica gabbroic rocks represent remnants of an extinct intra-
oceanic arc system formed in the Repno domain of the Neotethyan oceanic realm. A petrogenetic and geotectonic connec-
tion between the Repno and Maliak-Dinaric oceanic domains cannot be positively postulated on the basis of the presented
data.
Key words: Croatia, Zagorje-Mid-Transdanubian
Shear Zone, Mt Medvednica, island arc, amphibole gabbros, ophiolite
mélange.
The fragments of cumulus and isotropic gabbroic rocks
with magmatic amphibole are relatively widespread in the
Mt Medvednica ophiolite mélange (Crnković 1963; Slo-
venec & Pamić 2002). Amphibole mafic intrusives have par-
ticular importance in the study of ophiolites because they are
diagnostic for suprasubduction origin of an ancient oceanic
crust. However, detailed study of the petrological and
geochemical characteristics of the Mt Medvednica amphib-
ole gabbroic rocks have not been performed yet.
The aim of this paper is to give overall petrological and
geochemical characteristics of the Mt Medvednica amphib-
ole gabbroic rocks to determine their petrogenesis and to
suggest the probable geotectonic setting of their formation.
For the first time we present the evidence for the existence of
a proto-arc—island arc system in the ZMTDZ. Finally, we
correlate the Mt Medvednica gabbroic rocks with equivalent
rocks from the Szarvaskő ophiolites at Bükk Mts in NE Hun-
gary and the Meliata ophiolites at Jaklovce intending to fig-
ure out their potential geotectonic link.
Geological setting
Mt Medvednica is located on the ZMTDZ segment of the
Sava Zone (Fig. 1A,B). This part of the Sava Zone repre-
sents the area of the triple junction of the Southern-Eastern
Alps, Tisia block of the Pannonian Basin and the Internal Di-
278
SLOVENEC and LUGOVIĆ
narides and consists of mixed and superimposed Dinaric and
Alpine tectonostratigraphic and tectonometamorphic slices
of still debatable origin (Pamić & Tomljenović 1998; Herak
1999; Haas et al. 2000; Haas & Kovács 2001; Pamić 2002,
2003). The low-grade metamorphic unit of Early Aptian age
(Belak et al. 1995) are derived from the protoliths of Silurian
to Ladinian volcano-sedimentary successions and from Mid-
dle Jurassic—Lower Cretaceous island arc volcanics (Lugović
et al. 2006, and references therein). In the recent structural
assemblage this low-grade metamorphic complex thrusts
over the ophiolite mélange. The accretion age of the ophio-
lite mélange was constrained by palynomorph assemblages
as Middle Jurassic to Hauterivian (Babić et al. 2002). Both
units are covered by Cretaceous-Paleocene alluvial fan to
flysch sequences. These successions form the base of the
transgressive Neogene sedimentary sequence composed of
Miocene limestones, clastics and marls. The pre-Neogene Mt
Medvednica basement is believed to have experienced long
distance transport from the NW and was rotated during the
Oligocene by ca. 100° CW to the alignment perpendicular to
the Dinaric structural trend in the SE (Tomljenović 2002;
Tomljenović et al. 2008) (Fig. 1A).
Within the pelitic-siltous matrix the Mt Medvednica ophi-
olite mélange archives variously sized rock fragments of
very different tectonic settings. Gabbroic cumulus and iso-
tropic rocks form decimeter to hectometer large fragments
within the Mt Medvednica ophiolite mélange (Fig. 1B).
Large gabbroic blocks are locally intersected by gabbropeg-
matite veins, or may contain their segregations and ooze.
Most rock fragments are not suitable for paleontological or
geochemical dating (Halamić 1998).
The oldest blocks in the mélange are olistoliths of Illyri-
an-Fassanian pelagic limestones associated with MORB-
pillow lavas (Halamić et al. 1998) and, fragments of Middle
to Upper Triassic limestones interlayered with radiolarian
cherts (Halamić & Goričan 1995). The uppermost Bajocian
to Lower Callovian radiolarian cherts are also found in the
Mt Medvednica ophiolite mélange together with MORB-
pillow lavas as slices which were assumed to record an oce-
anic ridge relationship (Halamić et al. 1999). The age span
of the K-Ar data (196—179 Ma) performed on the MORB-
type gabbro and dolerite (Pamić 1997) revealed that the
oceanic crust formed within the same paleogeographical
domain also between the Pliensbachian and Bajocian. The
Middle Jurassic to Hauterivian ophiolitic mélange as a tec-
tonic formation was involved in Aptian to post-Paleocene
emplacement onto the eastern continental margin of the
Adria plate (Pamić & Tomljenović 1998; Pamić 2002).
However, the ophiolite emplacement which resulted in ob-
duction of mantle peridotite took place before the Senonian
as is clearly seen from the mantle peridotite clasts docu-
mented in the Campanian basal conglomerates (Halamić
1998) and by subaerial weathering to the Ni-lateritic crust
(Palinkaš et al. 2006).
Fig. 1. A – Geotectonic sketch map of the Alps, Dinarides and Hellenides showing 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 Dinar-
ide 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 – Panonian Basin. Faults: BL – Balaton; DF – Drava; PL – Periadriatic; SF – Sava;
SP – Skadar-Peć; SN – Sava Nape; VF – Vardar; ZZ – Zagreb-Zemplin. Mountains: I – Ivanščica; K – Kalnik; Ko – Kopaonik;
Md – Medvednica; SG – Samoborska Gora; SD – Szarvaskő-Darnó; B – Bódva valley; JK – Jaklovce. B – Simplified geological map
of Mt Medvednica (modified after Halamić 1998). Legend: 1 – Neogene and Pleistocene sedimentary rocks; 2 – Late Cretaceous-Paleocene
flysch including Senonian carbonate breccias; 3 – ophiolite mélange with blocks of: 4 – basalt intersected by dolerite dikes, 5 – gabbros,
6 – cumulate peridotites, 7 – Jurassic radiolarites and shales with olistoliths composed of basalts; 8 – Alb-Cenomanian limestones and clas-
tic rocks (shale, siltite and sandstone); 9 – Lower Cretaceous metamorphic complex; 10 – reverse or thrust faults; 11 – normal faults; 12 –
geological contact line; 13 – main creeks; 14 – sample location (1 = mc-16; 2 = mc-pg; 3 = vs-801; 4 = vs-335; 5 = m-16/2; 6 = vs-331;
7 = vs-367; 8 = vs-386; 9 = vk-298; 10 = mc-2/4; 11 = rn-13; 12 = vs-494; 13 = vh-617; 14 = vs-578).
279
GEOCHEMISTRY AND TECTONIC SETTING OF THE AMPHIBOLE GABBROIC ROCKS (NW CROATIA)
Analytical techniques
Eight samples were selected for the analysis of mineral
chemistry. The minerals were analysed with Camebax SX51
microprobe equipped with five wavelength spectrometers at
the Mineralogisches Institut, Universität Heidelberg. The el-
ements were measured by WDS with an accelerating voltage
of 15 kV, beam current of 20 nA, ~ 1 µm beam size and 10 s
counting time for all elements. For feldspars an analysis
beam size of 10 µm was used. Natural oxides (corundum,
spinels, hematite and rutile) and silicates (albite, orthoclase,
anorthite and wollastonite) were used as standards and for
calibration. Raw data for all analyses were corrected for ma-
trix effects with the PAP algorithm (Pouchou & Pichoir
1984, 1985) implemented by CAMECA. Calculations of the
structural chemical formulas were done using a software
package authorized by Hans-Peter Mayer (Mineralogisches
Institut University of Heidelberg).
The total of twelve representative samples were crushed in
a steel jaw. After splitting, rock chips free of visible vein and
crack fillings were ground in an agate ring-disc mill, and
powders were then dried at 110 °C. Three samples were
analysed for the major elements and trace elements Rb, Ba,
Sr, Zr, Y and Cr at XRAL Laboratories at Toronto (Canada)
by wavelength dispersive X-ray fluorescence (WDS XRF)
on lithium borate fusion pellets using international reference
samples for calibration. Trace elements (Th, Nb, Ta, Hf, Sc,
V, Co, Ni, Zn and REE) of those samples were determined
by inductively coupled plasma mass spectrometry (ICP-MS)
at ACME Laboratories in Vancouver, Canada. Another set of
nine samples were analysed by ICP for major elements, and
ICP-MS for all trace elements at Actlab Laboratories in An-
caster, Canada.
Amphibole separate from gabbropegmatite (sample mc-
pg) was prepared by crushing and sieving the grain fraction
200—400
µm followed by magnetic separation and final
handpicking under stereomicroscope. The amphibole sepa-
rate was wrapped in aluminium foil and stacked in an irradi-
ation capsule with similar-aged samples and neutron flux
monitors (Fish Canyon Tuff sanidine). Samples were irradi-
ated at the McMaster Nuclear Reactor in Hamilton, Ontario
and analysed at the Noble Gas Laboratory Pacific Centre for
Isotopic and Geochemical Research, University of British
Columbia, Vancouver, Canada. The samples were step-heat-
ed at incrementally higher powers in the defocused beam of
a 10 W CO
2
laser until fused. The gas evolved from each
step was analysed by a VG5400 mass spectrometer equipped
with an ion-counting electron multiplier.
Petrography and mineral chemistry
The gabbroic rocks of Mt Medvednica are medium- to
coarse-grained (1—4 mm) and may contain 1—5 cm large
grains in the gabbropegmatite. The rocks preserve igneous
fabric in spite of being successively altered. Amphibole gab-
bro dominates over amphibole- olivine-gabbro and minor
gabbropegmatite. The rocks are isotropic and only a few
samples show heteradcumulate poikilitic texture with large
oikocrystals of brown-reddish amphibole enclosing fresh cu-
mulus clinopyroxene (Fig. 2A) or albitized plagioclase
(Fig. 2B). Occasionally, olivine pseudomorphosed by ser-
pentine and magnetite is also embedded by amphibole. The
crystallization sequence includes olivine, augite, plagioclase,
reddish-brown amphibole and Fe-Ti oxide (magnetite-ul-
vöspinel). Plagioclase forms oikocrystals in cumulate oliv-
ine-gabbros. Poikilitic embedding of plagioclase and
amphibole suggests their cotectic crystallization. Detailed
petrography of these rocks may be found in Slovenec (1998).
The cumulus clinopyroxene embedded in the amphibole
oikocrysts (see Fig. 2) ranges in composition from augite to
diopside (Wo
42—49
En
42—50
Fs
6—14
; Fig. 3). A reaction relation
of augite with enclosing amphibole was not observed. The
clinopyroxene from the gabbroic cumulates shows Mg# of
75.8—88.9 suggesting that the most evolved composition
fractionated from a melt having Mg# of around 39 [calculat-
ed on K
d
( = cpx/bulk rock FeO
tot
/MgO molar ratio) of 0.20
after Grove et al. (1982); Baker & Eggler (1987)]. Abun-
dances of Ti, Al, Cr and Na are low ( < 0.64 wt. % TiO
2
;
Fig. 2. Back-scattered electron image of (A) cumulus gabbro, sample
mc-16, showing large oikocrystal of zoned tschermakite-magnesio-
hornblende enclosing cumulus clinopyroxene surrounded by various
alterations minerals. The numbers correspond to the microprobe spot
analyses as indicated in the tables of mineral chemistry (for example:
1 – tschermakite, 2 and 3 – magnesiohornblende, 4 – actinolite).
Legend: Ab = albite; Amp = amphibole; Cpx = clinopyroxene; Prh =
prehnite. (B) cumulus gabbro, sample vh-617, showing albitized cu-
mulus plagioclase (spots 3 and 4) enclosed in large intercumulus
oikocrystal of magnesiohornblende (spots 1 and 2).
280
SLOVENEC and LUGOVIĆ
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 gab-
broic rocks from the Mt Medvednica ophiolite mé-
lange. Fields for clinopyroxene compositions from iso-
tropic gabbros of Szarvaskő Ophiolite Complex (Balla
& Dobretsov 1984) and basalts from the Jaklovce For-
mation (Hovorka & Spišak 1988; Ivan 2002) plotted
for correlation constraints.
Fig. 4. Discriminant diagram (A) Ti—Al
IV
and (B) SiO
2
/100—Na
2
O—TiO
2
(simplified after Beccaluva et al. 1989) for clinopyroxene from
the Mt Medvednica gabbros. For convenience cumulus pyroxenes are also plotted although they were not considered in geotectonic con-
straints. IAT = island-arc tholeiites, MORB = mid-ocean ridge basalts, BARB = back-arc ridge basalts. Symbols and fields as in the Fig. 3.
< 2.41 wt. % Al
2
O
3
; < 0.51 wt. % Cr
2
O
3
; < 0.38 wt. % Na
2
O;
Table 1). The clinopyroxene from isotropic gabbros has a
composition of Wo
39—48
En
40—49
Fs
8—19
, similar to the composi-
tion of cumulus clinopyroxene (Fig. 3; Table 1). However,
they are slightly enriched in all non-quadrilateral compo-
nents (Fig. 4A,B) except Na
2
O which is identical
( < 0.97 wt. % TiO
2
; < 4.16 wt. % Al
2
O
3
; wt. % 1.01 Cr
2
O
3
;
Table 1). The Al
VI
/Al
IV
ratio of cumulus clinopyroxene does
not exceed 0.73 which is typical of clinopyroxene from low
to medium pressure igneous rocks (Aoki & Kushiro 1968;
Wass 1979). The Al
VI
/Al
IV
ratio of clinopyroxene from iso-
tropic gabbros is low ( < 0.56), consistent with their position
higher in the ophiolite pile.
All analysed amphiboles show Mg# [ = Mg/(Mg + Fe
2 +
)] of
0.591—0.868 and belong to the Ca-amphibole group (Ta-
ble 2; Fig. 5). Magnesiohornblende oikocrystal embaying al-
bitized plagioclase in a cumulus gabbro (Fig. 2B) has
homogeneous composition across the grain (Table 2) whilst
amphibole oikocrystal in the sample mc-16 (Fig. 2A) shows
three generations rimward: (1) reddish-brown igneous tscher-
makite enclosing cumulus augite, (2) brown magnesiohorn-
blende as the late stage crystallization to deuteric product,
and (3) pale green actinolite as low temperature alteration of
brown magnesiohornblende (Table 2, Fig. 2A and Fig. 5).
The zoning pattern of a tschermakite from the gabbropegma-
tite showing decreasing Ti, Al and Na towards the grain pe-
riphery (Fig. 6) is typical of fractional crystallization.
Amphiboles (1) and (2) in the sample mc-16 resemble the
fractionation pattern of tschermakite megacryst from the
gabbropegmatite confirming their igneous origin. Tscherma-
kite in the cumulus rocks is characterized by high Ti and Al
contents compared to the late magmatic magnesiohorn-
blende (2.84—3.77 wt. % TiO
2
vs. 1.22—2.66 wt. % TiO
2
and
9.43—11.14 wt. % Al
2
O
3
vs. 5.52—8.80 wt. % Al
2
O
3
), and par-
281
GEOCHEMISTRY AND TECTONIC SETTING OF THE AMPHIBOLE GABBROIC ROCKS (NW CROATIA)
Table 1: Selected microprobe analyses and formulae of clinopyroxene from the gabbroic rocks in the Mt Medvednica ophiolite mélange.
Fig. 5. Al
IV
—(Na + K)
A
plot of amphiboles from the Mt Medvednica
gabbroic rocks with the nomenclature fields of Leake et al. (1997).
Amphiboles from the Mt Medvednica ultramafic cumulates
(Lugović et al. 2007) are shown for comparison (shaded fields).
Fig. 6. Compositional zoning patterns of a tschermakite megacryst
from the Mt Medvednica gabbropegmatite (sample mc-pg). All spot
analyses from the profile resembling alteration composition were omit-
ted for simplicity. Oxides are in wt. % and grain size in millimeters.
ticularly to the low temperature alteration amphiboles con-
fined at the grain periphery (Table 2). In the isotropic gabbros
igneous magnesiohornblende was preserved only in the sam-
ples vs-617 and vs-331 showing similar composition to the
magnesiohornblende from cumulus rocks (Table 2, Fig. 5).
Most measured feldspars show albite composition (An
0.2—2.7
;
Table 3). Relic igneous plagioclase in cumulate gabbros
shows the most Ca-rich composition (An
88.7—92.9
). In the iso-
tropic gabbros plagioclase is systematically lower in Ca and,
although homogeneous, individual grains in a sample may
show significant composition variations (An
23.6—59.9
in the
sample vs-578).
Magmatic Fe-Ti-oxides are magnetite-ulvöspinel solid so-
lutions (Table 4). In most samples they now contain a net of
very fine exsolved lamellae and patches. EPMA spot mea-
surement yields mixed analyses for the exsolved minerals
which may be interpreted as magnetite and pseudobrookite.
The Mt Medvednica gabbroic rocks are severely altered
showing two distinct alteration assemblages. The chemical
Formulae calculated on the basis of 4 cations and 6 oxygens. CG = cumulus gabbro, IG = isotropic gabbro. Mg# = 100*Mg/(Mg + Fe
2+
).
282
SLOVENEC and LUGOVIĆ
Table
2:
Selected
microprobe
analyses
and
formulae
of
amphibole
from
th
e
gabbroic
rocks
in
the
Mt
Medvednica
ophiolite
mélange.
compositions of secondary amphiboles are
displayed in Table 2, other secondary miner-
als in Table 4. Older alteration is typical of
the greenschist facies and includes albite, acti-
nolite plus chlorite (clinochlore, penninite and
diabantite) and serpentine after magmatic pla-
gioclase, clinopyroxene and/or tschermakite-
magnesiohornblende and cumulus olivine.
Subsequent alteration comprises prehnite,
pumpellyite, titanite and muscovite (sericite).
Pumpellyite is Al-rich (Table 4), typical of
pumpellyite after plagioclase (Aldahan 1989;
Izhizuka 1991) indicating alteration grade be-
tween zeolite and greenschist facies. Titanite
shows a high rate of Al and lower of Fe
3+
sub-
stitution (Table 4), consistent with the compo-
sition of titanite from prehnite-pumpellyite
facies alterations (Coombs et al. 1976; Mével
1981).
Bulk-rock chemistry
Chemical analyses of the Mt Medvednica
gabbroic rocks are shown in Table 5. High
values for LOI (2.12—4.52 wt. %) combined
with petrographic evidence suggest that the
igneous composition of the rocks may have
been changed through successive alterations
including weathering.
In spite of the alterations, all analysed rocks
plot in the field of sub-alkali gabbros
(Fig. 7A). The diagram TiO
2
—Al
2
O
3
(Fig. 7B)
distinguishes between cumulates and basaltic
liquid composition, which is more suitable for
altered rocks. Cumulus rocks resemble the
composition of olivine gabbros and trocto-
lites, whereas isotropic rocks plot in the field
of clinopyroxene-plagioclase gabbros. The
rocks have low CaO/Al
2
O
3
ratio (0.44—0.82)
typical of ophiolitic gabbros (Werner 1984).
In the AFM diagram (Fig. 8) cumulates occu-
py the field of Mg-gabbros while isotropic
gabbros are scattered in the field of ophiolitic
basalt suggesting that the latter rocks may
represent liquid composition. High Mg#
(76.8—85.3 vs. 62.3—75.6) and wide range of
Al
2
O
3
(10.71—16.68 wt. %
vs.
15.19—
17.10 wt. %) in the cumulate rocks relative to
the isotropic varieties (Table 5) are strongly
controlled by the modal ratio of cumulus min-
erals and intercumulus amphibole in the
former, and by similar modal mineral abun-
dances in the latter.
Crystallization of amphibole in arc-related
cumulates will not deplete the liquid in Zr,
but will in the Sc, Ti and V (Meurer & Clae-
son 2002) and therefore the slight increase in
Zr in the Mt Medvednica isotropic gabbros
Formulae
calculated
on
the
basis
of
23
oxygens
and
fixed
number
of
13
cations
excluding
Ca,
Na
and
K
(6),
15
cations
excluding
Na
and
K
(5)
or
15
cations
excluding
K
(4)
to
match
the
best
amphibole
crystalochemical
parameters.
Estimated
H
2
O
corresponds
2
(OH)
per
formula
unit.
Mg#
=
Mg/(Mg
+
Fe
2+
).
CG
=
cumulate
gabbro,
IG
=
isotropic
gabbro,
GPG
=
gabbropeg
matite.
283
GEOCHEMISTRY AND TECTONIC SETTING OF THE AMPHIBOLE GABBROIC ROCKS (NW CROATIA)
Table 3: Selected microprobe analyses and formulae of feldspars from the gabbroic rocks in the Mt Medvednica ophiolite mélange.
Table 4: Selected microprobe analyses and formulae of chlorite, prehnite, pumpellyite, magnetite-ulvöspinel, muscovite and titanite from
the gabbroic rocks in the Mt Medvednica ophiolite mélange.
Formulae calculated on the basis of 8 oxygens and total Fe as trivalent. CG = cumulus gabbro, IG = isotropic gabbro, GPG = gabbropegmatite.
An = 100*Ca/(Ca + Na + K).
Mgt = magnetite-ulvöspinel, Ms = muscovite, Tnt = titanite. CG = cumulus gabbro, IG = isotropic gabbro, GPG = gabbropegmatite. Formulae cal-
culated on the basis of 14 oxygens and all Fe as divalent for chlorite; 11 oxygens and all Fe as trivalent for prehnite; 12.5 oxygens and all Fe as
trivalent for pumpellyite; 4 oxygens and 3 cations for magnetite-ulvöspinel; 11 oxygens and all Fe as divalent for muscovite; 5 oxygens and all Fe
as trivalent for titanite. H
2
O is calculated and corresponds to 8 (OH), 2 (OH), 3 (OH) and 2 (OH) per formula unit in chlorite, prehnite pumpellyite
and muscovite, respectively. Mg = 100*Mg/(Mg + Fe
2+
).
284
SLOVENEC and LUGOVIĆ
(Table 2) may reflect clinopyroxene fractionation. This is
corroborated by the Sc/Yb ratio ranging from 9 to 73. Thus,
we used Zr as a potential differentiation index to test the be-
haviour of selected compatible elements (Sc, V, Ni and Cr)
and incompatible elements (Ti and La) in the analysed rocks
(Fig. 9A—F). In the isotropic gabbros Sc is negatively corre-
lated with Zr (Fig. 9A) whilst V and Ti tend to be positively
correlated (Fig. 9B and 9C). Since V and Ti are highly com-
patible with clinopyroxene in tholeiitic melt their inconsis-
tent behaviour in the analysed rocks suggests that the
distribution of V is also strongly controlled by the crystalli-
zation of coexisting Fe-Ti-oxide. The amount of Ni decreas-
es in the evolved isotropic gabbros due to cessation of
olivine fractionation (Fig. 9D). The low concentration of Cr
Fig. 7. (A) TAS classification diagram with the nomenclature fields of intrusive rocks after Cox et al. (1979) and (B) Al
2
O
3
—TiO
2
discrimi-
nation diagram (adopted from Colombi 1989) for the gabbroic rocks from the Mt Medvednica ophiolite mélange. Symbols as in the Fig. 5.
Fig. 8. A—F—M [(Na
2
O + K
2
O)—FeO
total
—MgO] diagram (from Bian-
chi et al. 1998, slightly modified by Colombi 1989) for the gabbroic
rocks from the Mt Medvednica ophiolite mélange. Symbols as in
the Fig. 5.
in the isotropic gabbros (Fig. 9E) reflects the high partition
of Cr in spinel and clinopyroxene of underlying cumulus ul-
tramafic rocks (Lugović et al. 2007). La is strongly positive-
ly correlated with Zr (Fig. 9F) suggesting that La was not
significantly affected by alterations. In short, the HFSE and
REE may be used in petrogenetic and geotectonic con-
straints.
N-MORB normalized multielement concentrations of the
gabbroic rocks from the Mt Medvednica are displayed in the
spider diagrams (Fig. 10A). The samples show high LILE
(Cs, Ba, Rb, K) enrichment and, excluding strong Sr spike,
have a nearly flat profile for more compatible elements with
higher relative concentrations in isotropic gabbros, which is
consistent with their higher degree of fractionation. Com-
pared to the compatible elements, the LILE could be affected
during alterations and weathering. We plotted LILE concen-
trations against Zr and found scattered distributions (not
shown) suggesting that the LILE were not enriched to such
an extent only by magmatic processes. It particularly holds
true for Sr, at least for the isotropic gabbros, since they show
early plagioclase fractionation through negative Eu anomaly
(Fig. 10B2). Amongst the HFSE, Nb and Ta show significant
negative anomalies for cumulate and isotropic gabbros [(Nb/
La)
MORBN
= 0.40—0.72 and 0.28—0.35, respectively] typical for
subduction related rocks. Ti may be relatively enriched or de-
pleted (Fig. 10A). The cumulate and early fractionated isotro-
pic gabbros have low abundances of Ti (0.15 wt. % TiO
2
in
the sample vs-386 vs. 0.47 wt. % TiO
2
in the sample vs-494)
resulting from Ti enrichment in the amphibole oikocrystals of
underlying ultramafic cumulates (Lugović et al. 2007) which
is typical of suprasubduction cumulate gabbros (Saunders et
al. 1980). Positive Ti anomalies in late isotropic mafic rocks
(1.44 wt. % TiO
2
in the sample vs-331) are connected to the
onset of Fe-Ti-oxides (Elthon 1991).
The REE contents of the gabbroic rocks from the Mt
Medvednica normalized to N-MORB concentrations are
285
GEOCHEMISTRY AND TECTONIC SETTING OF THE AMPHIBOLE GABBROIC ROCKS (NW CROATIA)
Fig. 9. Variation diagrams for selected elements with Zr as an index of the fractionation index for the Mt Medvednica gabbroic rocks. A –
Sc—Zr, B – V—Zr, C – TiO
2
—Zr, D – Ni—Zr, E – Cr—Zr and F – La—Zr. Symbols as in the Fig. 5.
Fig. 10. N-MORB-normalized (A) multielement and (B) REE patterns for the Mt Medvednica gabbroic rocks. Normalization values are
from Sun & McDonough (1989).
286
SLOVENEC and LUGOVIĆ
Table 5: Chemical analyses of gabbroic rocks from the Mt Medvednica ophiolite mélange.
Table 6:
40
Ar/
39
Ar data of gabbropegmatite amphibole from the Mt Medvednica ophiolite mélange.
Major elements in wt.%, trace elements in ppm. GPG = gabbropegmatite. LOI = loss on ignition at 1100
°C. Mg# = 100*molar MgO/
(MgO+FeO
total
). Analyses obtained at Actlabs laboratories indicated by the italic sample lebels.
a
Corrected for background (mean values in mol: m/e40 = 1.4
× 10
—16
; m/e39 = 7.6
× 10
—17
; m/e38 = 3.5
× 10
—17
; m/e37 = 5.4
× 10
—17
; m/e36 =
5.5
× 10
—17
), mass discrimination (measured
40
Ar/
36
Ar = 293.5±0.5), abundance sensitivity (5 ppm), and radioactive decay.
b
Normalized to
100% delivery to mass spectrometer.
c
Includes static blank.
d
Corrected for atmospheric argon and nucleogenic interferences
40
Ar/
39
Ar
K
= 0.0306;
36
Ar/
37
Ar
Ca
= 0.00027;
39
Ar/
37
Ar
Ca
= 0.00077). J-factor = 0.004683 (assumed Fish Canyon Tuff sanidine = 28.02 Ma; Renne et al. 1998).
287
GEOCHEMISTRY AND TECTONIC SETTING OF THE AMPHIBOLE GABBROIC ROCKS (NW CROATIA)
shown in Fig. 10B. The normalizing REE patterns are parallel
at different concentration levels with the higher level in the
isotropic gabbros suggesting persistently constant modal com-
position of the fractioned phases and, consequently, accumu-
lation by different settling rate rather than by change in the
settling mineral assemblage. All samples show REE patterns
transitional between mid-ocean ridge and island arc plutonic
rocks. The cumulus gabbros have a slightly depleted HREE
profile whereas the early fractionated isotropic gabbros show
an almost flat HREE profile [(Tb/Lu)
MORBN
from 0.76 to 0.91
vs. 0.94—1.13]. The evolved isotropic gabbros display con-
cave-down HREE profile. The cumulus rocks exhibit flat to
slightly LREE enriched profiles [(La/Nd)
MORBN
= 0.85 to
1.05], while the isotropic gabbros show pronounced LREE en-
richment [(La/Nd)
MORBN
= 1.09—1.29]. The concave-down
shaped MREE profile of the REE patterns reflects amphibole
fractionation through the cumulate sequence. The intensity of
the Eu-anomaly (Eu/Eu*) decreases from the early cumulates
(1.07—1.18) through isotropic gabbros down to 0.86.
40
Ar/
39
Ar dating of gabbropegmatite
The analytical data from laser incremental heating of the
amphibole separate are detailed in Table 6 and
39
Ar release
spectra are shown in Fig. 11. The apparent ages increase
over the first heating step to attain maximum apparent ages
for the middle half of the age spectrum and then decrease
over the last few steps. Plateau age calculated by the criteria
of Ludwig (2003) is 161.1 ± 2.1 Ma. We interpreted the pla-
teau age as the age of gabbropegmatite crystallization. This
calendar age of gabbropegmatite crystallization corresponds
almost exactly to the boundary between the Callovian and
Oxfordian (see in Ogg 2004).
Discussion
The mafic cumulate fragments from the Mt Medvednica
ophiolite mélange comprise amphibole olivine gabbro and
amphibole gabbro. Amphibole gabbro intersected by minor
Fig. 11.
40
Ar/
39
Ar apparent age diagram of gabbropegmatite am-
phibole separate from the Mt Medvednica ophiolite mélange.
amphibole pegmatitic gabbro dominates amongst isotropic
(noncumulate) rocks. On account of cumulate rock mineral
composition and rock type the Mt Medvednica mafic intru-
sive association is an analogue of the Type III arc cumulate
suite defined by Beard (1986) from arc intrusive complexes
of Bear Mountain, Adak and Lesser Antilles. The observed
crystallization sequence is olivine, augite-diopside, Ca-pla-
gioclase and minor magnetite-ulvöspinel as cumulus phase
followed by tschermakite-magnesiohornblende and plagio-
clase as intercumulus fillings (Fig. 2A and 2B). The crystal-
lization sequence is typical of fractionation of tholeiite
basalts under low to medium pressure (Serri 1980) in supra-
subduction settings and is atypical for mafic rocks from an
ocean ridge (Pearce et al. 1984). Maximum crystallization
pressure and temperature were inferred as 0.55 GPa and
850 °C, respectively, using semiquantitative thermobarome-
ter calibrated by Ernst & Liu (1998) based on the Al- and Ti
contents in tschermakite. This all suggests that the magma
crystallized Mt Medvednica amphibole gabbro probably un-
derwent fractional crystallization in a high-level magma
chamber.
Abundant tschermakite-magnesiohornblende throughout
the gabbroic sequence as well as in the ultramafic cumulates
from Mt Medvednica (Lugović et al. 2007) indicates volatile
influenced early crystallization. In texture and chemical
composition these amphiboles undoubtedly resemble am-
phiboles defined as magmatic in the ophiolitic gabbros (e.g.
Coogan 2003). The ophiolitic mafic-ultramafic rocks associ-
ations crystallized from volatile-rich magmas are almost ex-
clusively found in subduction zones, either in island arcs or
continental margins (Conrad & Kay 1984; DeBari & Cole-
man 1989; Claeson & Meurer 2004; Kocak et al. 2005) sug-
gesting the formation of the Mt Medvednica ophiolite
plutonic sequences in an analogue of recent/ancient supra-
subduction setting. The normal zoning of igneous tscherma-
kite (Fig. 6) indicates crystallization in a closed system
(Stern 1979). The relatively low Ti- and Al abundances and
apparent chemical homogeneity of cumulus augite suggest
slow cooling at elevated pressure.
However, amphibole gabbros may also occur on the mid-
ocean ridges. In the MORB setting, formation of Ti-par-
gasite-tschermakite is confirmed in the late-magmatic
evolution of an intrusive sequence as low abundant intersti-
tial feelings, blebs and as replacive or vein minerals (Tribu-
zio et al. 2000; Coogan et al. 2001). In suprasubduction
ridge ophiolites like at Oman, magmatic amphibole may be
abundant in cumulate gabbros (Coogan 2003) or in gabbroic
dikes as large poikilitic patches associated with similar or-
thopyroxene and clinopyroxene patches (Bosch et al. 2004).
The Mt Medvednica amphibole cumulates and isotropic gab-
bros are thus in this respect more akin to the latter.
The peculiar lithology of the Mt Medvednica gabbroic suite
is pegmatitic gabbro composed of secondary albite and well
preserved igneous tschermakite. Such pegmatitic occurrences
were repeatedly attributed to the interaction of magma with
seawater infiltrated down to the magma chamber, which in the
case of the Trinity Ophiolite Complex (California) was as
deep as close to Mohorovičić discontinuity (Boudier et al.
1989). However, for augite-tschermakite gabbropegmatites
288
SLOVENEC and LUGOVIĆ
hosted in gabbro from Szarvaskő Ophiolite Complex, Bükk
Mountains in NE Hungary, assimilation of sedimentary rocks
increasing volatile content in the gabbro magma chamber was
attributed to cause local crystallization of gabbropegmatite
(Péntek et al. 2006). This gabbropegmatite shows deuteric and
low temperature amphibole alterations similar to Mt Medved-
nica gabbropegmatite but along with host gabbros shows less
intensity of albitization. We have found Boudier et al.’s
(1989) explanation more acceptable for the Mt Medvednica
oceanic crust and, moreover, suggest that the sea water,
warmed during downward percolation through the crust, is
also responsible for the severe alterations, particularly albiti-
Fig. 12. Discrimination diagrams for the gabbroic rocks from the Mt Medvednica ophiolite mélange. A – TiO
2
—FeO
total
/
(FeO
total
+MgO) diagram (Serri 1981). B – V—Ti/1000 diagram (Shervais 1982). 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. C – Th—Hf/3— Nb/16 diagram (Wood 1980). N-MORB = normal mid-ocean ridge basalts, E-MORB = enriched MORB,
WPT = within plate tholeiites, WPB = alkaline within plate basalts, CAB = calc-alkaline basalts and IAT = island-arc tholeiites. D – (Nb/
Zr)
pm
—(La/Sm)
pm
diagram (Révillon et al. 2000). Normalized values for primitive mantle (pm) are from Hofmann (1988). Symbols and fields
as in the Fig. 3. Fields for isotropic gabbros of Szarvaskő Ophiolite Complex (Aigner-Torres & Koller 1999 and Downes et al. 1990) and ba-
salts from the Jaklovce Formation (Ivan 2002) are plotted for correlation constraints.
zation of the entire rock pile to the mineral assemblage ana-
logue of greenschist facies. The prehnite-pumpellyite facies
alteration is clearly successive to the greenschist facies miner-
al paragenesis and we address its formation to the ophiolite
emplacement. In the Szarvaskő gabbropegmatite Péntek et al.
(2006) inferred a temperature of 250—400 °C for sea-floor al-
teration paragenesis, and 270—285 °C at 0.15—0.2 GPa for the
prehnite-pumpellyite facies alteration assemblage.
In severely altered ophiolitic rocks clinopyroxene is often
the only mineral that preserves an igneous composition. This
fact was proved to introduce clinopyroxene as a robust
geochemical tracer of the original tectonic setting of the host
289
GEOCHEMISTRY AND TECTONIC SETTING OF THE AMPHIBOLE GABBROIC ROCKS (NW CROATIA)
rock (Beccaluva et al. 1989). Clinopyroxene from the Mt
Medvednica amphibole gabbros plotted in discriminatory dia-
grams shows clear compositional correspondence with cli-
nopyroxenes from modern ocean island arc magmatic rocks
(Fig. 4A and 4B). The composition of the relic plagioclase in
the Mt Medvednica cumulus gabbros (Table 3) is consistent
with clinopyroxene composition. Such high Ca-plagioclase
coexisting with amphibole is typically found in the island arc
cumulates (e.g. Burns 1985) as a consequence of crystalliza-
tion from water saturated melts (Arculus & Wills 1980).
Many discriminant diagrams were introduced to distin-
guish between ophiolitic volcanic rocks from different tec-
tonic settings (Fig. 12). The diagrams are also used for
discrimination of plutonic rock but, only if they represent the
melt, that is they are not cumulates. The diagrams were de-
vised for fresh rocks of known tectonic position and should
be used with caution for altered rocks. Since the Mt Medved-
nica gabbroic rocks are severely altered we only use ele-
ments which were proved to resemble magmatic
interelement ratios. The majority of the analysed rocks corre-
spond to high Mg-gabbros (Fig. 8) and show geochemical
trend typical of low-Ti gabbro suites (Fig. 12A). Low-Ti
gabbros are assumed to be highly representative of supra-
subduction ophiolites and particularly of island arc ophio-
lites (Serri 1981). The island arc affinity of the Mt
Medvednica gabbros is confirmed in the classical discrimi-
nation diagram Ti-V (Shervais 1982) where the gabbros plot
within the field of island arc tholeiites (Fig. 12B). However,
beside gabbropegmatite, one gabbro (sample vs-331) is Ti-
and V-enriched and therefore more akin to the ocean ridge
gabbro. Since this sample contains an unusually high
amount of exsolved Fe-Ti-oxides (former magnetite-ul-
vöspinel) it is interpreted as an integral part of the supra-
subduction suite rather than ridge gabbro. The island arc
environment of analysed rocks is fully confirmed by their
plot in the diagram Th—Hf/3—Nb/16 (Fig. 12C) where the
analysed rocks plot in the tholeiitic field of island arc mag-
matic rocks. Mantle wedge source for these rocks was only
slightly depleted (Fig. 12D), which is also suggested by the el-
ement relations from the REE patterns (Fig. 10). Transitional
harzburgites representing mantle residium after approximately
20% partial melting exposed near Gornje Orešje several kilo-
meters to the east of the Mt Medvednica ophiolite mélange
(Lugović et al. 2007) appear to be the best candidate for the
mantle wedge of the Mt Medvednica island arc.
Ophiolite mélanges from the SW ZMTDZ exposed in the
Mts Medvednica, Kalnik and Ivanščica (Fig. 1) were regard-
ed by Haas et al. (2000) as a single tectonostratigraphic unit,
namely the Kalnik Unit (KU). We assumed that the KU con-
tains ophiolitic rock fragments of a discrete Mesozoic ocean-
ic domain, hereafter termed the Repno oceanic domain
(ROD), as originally proposed by Babić et al. (2002). The
formation of MORB-type oceanic crust in the ROD com-
menced in the late Ladinian (Halamić et al. 1998; Goričan et
al. 2005) and is clearly traced during the Upper Triassic
(Halamić & Goričan 1995). The formation of MORB-type
gabbros and dolerites continued from Pliensbachian to the
Bajocian as documented by K-Ar bulk-rock isotopic ages
(Pamić 1997). The youngest MORB-type crust may be rec-
ognized in the pillow lavas exposed adjacent to the Lower
Callovian cherts (Halamić et al. 1999). If the ophiolitic rock
fragments from the Mt Medvednica mélange resemble rem-
nants of the same slow-spreading ( ~ 1.5 mm/yr) oceanic do-
main which was continously producing oceanic crust from
the late Ladinian to the Early Callovian, then oceanic crust
about 900 km wide must have been formed.
An intraoceanic subduction leads to formation of an accre-
tionary wedge in front of a fore-arc—arc system (Wilson
1989). The commencement of intra-oceanic subduction in
the ROD is not clear. Most probably subduction/accretion-
ary wedge started to form soon after Bajocian-Early Callov-
ian when the youngest ridge crust was documented (Pamić
1997; Halamić et al. 1999). The accretionary wedge in the
ROD corresponds to the KU sensu Haas et al. (2000) and to
the chaotic Repno Complex sensu Babić et al. (2002). The
age of the Repno Complex was constrained to Early Callov-
ian to Late Valanginian (Babić et al. 2002) and the lower age
matches well the assumed age of the cessation of ridge crust
formation. However, Babić et al. (2002) do not link the Rep-
no Complex to an intra-oceanic setting but to the “eastern”
continental margin (Tisia?). The existence of a post Bajo-
cian-Lower Callovian fore-arc, that is a proto-arc sensu
Woodhead et al. (1998) in the ROD may be envisaged from
the ultramafic cumulate rock fragments archived in the Mt
Medvednica ophiolite mélange and transitional tectonite
peridotites from Gornje Orešje (Lugović et al. 2007). During
the time of around 15 Ma, a fore-arc progrades to an island
arc with coeval cognate back-arc basin (Stern & Bloomer
1992; Bloomer et al. 1995). Following this model we link
the Mt Medvednica gabbropegmatite, which crystallized
161.1 Ma ago, to an early suprasubduction stage, namely to
the proto-arc setting. Our amphibole gabbroic rocks clearly
testify for the progradation of that proto-arc and formation of
ophiolites in an island arc setting. The ROD island arc never
prograded to maturity as it may be concluded from the total
absence of calc-alkaline rocks in the mélange. We adopt the
age span of Babić et al. (2002) for the KU but doubt its pro-
posed setting. Assuming our approach is correct, the age of
duration of the subduction factory producing the ROD proto-
arc—island arc ophiolites may be extended to the Late Val-
anginian. Cognate back-arc ophiolites were not recognized
among the ophiolitic rock fragments in the ROD yet.
The closure of the ROD arc—back-arc system was complet-
ed rapidly during the Barremian—Aptian by the obduction of
the ROD island arc onto the Adria continental platform and re-
sulted in formation of the Mt Medvednica orthogreenschists
(Lugović et al. 2006) 118 Ma BP (Belak et al. 1995). The ini-
tiation of closure of the potential cognate ROD back-arc basin
may be inferred from the amphibolites in the metamorphic
sole in Mt Kalnik. These amphibolites metamorphosed
118 Ma BP from back-arc ridge tholeiite basaltic and gabbro
protoliths (Šegvić et al. 2005; Ignjatić 2007).
In modern island arc—back-arc systems there is a correla-
tion between the rate of subduction and the intensity of back-
arc volcanism (Rodkin & Rodkinvo 1996). Island arcs with
low subduction rates (3—7 cm/yr) show slight HFSE deple-
tion and low back-arc volcanism, whilst island arcs with
high subduction rates ( > 10 cm/yr) show high HFSE deple-
290
SLOVENEC and LUGOVIĆ
tion (Thirlwall et al. 1994). In the Mt Medvednica isotropic
gabbros the intensity of Ta-Nb depletion is significant
(Fig. 10A) advocating at least a medium subduction rate. As-
suming that the subduction/accretion processes in the ROD
were active from after the Early Callovian to the Late Val-
anginian (Babić et al. 2002), that is around 35 Myr, an im-
pressive amount ( ~ 900 km) of MORB-type oceanic crust
must have been subducted underneath the Mt Medvednica
island arc.
Our data document the establishment of an island arc in
the ROD positioned between the Meliata/Maliak ophiolites
in the NW and the Dinaric/Vardar ophiolites in the SE
(Fig. 1A). With the aim of testing the geochemical affinities
of these ophiolites, we have correlated the geochemical char-
acteristics of the amphibole gabbros from the Mt Medvedni-
ca ophiolite mélange with selected rocks, thought to be
possible equivalents, from the Szarvaskő Ophiolite Complex
in the NE part of ZMTDZ and from the Jaklovce Formation
of Meliata ophiolites. In the Figs. 3, 4 we compare clinopy-
roxene from the Mt Medvednica gabbros, Szarvaskő gabbros
(Balla & Dobretsov 1984) and Jaklovce basalts since these
gabbroic rock were not reported by analyses (Hovorka &
Spišak 1988; Ivan 2002). The liable data for the correlation
of gabbroic rocks from the Central Dinaric and Vardar Zone
ophiolites are not available. However, there is little doubt
about the formation of the Central Dinaric ophiolites in a
ridge, either in the middle of an ocean (Trubelja et al. 1995)
or possibly in a back-arc (Lugović et al. 1991). The Vardar
ophiolites seem to be related to a back-arc basin (see com-
piled in Pamić et al. 2002). The pyroxene from the Mt
Medvednica gabbros shows a distinctive chemical composi-
tion compared to clinopyroxene from the relevant ophiolites
(Fig. 3) showing a composition akin to clinopyroxene hosted
in island arc tholeiitic rocks (Fig. 4). The clinopyroxenes
from the Szarvaskő gabbros and Jaklovce basalts show com-
position similar to clinopyroxenes from magmatic rocks
formed in the mid-ocean ridge or back-arc ridge. These ob-
servations are consistent with relations in the discriminant
and spider diagrams where the Mt Medvednica gabbros are
characterized by strong suprasubduction geochemical signa-
tures, typical of island arcs, whereas the Szarvaskő gabbros
and Jaklovce basalt show insignificant to slight traces of
subduction component (Figs. 12A—C and 13). Therefore, a
petrogenetic and consequently geotectonic connection be-
tween the Mt Medvednica amphibole gabbros and Szarvaskő
ophiolites cannot be postulated. Mt Medvednica gabbros ap-
pear to be peculiar ophiolitic rocks which represent remnants
of an extinct island arc from the Repno oceanic domain and
are not comparable to either rocks reported up to now from
the Meliata/Maliak ophiolites and Dinaric/Vardar ophio-
lites. They should be considered as the marker rock type for
geotectonic correlations in the western Tethyan realm, par-
ticularly concerning the ophiolites of the Vardar Zone which
were assumed to have been formed in a back-arc setting
(Pamić et al. 2002). However, a fore-arc—island arc segment
of the oceanic system as the source of these ophiolites
should not be disregarded (Lugović et al. 2006a).
Conclusions
The analysed amphibole gabbroic rocks intersected by am-
phibole gabbropegmatite are fragments of a dismembered
ophiolite within the Mt Medvednica ophiolite mélange
formed between the Early Callovian and the Late Valangin-
ian and positioned in the SW tips of the ZMTDZ, this is the
NW part of the Sava Zone. The rocks belong to the Repno
oceanic domain located between the Maliak and Dinaric do-
mains.
The gabbros showing a crystallization sequence of olivine,
augit-diopside, Ca-plagioclase and magnetite-ulvöspinel em-
bedded in tschermakite-magnesiohornblende in the cumulate
Fig. 13. N-MORB-normalized (A) multielement and (B) REE patterns for the representative isotropic gabbros from the Mt Medvednica
ophiolite mélange compared with gabbroic rocks from Szarvaskő Ophiolite Complex and from the Jaklovce Formation basalts. Data for
Szarvaskő are from Aigner-Torres & Koller (1999) and Downes et al. (1990) and for basalts from Jaklovce basalts from Ivan (2002). Nor-
malization values are from Sun & McDonough (1989).
291
GEOCHEMISTRY AND TECTONIC SETTING OF THE AMPHIBOLE GABBROIC ROCKS (NW CROATIA)
rocks are interpreted to have been formed at low to moderate
pressure from wet magmas in a high-level magma chamber.
Geochemical data on mineral and rock chemistry and age
determination suggest a proto-arc—immature island arc as the
crystallization source of the analysed gabbroic rock frag-
ments. Formation of this system is linked to the Early Call-
ovian to Late Valanginian intra-oceanic subduction in the
Repno oceanic domain. The island arc most likely did not
evolve to maturity.
The Mt Medvednica amphibole gabbroic rocks experi-
enced ocean-floor greenschist facies alteration by percolat-
ing of descending solutions. Prehnite-pumpellyite alterations
are addressed to the ophiolite emplacement.
The island arc became extinct during obduction onto the
Adria platform and, besides the analysed fragments, was rec-
ognized in the Baremian-Aptian greenschist facies ortho-
metamorphic rocks from Mt Medvednica.
The peculiar geochemical characteristics of the analysed
amphibole gabbroic rocks promote them as an excellent
marker for petrogenetic and geotectonic correlations of su-
prasubduction ophiolites from the western Tethys, particu-
larly of the Vardar Zone.
Acknowledgments: The presented outcome is the result of the
scientific projects “Mesozoic magmatic, mantle and pyroclastic
rocks of northwestern Croatia”; Project No. 181-1951126-1141
and “Tectonomagmatic correlation of fragmented oceanic
lithosphere in the Dinarides”; Project No. 195-1951126-3205,
carried out with the support of the Croatian Ministry of Sci-
ence, Education and Sport. We thank H-P. Meyer for micro-
probe facilities and I. Fin for polished thin sections. We
appreciate the laboratory assistance of M. Valent, N. Čegec
and B. Prša. Critical comments by Z. Jovanović, P. Ivan, B.
Tomljenović and an anonymous reviewer greatly helped to
improve an earlier version of the manuscript.
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