Boninite volcanic rocks from the mélange of NW Dinaric...

ORIGINAL PAPER

Boninite volcanic rocks from the mélange of NW Dinaric-Vardarophiolite zone (Mt. Medvednica, Croatia) – record of Middle to LateJurassic arc-forearc system in the Tethyan subduction factory

Damir Slovenec1 & Branimir Šegvić2

Received: 13 September 2017 /Accepted: 17 September 2018# Springer-Verlag GmbH Austria, part of Springer Nature 2018

AbstractIn the Late Jurassic to Early Cretaceous ophiolite mélange from the Mt. Medvednica (Vardar Ocean) blocks of boninite rockshave been documented. They emerge as massive lavas made of augite, spinel, albite and secondary hydrous silicates (e.g.,chlorite, epidote, prehnite, and pumpellyite). An established crystallization sequence (spinel→clinopyroxene→plagioclase±Fe-Ti oxides) was found to be typical for the boninite series from the suprasubduction zones (SSZ). Augite crystallizationtemperatures and low pressures of ~1048 to 1260 °C and ~0.24 to 0.77 GPa, respectively, delineated the SSZ mantle wedge as aplausible source of boninite parental lavas. Their whole-rock geochemistry is characterised by low Ti, P2O5, Zr, Y, high-silica,and high Mg# and Cr# values. Low and U-shaped REE profiles are consistent with the negative Nb-Ta, P and Ti anomaliesindicative for SSZ. Thorium and LILE enrichment, and very low initial Nd-isotopic values (εNd(T = 150 Ma) + 0.49 to +1.27) act asvestiges of mantle-wedge metasomatism. The mantle source was likely depleted by the MORB and IAT melt extraction and wascontemporaneously affected by subduction fluids, prior to the large-scale adiabatic melting of the mantle hanging wall. Thiseventually gave rise to boninite lavas and an ultra-refractory harzburgite residiuum. The genesis of boninites is related to theTithonian mature forearc setting that evolved from an intra-oceanic, Callovian to Oxfordian, island-arc environment. The Mt.Medvednica boninite rocks stand for the youngest SSZ-related Jurassic oceanic crust from the NW segment of the Dinaric-VardarTethys that are nowadays obducted onto the passive margins of Adria. Taking into account the existence of similar rocks in theophiolite zones of Serbia, Albania and Greece, the boninites ofMt.Medvednica strongly favours the single Tethyan oceanic basinthat existed in this part of Europe during the Late Jurassic.

Keywords Boninite . Ophiolite mélange . Forearc . Suprasubduction zone . Dinaric-Vardar ophiolite zone .MountMedvednica

Introduction

The subduction zone volcanism results from the interaction offluids released from a subducted slab and mantle wedge over-lying a descending plate. The fluid has a decisive effect inlowering a melting temperature of the mantle and leading toa melt generation that rises upward to produce a chain ofvolcanoes known as an island arc (e.g., McCulloch and

Gamble 1991;Woodhead et al. 1998;Winter 2001). It is there-fore an imperative to clearly identify different components ofisland-arc and forearc magmatic systems necessary to producea quantitative model applicable to magma formation in suchsettings. It remains however a challenge to reveal a range ofcomponents entrained by subduction fluids into the mantlemelting portion. The influence of subduction-related fluxesis best recognizable in magmas derived from refractorymantlesources at low pressures and small crustal depths(Hawkesworth and Ellam 1989). Boninites can provide fun-damental information in that regard, as they are known to haveformed from the part of mantle wedge that had previouslybeen exposed to highest degrees of depletion in an island-arcgeotectonic regime (Crawford et al. 1989; Taylor et al. 1994;Dilek and Thy 2009; Resing et al. 2011; Escrig et al. 2012).Higher degrees of melting at high-temperatures (~1200–1350 °C) and shallow depths (ca. 25–50 km; pressures 1.0–

Editorial handling: Q. Wang

* Damir [email protected]

1 Croatian Geological Survey, Sachsova 2, HR-10 000 Zagreb, Croatia2 Department of Geosciences, Texas Tech University, 1200 Memorial

Circle, Lubbock, TX 79409, USA

Mineralogy and Petrologyhttps://doi.org/10.1007/s00710-018-0637-0

http://crossmark.crossref.org/dialog/?doi=10.1007/s00710-018-0637-0&domain=pdf

http://orcid.org/0000-0002-6485-7930

mailto:[email protected]

1.5 GPa) of hot mantle wedge are needed to produce boninitelavas (Umino and Kushiro 1989; Falloon and Danyushevsky2000; Kushiro 2007; Green et al. 2010). There is a generalagreement on boninite petrogenesis suggesting the mantlesource enrichment through the metasomatism of sub-forearcmantle by hydrous fluids or melts derived from a subductingplate (e.g., Hickey and Frey 1982; Murton et al. 1992;Ishikawa et al. 2002; Dilek and Thy 2009). Such processesthat affected the mantle source have already been documentedin post-subduction blocks of igneous rocks archived in theophiolite mélange of Mt. Medvednica (Lugović et al. 2007;Slovenec and Lugović 2009).

Boninite rocks present sensitive but powerful indicators ofmantle wedge processes in the suprasubduction zones andtheir appearance has always had important tectonic implica-tions (e.g., Meijer 1980; Crawford et al. 1989; Pearce et al.1992; Falloon et al. 2008). They have been reported in numer-ous Tethyan ophiolites (Troodos, Oman, Pindos -e.g.,Cameron 1985; Kostoeoulos and Murton 1992; Ishikawaet al. 2002; Pe-Piper et al. 2004; Saccani et al. 2017 andreferences therein) or in the Izu-Bonin and Tonga arcs (e.g.,Crawford et al. 1989) where they are frequently associated toophiolite rock suites. Most of the authors link modern boninitelavas to intra-oceanic forearc-arc systems, with boninites typ-ically forming at the basements of arc volcanoes (e.g.,Hawkins et al. 1984; Murton 1989; Falloon and Crawford1991; MacPherson and Hall 2001; Resing et al. 2011;Saccani and Tassinari 2015; Saccani et al. 2011, 2017).Boninite genesis is thought to have been related to the sub-duction inception, slow convergence and slab roll-back thateventually lead to forearc-arc splitting and/or forearc rapidextension and upwelling of the mantle allowing a volatile-fluxed decompression melting of severely depleted mantlerocks (e.g., Crawford et al. 1981, 1989; MacPherson andHall 2001; Ishikawa et al. 2002 and references therein; Dilekand Thy 2009). A similar genetic succession of oceanic crustformation during theMesozoic has already been suggested forthe ophiolitic sequence of Mt. Medvednica (Slovenec andLugović 2009).

Boninite rocks have rarely been documented in theophiolite zones of Albanides and Hellenides but their petro-genesis was thoroughly investigated (Beccaluva and Serri1988; Bébien et al. 2000; Bortolotti et al. 2002; Pe-Piperet al. 2004; Saccani and Tassinari 2015; Saccani et al. 2017and references therein; Fig. 1). However, in the Dinaric-Vardar ophiolite zone the boninite-type rocks are reportedsolely in the region of Kopaonik (southern Serbia; Marroniet al. 2004) and at the extremely west portion of the Dinaric-Vardar Ophiolitic Zone in the area of Mt. Medvednica, whichis a part of the Sava Unit (sensu Haas et al. 2000; Fig. 2). Thisstudy brings a first report on the occurrence of boninites fromthe ophiolite mélange of Mt. Medvednica in Northern Croatia(Fig. 3) . Having originated in the Late Jurassic

suprasubduction geological setting these rocks represent amissing piece of the puzzle allowing a more exhaustivegeodynamic reconstruction of this part of the Dinaric-Vardaroceanic space during the Middle to Late Jurassic time(Slovenec and Lugović 2009; Slovenec et al. 2011; Šegvićet al. 2016). Placed at western margins of the Dinaride-Albanide-Hellenide ophiolite realm, the boninite rocks ofMt. Medvednica (Fig. 1) are of high importance for the po-tential correlation with similar rocks known from the ophiolitezones in the southern part of the Balkans. The aim of this workis to propose a comprehensive petrological and geochemicalcharacterization of boninites from the ophiolite mélange ofMt. Medvednica and to report on its petrogenesis and geotec-tonic significance in the light of high-resolution geodynamicreconstruction of the evolution of the Jurassic north-westernbranch of the Dinaric-Vardar Tethys. Finally, an attempt wasmade to investigate the correspondence of analysed rocks withtheir analogues from the southern parts of the Dinaric-Vardar–Albanide–Hellenide ophiolite belts in order to test a hypothe-sis of the single oceanic space that existed in this part ofTethys during Mesozoic times.

Geological setting

Mount Medvednica, located in northern Croatia, is situated atthe triple junction of three major geotectonic units - the South-Eastern Alps, the Dinarides and the Tiszia continental block(Pamić and Tomljenović 1998; Slovenec and Pamić 2002;Fig. 1). Its northern slopes bear the record of the Triassicand Jurassic oceanic crust of the Neotethys (Lugović et al.2007; Slovenec and Lugović 2008, 2009, 2012; Slovenecet al. 2010) that belongs to the westernmost segment of theWestern Vardar Ophiolitic Unit (sensu Schmid et al. 2008).Along with other intra-Pannonian inselgebirge like the Mts.Kalnik, Ivanščica and Samoborska Gora, the Mt. Medvednicarepresents a part of the Sava Unit (sensu Haas et al. 2000;Fig. 2) or the Zagorje-Mid-Transdanubian Zone (ZMTDZ;sensu Pamić and Tomljenović 1998). The Sava Unit is anabout 100 km wide and approximately 400 km long shearedbelt sandwiched between the two regional fault systems: theZagreb-Zemplin (ZZL) and the Periadriatic-Balaton lineament(PL-BL). The PL-BL separates the SavaUnit to the north fromthe Austroalpine and Pelso Units, while the ZZL confines theSava Unit south-westward towards the Tiszia Mega-unit(Fig. 2). The geological history of the Sava Unit is complexwhich is mirrored in its composition consisted of amalgamat-ed Dinaric and South Alpine tectonostratigraphic fragments(e.g., Haas et al. 2000). The Paleotethys back-arc oceansknown in the literature as Meliata-Maliac or Meliata-Hallstatt (e.g., Stampfli et al. 2002) experienced a constantshortening during the Jurassic period due to the rotation ofAfrica with respect to Europe (Cavazza et al. 2004; Burke

D. Slovenec, B. Šegvić

2011). This eventually led to the opening of the Vardarsuprasubduction ocean during the roll-back of theMeliata-Maliac (e.g., Stampfli et al. 2002; Saccani et al.2008; Bortolotti et al. 2013). In the area of the Sava Unitan ensuing Middle to Late Jurassic intra-oceanic conver-gence of the Western Vardar realm was manifested by avivid tectonic activity that included a subduction of an ac-tive oceanic ridge, arc activity, and back-arc magmatism(Slovenec and Lugović 2009; Bortolotti et al. 2013). Thefinal closure of the Neotethys was suggested to correspondto the Barremian-Aptian, constrained in the Sava Unit bythe age of greenschist of the Mt. Medvednica (Belak et al.1995) nowadays obducted onto the Adria passive margins(Lugović et al. 2006). Despite a mixed composition of theSava Unit that belongs to both the Dinaric-Vardar andSouth Alpine tectonostratigraphic units (e.g., Haas et al.2000) its affiliation to the Dinarides and their ophiolites iswidely endorsed (e.g., Milovanović et al. 1995; Pamić andTomljenović 1998; Šegvić et al. 2016).

The geography of the studied area and a simplified geolog-ical map of the Mt. Medvednica northern slopes are shown inFig. 3. The mountain consists of pre-Neogene heterogeneousand superimposed Dinaric and Alpine tectonostratigraphicand tectonometamorphic units of both the continental andoceanic origin (e.g., Pamić and Tomljenović 1998; Tari andPamić 1998; Haas et al. 2000; Pamić 2002; Tomljenović et al.2008). The earliest Silurian to Middle Triassic (Ladinian)volcano-sedimentary successions were subjected to low-grade metamorphism during the Lower Aptian (Belak et al.1995 and references therein) and were, thereafter, tectonicallyoverlain by the ophiolite mélange unit (Kalnik Unit, sensuHaas et al. 2000; Fig. 2). The obduction of ophiolites furthercaused an Early Cretaceous metamorphism with Late Jurassicisland-arc basalts metamorphosed up to the greenschist meta-morphic facies conditions (Lugović et al. 2006). The ophiolitemélange that crops out abundantly along the northern slopesof the Mt. Medvednica is mainly found in the tectonic contactwith a series of Mesozoic sedimentary rocks or, in a lesser

Fig. 1 Geotectonic sketch map of the major tectonic units of the Alps,Carpathians and Dinarides (simplified after Schmid et al. 2008). Theinserted map (upper left corner): regional geographic overview. 1 =Adria derived far-travelled nappes Alps and W. Carpathians(ALCAPA); 2 = Europe-derived units (Dacia); 3 =mixed European andAdriatic affinities (Tisza); Ophiolites oceanic accretionary prisms: 4a =Meliata, Darnó-Sźarvaskö, Dinaric, Western Vardar, Mirdita; 4b =

Piemont-Liguria, Vahicum, Inacovce-Kriscevo, Szolnok, Sava; 4c =Transylvanian, South Apuseni, Eastern Vardar; 5 = Southern Alps; 6 =Adriatic Plate, High Karst and Dalmatian Zone; 7 = Pre-Karst andBosnian Flysch; 8 = East Bosnian-Durmitor; 9 = Drina-Ivanjica, Korab,Pelagonides; 10 = Bükk, Jadar, Kopaonik; 11 = black arrow indicates theMedvednica Mt. investigated area

Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –...

extent, it is thrusted on sedimentary rocks of Neogene andQuaternary age.

The Mt. Medvednica ophiolite mélange and analogue unitsdocumented at the mountains of the Samoborska Gora, Kalnikand Ivanščica form a joint tectonostratigraphic unit known asthe Kalnik Unit (Fig. 2). This unit also includes the remnantsof a specific oceanic realm reported in the literature as theRepno oceanic domain (ROD; sensu Babić et al. 2002). Themélange of the Kalnik Unit is characterized by the poorlypreserved original structural and depositional order. It showsstructural features delineated by block-in-matrix fabric, typi-cal for chaotic complexes from subduction-related tectonicmélanges (Festa et al. 2010). This olistoliths-dominated com-plex ismixedwith the fault-bounded heterogeneous fragmentsof different Mesozoic rocks. Their range varies from pebblesand slivers to hectometre-sized homogenous blocks incorpo-rated in the strongly sheared pelitic to siltous continent-derived matrix (Babić et al. 2002; Fig. 3). The mélange isdominantly composed of mafic intrusives (peridotites andgabbros) and extrusives (basalts), and fragments of sedimen-tary rocks (greywackes, minor shales, red and grey cherts andscarce limestones) (Slovenec and Lugović 2008, 2009;

Slovenec et al. 2010). Igneous components of mélange showvarious geochemical affinities consistent with their distinctnascent geotectonic environments that existed from theIllyrian to the late Oxfordian (Slovenec and Lugović 2009;Slovenec et al. 2010). However, among those igneous com-ponents of the Mt. Medvednica ophiolite mélange twodecametre-sized blocks of Bexotic^ volcanic rocks (i.e.boninite) have been recovered (Fig. 4). Their geochemistryclearly differentiates from the above-mentioned igneous ex-trusives. The positions of boninites are indicated in Fig. 3.Regardless of a relatively small area of exposure of boniniterocks their appearance in ophiolite mélange of the Kalnik Unitpresents a valuable piece of information essential for an in-depth comprehension of the geodynamic evolution of thenorth-western oceanic branch of the Dinaric-Vardar Tethys.

Analytical techniques

Chemical composition of mineral phases from two sampleswere analysed using a CAMECA SX51 electron microprobeequipped with five wavelength-dispersive spectrometers. The

Fig. 2 Sketch map of the structural units and major lineaments (modifiedafter Haas et al. 2000). 1 = Austroalpine units; 2 = Pelso Unit; 3 = SouthAlpine units and Julian-Savinja and South Karawanken units; 4 = SouthZala Unit; 5 = Central Slovenian and Bosnian units; 6 =MedvednicaUnit; 7 = Kalnik Unit; 8 = Internal Dinaridic Unit (Vardar Unit); 9 =

External Dinaridic Unit; 10 = Tisza Mega-Unit; 11 = black arrowindicates the Medvednica Mt. study area; 12 = box indicates the areashown on the Fig. 3; BL =Balaton Lineament; ZZL = Zagreb-ZemplinLineament; PL = Periadriatic Lineament


operating parameters included 15 kV accelerating voltage,20 nA beam current, and ∼ 1 μm beam size (~ 10 μm forfeldspars). Counting times of 20 s on peak and 10 s on back-ground on both sides of the peak were used for all elements.Limits of detection (LOD) were calculated as the minimumconcentration required to produce count rates three timeshigher than the square root of the background (3 s; 99 wt.%degree of confidence at the lowest detection limit)., and 10 scounting time for all elements (peaks and backgrounds).Natural minerals, oxides (corundum, spinel, hematite, and ru-tile), and silicates (albite, orthoclase, anorthite, and wollaston-ite) were used for calibration. The measurements relative errorwas less than 1%. Raw data were corrected for matrix effectsusing the PAP algorithm (Pouchou and Pichoir 1984, 1985)implemented by CAMECA. Mineral phase formula calcula-tions were done using a software package designed by Hans-Peter Meyer (personal communication).

Bulk-rock powders for chemical analyses of six sampleswere obtained from rock chips free of veins. The samples wereanalysed by ICP-OES for major elements and ICP-MS for all

trace elements at Actlab Laboratories. International maficrocks were used as standards. Major element and trace ele-ment concentrations were measured with accuracy better than1 and 5%, respectively.

Isotopic compositions of two bulk rock samples were mea-sured in CRPG using a Triton Plus mass spectrometer.Normalizing ratios of 86Sr/88Sr = 0.1194 and 146Nd/144Nd =0.7219 were assumed. The 87Sr/86Sr ratio for the NBS 987 Srstandard for the period of measurement was 0.710242 ±0.000030 (2σ). The 143Nd/144Nd ratio for the La Jolla standardwas 0.5118451 ± 0.000010 (2σ). Total procedural blanks were~500 pg and ~150 pg for Sr and Nd, respectively.

Results

Petrography and mineral chemistry

Analysed boninite rocks from the Mt. Medvednica ophiolitemélange dominantly emerge as fine-grained massive lavas

Fig. 3 Simplified geological map and stratigraphic column of Mt.Medvednica (modified after Šikić et al. 1978; Basch 1981 and Halamić1998). 1 = Neogene and Quaternary sedimentary rocks; 2 = LateCretaceous-Paleogens flysch including Senonian carbonate breccias;3 = Alb-Cenomanian limestones and clastic rocks (shale, siltite andsandstone); 4 = Jurassic/Early Cretaceous ophiolite mélange (pelitic tosiltous matrix) with blocks and slices of: 4a = basalts (intersected by

diabase dikes) and boninites, 4b = gabbros (intersected by diabasedikes), 4c = cumulate peridotites, 4d = Triassic (squares field) - Jurassic(triangles field) radiolarites, shales and basalts (light gray fields); 5 =Palaeozoic metamorphic complex; 6 = reverse or thrust faults; 7 = normalfaults; 8 = geological line; 9 = sample locations: 1 =mt-18/1, −18/2, −18/3; 2 = vh-49/1, −49/2, −49/3; 10 = picture break


(Fig. 4b). Their holocrystalline massive base mass consists ofclinopyroxene microlites accompanied by rare laths of alteredplagioclase, spinel, accessory and interstitial Fe-Ti oxides anda range of hydrous secondary phases (Fig. 4c–d). Subtle trans-versal veins are filled with chlorite, epidote, prehnite, calciteand/or quartz. Clinopyroxene is omnipresent and is partiallysaussuritized or altered to distinguishable secondary phasessuch as chlorite, prehnite, and low-Al pumpellyite (sensuIshizuka 1999) (Fig. 4c). High fluid content hampered alarge-scale plagioclase phenocryst crystall ization(Ohnenstetter and Brown 1992), which in analysed samplesis entirely re-crystallized into secondary albite. Primary darkgreyish to blackish low-Ti (TiO2 = 0.13–0.22 wt.%) Al-chromite (Fig. 5a; Table 1) is frequently changed to ilmenite,leucoxene, a mixture of titanite and rutile, and may sporadi-cally be encountered as inclusions in clinopyroxene (Fig. 4d).Taking into account that the Mt. Medvednica boninite rocksare altered to a certain extent, their igneous aphyric (rarelyporphyric) sub-ophitic to intergranular texture (Fig. 4c–d)made of plagioclase needles and equigranular clinopyroxenehas been totally preserved. Petrographic evidences suggest afollowing crystallization order: spinel → clinopyroxene →plagioclase ± Fe-Ti oxides (Fig. 4d). This sequence is fullyin line with the crystallization order documented in boniniterocks (e.g., Ohnenstetter and Brown 1996). In the studiedsamples very low-Ti lavas clinopyroxene precedes

plagioclase, which is generally accepted as an important char-acteristic of SSZ lavas (e.g., Beccaluva et al. 1980, 1989). Theabsence of low-Ca clinopyroxene (clinoenstatite) in Mt.Medvednica boninites on one hand and presence ofplagioclase-rich matrix on the other hand, along with theCr# and Mg# values of analysed chromite are all parametersthat are very much alike to those of other Tethyan boniniterocks (e.g., from Troodos ophiolites; Cameron 1985;Beccaluva and Serri 1988).

High Cr# (0.73–0.79) and Mg# (0.45–0.50) coupled withlow Fe# (2–12) and TiO2 values that are reported from thestudied Al-chromite are typically reported in boninite rocks(Fig. 5). These values further delineate a primitive nature ofboninite melts and at the same time testify on their high initialCr concentrations. Phase chemistry of analysed chromite cor-responds well to those provenancing from the Albanide-Hellenide boninite series (Fig. 5b; Saccani and Tassinari2015). Representative clinopyroxene compositions fromanalysed rocks are provided in Table 1. Clinopyroxene ismainly represented by augite and Mg-rich augite (Wo30.75–39.93En49.34–56.75Fs7.84–13.17; after Morimoto (1988) classifica-tion diagram – not shown) that both show a trend of Mg-Caenrichment (Table 1). In all samples the analysedclinopyroxene has a homogenous core and depict a normalzonation defined by a continuous decrease in Mg#, AlVI/AlIV, Cr and Ca content toward the rims, while at the same

Fig. 4 a Decametre-sized blockin a strongly sheared pelitic-siltous continent derivedmatrix ofthe Mt. Medvednica ophiolitemélange (Location 1 from the Fig.3). 1 = pelitic-siltous matrix; 2 =boninite lavas. b Massiveboninite lavas from the Mt.Medvednica ophiolite mélange(Location 2 from the Fig. 3).Microphotographs of thin sectionof the Mt. Medvednica boniniticrocks c sample mt-18/1 N+ and dsample vh-49/1 N–. cpx =clinopyroxene; sp. = spinel; ab =albite; chl = chlorite; pmp =pumpellyite; ti = titanite; cal =calcite; qz = quartz


time the Ti abundances increase. Such a zonation pattern istypical for rapid cooling in closed magmatic systems (Stern1979; Nakagawa et al. 2002). Clinopyroxene is further fea-tured by a low-TiO2 content (0.01–0.23 wt.%; Fig. 6), highMg# (81.06–88.55) and total absence of Fe enrichment(Table 1) implying they were derived from a depleted mantlesource. The Cr2O3 content in pyroxene is high (0.11 to1.10 wt.%) and is well correlated with the Mg# (not shown).In general, the composition of analysed clinopyroxene is sim-ilar to those reported in SSZ ophiolites and their modern an-alogues (e.g., Beccaluva et al. 1989). Clinopyroxene chemis-t ry i s a l so known as an exce l l en t ind ica to r o fthermobarometric regime that prevailed in the magmatic res-ervoir at the time of its crystallization (Wass 1979; Shiffmanand Lofgren 1982). The AlVI/AlIV ratio is constantly below1.7, which is typical for clinopyroxene originating from low-to medium-pressure magmatic rocks (Wass 1979; Coish andTaylor 1979; Shiffman and Lofgren 1982). Maximal crystal-lization temperatures of analysed augite were estimated to fitthe range from 1048 to 1260 (± 30) °C (after Lindsley 1983),while the geobarometer of Nimis and Ulmer (1998) and Nimis(1999) resulted with low equilibration pressures from 0.24 to0.77 (± 0.31) GPa.

Bulk rock geochemistry

Representative major and trace element geochemical analysesof six boninite samples from the Mt. Medvednica boninite areprovided in Table 2. Isotopic compositions of Nd and Sr fromtwo representative boninite rocks are given in Table 3. Loss onignition (LOI), used to monitor the extent of element

redistribution during alteration (Polat et al. 2002; Polat andHofmann 2003), is featured by moderate low values (up to3.95 wt.%), pointing to low degrees of secondary alterationsthat are normally accounted for the low-temperature ocean-floor hydrothermal metasomatism under prehnite-pumpellyiteto lower greenschist facies conditions (e.g., Mevel 1981;Peacock 1987; Erzinger 1989; Topuz et al. 2013).Notwithstanding the less pronounced hydrothermal alter-ations, magmatic textures of analysed rocks have been pre-served as well as their pristine geochemistry that reflects theoriginal chemistry of parent magmas. This may be inferredfrom Na2O vs. CaO ratio (Graham 1976; Stillman andWilliams 1979 – not shown), which is traditionally employedto screen igneous rocks for the low-grade spilitization effects.Analysed rocks fromMt. Medvednica are discriminated in thefield of unchanged basalts thus clearly suggesting a minimalalteration during hydrothermal alterations and low-grademetamorphism. In the SiO2 vs. MgO (Fig. 7a) and Ti/1000vs. V (Ti/V = 3.7–5.3; Fig. 7b) classification diagrams theanalysed volcanic rocks are distinctly defined as boninites.Because of the high CaO content (>9.57 wt.%; Fig. 7c),CaO/Al2O3 ratio that fits the range between 0.76 and 0.84,SiO2 abundances that do not exceed 56 wt.%, and total alkalicontent that is below 2.07 wt.% the Mt. Medvednica boniniterocks can be classified as high-Ca boninites (Crawford et al.1989) clearly belonging to the calc-alkaline igneous rock se-ries (Fig. 7d). Yet, the major chemistry of analysed rockscorresponds to basaltic andesites (TAS classification, afterLe Bas 2000; Le Maitre 2002 – not shown) but no plagioclasephenocrysts were documented, thus favouring their classifica-tion as boninites (Beccaluva and Serri 1988). They are rich in

Fig. 5 Classification and discrimination diagrams for spinels from theboninitic rocks from the Mt. Medvednica ophiolite mélange. a TrivalentCr–Al–Fe3+ ternary cation plot (Stevens 1944). Fields for mid-oceanridge (MORB) setting and boninites (BON), as well as boninites fromthe Albanide-Hellenide ophiolites are from Saccani and Tassinari (2015)

and references therein. b Cr# – Mg# diagram. Cr# = 100*(Cr/(Cr + Al));Mg# = 100*(Mg/(Mg + Fe2+)). Fields for spinels in boninites, forearcperidotites and abyssal peridotites, as well as Albanide-Hellenideophiolites are from Saccani and Tassinari (2015) and references therein


Table1

Representativechem

icalcompositio

ns,calculatedmineralform

ulae

andcalculated

modalfractio

nsof

spinelandclinopyroxenefrom

theboninitic

rocksintheMt.Medvednicaophiolite

mélange

Mineral

Spinel

Clin

opyroxene

Sam

ple

Site

mt-18/

1mt-18/

1mt-18/

1mt-18/

1vh-49/

1vh-49/

1vh-49/

1vh-49/

1mt-18/

1mt-18/

1mt-18/

1mt-18/

1vh-49/

1vh-49/

1vh-49/

1vh-49/

1vh-49/

1vh-49/

1vh-49/1

gmgm

gmgm

gmgm

gmgm

gm,c

gm,r

gmgm

gmgm

gmgm

gmgm

,cgm

,r

Oxides(w

t%)

SiO

20.08

0.11

0.08

0.06

0.05

0.06

0.05

0.09

54.13

52.93

52.70

53.59

53.20

54.32

53.00

53.10

53.02

52.75

53.01

TiO

20.14

0.10

0.15

0.16

0.15

0.16

0.11

0.13

0.04

0.13

0.15

0.06

0.08

0.01

0.07

0.03

0.11

0.11

0.16

Al 2O3

12.07

10.05

12.40

12.24

12.83

12.52

10.60

11.16

1.68

3.44

3.83

2.14

2.97

1.72

1.99

2.43

2.46

2.30

3.31

Cr 2O3

53.51

56.80

52.78

53.50

52.90

53.25

56.79

56.48

0.61

0.14

0.19

1.10

0.27

1.01

1.09

0.98

0.11

0.60

0.47

FeO

22.60

21.92

22.90

22.04

22.06

22.80

22.21

20.28

5.35

6.80

8.08

4.84

7.33

4.93

5.01

5.47

7.84

6.00

6.93

MnO

0.11

0.06

0.07

0.12

0.12

0.16

0.11

0.14

0.21

0.19

0.24

0.18

0.18

0.17

0.12

0.16

0.23

0.17

0.12

MgO

10.86

10.62

11.05

11.03

10.68

10.78

10.14

11.43

19.35

17.55

19.70

18.95

19.38

18.89

18.65

18.15

17.95

17.46

18.07

CaO

0.18

0.14

0.13

0.69

0.30

0.20

0.13

0.15

19.47

19.41

15.41

19.74

16.41

19.48

19.91

19.57

18.01

19.28

17.38

Na 2O

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.05

0.08

0.05

0.08

0.06

0.07

0.06

0.05

0.16

0.08

0.03

Total

99.55

99.80

99.56

99.84

99.09

100.01

100.14

99.86

100.91

100.67

100.34

100.68

99.86

100.60

99.89

99.94

99.89

98.73

99.47

Calculatedmineralform

ulae

(apfu)

a

Si

0.003

0.004

0.003

0.002

0.002

0.002

0.002

0.003

1.949

1.922

1.909

1.935

1.937

1.964

1.931

1.938

1.942

1.952

1.947

Ti

0.003

0.003

0.004

0.004

0.004

0.004

0.003

0.003

0.001

0.004

0.004

0.002

0.002

0.001

0.002

0.001

0.003

0.003

0.004

Al

0.472

0.395

0.484

0.476

0.501

0.487

0.415

0.433

0.072

0.147

0.163

0.091

0.128

0.074

0.085

0.104

0.106

0.101

0.143

AlIV

––

––

––

––

0.051

0.078

0.091

0.065

0.063

0.036

0.069

0.062

0.058

0.048

0.053

AlV

I–

––

––

––

–0.021

0.069

0.073

0.026

0.065

0.038

0.016

0.042

0.048

0.053

0.090

Cr

1.402

1.497

1.381

1.395

1.386

1.388

1.491

1.470

0.017

0.004

0.005

0.031

0.008

0.028

0.031

0.028

0.003

0.018

0.014

Fe3

+0.134

0.114

0.149

0.105

0.110

0.132

0.095

0.098

0.014

0.004

0.008

0.011

0.000

0.000

0.021

0.000

0.011

0.000

0.000

Fe2

+0.481

0.489

0.473

0.492

0.492

0.487

0.513

0.453

0.147

0.203

0.237

0.135

0.223

0.149

0.131

0.167

0.229

0.186

0.213

Mn

0.003

0.002

0.002

0.003

0.003

0.004

0.003

0.004

0.006

0.006

0.007

0.006

0.006

0.005

0.004

0.005

0.007

0.005

0.004

Mg

0.537

0.528

0.546

0.543

0.528

0.530

0.503

0.562

1.039

0.950

1.064

1.020

1.052

1.018

1.013

0.988

0.980

0.964

0.989

Ca

0.006

0.005

0.005

0.024

0.011

0.007

0.005

0.005

0.751

0.755

0.598

0.764

0.640

0.755

0.777

0.765

0.707

0.765

0.684

Na

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.003

0.006

0.004

0.006

0.004

0.005

0.004

0.004

0.011

0.006

0.002

Mg#

46.13

46.31

46.27

47.14

46.32

45.65

44.91

50.13

87.61

85.11

81.78

88.31

82.51

87.23

88.55

85.54

81.06

83.83

82.28

Cr#

74.81

79.12

77.05

74.56

73.45

74.03

78.22

77.25

––

––

––

––

––

–AlV

I /AlIV

––

––

––

––

0.41

0.88

0.80

0.40

1.03

1.06

0.23

0.68

0.83

1.10

1.70

Modalmineralfractions

(mol%)

Wo

––

––

––

––

38.38

39.38

31.24

39.46

33.33

39.15

39.93

39.76

36.55

39.84

36.19

En

––

––

––

––

53.07

49.55

55.59

52.71

54.76

52.84

52.04

51.31

50.67

50.20

52.35

Fs

––

––

––

––

8.56

11.07

13.17

7.84

11.91

8.01

8.03

8.93

12.78

9.96

11.47

Mg#

(mg-number)=100×[M

g/(M

g+Fe2

+)];C

r#(cr-number)=100×[Cr/(Cr+

Al)]

gmgroundmass,ccore,r

rim

Wowollastonite

(Ca 2Si 2O6),Enenstatite

(Mg 2Si 2O6),Fsferrosilite(Fe 2Si 2O6)

aCalculatedon

thebasisof

3catio

nsand4oxygensforspineland4catio

nsand6oxygensforclinopyroxene


compatible mantle-derived trace elements, like NiO(>97 ppm) and Cr (>413 ppm), that were likely derived fromrefractory mantle peridotites (Crawford et al. 1989). High Crcontent (Cr >400 ppm) permits one to classify these rocks ashigh-Cr boninites (Pe-Piper et al. 2004). Boninite lavas of theMt. Medvednica are relatively primitive with highMg# (~72),Cr# (~85), showing an extreme depletion in TiO2

(<0.23 wt.%), P2O5 (≤0.02 wt.%), Zr (<27 ppm), Y(<5.9 ppm) and other HFSE as well as rare earth elements(REE) (Table 2). Because of the low TiO2 content(<0.3 wt.%) these rocks may be defined as low-Ti boniniteswith Al2O3/TiO2 ratios (60–100) that are considerably higherwith regard to the primitive MORB magmas (up to 32;Crawford and Cameron 1985). High CaO/Al2O3 ratio (0.76–0.84) that approaches the values of chondrite and primaryupper mantle (0.90; Clague and Frey 1982) and the presenceof plagioclase microlites clearly distinguish the Mt.Medvednica boninite series from the Tertiary boninites sensusstricto which originate from the Bonin Island – MarianaTrench (CaO/Al2O3 = 0.72–0.38). It follows that the rocksanalysed herein better correspond to the Tethyan boninite(e.g., Troodos, Beccaluva and Serri 1988).

In spider diagram, LIL elements (i.e., Cs, Rb, Ba, and K;Fig. 8a) depict a selective mobility and will therefore be ex-cluded from further petrogenetic considerations. These ele-ments were likely entrained in the mantle wedge by the fluidsreleased from the subducting slab (Ellam and Hawkesworth1988; Pearce and Peate 1995). In line with earlier studies onsimilar rocks (e.g., Pearce and Norry 1979; Saunders et al.1979; Shervais 1982; Beccaluva et al. 1983), the HFS ele-ments (Nb, Ta, Ti, Hf, P, and Y) and most of the REE(middle REE and heavy REE) measured in the boninitesof Mt. Medvednica seem to have remained immobile dur-ing the mantle wedge metasomatism of subducted slab as

well as during a very low-grade metamorphism that theserocks were presumably exposed to. Therefore, these ele-ments are considered useful in tracing the nature of mantlesource. Only LREE might have been affected by mobiliza-tion during alteration; however, good correlations betweenLREE and many immobile elements (not shown) indicatesolely a slight mobilization of LREE. Still, their petroge-netic application requires a great care. Relatively good cor-relation between transitional metals (V, Cr, Mn, Fe, Ni andZn) and Zr argues on their magmatic abundances inanalysed rocks (Canil 1987). Vanadium and Ni are, there-fore, used in further geochemical and petrogenetic discus-sions. Good and positive correlation of Ti and Zr indicatesa low amount of early crystallization of Fe-Ti oxides. Theinput of Th and La in the generation of mafic lavas wasassessed using trace-element diagrams normalised to thehighly incompatible Yb (Pearce and Peate 1995; Peateet al. 1997). The contribution of Th and La from thesubducting slab was up to 85% and ~25%, respectively(Fig. 9; Pearce 1983). The suprasubduction zone origin ofthe Mt. Medvednica boninites is further indicated by thefield geological data placing them as blocks in theophiolite mélange.

In multi-element spider patterns normalized to N-typeMORB analysed boninites shows the moderate oscillationsof HFS elements (Fig. 8a). The LIL elements (Cs, Ba andRb) depict the selective enrichments only due to post-magmatic alterations while Th remains as a relatively stableigneous indicator. Its enrichment relative to other incompati-ble elements eflects the fluid input from subduction zone(Wood et al. 1979; Pearce 1983). The HFS element relativeconcentration profile is characterized by humped patters atvery low concentration levels (0.1 to 0.6 times relative to N-MORB). All samples show negative Nb-Ta, P, and Ti

Fig. 6 Discriminant diagrams for clinopyroxene from the Mt.Medvednica boninitic rocks. a SiO2/100 – Na2O – TiO2 diagram and bTi –AlIV diagram (simplified after Beccaluva et al. 1989). MORB =mid-ocean ridge basalts; BABB = back-arc basin basalts; IAT = island-arc

tholeiites; B-A = intra-oceanic forearc basalts and andesites; BON =boninite. Fields for clinopyroxene compositions from high-, medium-and low-Ti tholeiitic SSZ basalts of the Mt. Medvednica ophiolitemélange (Slovenec and Lugović 2009) plotted for correlation constraints


anomalies [(Nb/La)N = 0.67–0.71; (P/Nd)N = 0.45–0.81; (Ti/Sm)N = 0.72–0.87] thus suggesting average to high inputs ofsubduction fluids released from the down-going slab (Pearce1982; Tatsumi and Kogiso 2003). Moreover, the positiveanomaly of Sr [(Sr/Nd)N = 1.85–2.96] is a typical feature ofsubduction-related magmas from recent as well as ancient arcs(e.g., Pearce et al. 1984; McCulloch and Gamble 1991).Analysed boninite rocks are also characterized by the positiveanomaly of Zr-Hf [(Zr/Sm)N = 1.55–1.95], which is a charac-teristic of boninite magmas from the Western Pacific boninitesuites (e.g., Crawford et al. 1989; Pearce et al. 1992).

Chondrite-normalized REE patterns show typical trendscharacteristic for boninites from forearc regions (e.g.,Crawford et al. 1989). In addition they show similaritieswith boninites from many ophiolite complexes (e.g.,Beccaluva and Serri 1988; Bédard 1999) emerging in formsof characteristic concave-upward (U-shape) profiles(Hickey and Frey 1982), showing a slight LREE enrich-ment [(La/Sm)CN = 1.49–1.66)] and negatively fractionatedHREE [(Tb/Lu)CN = 0.43–0.54] at levels of 1.3 to 1.5 timesrelative to chondrite as well as depleted MREE with regardto HREE (Fig. 8b). The LaCN/YbCN ratio was reported to bebetween 0.80 and 0.86. Such a discordant LREE enrich-ment in the worldwide boninite suites is commonly as-cribed to the second-stage re-enrichment process that nor-mally takes place prior or during boninite generation (e.g.,Xia et al. 2012 and references therein). The REE concen-tration levels reported herein indicate an extremely deplet-ed nature of mantle source area of analysed boninites.Estimated Eu and Sr anomalies (Eu/Eu* = 0.90–1.52 andSr/Sr* = 1.42–2.37, respectively) reflect the fractionationof plagioclase and its low accumulation during magmaticdifferentiation (Hawkesworth et al. 1977; West et al. 1992),which requires a moderately to high degrees of partial melt-ing (e.g., Saccani and Photiades 2004; Beccaluva et al.2005).

Measured 143Nd/144Nd ratios of the two boninite rocksfrom the Mt. Medvednica ophiolite mélange do not showsignificant discrepancies and fit the range from 0.512662 to0.512702, which corresponds to the initial εNd(150 Ma) valuesfrom the range of +0.49 to +1.27 (Table 3). This fits well theεNd of the pristine mantle (εNd ~0). Further on, the measured143Nd/144Nd values are relatively low compared to the localMORB sources and MORB/IAT-like SSZ basalts (Fig. 10;Slovenec and Lugović 2009). The ratios of 87Sr/86Sr are be-tween 0.706427 and 0.707422, corresponding to initial ratiosof 0.706208 to 0.707011 calculated for the age of 150 Ma.This age stands for the anticipated crystallization ages ofanalysed boninite rocks. Nevertheless, the elevated 87Sr/86Srratio values may indicate a seawater alteration at the time ofthe ocean-floor hydrothermal metamorphism (e.g., Karpenkoet al. 1985; Bach et al. 2003). These values will therefore beexcluded from further petrogenetic considerations.

Table 2 Chemical compositions of boninitic rocks from the Mt.Medvednica ophiolite mélange

Sample mt-18/1 mt-18/2 mt-18/3 vh-49/1 vh-49/2 vh-49/3

Major oxides (wt%)

SiO2 53.41 54.83 53.73 55.11 54.02 53.98

TiO2 0.13 0.22 0.16 0.20 0.19 0.16

Al2O3 12.95 13.25 12.73 12.64 12.70 12.68

Fe2O3total 7.63 7.81 7.72 7.69 7.88 7.71

MnO 0.15 0.13 0.15 0.14 0.15 0.16

MgO 8.99 8.52 8.93 8.79 8.81 8.92

CaO 10.86 10.09 10.41 9.57 10.43 10.44

Na2O 2.01 1.88 1.88 1.92 1.79 1.96

K2O 0.05 0.05 0.04 0.12 0.07 0.04

P2O5 0.01 0.02 0.01 0.02 0.01 0.01

LOI 3.02 3.27 3.58 3.69 3.42 3.95

Total 99.21 100.07 99.34 99.89 99.47 100.01

Mg# 72.75 71.67 72.62 72.30 72.35 72.60

Trace elements (ppm)

Cs 0.4 0.6 1.3 0.1 1.0 0.2

Rb 2 3 5 3 5 1

Ba 67 32 37 49 46 28

Th 0.38 0.76 0.62 0.73 0.64 0.57

Ta 0.03 0.05 0.04 0.05 0.05 0.04

Nb 0.52 0.86 0.72 0.83 0.80 0.70

Sr 41 48 39 42 56 30

Zr 17 26 20 23 21 19

Hf 0.41 0.65 0.56 0.62 0.59 0.50

Y 5.0 5.8 5.3 5.5 5.5 5.1

Sc 26 39 34 36 36 33

V 212 251 235 243 239 227

Cr 452 688 562 554 412 540

Co 27 32 27 35 29 31

Ni 98 135 141 132 105 110

Rare-earth elements (ppm)

La 0.82 1.35 1.15 1.29 1.26 1.06

Ce 1.94 3.22 2.57 2.81 2.66 2.48

Pr 0.22 0.37 0.30 0.34 0.33 0.28

Nd 1.14 1.76 1.38 1.56 1.53 1.33

Sm 0.31 0.57 0.46 0.52 0.48 0.43

Eu 0.184 0.215 0.174 0.209 0.191 0.153

Gd 0.44 0.83 0.68 0.79 0.75 0.62

Tb 0.08 0.15 0.12 0.14 0.14 0.11

Dy 0.57 1.09 0.86 0.99 0.94 0.80

Ho 0.15 0.26 0.21 0.24 0.22 0.19

Er 0.54 0.81 0.69 0.77 0.73 0.64

Tm 0.090 0.149 0.126 0.146 0.132 0.119

Yb 0.67 1.13 0.92 1.08 0.99 0.88

Lu 0.121 0.188 0.163 0.174 0.169 0.152

LOI = loss on ignition at 1100 °C

Mg# (mg-number) = 100 ×molar [MgO/(MgO+ FeOtotal)]


Discussion and conclusions

In the ophiolite mélange of the Mt. Medvednica the most com-monly occurring magmatic rocks are hectometre to kilometrelarge slices or blocks of composite fragments of the SSZ upperoceanic crust, represented by Middle-Late Jurassic extrusiverocks (Fig. 3) of N-MORB-like affinity (‘MORB with arc

signatures’ – sensu Shervais 2001). They were supposedlyformed at an active subducting ridge. The IAT-like rocks ofBAB affinity generated in various suprasubduction geotecton-ic environments are, on the other hand, less numerous(Slovenec and Lugović 2009), while boninite rocks are theleast abundant igneous fragments recovered from the ophiolitemélange of the Mt. Medvednica (Figs. 3 and 4).

Fig. 7 Discriminant diagrams for the boninitic rocks from the Mt.Medvednica ophiolite mélange. a SiO2 – MgO diagram (Le Bas 2000).b V – Ti/1000 diagram (Shervais 1982). BON = boninites; IAT = island-arc tholeiites; MORB=mid-ocean ridge basalts; BABB= back-arc basinbasalts; CAB = calc-alkaline basalts; CFB = continental flood basalts;OIB = ocean-island basalts; AB = alkali basalts. Fields for high-,medium- and low-Ti tholeiitic SSZ basalts of the Mt. Medvednica

ophiolite mélange (Slovenec and Lugović 2009), and boninites from theKopaonik area - Vardar Zone (Marroni et al. 2004), and Albanide-Hellenide boninites (Saccani and Photaides 2004, 2005; Beccaluvaet al. 2005; Saccani et al. 2008, 2011 and references therein) plotted forcorrelation constraints. c MgO – CaO diagram (Maehara and Maeda2004) and d SiO2 – FeOT/MgO diagram (Miyashiro 1974)

Table 3 Nd and Sr isotope data of boninitic rocks from the Mt. Medvednica ophiolite mélange

Sample Location Rock type 143Nd/144Nda 147Sm/144Nd 87Sr/86Sra εNd (150 Ma)b 87Sr/86Sr (150 Ma)

c

mt-18/2 1 Vl-Ti, MB 0.512702 (6) 0.195802 0.707422 (15) +1.27 0.707011

vh-49/3 2 Vl-Ti, MB 0.512662 (9) 0.195463 0.706427 (11) +0.49 0.706208

Location number corresponds to the locations in Fig. 3

Vl-Ti very low-Ti, MB massive basalta Errors in brackets for Nd and Sr isotopic ratios are given at the 2σ-level. 147 Sm/144 Nd calculated from the ICP-MS concentrations of Sm and Ndfollowing equation: 147 Sm/144 Nd = (Sm/Nd) × [0.53151 + 0.14252 × 143 Sm/144 Nd]b initial εNd (150 Ma) calculated assuming Io CHUR = 0.512638, (147 Sm/144 Nd)o CHUR = 0.1966, and λSm = 6.54 × 10−12 a−1

c87 Sr/86 Sr(150 Ma) calculated using ICP-MS Rb and Sr concentrations and assuming λRb = 1.42 × 10−11 a−1


High values of the CaO/Al2O3 ratio as well as the CaOcontent suggest the origin of analysed boninite from a portionof mantle wedge that also contained a paragenetic Ca-pyrox-ene. This type of pyroxene is not documented in the sensu-stricto boninites from the Bonin Island – Mariana Trench re-gion. There, both orthopyroxene and Ca-poor clinopyroxene(clinoenstatite) preponderate (e.g., Hawkins 2003). A narrowrange of Ti-Zr ratio values presumably reflects the absence oflarge-scale precipitation of Fe-Ti oxides during the crystalli-zation of boninite lavas. Similar effect has been reported frommodern boninites of West Pacific (Crawford and Cameron1985). The enrichment of LIL elements with regard to theclearly depleted suite of HFS elements as well as negativeTa-Nb and Ti anomalies documented in the Mt. Medvednicaboninites (Fig. 8a) stem from the melting of the previouslymetasomatized mantle. Alternatively, the late subduction pro-cesses may be held responsible for such geochemical

signature. Namely, the components entrained in the boninitemantle source are generally related to the subduction and re-lated higher degrees of partial melting (e.g., Beccaluva et al.2005). The isotopic data from the Mt. Medvednica boninitesshown in the 147Sm/144Nd – (εNd)i diagram (Fig. 10) indicate acomplex mixing of the subduction related components intro-duced into the mantle (i.e., metasomatic fluids released fromthe subducted juvenile/crustal material) with the variably de-pleted mantle melts. The influence of metasomatic flux tomantle melts parental to the studied boninite was moderatelyhigh as suggested by the low εNd values (+0.49 do +1.27). Theenrichment in both the LILE and partially LREE is also relatedto the influence of the subduction-derived aqueous fluxes(Fig. 9), whereas the HFSE concentration presents an originalmantle wedge content of the low-soluble and immobile traceelements. Negative Nb-Ta and positive Th anomalies are infavour of SSZ geotectonic environment. Despite a generally

Fig. 8 a N-MORB-normalised multielement patterns (Sun andMcDonough 1989); b REE-normalized patterns (Taylor and McLennan1985) for the Mt. Medvednica boninitic rocks. Fields for high-, medium-and low-Ti tholeiitic SSZ basalts of the Mt. Medvednica ophiolitemélange (Slovenec and Lugović 2009), and boninites from the

Kopaonik area - Vardar Zone (Marroni et al. 2004), and Albanide-Hellenide boninites (Saccani and Photaides 2004, 2005; Beccaluvaet al. 2005; Saccani et al. 2008, 2011 and references therein) plotted forcorrelation constraints

Fig. 9 Discriminant diagram aNb/Yb – Th/Yb and bNb/Yb – La/Yb forthe boninitic rocks from the Mt. Medvednica ophiolite mélange. Theslopes solid double-headed arrow shows array patterns of enrichment

and depletion of an average N-MORB mantle (Sun and McDonough1989), and the broken lines represent contours of % subduction zonecontribution for given element in the mantle (Pearce and Peate 1995)


low concentrations of incompatible elements, an arc-derivedenrichment has been recognised (i.e., high Th/Ta = 12.7–15.5and Th/Nb = 0.7–0.9). In the NbN – ThN diagram, studiedvolcanic rocks are plotted above the mantle array in the arc,i.e. boninite field, suggesting an intermediate to strong influ-ence of subduction-zone fluids (Fig. 11a). The markedly lowratios of SmN/ZrN (~0.5) and Ti/Zr (~50), lowTiN/Ti*N (<1)and ZrN/Zr*N (~1.5), along with the distinctively low Ti/V(<5.4) and Y (<6 ppm) concentrations characterise analysedrocks as boninites and clearly distinguish them from theMORB, IAT and OIB geotectonic realms (Figs. 7b, 11b, andc). Such findings are corroborated by spinel andclinopyroxene geochemistry (Figs. 5 and 6). Furthermore, rel-atively high ZrN/SmN (>1) and low NbN/ThN (~0.6) reportedin the Mt. Medvednica boninites are indicative for the forearcorigin of boninite lavas (Fig. 11d). A hypothesizedsubduction-zone forearc environment of the formation ofanalysed rocks (probably close to the trench) is corroboratedby their high Zr/Sm (45–55) and Ti/Zr (46–54) ratios as wellas the enrichment in LREE-Zr-Hf and Th (Figs. 8 and 11a).The calc-alkaline character of the Mt. Medvednica boninites(Fig. 7d and Th – Nb/16 – Hf/3 diagram (Wood 1980 – notshown)) further suggests that these rocks were not formed inan incipient stage of the forearc evolution but rather in itsmature phase (probably during the Tithonian), most likelyfollowing the emergence of an intra-oceanic nascent island

arc, which in this part of the Dinaric-Vardar Tethys appearedduring the Callovian to Oxfordian times (Slovenec andLugović 2009; Šegvić et al. 2014; Lugović et al. 2015). Itfollows that the boninite rocks of the Mt. Medvednica standfor the youngest, up to now known, the subduction-relatedoceanic crust formed during the Late Jurassic in this oceanicsegment of the Dinaric-Vardar Tethys.

Various authors implemented different geochemical vari-ables to infer on the amount of partial melts extracted from aresidual mantle source. Pearce (1983) used compatible (Cr)versus incompatible (Y) element to estimate the compositionof mantle sources and degree of partial melting that generateddifferent magma types (Fig. 11b). In the Cr vs. Y diagramanalysed boninite lavas could have been produced by about20 to 28% of partial melting of the highly depleted MORB-type mantle source (S3). Such source would correspond to therefractory mantle of harzburgite nature (Murton 1989).Similar highly depleted tectonite peridotites are known tohave been formed in the forearc environment documented inthe Mt. Medvednica ophiolite mélange (Lugović et al. 2007).The transitional harzburgites from the locality of GornjeOrešje in the easternmost part of the Mt. Medvednica depicta slightly depleted spinel composition (Mg# = 0.64–0.56;Cr# = 0.45–0.42) consistent with ~20% of partial melting.These rocks are plotted close to the theoretical S3 sourceand may represent mantle residuum remained after extractionof the Mt. Medvednica medium-Ti basalts and low-Ti island-arc tholeiites (Fig. 11b). Thus, they define a perfect source ofthe boninitic parental melts. Assumed origin of the Mt.Medvednica boninites that advocates a partial melting of thestrongly depleted mantle harzburgites is also in line withboninite depletion in HFS elements (Figs. 7b, 8a, 11b, and c).Such features were also reported in the boninites from VardarZone (Kopaonik area, southern Serbia; Marroni et al. 2004), aswell as in those from the area of the Albanide-HellenideOphiolitic Belt (e.g., Saccani and Tassinari 2015; Saccaniet al. 2017 and references therein). Cr-rich spinel renders an-other indicator of highly depleted mantle source (Fig. 5).Namely, according to Dick and Bullen (1984) spinel fromanalysed boninites has compositional counterparts only in al-pine peridotites (Type III) formed after an excessive depletionwith all diopside being consumed by partial melting.

The moderately high melting rates of analysed lavas maybe a result of melting in the hot thermal regime of a shallowforearc mantle wedge or/and these lavas were formed bymelt-ing of the mantle portion of the young intra-oceanic subduc-tion-related system (Tatsumi and Eggins 1995). Hot thermalregime is indicated by the maximal crystallization tempera-tures of augite between 1048 and 1260 (±30) °C (afterLindsley 1983) at low pressures that ranged from 0.24 to0.77 (±0.31) GPa (after Nimis and Ulmer 1998; Nimis1999). This further suggests that the magmas parental toboninite lavas of the Mt. Medvednica formed at moderately

Fig. 10 147Sm/144Nd – εNd(150Ma) diagram for the boninitic rocks fromMt. Medvednica ophiolite mélange showing the various mantle andsubduction components interpreted to be involved in its petrogenesis.Hypothetical mantle sources: DM = depleted mantle (not refracrory);VDM = very depleted mantle (refracrory); SJM = subducted juvenilematerial (subducted oceanic crust; slab with little pelagic sediment);SCM= subducted continental material. The observed compositions andhypothetical end-members sources calculated for the Middle Jurassicfollowing Swinden et al. (1990). Field for high-, medium- and low-Titholeiitic SSZ basalts of the Mt. Medvednica ophiolite mélange(Slovenec and Lugović 2009) plotted for correlation constraints


high temperatures and pressures inferior to 1 GPa (~ 0.5 GPa)and were likely segregated at moderately shallow depths (~15 km), thus corresponding to the conditions at hot mantlewedges where boninite magmas usually form (e.g., Cameronet al. 1983; Beccaluva and Serri 1988; Crawford et al. 1989;Sobolev and Danyushevsky 1994; Taylor et al. 1994). Hightemperature conditions at the shallow mantle wedge can beexplained by the subduction of a still active ridge below younglithosphere (Van der Laan et al. 1989; Pearce et al. 1992).

Thermal conditions necessary for the generation of the Mt.Medvednica boninite lavas are believed to have been met byentrainment of the still young lithosphere, formed near thespreading axis, in the subduction zone factory. Suchgeodynamic scenario fits well with geotectonic model pro-posed for the western segment of the Dinaric-Vardar Tethys(Slovenec and Lugović 2009; Slovenec et al. 2011).

Recent research, including this study with a focus set to thenorth-western branch of the Dinaric-Vardar Tethys (Slovenec

Fig. 11 Discrimination diagrams for the boninitic rocks from the Mt.Medvednica ophiolite mélange. a Simplified ThN – NbN diagram(Saccani 2014). PM= primitive mantle. Th and Nb normalized to theN-MORB composition (Sun and McDonough 1989). b Cr – Y diagram(modified after Pearce 1983 and Pearce et al. 1981). Mantle sourcecompositions and melting paths for incremental batch melting are fromMurton (1989). S1: calculated mid-ocean ridge basalt (MORB) source;S2: residue after 20 % MORB extraction; S3: residue after 12 % meltextraction from S2. BON = boninites; IAT = island-arc tholeiites;MORB = mid-ocean ridge basalts. c TiN/Ti*N – ZrN/Zr*N diagram(Taylor et al. 1994) and d NbN/ThN – ZrN/SmN diagram (Godard et al.

2003). BON = boninites; IAB = island-arc basalts; MORB=mid-oceanridge basalts; OIB = ocean island basalts; BABB = back-arc basinbasalts. Ti* = (GdN +DyN)/2, Zr* = (NdN + SmN)/2. Ti/Ti*, Zr/Zr*, Nb/Th and Zr/Sm ratios normalized to the N-MORB composition (Sun andMcDonough 1989). Fields for high-, medium- and low-Ti tholeiitic SSZbasalts of the Mt. Medvednica ophiolite mélange (Slovenec and Lugović2009), and boninites from the Kopaonik area - Vardar Zone (Marroniet al. 2004), and Albanide-Hellenide boninites (Saccani and Photiades2004, 2005; Beccaluva et al. 2005; Saccani et al. 2008, 2011 andreferences therein) plotted for correlation constraints


and Lugović 2009; Slovenec et al. 2011) had all shown aprogressive evolution of ophiolite magmas from the MORB-like to IAT-like, ultimately reaching a boninite composition inthe segment of the oceanic trench. Boninite magmatism musthave taken place following the emplacement of island-arctholeiites. Analogue sequence is reported from the ophiolitesof the Albanide-Hellenide belt (Mirdita, Pindos and Troodos)reflecting a progressive melting of mantle source that, in turn,becomes increasingly depleted (e.g., Beccaluva and Serri1988; Saccani and Photiades 2004, 2005; Dilek and Thy2009; Saccani and Tassinari 2015; Saccani et al. 2017 andreferences therein). The area of the Mt. Medvednica investi-gated in this research renders a corner-stone segment of thenorth-western branch of the Dinaric-Vardar Tethys, and to-gether with the Meliata–Maliac oceanic system, makes anintegral part of one and the same oceanic basin (Schmidet al. 2008) featured by a common tectono-magmatic evolu-tion during the Mesozoic times (Bortolotti et al. 2005, 2013).However, some regional particularities and diachronous ap-pearances are reported (Slovenec and Lugović 2009;Slovenec et al. 2010). The Mesozoic geodynamic evolutionof this part of the Dinaric-Vardar Tethys commenced with anopening of ensialic back-arc basin next to the peri-continentalvolcanic arc related to the Andean-type subduction of thePaleo-Tethyan lithosphere beneath the European continentalmargin (e.g., Stampfli and Borel 2004, Stampfli et al. 2002;Goričan et al. 2005). Initial phase in the development of thismarginal basin included intra-continental rifting during theAnisian coupled with the generation of alkaline volcanic rocks(Slovenec et al. 2010, 2011). The newly formed oceanic realmhas continuously expanded during the Triassic (Ladinian toNorian) and Early Jurassic (Pliensbachian to Toarcian) leadingto the formation of the new E-, T- and N-MORB-types ofoceanic crusts between the Apulian plate (future Adria micro-plate) and continental margin of the southern Laurussia (futureTiszia mega-block). In the Middle Jurassic the MORB-typelithosphere collapsed due to the onset of the intra-oceanicconvergence followed by the subduction of an active oceanicridge crust. In the initial phase of subduction the MORB-type magmatism is progressively replaced by IAT-type ofvolcanic activity, whereas in the Callovian/Oxfordian timea nascent island-arc has emerged (Fig. 12a and b)(Slovenec and Lugović 2008, 2009, 2012; Slovenec et al.2010, 2011; Lugović et al. 2015). A continuous productionof basaltic magmas created a large volume of SSZ-typeoceanic crust in an intra-oceanic arc-forearc system. Anelemental characteristic of the ophiolites from this segmentof the Dinaric-Vardar Ocean is the similarity and continu-ous evolut ion from the MORB and trans i t ionalMORB/IAT-like rocks toward IAT volcanites andboninites, suggesting a penecontemperaneous activity oftheir respective magmatic sources in a relatively narrowpo r t i o n o f t h e D i n a r i c -Va r d a r i n t r a - o c e a n i c

suprasubduction zone (Fig. 12b). Shortly after the extru-sion of island-arc volcanites began the hinge roll-back sub-duction of the oceanic slab (Fig. 12c and d). Such tectonicsetting is exclusively related to the fast slab roll-back in anintensive extension-dominated tectonic regime thatallowed an adiabatic upwelling of asthenosphere melt fromthe hot and hydrous, highly depleted, residual (harzburgite)mantle (e.g., Hamilton 2007; Xia et al. 2012; Boutelier andCruden 2013). This further means that the extension rate inthe upper plate is gradually balanced by the roll-back andretreat of the subducted slab. The progressive sinking ofthe down-going slab and its further retreat give rise to theupwelling of the asthenospheric mantle causing a temper-ature raise and high degrees of partial melting through outthe mantle wedge, from the arc axis to the forearc region.Tensions created by the slab roll-back likely triggered arifting, and then initiation of the seafloor splitting duringthe Late Jurassic (Kimeridgian to Tithonian). These pro-cesses eventually lead to large-scale spreading in theforearc region coupled with partial melting of hydrousand progressively depleted refractory peridotites of sub-arc mantle, eventually causing a generation of boninitemagmas (Fig. 12c and d). Based on the evidences providedherein, fast magmatic progression is hypothesized, fromIA-like tholeiites produced in the relatively depleted man-tle to extremely depleted low-Ti boninites Bmoderately^influenced by the slab-derived fluids/melts in a rapidlyevolving suprasubduction mantle wedge.

Not only different phases of IAT and boninitemagmatism took place in a spatially constrained area,these effusions also likely happened in a short intervalof time, possibly within several millions of years duringthe Late Jurassic (Oxfordian to Tithonian). Modernforearcs require about 15 Ma to achieve their maturitywhich presumes a development of the coeval and cog-nate back-arc basins (Stern and Bloomer 1992; Bloomeret al. 1995). Calc-alkaline boninites of Mt. Medvednicafits a mature arc-stage, analogous to similar rocks fromthe Mirdita Eastern Belt and Vourinos (Hellenides)(Beccaluva et al. 1978; Hoeck et al. 2002; Bortolottiet al. 2002; Saccani and Tassinari 2015; Saccani et al.2017). Obviously, the process of the subduction roll-backtriggers extension in the arc region thus eventually lead-ing to the formation and evolution of back-arc basins(e.g., Uyeda and Kanamori 1979). With the ongoingforearc extension in this segment of the Dinaric-Vardarocean, the spreading centre as well as the mantle portionproducing upwelling melts moved away from the con-verging margin toward the region of deeper mantle (farfrom mantle wedge). This is the zone of less refractoryperidotites where influence of subducting slab on thenewly produced magma is gradually decreasing(Lugović et al. 2007), thus giving rise to the formation


of the back-arc basin basalts with intermediate MORB-IAT composition (Lugović et al. 2015; Fig. 12c). Thiscognate back-arc (ensimatic) marginal basin must havebeen fully formed during the Early Cretaceous, howeverthe nascent phase of its development took place earlier at

the time of the latest Jurassic (Kimeridgian to Tithonian)(Slovenec and Lugović 2009; Šegvić et al. 2016;Fig. 12c). During the latest Jurassic, and especially inthe Early Cretaceous at the SW corner of the Dinaric-Vardar oceanic segment a significant consummation of


the crust commenced with a concomitant development ofthe accretionary wedge (Slovenec and Pamić 2002;Pamić 2002). Thereupon, the formation of mélange en-abled a preservation of the remnants of an ancient crustin the form of slices and blocks (including the boninitesof Mt. Medvednica) that were obducted during the colli-sion phase onto the northern passive continental marginof the Adria microplate (Fig. 12e) and were thereuponincorporated in the present-day ophiolite mélange of theSava Unit - thought to be the suture zone between theDinarides and fragments of the European continentalmargin (e.g., Lugović et al. 2015; Šegvić et al. 2016).

Although geographically distant, documented in the Mt.Medvednica ophiolites at the very western corner of the WestVardar Ophiolitic Unit, analysed rocks may be compositionallycorrelated with boninite rocks recovered from eastern portionsof this mega-tectonic unit (boninites of the Kopaonik Mt.,southern Serbia) as well as with those documented along theAlbanide-Hellenide ophiolitic range (Figs. 5, 7b, 8, and 11).This strongly argues for a common origin, petrogenesis andsimilar geotectonic environment where all these rocks haveformed within a single branch of the Neotethys Ocean (sensuBortolotti et al. 2004, 2005, 2013; Schmid et al. 2008) duringthe Middle to Late Jurassic.

Acknowledgments We thank Ilona Fin for producing excellent polishedthin sections. Our appreciation is further extended to Boško Lugović andHans-Peter Meyer for their assistance with microprobe measurements atthe Institute of Geosciences (University of Heidelberg, Germany). SamCarmalt and Aleksandar Ristić helped to improve the quality of theEnglish. Critical comments and constructive reviews by DraganMilovanović, Shuguang Song and an anonymous expert, as well as edi-torial comments of journal editors Qiang Wang and Lutz Nasdala con-tributed significantly to the manuscript quality. The presented work is thecontribution to the scientific project BMesozoic magmatic, mantle andpyroclastic rocks of north-western Croatia^ (grant no. 181-1951126-1141 to D. S.) carried out under the support of the Croatian Ministry ofScience, Education and Sport.

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