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


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

Damir Slovenec1 & Branimir Šegvić2

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

Abstract In the Late Jurassic to Early ophiolite mélange from the Mt. Medvednica (Vardar Ocean) blocks of boninite rocks have been documented. They emerge as massive 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 crystallization temperatures and low pressures of ~1048 to 1260 °C and ~0.24 to 0.77 GPa, respectively, delineated the SSZ mantle wedge as a plausible source of boninite parental lavas. Their whole- 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 anomalies indicative for SSZ. Thorium and LILE enrichment, and very low initial Nd-isotopic values (εNd(T = 150 Ma) +0.49to+1.27)actas vestiges of mantle-wedge . The mantle source was likely depleted by the MORB and IAT melt extraction and was contemporaneously affected by subduction fluids, prior to the large-scale adiabatic melting of the mantle hanging wall. This eventually gave rise to boninite lavas and an ultra-refractory residiuum. The genesis of boninites is related to the Tithonian 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 from the NW segment of the Dinaric-Vardar Tethys that are nowadays obducted onto the passive margins of Adria. Taking into account the existence of similar rocks in the ophiolite zones of Serbia, Albania and Greece, the boninites of Mt. Medvednica strongly favours the single Tethyan oceanic basin that existed in this part of Europe during the Late Jurassic.

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

Introduction Gamble 1991; Woodhead et al. 1998; Winter 2001). It is there- fore an imperative to clearly identify different components of The subduction zone volcanism results from the interaction of island-arc and forearc magmatic systems necessary to produce fluids released from a subducted slab and mantle wedge over- a quantitative model applicable to formation in such lying a descending plate. The fluid has a decisive effect in settings. It remains however a challenge to reveal a range of lowering a melting temperature of the mantle and leading to components entrained by subduction fluids into the mantle a melt generation that rises upward to produce a chain of melting portion. The influence of subduction-related fluxes volcanoes known as an (e.g., McCulloch and is best recognizable in derived from refractory mantle sources at low pressures and small crustal depths Editorial handling: Q. Wang (Hawkesworth and Ellam 1989). Boninites can provide fun- damental information in that regard, as they are known to have * Damir Slovenec formed from the part of mantle wedge that had previously [email protected] been exposed to highest degrees of depletion in an island-arc geotectonic regime (Crawford et al. 1989; Taylor et al. 1994; 1 Croatian Geological Survey, Sachsova 2, HR-10 000 Zagreb, Croatia Dilek and Thy 2009; Resing et al. 2011; Escrig et al. 2012). 2 Department of Geosciences, Texas Tech University, 1200 Memorial Higher degrees of melting at high-temperatures (~1200– Circle, Lubbock, TX 79409, USA 1350 °C) and shallow depths (ca. 25–50 km; pressures 1.0– D. Slovenec, B. Šegvić

1.5 GPa) of hot mantle wedge are needed to produce boninite suprasubduction geological setting these rocks represent a lavas (Umino and Kushiro 1989; Falloon and Danyushevsky missing piece of the puzzle allowing a more exhaustive 2000; Kushiro 2007; Green et al. 2010). There is a general geodynamic reconstruction of this part of the Dinaric-Vardar agreement on boninite petrogenesis suggesting the mantle oceanic space during the Middle to Late Jurassic time source enrichment through the metasomatism of sub-forearc (Slovenec and Lugović 2009; Slovenec et al. 2011; Šegvić mantle by hydrous fluids or melts derived from a subducting et al. 2016). Placed at western margins of the Dinaride- plate (e.g., Hickey and Frey 1982;Murtonetal.1992; Albanide-Hellenide ophiolite realm, the boninite rocks of Ishikawa et al. 2002; Dilek and Thy 2009). Such processes Mt. Medvednica (Fig. 1) are of high importance for the po- that affected the mantle source have already been documented tential correlation with similar rocks known from the ophiolite in post-subduction blocks of igneous rocks archived in the zones in the southern part of the Balkans. The aim of this work ophiolite mélange of Mt. Medvednica (Lugović et al. 2007; is to propose a comprehensive petrological and geochemical Slovenec and Lugović 2009). characterization of boninites from the ophiolite mélange of Boninite rocks present sensitive but powerful indicators of Mt. Medvednica and to report on its petrogenesis and geotec- mantle wedge processes in the suprasubduction zones and tonic significance in the light of high-resolution geodynamic their appearance has always had important tectonic implica- reconstruction of the evolution of the Jurassic north-western tions (e.g., Meijer 1980;Crawfordetal.1989; Pearce et al. branch of the Dinaric-Vardar Tethys. Finally, an attempt was 1992; Falloon et al. 2008). They have been reported in numer- made to investigate the correspondence of analysed rocks with ous Tethyan (Troodos, Oman, Pindos -e.g., their analogues from the southern parts of the Dinaric-Vardar– Cameron 1985; Kostoeoulos and Murton 1992;Ishikawa Albanide–Hellenide ophiolite belts in order to test a hypothe- et al. 2002;Pe-Piperetal.2004; Saccani et al. 2017 and sis of the single oceanic space that existed in this part of references therein) or in the Izu-Bonin and Tonga arcs (e.g., Tethys during times. Crawford et al. 1989) where they are frequently associated to ophiolite rock suites. Most of the authors link modern boninite lavas to intra-oceanic forearc-arc systems, with boninites typ- Geological setting ically forming at the basements of arc volcanoes (e.g., Hawkins et al. 1984; Murton 1989; Falloon and Crawford Mount Medvednica, located in northern Croatia, is situated at 1991; MacPherson and Hall 2001; Resing et al. 2011; the triple junction of three major geotectonic units - the South- Saccani and Tassinari 2015; Saccani et al. 2011, 2017). , the Dinarides and the Tiszia continental block Boninite genesis is thought to have been related to the sub- (Pamić and Tomljenović 1998; Slovenec and Pamić 2002; duction inception, slow convergence and slab roll-back that Fig. 1). Its northern slopes bear the record of the eventually lead to forearc-arc splitting and/or forearc rapid and Jurassic oceanic crust of the Neotethys (Lugović et al. extension and upwelling of the mantle allowing a volatile- 2007; Slovenec and Lugović 2008, 2009, 2012; Slovenec fluxed decompression melting of severely depleted mantle et al. 2010) that belongs to the westernmost segment of the rocks (e.g., Crawford et al. 1981, 1989; MacPherson and Western Vardar Ophiolitic Unit (sensu Schmid et al. 2008). Hall 2001;Ishikawaetal.2002 and references therein; Dilek Along with other intra-Pannonian inselgebirge like the Mts. and Thy 2009). A similar genetic succession of oceanic crust Kalnik, Ivanščica and Samoborska Gora, the Mt. Medvednica formation during the Mesozoic has already been suggested for represents a part of the Sava Unit (sensu Haas et al. 2000; the ophiolitic sequence of Mt. Medvednica (Slovenec and Fig. 2) or the Zagorje-Mid-Transdanubian Zone (ZMTDZ; Lugović 2009). sensu Pamić and Tomljenović 1998). The Sava Unit is an Boninite rocks have rarely been documented in the about 100 km wide and approximately 400 km long sheared ophiolite zones of Albanides and Hellenides but their petro- belt sandwiched between the two regional systems: the genesis was thoroughly investigated (Beccaluva and Serri Zagreb-Zemplin (ZZL) and the Periadriatic-Balaton lineament 1988;Bébienetal.2000; Bortolotti et al. 2002;Pe-Piper (PL-BL). The PL-BL separates the Sava Unit to the north from et al. 2004; Saccani and Tassinari 2015; Saccani et al. 2017 the Austroalpine and Pelso Units, while the ZZL confines the and references therein; Fig. 1). However, in the Dinaric- Sava Unit south-westward towards the Tiszia Mega-unit Vardar ophiolite zone the boninite-type rocks are reported (Fig. 2). The geological history of the Sava Unit is complex solely in the region of Kopaonik (southern Serbia; Marroni which is mirrored in its composition consisted of amalgamat- et al. 2004) and at the extremely west portion of the Dinaric- ed Dinaric and South Alpine tectonostratigraphic fragments Vardar Ophiolitic Zone in the area of Mt. Medvednica, which (e.g., Haas et al. 2000). The Paleotethys back-arc oceans is a part of the Sava Unit (sensu Haas et al. 2000;Fig.2). This known in the literature as Meliata-Maliac or Meliata- study brings a first report on the occurrence of boninites from Hallstatt (e.g., Stampfli et al. 2002) experienced a constant the ophiolite mélange of Mt. Medvednica in Northern Croatia shortening during the Jurassic period due to the rotation of (Fig. 3). Having originated intheLateJurassic Africa with respect to Europe (Cavazza et al. 2004;Burke Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –...

Fig. 1 Geotectonic sketch map of the major tectonic units of the Alps, Piemont-Liguria, Vahicum, Inacovce-Kriscevo, Szolnok, Sava; 4c = Carpathians and Dinarides (simplified after Schmid et al. 2008). The Transylvanian, South Apuseni, Eastern Vardar; 5 = ; 6 = inserted map (upper left corner): regional geographic overview. 1 = , High Karst and Dalmatian Zone; 7 = Pre-Karst and Adria derived far-travelled nappes Alps and W. Carpathians Bosnian Flysch; 8 = East Bosnian-Durmitor; 9 = Drina-Ivanjica, Korab, (ALCAPA); 2 = Europe-derived units (Dacia); 3 = mixed European and Pelagonides; 10 = Bükk, Jadar, Kopaonik; 11 = black arrow indicates the Adriatic affinities (Tisza); Ophiolites oceanic accretionary prisms: 4a = Medvednica Mt. investigated area Meliata, Darnó-Sźarvaskö, Dinaric, Western Vardar, Mirdita; 4b =

2011). This eventually led to the opening of the Vardar The geography of the studied area and a simplified geolog- suprasubduction ocean during the roll-back of the ical map of the Mt. Medvednica northern slopes are shown in Meliata-Maliac (e.g., Stampfli et al. 2002;Saccanietal. Fig. 3. The mountain consists of pre-Neogene heterogeneous 2008;Bortolottietal.2013). In the area of the Sava Unit and superimposed Dinaric and Alpine tectonostratigraphic an ensuing Middle to Late Jurassic intra-oceanic conver- and tectonometamorphic units of both the continental and gence of the Western Vardar realm was manifested by a oceanic origin (e.g., Pamić and Tomljenović 1998;Tariand vivid tectonic activity that included a subduction of an ac- Pamić 1998;Haasetal.2000; Pamić 2002; Tomljenović et al. tive oceanic ridge, arc activity, and back-arc magmatism 2008). The earliest Silurian to Middle Triassic (Ladinian) (Slovenec and Lugović 2009;Bortolottietal.2013). The -sedimentary successions were subjected to low- final closure of the Neotethys was suggested to correspond grade metamorphism during the Lower Aptian (Belak et al. to the Barremian-Aptian, constrained in the Sava Unit by 1995 and references therein) and were, thereafter, tectonically the age of greenschist of the Mt. Medvednica (Belak et al. overlain by the ophiolite mélange unit (Kalnik Unit, sensu 1995) nowadays obducted onto the Adria passive margins Haas et al. 2000;Fig.2). The obduction of ophiolites further (Lugović et al. 2006). Despite a mixed composition of the caused an Early Cretaceous metamorphism with Late Jurassic Sava Unit that belongs to both the Dinaric-Vardar and island-arc metamorphosed up to the greenschist meta- South Alpine tectonostratigraphic units (e.g., Haas et al. morphic facies conditions (Lugović et al. 2006). The ophiolite 2000) its affiliation to the Dinarides and their ophiolites is mélange that crops out abundantly along the northern slopes widely endorsed (e.g., Milovanović et al. 1995;Pamić and of the Mt. Medvednica is mainly found in the tectonic contact Tomljenović 1998; Šegvić et al. 2016). with a series of Mesozoic sedimentary rocks or, in a lesser D. Slovenec, B. Šegvić

Fig. 2 Sketch map of the structural units and major lineaments (modified External Dinaridic Unit; 10 = Tisza Mega-Unit; 11 = black arrow after Haas et al. 2000). 1 = Austroalpine units; 2 = Pelso Unit; 3 = South indicates the Medvednica Mt. study area; 12 = box indicates the area Alpine units and Julian-Savinja and South Karawanken units; 4 = South shown on the Fig. 3; BL = Balaton Lineament; ZZL = Zagreb-Zemplin Zala Unit; 5 = Central Slovenian and Bosnian units; 6 = Medvednica Lineament; PL = Periadriatic Lineament Unit; 7 = Kalnik Unit; 8 = Internal Dinaridic Unit (Vardar Unit); 9 = extent, it is thrusted on sedimentary rocks of Neogene and Slovenec et al. 2010). Igneous components of mélange show Quaternary age. various geochemical affinities consistent with their distinct The Mt. Medvednica ophiolite mélange and analogue units nascent geotectonic environments that existed from the documented at the mountains of the Samoborska Gora, Kalnik Illyrian to the late Oxfordian (Slovenec and Lugović 2009; and Ivanščica form a joint tectonostratigraphic unit known as Slovenec et al. 2010). However, among those igneous com- the Kalnik Unit (Fig. 2). This unit also includes the remnants ponents of the Mt. Medvednica ophiolite mélange two of a specific oceanic realm reported in the literature as the decametre-sized blocks of Bexotic^ volcanic rocks (i.e. Repno oceanic domain (ROD; sensu Babić et al. 2002). The boninite) have been recovered (Fig. 4). Their geochemistry mélange of the Kalnik Unit is characterized by the poorly clearly differentiates from the above-mentioned igneous ex- preserved original structural and depositional order. It shows trusives. The positions of boninites are indicated in Fig. 3. structural features delineated by block-in-matrix fabric, typi- Regardless of a relatively small area of exposure of boninite cal for chaotic complexes from subduction-related tectonic rocks their appearance in ophiolite mélange of the Kalnik Unit mélanges (Festa et al. 2010). This olistoliths-dominated com- presents a valuable piece of information essential for an in- plex is mixed with the fault-bounded heterogeneous fragments depth comprehension of the geodynamic evolution of the of different Mesozoic rocks. Their range varies from pebbles north-western oceanic branch of the Dinaric-Vardar Tethys. and 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 is Analytical techniques dominantly composed of intrusives ( and gabbros) and extrusives (basalts), and fragments of sedimen- Chemical composition of mineral phases from two samples tary rocks (greywackes, minor shales, red and grey cherts and were analysed using a CAMECA SX51 electron microprobe scarce ) (Slovenec and Lugović 2008, 2009; equipped with five wavelength-dispersive spectrometers. The Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –...

Fig. 3 Simplified geological map and stratigraphic column of Mt. diabase dikes) and boninites, 4b = gabbros (intersected by diabase Medvednica (modified after Šikić et al. 1978;Basch1981 and Halamić dikes), 4c = cumulate peridotites, 4d = Triassic (squares field) - Jurassic 1998). 1 = Neogene and Quaternary sedimentary rocks; 2 = Late (triangles field) radiolarites, shales and basalts (light gray fields); 5 = Cretaceous-Paleogens flysch including Senonian carbonate ; Palaeozoic metamorphic complex; 6 = reverse or thrust faults; 7 = normal 3 = Alb-Cenomanian limestones and clastic rocks (shale, siltite and faults; 8 = geological line; 9 = sample locations: 1 = mt-18/1, −18/2, −18/ sandstone); 4 = Jurassic/Early Cretaceous ophiolite mélange (pelitic to 3; 2 = vh-49/1, −49/2, −49/3; 10 = picture break siltous matrix) with blocks and slices of: 4a = basalts (intersected by operating parameters included 15 kV accelerating voltage, trace elements at Actlab Laboratories. International mafic 20 nA beam current, and ∼ 1 μmbeamsize(~10μmfor rocks were used as standards. Major element and trace ele- feldspars). Counting times of 20 s on peak and 10 s on back- ment concentrations were measured with accuracy better than ground on both sides of the peak were used for all elements. 1and5%,respectively. Limits of detection (LOD) were calculated as the minimum Isotopic compositions of two bulk rock samples were mea- concentration required to produce count rates three times sured in CRPG using a Triton Plus mass spectrometer. higher than the square root of the background (3 s; 99 wt.% Normalizing ratios of 86Sr/88Sr = 0.1194 and 146Nd/144Nd = degree of confidence at the lowest detection limit)., and 10 s 0.7219 were assumed. The 87Sr/86Sr ratio for the NBS 987 Sr counting time for all elements (peaks and backgrounds). standard for the period of measurement was 0.710242 ± Natural minerals, oxides (corundum, spinel, hematite, and ru- 0.000030 (2σ). The 143Nd/144Nd ratio for the La Jolla standard tile), and silicates (albite, orthoclase, anorthite, and wollaston- was 0.5118451 ± 0.000010 (2σ). Total procedural blanks were ite) were used for calibration. The measurements relative error ~500 pg and ~150 pg for Sr and Nd, respectively. was less than 1%. Raw data were corrected for matrix effects using the PAP algorithm (Pouchou and Pichoir 1984, 1985) implemented by CAMECA. Mineral phase formula calcula- Results tions were done using a software package designed by Hans- Peter Meyer (personal communication). Petrography and mineral chemistry Bulk-rock powders for chemical analyses of six samples were obtained from rock chips free of veins. The samples were Analysed boninite rocks from the Mt. Medvednica ophiolite analysed by ICP-OES for major elements and ICP-MS for all mélange dominantly emerge as fine-grained massive lavas D. Slovenec, B. Šegvić

Fig. 4 a Decametre-sized block in a strongly sheared pelitic- siltous continent derived matrix of the Mt. Medvednica ophiolite mélange (Location 1 from the Fig. 3). 1 = pelitic-siltous matrix; 2 = boninite lavas. b Massive boninite lavas from the Mt. Medvednica ophiolite mélange (Location 2 from the Fig. 3). Microphotographs of thin section of the Mt. Medvednica boninitic rocks c sample mt-18/1 N+ and d sample vh-49/1 N–. cpx = clinopyroxene; sp. = spinel; ab = albite; chl = chlorite; pmp = pumpellyite; ti = titanite; cal = calcite; qz = quartz

(Fig. 4b). Their holocrystalline massive base mass consists of plagioclase, which is generally accepted as an important char- clinopyroxene microlites accompanied by rare laths of altered acteristic of SSZ lavas (e.g., Beccaluva et al. 1980, 1989). The plagioclase, spinel, accessory and interstitial Fe-Ti oxides and absence of low-Ca clinopyroxene (clinoenstatite) in Mt. a range of hydrous secondary phases (Fig. 4c–d). Subtle trans- Medvednica boninites on one hand and presence of versal veins are filled with chlorite, epidote, prehnite, calcite plagioclase-rich matrix on the other hand, along with the and/or quartz. Clinopyroxene is omnipresent and is partially Cr# and Mg# values of analysed chromite are all parameters saussuritized or altered to distinguishable secondary phases that are very much alike to those of other Tethyan boninite such as chlorite, prehnite, and low-Al pumpellyite (sensu rocks (e.g., from Troodos ophiolites; Cameron 1985; Ishizuka 1999)(Fig.4c). High fluid content hampered a Beccaluva and Serri 1988). large-scale plagioclase crystallization High Cr# (0.73–0.79) and Mg# (0.45–0.50) coupled with

(Ohnenstetter and Brown 1992), which in analysed samples low Fe# (2–12) and TiO2 values that are reported from the is entirely re-crystallized into secondary albite. Primary dark studied Al-chromite are typically reported in boninite rocks greyish to blackish low-Ti (TiO2 =0.13–0.22 wt.%) Al- (Fig. 5). These values further delineate a primitive nature of chromite (Fig. 5a; Table 1) is frequently changed to ilmenite, boninite melts and at the same time testify on their high initial leucoxene, a mixture of titanite and rutile, and may sporadi- Cr concentrations. Phase chemistry of analysed chromite cor- cally be encountered as inclusions in clinopyroxene (Fig. 4d). responds well to those provenancing from the Albanide- Taking into account that the Mt. Medvednica boninite rocks Hellenide boninite series (Fig. 5b; Saccani and Tassinari are altered to a certain extent, their igneous aphyric (rarely 2015). Representative clinopyroxene compositions from porphyric) sub-ophitic to intergranular texture (Fig. 4c–d) analysed rocks are provided in Table 1. Clinopyroxene is made of plagioclase needles and equigranular clinopyroxene mainly represented by augite and Mg-rich augite (Wo30.75– has been totally preserved. Petrographic evidences suggest a 39.93En49.34–56.75Fs7.84–13.17; after Morimoto (1988)classifica- following crystallization order: spinel → clinopyroxene → tion diagram – not shown) that both show a trend of Mg-Ca plagioclase ± Fe-Ti oxides (Fig. 4d). This sequence is fully enrichment (Table 1). In all samples the analysed in line with the crystallization order documented in boninite clinopyroxene has a homogenous core and depict a normal rocks (e.g., Ohnenstetter and Brown 1996). In the studied zonation defined by a continuous decrease in Mg#, AlVI/ samples very low-Ti lavas clinopyroxene precedes AlIV, Cr and Ca content toward the rims, while at the same Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –...

Fig. 5 Classification and discrimination diagrams for spinels from the and references therein. b Cr# – Mg# diagram. Cr# = 100*(Cr/(Cr + Al)); boninitic rocks from the Mt. Medvednica ophiolite mélange. a Trivalent Mg# = 100*(Mg/(Mg + Fe2+)). Fields for spinels in boninites, forearc Cr–Al–Fe3+ ternary cation plot (Stevens 1944). Fields for mid-ocean peridotites and abyssal peridotites, as well as Albanide-Hellenide ridge (MORB) setting and boninites (BON), as well as boninites from ophiolites are from Saccani and Tassinari (2015) and references therein the Albanide-Hellenide ophiolites are from Saccani and Tassinari (2015) time the Ti abundances increase. Such a zonation pattern is redistribution during alteration (Polat et al. 2002; Polat and typical for rapid cooling in closed magmatic systems (Stern Hofmann 2003), is featured by moderate low values (up to 1979; Nakagawa et al. 2002). Clinopyroxene is further fea- 3.95 wt.%), pointing to low degrees of secondary alterations tured by a low-TiO2 content (0.01–0.23 wt.%; Fig. 6), high that are normally accounted for the low-temperature ocean- Mg# (81.06–88.55) and total absence of Fe enrichment floor hydrothermal metasomatism under prehnite-pumpellyite (Table 1) implying they were derived from a depleted mantle to lower greenschist facies conditions (e.g., Mevel 1981; source. The Cr2O3 content in is high (0.11 to Peacock 1987;Erzinger1989; Topuz et al. 2013). 1.10 wt.%) and is well correlated with the Mg# (not shown). Notwithstanding the less pronounced hydrothermal alter- In general, the composition of analysed clinopyroxene is sim- ations, magmatic textures of analysed rocks have been pre- ilar to those reported in SSZ ophiolites and their modern an- served as well as their pristine geochemistry that reflects the alogues (e.g., Beccaluva et al. 1989). Clinopyroxene chemis- original chemistry of parent magmas. This may be inferred try is also known as an excellent indicator of from Na2Ovs.CaOratio(Graham1976; Stillman and thermobarometric regime that prevailed in the magmatic res- Williams 1979 – not shown), which is traditionally employed ervoir at the time of its crystallization (Wass 1979; Shiffman to screen igneous rocks for the low-grade spilitization effects. and Lofgren 1982). The AlVI/AlIV ratio is constantly below Analysed rocks from Mt. Medvednica are discriminated in the 1.7, which is typical for clinopyroxene originating from low- field of unchanged basalts thus clearly suggesting a minimal to medium-pressure magmatic rocks (Wass 1979; Coish and alteration during hydrothermal alterations and low-grade

Taylor 1979; Shiffman and Lofgren 1982). Maximal crystal- metamorphism. In the SiO2 vs. MgO (Fig. 7a) and Ti/1000 lization temperatures of analysed augite were estimated to fit vs. V (Ti/V = 3.7–5.3; Fig. 7b) classification diagrams the the range from 1048 to 1260 (± 30) °C(afterLindsley1983), analysed volcanic rocks are distinctly defined as boninites. while the geobarometer of Nimis and Ulmer (1998)andNimis Because of the high CaO content (>9.57 wt.%; Fig. 7c),

(1999) resulted with low equilibration pressures from 0.24 to CaO/Al2O3 ratio that fits the range between 0.76 and 0.84, 0.77 (± 0.31) GPa. SiO2 abundances that do not exceed 56 wt.%, and total alkali content that is below 2.07 wt.% the Mt. Medvednica boninite Bulk rock geochemistry rocks can be classified as high-Ca boninites (Crawford et al. 1989) clearly belonging to the calc-alkaline igneous rock se- Representative major and trace element geochemical analyses ries (Fig. 7d). Yet, the major chemistry of analysed rocks of six boninite samples from the Mt. Medvednica boninite are corresponds to basaltic (TAS classification, after provided in Table 2. Isotopic compositions of Nd and Sr from Le Bas 2000;LeMaitre2002 – not shown) but no plagioclase two representative boninite rocks are given in Table 3.Losson were documented, thus favouring their classifica- ignition (LOI), used to monitor the extent of element tion as boninites (Beccaluva and Serri 1988). They are rich in Table 1 Representative chemical compositions, calculated mineral formulae and calculated modal fractions of spinel and clinopyroxene from the boninitic rocks in the Mt. Medvednica ophiolite mélange

Mineral Spinel Clinopyroxene

Sample mt-18/ mt-18/ mt-18/ mt-18/ vh-49/ vh-49/ vh-49/ vh-49/ mt-18/ mt-18/ mt-18/ mt-18/ vh-49/ vh-49/ vh-49/ vh-49/ vh-49/ vh-49/ vh-49/1 Site 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 gm gm gm gm gm gm gm gm gm,c gm, r gm gm gm gm gm gm gm gm, c gm, r

Oxides (wt%) SiO2 0.08 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 TiO2 0.14 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 Al2O3 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 Cr2O3 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 Na2O 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 Calculated mineral formulae (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 AlVI ––––––––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 ––––––––––– AlVI/AlIV ––––––––0.41 0.88 0.80 0.40 1.03 1.06 0.23 0.68 0.83 1.10 1.70 Modal mineral fractions (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 × [Mg/(Mg + Fe2+ )]; Cr# (cr-number) = 100 × [Cr/(Cr + Al)] gm groundmass, c core, r rim .Soee,B. Slovenec, D.

Wo wollastonite (Ca2Si2O6), En enstatite (Mg2Si2O6), Fs ferrosilite (Fe2Si2O6) a Calculated on the basis of 3 cations and 4 oxygens for spinel and 4 cations and 6 oxygens for clinopyroxene Š egvi ć Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –...

Fig. 6 Discriminant diagrams for clinopyroxene from the Mt. tholeiites; B-A = intra-oceanic forearc basalts and andesites; BON = Medvednica boninitic rocks. a SiO2/100 – Na2O – TiO2 diagram and b boninite. Fields for clinopyroxene compositions from high-, medium- Ti – AlIV diagram (simplified after Beccaluva et al. 1989). MORB = mid- and low-Ti tholeiitic SSZ basalts of the Mt. Medvednica ophiolite ocean ridge basalts; BABB = back-arc basin basalts; IAT = island-arc mélange (Slovenec and Lugović 2009) plotted for correlation constraints compatible mantle-derived trace elements, like NiO well as during a very low-grade metamorphism that these (>97 ppm) and Cr (>413 ppm), that were likely derived from rocks were presumably exposed to. Therefore, these ele- refractory mantle peridotites (Crawford et al. 1989). High Cr ments are considered useful in tracing the nature of mantle content (Cr >400 ppm) permits one to classify these rocks as source. Only LREE might have been affected by mobiliza- high-Cr boninites (Pe-Piper et al. 2004). Boninite lavas of the tion during alteration; however, good correlations between Mt. Medvednica are relatively primitive with high Mg# (~72), LREE and many immobile elements (not shown) indicate

Cr# (~85), showing an extreme depletion in TiO2 solely a slight mobilization of LREE. Still, their petroge- (<0.23 wt.%), P2O5 (≤0.02 wt.%), Zr (<27 ppm), Y netic application requires a great care. Relatively good cor- (<5.9 ppm) and other HFSE as well as rare earth elements relation between transitional metals (V, Cr, Mn, Fe, Ni and

(REE) (Table 2). Because of the low TiO2 content Zn) and Zr argues on their magmatic abundances in (<0.3 wt.%) these rocks may be defined as low-Ti boninites analysed rocks (Canil 1987). Vanadium and Ni are, there- with Al2O3/TiO2 ratios (60–100) that are considerably higher fore, used in further geochemical and petrogenetic discus- with regard to the primitive MORB magmas (up to 32; sions. Good and positive correlation of Ti and Zr indicates

Crawford and Cameron 1985). High CaO/Al2O3 ratio (0.76– a low amount of early crystallization of Fe-Ti oxides. The 0.84) that approaches the values of chondrite and primary input of Th and La in the generation of mafic lavas was upper mantle (0.90; Clague and Frey 1982) and the presence assessed using trace-element diagrams normalised to the of plagioclase microlites clearly distinguish the Mt. highly incompatible Yb (Pearce and Peate 1995;Peate Medvednica boninite series from the Tertiary boninites sensus et al. 1997). The contribution of Th and La from the stricto which originate from the Bonin Island – Mariana subducting slab was up to 85% and ~25%, respectively

Trench (CaO/Al2O3 = 0.72–0.38). It follows that the rocks (Fig. 9; Pearce 1983). The suprasubduction zone origin of analysed herein better correspond to the Tethyan boninite the Mt. Medvednica boninites is further indicated by the (e.g., Troodos, Beccaluva and Serri 1988). field geological data placing them as blocks in the In spider diagram, LIL elements (i.e., Cs, Rb, Ba, and K; ophiolite mélange. Fig. 8a) depict a selective mobility and will therefore be ex- In multi-element spider patterns normalized to N-type cluded from further petrogenetic considerations. These ele- MORB analysed boninites shows the moderate oscillations ments were likely entrained in the mantle wedge by the fluids of HFS elements (Fig. 8a). The LIL elements (Cs, Ba and released from the subducting slab (Ellam and Hawkesworth Rb) depict the selective enrichments only due to post- 1988;PearceandPeate1995). In line with earlier studies on magmatic alterations while Th remains as a relatively stable similar rocks (e.g., Pearce and Norry 1979;Saundersetal. igneous indicator. Its enrichment relative to other incompati- 1979;Shervais1982; Beccaluva et al. 1983), the HFS ele- ble elements eflects the fluid input from subduction zone ments(Nb,Ta,Ti,Hf,P,andY)andmostoftheREE (Wood et al. 1979; Pearce 1983). The HFS element relative (middle REE and heavy REE) measured in the boninites concentration profile is characterized by humped patters at of Mt. Medvednica seem to have remained immobile dur- very low concentration levels (0.1 to 0.6 times relative to N- ing the mantle wedge metasomatism of subducted slab as MORB). All samples show negative Nb-Ta, P, and Ti D. Slovenec, B. Šegvić

Table 2 Chemical compositions of boninitic rocks from the Mt. anomalies [(Nb/La)N =0.67–0.71; (P/Nd)N = 0.45–0.81; (Ti/ Medvednica ophiolite mélange Sm)N =0.72–0.87] thus suggesting average to high inputs of Sample mt-18/1 mt-18/2 mt-18/3 vh-49/1 vh-49/2 vh-49/3 subduction fluids released from the down-going slab (Pearce 1982; Tatsumi and Kogiso 2003). Moreover, the positive Major oxides (wt%) anomaly of Sr [(Sr/Nd)N =1.85–2.96] is a typical feature of SiO2 53.41 54.83 53.73 55.11 54.02 53.98 subduction-related magmas from recent as well as ancient arcs TiO2 0.13 0.22 0.16 0.20 0.19 0.16 (e.g., Pearce et al. 1984; McCulloch and Gamble 1991). Al2O3 12.95 13.25 12.73 12.64 12.70 12.68 Analysed boninite rocks are also characterized by the positive

Fe2O3total 7.63 7.81 7.72 7.69 7.88 7.71 anomaly of Zr-Hf [(Zr/Sm)N =1.55–1.95], which is a charac- MnO 0.15 0.13 0.15 0.14 0.15 0.16 teristic of boninite magmas from the Western Pacific boninite MgO 8.99 8.52 8.93 8.79 8.81 8.92 suites (e.g., Crawford et al. 1989; Pearce et al. 1992). CaO 10.86 10.09 10.41 9.57 10.43 10.44 Chondrite-normalized REE patterns show typical trends

Na2O 2.01 1.88 1.88 1.92 1.79 1.96 characteristic for boninites from forearc regions (e.g.,

K2O 0.05 0.05 0.04 0.12 0.07 0.04 Crawford et al. 1989). In addition they show similarities

P2O5 0.01 0.02 0.01 0.02 0.01 0.01 with boninites from many ophiolite complexes (e.g., LOI 3.02 3.27 3.58 3.69 3.42 3.95 Beccaluva and Serri 1988; Bédard 1999) emerging in forms Total 99.21 100.07 99.34 99.89 99.47 100.01 of characteristic concave-upward (U-shape) profiles Mg# 72.75 71.67 72.62 72.30 72.35 72.60 (Hickey and Frey 1982), showing a slight LREE enrich-

Trace elements (ppm) ment [(La/Sm)CN =1.49–1.66)] and negatively fractionated Cs 0.4 0.6 1.3 0.1 1.0 0.2 HREE [(Tb/Lu)CN =0.43–0.54] at levels of 1.3 to 1.5 times Rb235351 relative to chondrite as well as depleted MREE with regard Ba 67 32 37 49 46 28 to HREE (Fig. 8b). The LaCN/YbCN ratio was reported to be Th 0.38 0.76 0.62 0.73 0.64 0.57 between 0.80 and 0.86. Such a discordant LREE enrich- Ta 0.03 0.05 0.04 0.05 0.05 0.04 ment in the worldwide boninite suites is commonly as- Nb 0.52 0.86 0.72 0.83 0.80 0.70 cribed to the second-stage re-enrichment process that nor- Sr 41 48 39 42 56 30 mally takes place prior or during boninite generation (e.g., Zr 17 26 20 23 21 19 Xia et al. 2012 and references therein). The REE concen- Hf 0.41 0.65 0.56 0.62 0.59 0.50 tration levels reported herein indicate an extremely deplet- Y 5.0 5.8 5.3 5.5 5.5 5.1 ed nature of mantle source area of analysed boninites. – Sc 26 39 34 36 36 33 Estimated Eu and Sr anomalies (Eu/Eu* = 0.90 1.52 and – V 212 251 235 243 239 227 Sr/Sr* = 1.42 2.37, respectively) reflect the fractionation Cr 452 688 562 554 412 540 of plagioclase and its low accumulation during magmatic Co 27 32 27 35 29 31 differentiation (Hawkesworth et al. 1977;Westetal.1992), Ni 98 135 141 132 105 110 which requires a moderately to high degrees of partial melt- Rare-earth elements (ppm) ing (e.g., Saccani and Photiades 2004; Beccaluva et al. 2005). La 0.82 1.35 1.15 1.29 1.26 1.06 Measured 143Nd/144Nd ratios of the two boninite rocks Ce 1.94 3.22 2.57 2.81 2.66 2.48 from the Mt. Medvednica ophiolite mélange do not show Pr 0.22 0.37 0.30 0.34 0.33 0.28 significant discrepancies and fit the range from 0.512662 to Nd 1.14 1.76 1.38 1.56 1.53 1.33 0.512702, which corresponds to the initial ε values Sm 0.31 0.57 0.46 0.52 0.48 0.43 Nd(150 Ma) from the range of +0.49 to +1.27 (Table 3). This fits well the Eu 0.184 0.215 0.174 0.209 0.191 0.153 ε of the pristine mantle (ε ~0). Further on, the measured Gd 0.44 0.83 0.68 0.79 0.75 0.62 Nd Nd 143Nd/144Nd values are relatively low compared to the local Tb 0.08 0.15 0.12 0.14 0.14 0.11 MORB sources and MORB/IAT-like SSZ basalts (Fig. 10; Dy 0.57 1.09 0.86 0.99 0.94 0.80 Slovenec and Lugović 2009). The ratios of 87Sr/86Sr are be- Ho 0.15 0.26 0.21 0.24 0.22 0.19 tween 0.706427 and 0.707422, corresponding to initial ratios Er 0.54 0.81 0.69 0.77 0.73 0.64 of 0.706208 to 0.707011 calculated for the age of 150 Ma. Tm 0.090 0.149 0.126 0.146 0.132 0.119 This age stands for the anticipated crystallization ages of Yb 0.67 1.13 0.92 1.08 0.99 0.88 analysed boninite rocks. Nevertheless, the elevated 87Sr/86Sr Lu 0.121 0.188 0.163 0.174 0.169 0.152 ratio values may indicate a seawater alteration at the time of LOI = loss on ignition at 1100 °C the ocean-floor hydrothermal metamorphism (e.g., Karpenko et al. 1985;Bachetal.2003). These values will therefore be Mg# (mg-number) = 100 × molar [MgO/(MgO + FeOtotal)] excluded from further petrogenetic considerations. Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –...

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

143 144 a 147 144 87 86 a b8786 c Sample Location Rock type Nd/ Nd Sm/ Nd Sr/ Sr εNd (150 Ma) Sr/ Sr (150 Ma) 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 a 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 Nd following equation: 147 Sm/144 Nd = (Sm/Nd) × [0.53151 + 0.14252 × 143 Sm/144 Nd] b o 147 144 o −12 −1 initial εNd (150 Ma) calculated assuming I CHUR = 0.512638, ( Sm/ Nd) CHUR = 0.1966, and λSm =6.54×10 a c87 86 −11 −1 Sr/ Sr(150 Ma) calculated using ICP-MS Rb and Sr concentrations and assuming λRb =1.42×10 a

Discussion and conclusions signatures’–sensu Shervais 2001). They were supposedly formed at an active subducting ridge. The IAT-like rocks of In the ophiolite mélange of the Mt. Medvednica the most com- BAB affinity generated in various suprasubduction geotecton- monly occurring magmatic rocks are hectometre to kilometre ic environments are, on the other hand, less numerous large slices or blocks of composite fragments of the SSZ upper (Slovenec and Lugović 2009), while boninite rocks are the oceanic crust, represented by Middle-Late Jurassic extrusive least abundant igneous fragments recovered from the ophiolite rocks (Fig. 3) of N-MORB-like affinity (‘MORB with arc mélange of the Mt. Medvednica (Figs. 3 and 4).

Fig. 7 Discriminant diagrams for the boninitic rocks from the Mt. ophiolite mélange (Slovenec and Lugović 2009), and boninites from the Medvednica ophiolite mélange. a SiO2 – MgO diagram (Le Bas 2000). Kopaonik area - Vardar Zone (Marroni et al. 2004), and Albanide- b V – Ti/1000 diagram (Shervais 1982). BON = boninites; IAT = island- Hellenide boninites (Saccani and Photaides 2004, 2005; Beccaluva arc tholeiites; MORB = mid-ocean ridge basalts; BABB = back-arc basin et al. 2005; Saccani et al. 2008, 2011 and references therein) plotted for basalts; CAB = calc-alkaline basalts; CFB = continental flood basalts; correlation constraints. c MgO – CaO diagram (Maehara and Maeda OIB = ocean-island basalts; AB = alkali basalts. Fields for high-, 2004)andd SiO2 – FeOT/MgO diagram (Miyashiro 1974) medium- and low-Ti tholeiitic SSZ basalts of the Mt. Medvednica D. Slovenec, B. Šegvić

Fig. 8 a N-MORB-normalised multielement patterns (Sun and Kopaonik area - Vardar Zone (Marroni et al. 2004), and Albanide- McDonough 1989); b REE-normalized patterns (Taylor and McLennan Hellenide boninites (Saccani and Photaides 2004, 2005; Beccaluva 1985) for the Mt. Medvednica boninitic rocks. Fields for high-, medium- et al. 2005; Saccani et al. 2008, 2011 and references therein) plotted for and low-Ti tholeiitic SSZ basalts of the Mt. Medvednica ophiolite correlation constraints mélange (Slovenec and Lugović 2009), and boninites from the

High values of the CaO/Al2O3 ratio as well as the CaO signature. Namely, the components entrained in the boninite content suggest the origin of analysed boninite from a portion mantle source are generally related to the subduction and re- of mantle wedge that also contained a paragenetic Ca-pyrox- lated higher degrees of partial melting (e.g., Beccaluva et al. ene. This type of pyroxene is not documented in the sensu- 2005). The isotopic data from the Mt. Medvednica boninites 147 144 stricto boninites from the Bonin Island – re- shown in the Sm/ Nd – (εNd)i diagram (Fig. 10)indicatea gion. There, both orthopyroxene and Ca-poor clinopyroxene complex mixing of the subduction related components intro- (clinoenstatite) preponderate (e.g., Hawkins 2003). A narrow duced into the mantle (i.e., metasomatic fluids released from range of Ti-Zr ratio values presumably reflects the absence of the subducted juvenile/crustal material) with the variably de- large-scale precipitation of Fe-Ti oxides during the crystalli- pleted mantle melts. The influence of metasomatic flux to zation of boninite lavas. Similar effect has been reported from mantle melts parental to the studied boninite was moderately modern boninites of West Pacific (Crawford and Cameron high as suggested by the low εNd values (+0.49 do +1.27). The 1985). The enrichment of LIL elements with regard to the enrichment in both the LILE and partially LREE is also related clearly depleted suite of HFS elements as well as negative to the influence of the subduction-derived aqueous fluxes Ta-Nb and Ti anomalies documented in the Mt. Medvednica (Fig. 9), whereas the HFSE concentration presents an original boninites (Fig. 8a) stem from the melting of the previously mantle wedge content of the low-soluble and immobile trace metasomatized mantle. Alternatively, the late subduction pro- elements. Negative Nb-Ta and positive Th anomalies are in cesses may be held responsible for such geochemical favour of SSZ geotectonic environment. Despite a generally

Fig. 9 Discriminant diagram a Nb/Yb – Th/Yb and b Nb/Yb – La/Yb for and depletion of an average N-MORB mantle (Sun and McDonough the boninitic rocks from the Mt. Medvednica ophiolite mélange. The 1989), and the broken lines represent contours of % subduction zone slopes solid double-headed arrow shows array patterns of enrichment contribution for given element in the mantle (Pearce and Peate 1995) Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –...

arc, which in this part of the Dinaric-Vardar Tethys appeared during the Callovian to Oxfordian times (Slovenec and Lugović 2009; Šegvić et al. 2014; Lugović et al. 2015). It follows that the boninite rocks of the Mt. Medvednica stand for the youngest, up to now known, the subduction-related oceanic crust formed during the Late Jurassic in this oceanic segment of the Dinaric-Vardar Tethys. Various authors implemented different geochemical vari- ables to infer on the amount of partial melts extracted from a residual mantle source. Pearce (1983) used compatible (Cr) versus incompatible (Y) element to estimate the composition of mantle sources and degree of partial melting that generated different magma types (Fig. 11b). In the Cr vs. Y diagram analysed boninite lavas could have been produced by about 20 to 28% of partial melting of the highly depleted MORB- type mantle source (S3). Such source would correspond to the refractory mantle of harzburgite nature (Murton 1989). 147 144 – ε Fig. 10 Sm/ Nd Nd(150 Ma) diagram for the boninitic rocks from Similar highly depleted tectonite peridotites are known to Mt. Medvednica ophiolite mélange showing the various mantle and have been formed in the forearc environment documented in subduction components interpreted to be involved in its petrogenesis. ć Hypothetical mantle sources: DM = depleted mantle (not refracrory); the Mt. Medvednica ophiolite mélange (Lugovi et al. 2007). VDM = very depleted mantle (refracrory); SJM = subducted juvenile The transitional from the locality of Gornje material (subducted oceanic crust; slab with little pelagic sediment); Orešje in the easternmost part of the Mt. Medvednica depict SCM = subducted continental material. The observed compositions and a slightly depleted spinel composition (Mg# = 0.64–0.56; hypothetical end-members sources calculated for the Middle Jurassic – following Swinden et al. (1990). Field for high-, medium- and low-Ti Cr# = 0.45 0.42) consistent with ~20% of partial melting. tholeiitic SSZ basalts of the Mt. Medvednica ophiolite mélange These rocks are plotted close to the theoretical S3 source (Slovenec and Lugović 2009) plotted for correlation constraints and may represent mantle residuum remained after extraction of the Mt. Medvednica medium-Ti basalts and low-Ti island- low concentrations of incompatible elements, an arc-derived arc tholeiites (Fig. 11b). Thus, they define a perfect source of enrichment has been recognised (i.e., high Th/Ta = 12.7–15.5 the boninitic parental melts. Assumed origin of the Mt. and Th/Nb = 0.7–0.9). In the NbN – ThN diagram, studied Medvednica boninites that advocates a partial melting of the volcanic rocks are plotted above the mantle array in the arc, strongly depleted mantle harzburgites is also in line with i.e. boninite field, suggesting an intermediate to strong influ- boninite depletion in HFS elements (Figs. 7b, 8a, 11b, and c). ence of subduction-zone fluids (Fig. 11a). The markedly low Such features were also reported in the boninites from Vardar ratios of SmN/ZrN (~0.5) and Ti/Zr (~50), lowTiN/Ti*N (<1) Zone (Kopaonik area, southern Serbia; Marroni et al. 2004), as and ZrN/Zr*N (~1.5), along with the distinctively low Ti/V well as in those from the area of the Albanide-Hellenide (<5.4) and Y (<6 ppm) concentrations characterise analysed Ophiolitic Belt (e.g., Saccani and Tassinari 2015; Saccani rocks as boninites and clearly distinguish them from the et al. 2017 and references therein). Cr-rich spinel renders an- MORB, IAT and OIB geotectonic realms (Figs. 7b, 11b, and other indicator of highly depleted mantle source (Fig. 5). c). Such findings are corroborated by spinel and Namely, according to Dick and Bullen (1984) spinel from clinopyroxene geochemistry (Figs. 5 and 6). Furthermore, rel- analysed boninites has compositional counterparts only in al- atively high ZrN/SmN (>1) and low NbN/ThN (~0.6) reported pine peridotites (Type III) formed after an excessive depletion in the Mt. Medvednica boninites are indicative for the forearc with all diopside being consumed by partial melting. origin of boninite lavas (Fig. 11d). A hypothesized The moderately high melting rates of analysed lavas may subduction-zone forearc environment of the formation of be a result of melting in the hot thermal regime of a shallow analysed rocks (probably close to the trench) is corroborated forearc mantle wedge or/and these lavas were formed by melt- by their high Zr/Sm (45–55) and Ti/Zr (46–54) ratios as well ing of the mantle portion of the young intra-oceanic subduc- as the enrichment in LREE-Zr-Hf and Th (Figs. 8 and 11a). tion-related system (Tatsumi and Eggins 1995). Hot thermal The calc-alkaline character of the Mt. Medvednica boninites regime is indicated by the maximal crystallization tempera- (Fig. 7dandTh– Nb/16 – Hf/3 diagram (Wood 1980 – not tures of augite between 1048 and 1260 (±30) °C(after shown)) further suggests that these rocks were not formed in Lindsley 1983) at low pressures that ranged from 0.24 to an incipient stage of the forearc evolution but rather in its 0.77 (±0.31) GPa (after Nimis and Ulmer 1998; Nimis mature phase (probably during the Tithonian), most likely 1999). This further suggests that the magmas parental to following the emergence of an intra-oceanic nascent island boninite lavas of the Mt. Medvednica formed at moderately D. Slovenec, B. Šegvić

Fig. 11 Discrimination diagrams for the boninitic rocks from the Mt. 2003). BON = boninites; IAB = island-arc basalts; MORB = mid-ocean Medvednica ophiolite mélange. a Simplified ThN – NbN diagram ridge basalts; OIB = ocean island basalts; BABB = back-arc basin (Saccani 2014). PM = primitive mantle. Th and Nb normalized to the basalts. Ti* = (GdN +DyN)/2, Zr* = (NdN +SmN)/2. Ti/Ti*, Zr/Zr*, Nb/ N-MORB composition (Sun and McDonough 1989). b Cr – Y diagram Th and Zr/Sm ratios normalized to the N-MORB composition (Sun and (modified after Pearce 1983 and Pearce et al. 1981). Mantle source McDonough 1989). Fields for high-, medium- and low-Ti tholeiitic SSZ compositions and melting paths for incremental batch melting are from basalts of the Mt. Medvednica ophiolite mélange (Slovenec and Lugović Murton (1989). S1: calculated mid-ocean ridge basalt (MORB) source; 2009), and boninites from the Kopaonik area - Vardar Zone (Marroni S2: residue after 20 % MORB extraction; S3: residue after 12 % melt et al. 2004), and Albanide-Hellenide boninites (Saccani and Photiades extraction from S2. BON = boninites; IAT = island-arc tholeiites; 2004, 2005; Beccaluva et al. 2005;Saccanietal.2008, 2011 and MORB = mid-ocean ridge basalts. c TiN/Ti*N – ZrN/Zr*N diagram references therein) plotted for correlation constraints (Taylor et al. 1994)andd NbN/ThN – ZrN/SmN diagram (Godard et al. high temperatures and pressures inferior to 1 GPa (~ 0.5 GPa) Thermal conditions necessary for the generation of the Mt. and were likely segregated at moderately shallow depths (~ Medvednica boninite lavas are believed to have been met by 15 km), thus corresponding to the conditions at hot mantle entrainment of the still young lithosphere, formed near the wedges where boninite magmas usually form (e.g., Cameron spreading axis, in the subduction zone factory. Such et al. 1983; Beccaluva and Serri 1988; Crawford et al. 1989; geodynamic scenario fits well with geotectonic model pro- Sobolev and Danyushevsky 1994; Taylor et al. 1994). High posed for the western segment of the Dinaric-Vardar Tethys temperature conditions at the shallow mantle wedge can be (Slovenec and Lugović 2009; Slovenec et al. 2011). explained by the subduction of a still active ridge below young Recent research, including this study with a focus set to the lithosphere (Van der Laan et al. 1989; Pearce et al. 1992). north-western branch of the Dinaric-Vardar Tethys (Slovenec Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –... and Lugović 2009; Slovenec et al. 2011) had all shown a suprasubduction zone (Fig. 12b). Shortly after the extru- progressive evolution of ophiolite magmas from the MORB- sion of island-arc volcanites began the hinge roll-back sub- like to IAT-like, ultimately reaching a boninite composition in duction of the oceanic slab (Fig. 12c and d). Such tectonic the segment of the . Boninite magmatism must setting is exclusively related to the fast slab roll-back in an have taken place following the emplacement of island-arc intensive extension-dominated tectonic regime that tholeiites. Analogue sequence is reported from the ophiolites allowed an adiabatic upwelling of asthenosphere melt from of the Albanide-Hellenide belt (Mirdita, Pindos and Troodos) the hot and hydrous, highly depleted, residual (harzburgite) reflecting a progressive melting of mantle source that, in turn, mantle (e.g., Hamilton 2007; Xia et al. 2012; Boutelier and becomes increasingly depleted (e.g., Beccaluva and Serri Cruden 2013). This further means that the extension rate in 1988; Saccani and Photiades 2004, 2005; Dilek and Thy the upper plate is gradually balanced by the roll-back and 2009; Saccani and Tassinari 2015; Saccani et al. 2017 and retreat of the subducted slab. The progressive sinking of references therein). The area of the Mt. Medvednica investi- the down-going slab and its further retreat give rise to the gated in this research renders a corner-stone segment of the upwelling of the asthenospheric mantle causing a temper- north-western branch of the Dinaric-Vardar Tethys, and to- ature raise and high degrees of partial melting through out gether with the Meliata–Maliac oceanic system, makes an the mantle wedge, from the arc axis to the forearc region. integral part of one and the same oceanic basin (Schmid Tensions created by the slab roll-back likely triggered a et al. 2008) featured by a common tectono-magmatic evolu- rifting, and then initiation of the seafloor splitting during tion during the Mesozoic times (Bortolotti et al. 2005, 2013). theLateJurassic(KimeridgiantoTithonian).Thesepro- However, some regional particularities and diachronous ap- cesses eventually lead to large-scale spreading in the pearances are reported (Slovenec and Lugović 2009; forearc region coupled with partial melting of hydrous Slovenec et al. 2010). The Mesozoic geodynamic evolution and progressively depleted refractory peridotites of sub- of this part of the Dinaric-Vardar Tethys commenced with an arc mantle, eventually causing a generation of boninite opening of ensialic back-arc basin next to the peri-continental magmas (Fig. 12c and d). Based on the evidences provided volcanic arc related to the Andean-type subduction of the herein, fast magmatic progression is hypothesized, from Paleo-Tethyan lithosphere beneath the European continental IA-like tholeiites produced in the relatively depleted man- margin (e.g., Stampfli and Borel 2004,Stampflietal.2002; tle to extremely depleted low-Ti boninites Bmoderately^ Goričan et al. 2005). Initial phase in the development of this influenced by the slab-derived fluids/melts in a rapidly marginal basin included intra-continental rifting during the evolving suprasubduction mantle wedge. Anisian coupled with the generation of alkaline volcanic rocks Not only different phases of IAT and boninite (Slovenec et al. 2010, 2011). The newly formed oceanic realm magmatism took place in a spatially constrained area, has continuously expanded during the Triassic (Ladinian to these effusions also likely happened in a short interval Norian) and Early Jurassic (Pliensbachian to Toarcian) leading of time, possibly within several millions of years during to the formation of the new E-, T- and N-MORB-types of the Late Jurassic (Oxfordian to Tithonian). Modern oceanic crusts between the Apulian plate (future Adria micro- forearcs require about 15 Ma to achieve their maturity plate) and continental margin of the southern Laurussia (future which presumes a development of the coeval and cog- Tiszia mega-block). In the Middle Jurassic the MORB-type nate back-arc basins (Stern and Bloomer 1992;Bloomer lithosphere collapsed due to the onset of the intra-oceanic et al. 1995). Calc-alkaline boninites of Mt. Medvednica convergence followed by the subduction of an active oceanic fits a mature arc-stage, analogous to similar rocks from ridge crust. In the initial phase of subduction the MORB- the Mirdita Eastern Belt and Vourinos (Hellenides) type magmatism is progressively replaced by IAT-type of (Beccaluva et al. 1978; Hoeck et al. 2002; Bortolotti volcanic activity, whereas in the Callovian/Oxfordian time et al. 2002; Saccani and Tassinari 2015; Saccani et al. a nascent island-arc has emerged (Fig. 12a and b) 2017). Obviously, the process of the subduction roll-back (Slovenec and Lugović 2008, 2009, 2012;Slovenecetal. triggers extension in the arc region thus eventually lead- 2010, 2011;Lugović et al. 2015). A continuous production ing to the formation and evolution of back-arc basins of basaltic magmas created a large volume of SSZ-type (e.g., Uyeda and Kanamori 1979). With the ongoing oceanic crust in an intra-oceanic arc-forearc system. An forearc extension in this segment of the Dinaric-Vardar elemental characteristic of the ophiolites from this segment ocean, the spreading centre as well as the mantle portion of the Dinaric-Vardar Ocean is the similarity and continu- producing upwelling melts moved away from the con- ous evolution from the MORB and transitional verging margin toward the region of deeper mantle (far MORB/IAT-like rocks toward IAT volcanites and from mantle wedge). This is the zone of less refractory boninites, suggesting a penecontemperaneous activity of peridotites where influence of subducting slab on the their respective magmatic sources in a relatively narrow newly produced magma is gradually decreasing portion of the Dinaric-Vardar intra-oceanic (Lugović et al. 2007), thus giving rise to the formation D. Slovenec, B. Šegvić

of the back-arc basin basalts with intermediate MORB- the time of the latest Jurassic (Kimeridgian to Tithonian) IAT composition (Lugović et al. 2015;Fig.12c). This (Slovenec and Lugović 2009; Šegvić et al. 2016; cognate back-arc (ensimatic) marginal basin must have Fig. 12c). During the latest Jurassic, and especially in been fully formed during the Early Cretaceous, however the Early Cretaceous at the SW corner of the Dinaric- the nascent phase of its development took place earlier at Vardar oceanic segment a significant consummation of Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) –...

RFig. 12 Schematic geodynamic model showing a development of SSZ References oceanic lithosphere in the north-western branch of Dinaric-Vardar Tethys. Scale is approximate. a The early stage of intra-oceanic subduction and Babić LJ, Hochuli PA, Zupanić J (2002) The Jurassic ophiolitic mélange formation of an infant proto-arc with accompanying IAT magmatism in the NE Dinarides: dating, internal structure and geotectonic im- from the ascending asthenospheric diapirs in the future Dinaric-Vardar plications. Eclogae Geol Helv 95:263–257 Ocean. b An enlarged portion of figure a. c The evolved subduction stage Bach W, Peucker-Ehrenbrink B, Hart SR, Blusztajn J (2003) with continuous slab sinking and retreat, during which the initial hingo Geochemistry of hydrotermally altered oceanic crust: DSDP/ODP roll-back subduction oceanic slab occurs. Tension created by slab roll- hole 504B – implications for seawater-crust excange budgets and Sr- back triggers the initiation of seafloor spreading in a forearc region and and Pb-isotopic evolution of the mantle. Geochem Geophys Geosyst generation of boninite magmatism by shallow partial melting of wedged 4:8904 wet mantle residual after IAT extraction, leaving residual depleted ć harzburgites. Consequently, these processes over the time lead to the Basch O (1981) Basic geological map of SFRJ 1:100.000. Sheet Ivani – Grad (L 38-81), Institut za geološka istraživanja Zagreb - Savezni initial formation of an ensimatic marginal (back-arc) basin Dinaric- š Vardar ocean. d An enlarged portion of figure c. e The closure of the geolo ki zavod Beograd (in Croatian) Dinaric-Vardar Ocean with the formation of ophiolite mélange. 1 = Bébien J, Dimo-Lahitte A, Vergély P, Insergueix-Filippi D, Dupeyrat L partially melted subducted oceanic lithosphere; 2 = oceanic crust with (2000) Albanian ophiolites. I - Magmatic and metamorphic process- – radiolarian cherts; 3 = fluids from subducted slab; BAB = back-arc basin; es associated with the initiation of a subduction. Ofioliti 25:39 45 AP = accretionary prism Beccaluva L, Serri G (1988) Boninitic and low-Ti subduction-related lavas from intra-oceanic arc-back-arc systems and low-Ti ophiolites: a reappraisal of their petrogenesis and original tectonic setting. Tectonophysics 146:291–315 Beccaluva L, Ohnenstetter D, Ohnenstetter M, Paupy M (1978) The the crust commenced with a concomitant development of Vourinos ophiolitic complex has been created in an island arc set- the accretionary wedge (Slovenec and Pamić 2002; ting: petrographic and geochemical evidences. Ofioliti 3:62–63 Pamić 2002). Thereupon, the formation of mélange en- Beccaluva L, Piccardo GB, Serri G (1980) Petrology of northern abled a preservation of the remnants of an ancient crust Apennine ophiolites and comparision with other Tethyan ophiolites. In: Panayiotou A (ed) Ophiolites, Proceedings of International in the form of slices and blocks (including the boninites Ophiolite Symposium, 1979. Geological Survey of Mt. Medvednica) that were obducted during the colli- Department, Nicosia, pp 314–331 sion phase onto the northern passive continental margin Beccaluva L, Di Girolamo P, Macciota G, Morra V (1983) Magma affin- – of the Adria microplate (Fig. 12e) and were thereupon ities and fractionation trends in opholites. Ofioliti 8:307 324 Beccaluva L, Macciotta G, Piccardo GB, Zeda O (1989) Clinopyroxene incorporated in the present-day ophiolite mélange of the composition of ophiolite basalts as petrogenetic indicator. Chem Sava Unit - thought to be the suture zone between the Geol 77:165–182 Dinarides and fragments of the European continental Beccaluva L, Coltorti M, Saccani E, Siena F (2005) Magma generation margin (e.g., Lugović et al. 2015; Šegvić et al. 2016). and crustal accretion as evidenced by supra-subduction ophiolites of the Albanide–Hellenide Subpelagonian zone. Island Arc 14:551– Although geographically distant, documented in the Mt. 563 Medvednica ophiolites at the very western corner of the West Bédard JH (1999) Petrogenesis of boninites from the Betts cove ophiolite, Vardar Ophiolitic Unit, analysed rocks may be compositionally Newfoundland, Canada: identification of subducted source compo- correlated with boninite rocks recovered from eastern portions nents. J Petrol 40:1853–1889 ć š of this mega-tectonic unit (boninites of the Kopaonik Mt., Belak M, Pami J, Kolar-Jurkov ek T, Pescaskay Z, Karan D (1995) Alpine low-grade regional metamorphic complex of Mt. southern Serbia) as well as with those documented along the Medvednica (northwestern Croatia). In: Vlahović I, Velić I, Albanide-Hellenide ophiolitic range (Figs. 5, 7b, 8,and11). Šparica M (eds) Proceedings of 1st Croatian Geological Congress This strongly argues for a common origin, petrogenesis and Opatija october 18-21, 1995, Institut za Geološka istraživanja, – similar geotectonic environment where all these rocks have Zagreb, pp 67 70 (in Croatian) Bloomer SH, Taylor B, MacLeod CJ, Stern RJ, Fryer P, Hawkins JW, formed within a single branch of the Neotethys Ocean (sensu Johnson L (1995) Early arc volcanism and the ophiolite problem: a Bortolotti et al. 2004, 2005, 2013;Schmidetal.2008) during perspective from drilling in the western Pacific. In Taylor B, Natland the Middle to Late Jurassic. JH (eds) Active margins and marginal basins of the western Pacific. Am Geophys Un Geophys Monog 88:1–30 Bortolotti V, Marroni M, Pandolfi L, Principi G, Saccani E (2002) Acknowledgments We thank Ilona Fin for producing excellent polished Interaction between mid-ocean ridge and subduction magmatism thin sections. Our appreciation is further extended to Boško Lugović and in Albanian ophiolites. J Geol 110:561–576 Hans-Peter Meyer for their assistance with microprobe measurements at Bortolotti V, Chiari M, Marcucci M, Marroni M, Pandolfi L, Principi G, the Institute of Geosciences (University of Heidelberg, Germany). Sam Saccani E (2004) Comparison among the Albanian and Greek Carmalt and Aleksandar Ristić helped to improve the quality of the ophiolites, in search of constraints for the evolution of the English. Critical comments and constructive reviews by Dragan Mesozoic . Ofioliti 29:19–35 Milovanović, Shuguang Song and an anonymous expert, as well as edi- Bortolotti V, Marroni M, Pandolfi L, Principi G (2005) Mesozoic to torial comments of journal editors Qiang Wang and Lutz Nasdala con- tertiary tectonic history of the Mirdita ophiolites, northern Albania. tributed significantly to the manuscript quality. The presented work is the Island Arc 14:471–493 contribution to the scientific project BMesozoic magmatic, mantle and Bortolotti V, Chiari M, Marroni M, Pandolfi L, Principi G, Saccani E pyroclastic rocks of north-western Croatia^ (grant no. 181-1951126- (2013) The geodynamic evolution of the ophiolites from Albania 1141 to D. S.) carried out under the support of the Croatian Ministry of and Greece, Dinaric-Hellenic Belt: one, two, or more oceanic ba- Science, Education and Sport. sins? Int J Earth Sci 102:783–811 D. Slovenec, B. Šegvić

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