1. INTRODUCTION Fragments of ancient oceanic lithosphere are terrestrially ex- posed in an orogenic belt, due to relative coherent thrust sheets that form an ophiolite complex that is generally as- sociated with underlying ophiolite mélange. Most of the oce- anic lithosphere, with a geochemical affinity highlighted by proper (normal) middle ocean ridge basalts (N-MORB), was formed at the mid oceanic ridge during several tens of mil- lions of years of spreading. The major part of the mid oce- anic ridge succession has been subducted, and only a minor part has been preserved by being removed from the subduct- ing slab, in trench sediments of the accreationary prism in front of the overriding plate. However, a significant propor- Ab sTRA CT The Eastern Tethyan Repno oceanic domain (ROD), from the Zagorje-Mid-Transdanubian Shear Zone, borders the Meliata-Maliak and Dinaric-Vardar oceanic realms. The domain lacks ophiolite complexes and oceanic rocks are restricted to four Early Callovian to Late Valanginian ophiolite mélange sectors integrated in the Kalnik Unit. Previous research on ophiolitic rocks from the Kalnik Unit provided a detailed tectonomagmatic evolution of crust formation at different settings from the Late Anisian to the Cretaceous. N-MORB-type crust with peculiar supra-subduction signatures formed on a spreading ridge from the Late Carnian (~225 Ma) to the Late Pliensbachian (~185 Ma), and the first intraoceanic subduction rocks were dated as Late Bathonian (165 Ma). The newly discovered basaltic rocks, in coherent slices with the Latest Bajocian-Early Bathonian (~169 Ma) radiolarin cherts from the Mt. Medvednica ophiolite mélange sector, completed the Late Pliensbachian-Late Bathonian chemostratigraphic gap in the domain. The analysed unfractionated extrusive rocks lack any influence of a subduction related component, [(Nb/La)n = 1.16– 1.34; Th/Ta = 0.99–1.05] and are akin to proper N-MORB compositions [(Nd/Lu)N-MORB ~ 1.1; (La/Lu)cn = 0.82– 0.92] that were derived from a slightly depleted mantle source [εNd(T=170 Ma) = +6.21 to +6.27; (87Sr/86Sr)i = 0.703365 to 0.703511]. The rocks are interpreted as vestiges of the middle oceanic ridge crust formed during the culmination of spreading in the ROD. Keywords: petrology, geochemistry, N-MORB extrusives, ophiolite mélange, Bajocian-Bathonian, Zagorje- Mid-Transdanubian Zone, Mt. Medvednica, Croatia Geologia CroaticaGeologia Croatica Geologia Croatica 65/3 435–446 10 Figs. 3 Tabs. Zagreb 2012 Evidence of the spreading culmination in the Eastern Tethyan Repno oceanic domain, assessed by the petrology and geochemistry of N-MORB extrusive rocks from the Mt. Medvednica ophiolite mélange (NW Croatia)  Damir Slovenec1 and Boško Lugović2 1 Croatian Geological Survey, Sachsova 2, HR-10 000 Zagreb, Croatia (e-mail: damir.slovenec@hgi-cgs.hr) 2 Institute of Mineralogy, Petrology and Mineral Deposits, Faculty of Mining, Geology, and Petroleum Engineering, University of Zagreb, Pierottijeva 6, HR-10 000 Zagreb, Croatia (e-mail: blugovic@rgn.hr) doi: 104154/gc.2012.32 Geologia Croatica 65/3Geologia Croatica 436 tion of the youngest N-MORB lithosphere related to the spreading ridge has been obducted and created an example of pristine crust of the intraoceanic upper plate, which was replaced by ongoing formation of true SSZ crust derived from a depleted and metasomatized mantle wedge, that ex- perienced a long term complex history of partial melting. Consequently, the amount of proper N-MORB lithologies is relatively low in the ophiolite mélange. The culminating stage of ocean spreading, represented by the youngest N- MORB crust, may therefore be poorely documented in an ophiolite mélange and only a systematic petrological and geochemical study of magmatic inclusions of the mélange may provide more opportunity to characterize oceanic tec- tomagmatic evolution at this stage. This work provides more detailed knowledge of the tec- tonomagmatic evolution of an Eastern Tethyan oceanic do- main, colloquially termed the Repno Oceanic Domain (ROD; BABIĆ et al., 2002), which borders the Meliata-Maliak and Dinaric-Vardar ocean systems. The domain lacks ophiolite complexes and ophiolitic rocks are exposed in only four sep- arate ophiolite mélange sectors, integrated in the Kalnik Unit by HAAS et al., (2000). The various magmatic blocks and juxtaposed fragments of sedimentary rocks, vary in both age and lithology, and are mostly tectonically included within the matrix of mélange, thus obscuring their original geolog- ical setting of formation (HALAMIĆ, 1998; SLOVENEC & PAMIĆ, 2002). Previous research on the magmatic blocks in the Kalnik Unit, provided constraints on the detailed geody- namic and tectonomagmatic evolution of the ROD, based ex- clusively on geochemical and petrological data. The evolution commenced in the Anisian by intra-continental rifting, via for- mation of proto-oceanic crust in the Ladinian and the onset of crust formation at a spreading ridge until the Middle Jurassic. Initial SSZ crust formed in the Late Bathonian, with forma- tion of infant forearc-proto-arc crust in the Callovian-Oxfordian, and formation of the youngest crust in a Cretaceous back-arc basin (LUGOVIĆ et al., 2007; SLOVENEC & LUGOVIĆ, 2008, 2009; SLOVENEC et al., 2010, 2011; KISS et al., 2012; LUGOVIĆ et al., in rewiev). The peculiar characteristic of MORB-type crust in the ROD that commenced in the Middle Carnian and was prolonged to the Late Pliensbachian is its SSZ geochemical flavour. The evolutionary stage of the ROD that may be reflected by proper N-MORB lithosphere re- mained unsolved by previous research. Relatively rare occur- rences of the relevant rocks were recently encountered only in the Mt. Medvednica ophiolite sector near Poljanica (locus tipicus). Petrological and geochemical research on these rocks and particularly on the coherent blocks composed of N-MORB pillow lavas and radiolarian cherts, provided age determina- tion of the culminating phase of spreading in the ROD, thus improving knowledge of the high-resolution tectomomag- matic evolution of the domain. 2. OUTLINE OF REGIONAL GEOLOGY Mt. Medvednica along with Mts. Ivanščica and Kalnik is lo- cated at the southwestern tip of the SW-NE trending Zago- rje-Mid-Transdanubian Zone (ZMTDZ; PAMIĆ & TOM- LJENOVIĆ, 1998), within the triple junction zone between three complex tectonic units represented by the Southern- Eastern Alps Unit, Internal Dinarides, and the Tisia conti- nental block (Fig. 1A). The ZMTDZ approximately corre- sponds to the Sava Unit defined by HAAS et al. (2000) and comprises a 100 km wide and 400 km long area between the Periadriatic-Balaton lineament to the north and the Zagreb- Zemplin lineament to the south. Although the structural pa- ttern of these intra-Panonnian inselbergs is obscured by Ter- tiary displacements and Neogene sedimentary cover, a Dinaride characteristic has been recognized (e.g. HAAS & KOVáCS, 2001). The true Dinarides which are traditionaly divided into the External (Outer) and Internal (Inner) Dina- rides, stretch southeastwards from the Zagreb-Zemplin lin- eament as 700 km long tectonostratigraphic units bounded by the Skutari-Peć transform fault to the southeast (Fig. 1A). Dinaric tectonostratigraphic units extend further into the Al- banides and continue southwards into the Hellenides. The External Dinarides mainly consist of Mesozoic carbonate platform that is structurally overlain by allochthonous, con- tinentally derived units showing Palaeozoic–Triassic strati- graphic successions; the Internal Dinarides comprise several zones that regularly reflect the transition from platform sed- iments to the Mesozoic oceanic realm, with the outermost zone sutured to the Eurasian continental lithospheric plate, (see compilation in PAMIĆ et al., 2002 and ROBERTSON et al., 2009). Most internal zones are dominated by ophiolite units and are sensu PAMIĆ (2002) subdivided into the Cen- tral Dinaridic Ophiolite Zone (CDOZ) and more internal Sava-Vardar Suture Zone (SVSZ). Structural and palaeomagnetic data indicate that the tec- tonic block comprising the Medvednica, Ivanščica and Kalnik Mts. experienced ca. 130° clock-wise rotation and eastwards escape during the Oligocene-earliest Miocene, (TOMLJENOVIĆ et al., 2008) that resulted in a structural trend almost perpendicular to the NW-SE Dinaric trend. De- spite this dramatic change of trend in respect to the overall Dinaric structural trend, SCHMID et al. (2008) included the ZMTDZ in the Western Vardar Ophiolite Unit, based on the similar tectonostratigraphic evolution, particularly for the ophiolitic rocks. Ophiolite mélanges from the southwestern ZMTDZ exposed in these three mountain sectors, and pos- sibly those in the Mt. Samoborska Gora sector, are integrated into a combined tectonostratigraphic unit termed the Kalnik Unit (HAAS et al., 2000). The Kalnik Unit incorporates vari- ous ophiolitic, lithic vestiges related to the ROD, that accord- ing to BORTOLOTTI & PRINCIPI (2005) may represent the westernmost oceanic segment of Eastern Tethys. Therefore, the ROD is an important oceanic domain that borders the Me- liata-Maliak oceanic segment located to the northeast and the Dinaric-Vardar oceanic system to the southeast. 3. GEOLOGY OF MEDVEDNICA MT. The geographic position of the locations relevant for this work and a simplified geological map of the northwestern part of Mt. Medvednica are displayed in Fig. 1B–C. Damir slovenec and boško Lugović: Evidence of the spreading culmination in the Eastern Tethyan Repno oceanic domain... Geologia Croatica 437 Structurally, the lowermost tectonic unit of Mt. Medved- nica consists of lower greenschist facies para- and ortho- metamorphic rocks represented by slate-phyllites, quarzites, marbles metasandstones and greenschists, respectively (ŠI- KIĆ et al., 1978, 1979; BASCH, 1981, 1983), that corre- spond to the ZMTDZ metamorphic complex or Medvednica Unit (HAAS et al., 2000). The Medvednica Unit comprises a thick sedimentary succesion deposited from the Silurian to the Ladinian, that was metamorphosed in the Lower Aptian (BELAK et al., 1995) by emplacement of a Late Jurassic island-arc onto the Adria continental margin (LUGOVIĆ et al., 2006). The Medvednica Unit is thrusted by an ophiolite mélange, that is, by the Mt. Medvednica sector of the Kalnik Unit. The accretionary age of the Kalnik Unit can be con- strained from the Early Callovian to the Late Valanginian (BABIĆ et al., 2002) and represents the period of accumu- lation of lithostratigraphically different materials in the intra- oceanic trench (SLOVENEC et al., 2011). Both units are unconformably overlain by a Late Cretaceous-Palaeocene Gosau-type sedimentary sequence, composed of clastic-car- bonate and flysch sediments (ŠIKIĆ et al., 1979). Neogene and Pleistocene sedimentary sequences unconformably over- lie pre-Neogene basement rocks along the southern slopes of the mountain, whilst on the northwestern slopes they are tectonically superimposed. The Kalnik Unit in the Mt. Medvednica sector is char- acterized by block-in matrix fabric, typical for chaotic com- plexes from subduction-related tectonic mélanges (FESTA et al., 2010). The primary depositional structural features are obliterated during emplacement and incorporation of large fault-bounded olistoliths (Fig. 1C), that resulted in strongly sheared pelitic-silteous continent derived matrix. However, the matrix of the ophiolite mélange does not show any meta- morphic overprint after diagenetic equilibration (JUDIK et al., 2008). The Mt. Medvednica ophiolite mélange incorporates fragments of various Mesozoic sedimentary rocks, (grey wacke, minor shale, red and grey cherts and scarce limestones), that are randomly mixed, along with fragments of igneous pluto- nics (peridotite cumulates and gabbros) and extrusive basalts and mafic dyke rocks (SLOVENEC & LUGOVIĆ, 2008, 2009; SLOVENEC et al., 2010 and references). Fragments of basaltic rocks are fairly dominated by magmatic compo- Figure 1: (A) Geotectonic sketch map of Alps, Dinarides and Hellenides showing the position of the Periadriatic-Sava-Vardar suture zone (modified after PAMIĆ, 2000). Legend: 1 – External Dinarides and Alps; 2 – Internal units [a: Central Dinaride Ophiolite Belt (CDOB); b: Passive continental margin and Mirdita Zone]; 3 – Periadriatic-Sava-Vardar Zone; 4 – Serbo-Macedonian Massif; 5 – Pelagonide metamorphic complex; 6 – Golija Paleozoic Zone; 7 – Za- gorje-Mid-Transdanubian Zone; 8 – Panonian Basin. Faults: BL – Balaton; DF – Drava; PL – Periadriatic; SF – Sava; SP – Scutari-Peć; SN – Sava Nape; ZZ – Zagreb-Zemplin. Mountains: I – Ivanščica; K – Kalnik; Md – Medvednica; SgŽ – Samoborska gora and Mts. Žumberak; SD – Szarvaskö-Darnó. B – Bódva valley; JK – Jaklovce. (b) Geographical location of the study area (gray shaded). (C) Simplified geological map of Mt. Medvednica (modified after HALAMIĆ, 1998). Legend: 1 – Neogene and Pleistocene sedimentary rocks; 2 – Late Cretaceous-Paleocene flysch including Senonian carbonate breccias; 3 – ophio- lite mélange with blocks of: 4 – Middle Triassic radiolarites, shales, limestones, pyroclastites and basalts (black fields), 5 – Middle Jurassic radiolarites, shales and basalts (dark gray fields), 6 – Alb-Cenomanian limestones and clastic rocks (shale, siltite and sandstone); 7 – Lower Cretaceous metamorphic com- plex; 8 – reverse or thrust faults; 9 – normal faults; 10 – geological contact line; 11 – sample locations: 1 = vs-113/2, vs-113A4; 2 = vh-49B; 3 = vs-94/1; 4 = vs-85/2; 5 = vh-1001/1; 6 = vs-307/1. Geologia Croatica 65/3Geologia Croatica 438 nents in this ophiolite mélange. Excluding fragments of Il- lyrian-Fassanian pre-oceanic within-plate alkali basalts, all other magmatic rocks display oceanic supra-subduction af- finity, i.e., are scraped from an upper plate. In Mt. Medved- nica, the oceanic crust fragments show a wide range of geo- chemical signatures consistent with their geotectonic setting, but show a relatively narrow range of corresponding ages of formation (SLOVENEC & LUGOVIĆ, 2009). The oldest oceanic rocks in the Mt. Medvednica ophiolite mélange are the latest Bathonian N-MORB-like basalt, related to the in- itial intraoceanic subduction rocks that were followed by true subduction IAT-type basalts of the Early Oxfordian. Despite age or geotectonic provenance, these MORB-type or MORB- like lithologies show peculiar subduction related compo- nents, or, in another words, correspond to “MORBs with arc-signatures” sensu SHERVAIS (2001). However, only six blocks of proper N-MOR basalts were found in the Poljanica area of the Mt. Medvednica ophiolite mélange (Fig. 1C) which is a unique occurrence of this rock type in the entire Kalnik Unit. These hectometre large blocks consist of pillow lavas and massive extrusives (Fig. 1C, lo- cations 1, 4, 5, 6 and 2–3, respectively). Pillow lavas at lo- cation 1 and location 5 form a coherent slice along with ra- diolarian cherts, sometimes mixed with silicified shales which were (on account of the radiolarian assemblage) dated to the Latest Bajocian-Early Bathonian (HALAMIĆ et al., 1999). These extrusive rocks are the subject of this work. 4. ANALYTICAL TECHNIQUEs Minerals were analyzed at the Mineralogisches Institut, Uni- versität of Heidelberg, using a CAMECA SX51 electron mi- croprobe equipped with five wavelength-dispersive spectrom- eters. Measurements were performed using an accelerating voltage of 15 kV, beam current of 20 nA, beam size of ~ 1 µm (for feldspars 10 µm) and 10 s counting time for all elements. Natural minerals, oxides and silicates were used for calibra- tion. Raw data for all analyses were corrected for matrix ef- fects with the PAP algorithm (POUCHOU & PICHOIR, 1984, 1985) implemented by CAMECA. Formula calculations were Figure 2: Plot of clinopyroxene compositions in the En–Wo–Fs (Mg- 2Si2O6–Ca2Si2O6–Fe2Si2O6) diagram with the nomenclature fields of MO- RIMOTO (1988) for N-MORB volcanic rocks from the Mt. Medvednica ophiolite mélange. Field of clinopy- roxene compositions (dark gray shaded) from high-, medium- and low-Ti tholeiitic SSZ basalts in the Mt. Medvednica ophiolite mélange (SLOVENEC & LUGOVIĆ, 2009) plot- ted for correlation constraints. Table 1: Selected microprobe analyses and structural formulae of clinopy- roxene from the N-MORB extrusive rocks in the Mt. Medvednica ophiolite mélange Sample vs-113/2 Anal. nr. 6 8 11 13 17 18 21 23 SiO2 49.94 47.98 49.56 48.98 51.77 50.60 49.44 49.54 TiO2 1.31 2.29 1.57 1.64 1.09 1.44 1.31 1.53 Al2O3 2.55 4.26 3.13 3.68 2.06 2.11 2.93 3.11 Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 FeO 12.69 12.56 10.12 10.65 9.58 13.90 13.73 9.97 MnO 0.60 0.45 0.31 0.33 0.30 0.43 0.43 0.23 MgO 12.05 11.37 14.61 13.67 14.83 12.19 11.75 13.92 CaO 19.83 20.38 19.73 20.23 20.09 19.06 19.59 20.42 Na2O 0.45 0.43 0.68 0.41 0.32 0.19 0.40 0.46 Total 99.42 99.72 99.73 99.59 100.04 99.92 99.65 99.23 Si 1.900 1.824 1.846 1.838 1.926 1.925 1.882 1.862 Ti 0.037 0.065 0.044 0.046 0.030 0.041 0.038 0.043 AlIV 0.100 0.176 0.137 0.162 0.074 0.075 0.118 0.138 AlVI 0.014 0.015 0.001 0.001 0.016 0.020 0.014 0.001 Cr 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.001 Fe3+ 0.043 0.062 0.131 0.098 0.020 0.000 0.058 0.085 Fe2+ 0.360 0.338 0.184 0.236 0.278 0.442 0.379 0.229 Mn 0.019 0.014 0.010 0.010 0.009 0.014 0.016 0.006 Mg 0.683 0.644 0.811 0.765 0.882 0.692 0.667 0.780 Ca 0.808 0.830 0.787 0.813 0.801 0.777 0.799 0.822 Na 0.033 0.032 0.049 0.030 0.023 0.014 0.030 0.034 Total 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Mg# 65.5 65.6 81.5 76.4 74.7 61.0 63.8 77.3 AlVI/AlIV 0.14 0.09 0.01 0.01 0.21 0.27 0.12 0.01 Wo 42.21 43.96 40.93 42.30 41.48 40.37 41.64 42.76 En 35.69 34.13 42.17 39.77 42.60 35.93 34.75 40.56 Fs 22.09 21.91 16.89 17.93 15.93 23.70 23.62 16.68 Formulae calculated on the basis of 4 cations and 6 oxygens. Mg# = 100*Mg/ (Mg + Fe2+). Damir slovenec and boško Lugović: Evidence of the spreading culmination in the Eastern Tethyan Repno oceanic domain... Geologia Croatica 439 done by using a software package designed by Hans-Peter Meyer from the Mineralogisches Institut, Universität of Hei- delberg, Germany. Bulk-rock powders for chemical analyses of seven sam- ples were obtained from rock chips free of veins and amy- gdales. The samples were analysed by ICP for major elements, and ICP-MS for all trace elements at Actlab Laboratories in Ancaster, Canada. International mafic rocks were used as standards. Major element and trace element concentrations were measured with accuracy better than 1% and 5%, respec- tively. Isotopic compositions of 2 bulk rock samples were measured in CRPG in Vandoeuvre, France, on 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 Sr standard for the period of measurement was 0.710242 ± 0.000030 (2σ). The 143Nd/144Nd ratio for the La Jolla standard was 0.5118451 ± 0.000010 (2σ). Total pro- cedural blanks were ~500 pg and ~150 pg for Sr and Nd, re- spectively. 5. PETROGRAPHY AND MINERAL CHEMIsTRY The analysed rocks are represented by green to red basaltic massive or amygdaloidal pillow lavas, characterized by ir- regular fracturing. Monomineralic vesicle infillings are com- posed of calcite, chlorite or fine-grained quarz. Near the con- tact with the shale, the pillows exhibit a fine-grained haematite pigmented zone with divergent-radial texture. Both types of lavas are severely altered but still preserve fine- to medium- grain ophitic, or intergranular texture occassionally with mi- nor plagioclase phenocrysts. Magmatic plagioclase is replaced by albite (An0.2–3.1) and peristerite (An~8) and contains small inclusions of chlorite, aggregated sericite-pumpellyite and zoisite-group minerals. Small relics of skeletal clinopyroxene are rare, usually pseudomorphosed by pycnochlorite-diaban- tite and epidote. Fe-Ti oxides (magnetite and skeletal ilmenite) are accessory minerals. Such altered rocks are usually called spilite. Petrographic evidence suggests the following order of crystallization: plagioclase → clinopyroxene + plagioclase ± Fe-Ti oxides, that is traditionally assumed as typical for vol- canic rocks formed at an ocean ridge (e.g. BECCALUVA et al., 1980). Microtextures of an alteration assemblage with prehnite, pumpellyite, chlorite and calcite indicate ocean floor metamorphism. The clinopyroxene from the Poljanica rocks shows an augite composition (Wo40.4–43.9En34.1–42.6Fs15.93–23.7; Table 1) that is significantly different compared to the clino- pyroxenes from the Mt. Medvednica SSZ volcanic rocks (Fig. 2). They are characterized by a comparatively higher content of non-quadrilateral elements (Fig. 3) and their Ti/Al ratio (0.28–0.43). The TiO2 content is generally high but variable in individual grains (1.09–2.29 wt.%), whilst the range of Mg# is limited to 81.5–61.0 (Table 1) reflecting the lack of signifi- cant Fe-enrichment in the analyzed clinopyroxenes (Fig. 2). The clinopyroxene AlVI/AlIV ratio ≤ 0.27 suggests low pres- sure crystallization consistent with the observed order of crys- tallization. 6. bULK ROCK CHEMIsTRY Chemical compositions of the analyzed rocks are shown in Table 2. High LOI (up to 8.32 wt.%) combined with petro- graphic evidence confirm the severe alteration of the rocks that might have affected the magmatic content of certain el- ements. The relative mobility of elements was proven by plotting their concentration against Zr as a differentiation index (not shown). Large ion lithophile elements (= LILE) Cs, Rb, K, Ba and Sr revealed random and selective migra- tion caused by alteration and were consequently excluded from petrogenetic constraints. However, high field strength Figure 3: (A) Diagram SiO2/100 – Na2O – TiO2 and (b) Ti – AlIV (simplifed after BECCALUVA et al., 1989) for clinopyroxene from the Mt. Medvednica N-MORB volcanic rocks. MORB – mid-ocean ridge basalts; BABB – back-arc basin basalts; IAT – island-arc tholeiites; BON – boninite. Fields of clinopyroxene com- positions (gray shaded) from high-, medium- and low-Ti tholeiitic SSZ basalts in the Mt. Medvednica ophiolite mélange (SLOVENEC & LUGOVIĆ, 2009) plotted for correlation constraints. Geologia Croatica 65/3Geologia Croatica 440 elements (= HFSE) Ti, Th, Hf, Nb, Ta, P and Y and rare earth elements (= REE) from La to Lu showed magmatic cor re- lation and therefore were utilized as suitable tools for geo- che mical and petrogenetic considerations, as previously used for analogue rocks elsewhere (e.g. PEARCE & NORRY, 1979; SHERVAIS, 1982; BECCALUVA et al., 1983). This also holds true for the transitional metals V, Cr, Mn, Fe, Ni and Zn, though their measured concentrations are strongly de- pendent on the amount of related mineral phase in a rock. Figure 4: Ni – Ti/Cr diagram (BECCALUVA et al., 1983) for the N-MORB vol- canic rocks from the Mt. Medvednica ophiolite mélange. Field (gray shad- ed) for high-, medium- and low-Ti tholeiitic SSZ basalts from the Mt. Medvednica ophiolite mélange (SLOVENEC & LUGOVIĆ, 2009) plotted for correlation constraints. Figure 5: V – Ti/1000 diagram (SHERVAIS, 1982) for the N-MORB volcanic rocks from the Mt. Medvednica ophiolite mélange. IAT – island-arc tholei- ites, MORB – mid-ocean ridge basalts, BABB – back-arc basin basalts, OIB – ocean-island basalts and AB – alkali basalts. Fields for high-, medium- and low-Ti tholeiitic SSZ basalts from the Mt. Medvednica ophiolite mélange (SLOVENEC & LUGOVIĆ, 2009), Late Pliensbachian gabbros (LUGOVIĆ et al., in rewiev) and Late Carnian basalts (SLOVENEC et al., 2011) from Mt. Kalnik plotted for correlation constraints. Table 2: Chemical analyses of N-MORB extrusive rocks from the Mt. Med- vednica ophiolite mélange. Sample vs- 113/2 vs- 113A4 vh- 49B vs- 94/1 vs- 85/2 vh- 1001/1 vs- 307/1 Rock type PB PB MB MB PB PB PB SiO2 51.32 46.43 49.62 47.12 48.23 47.89 47.06 TiO2 1.20 1.34 1.24 1.38 1.42 1.57 1.64 Al2O3 16.68 15.67 17.46 15.02 15.94 14.96 13.78 Fe2O3 total 8.32 9.05 6.84 12.43 9.21 13.05 13.88 MnO 0.32 0.71 0.15 1.26 0.29 0.33 0.25 MgO 6.65 7.06 7.13 8.01 6.92 7.63 7.78 CaO 5.92 6.98 7.91 4.03 6.28 5.39 5.99 Na2O 4.75 3.99 4.51 3.35 3.56 3.21 2.83 K2O 0.32 0.15 0.65 0.16 0.32 0.18 0.16 P2O5 0.10 0.10 0.11 0.12 0.11 0.13 0.15 LOI 3.96 8.32 3.93 6.89 6.64 5.31 5.94 Total 99.54 99.80 99.55 99.77 99.92 99.65 99.46 Mg# 62.9 61.1 67.9 56.5 60.7 55.3 53.6 Cs 0.5 1.2 0.7 1.6 1.4 2.1 1.8 Rb 12 7 12 15 11 10 9 Ba 225 108 257 203 192 202 187 Th 0.24 0.21 0.26 0.27 0.23 0.34 0.37 Ta 0.24 0.20 0.25 0.27 0.22 0.33 0.36 Nb 4.0 3.4 4.1 4.5 3.7 5.5 5.8 Sr 92 101 131 105 112 101 132 Zr 72 70 83 92 75 98 111 Hf 2.0 1.8 2.2 2.5 1.9 2.7 3.0 Y 25 24 27 31 25 34 38 Sc 35 34 40 32 39 41 48 V 206 196 226 214 218 221 254 Cr 575 598 497 372 524 126 187 Ni 191 294 195 210 233 122 156 La 3.47 3.14 3.76 3.98 3.25 4.51 4.63 Ce 9.92 8.78 10.51 11.16 9.02 12.48 12.65 Pr 1.38 1.22 1.55 1.64 1.31 1.73 1.85 Nd 7.24 6.67 7.89 8.99 6.98 9.20 10.28 Sm 2.56 2.21 2.65 3.04 2.31 3.25 3.54 Eu 0.981 0.872 1.010 1.139 0.940 1.219 1.412 Gd 3.31 3.08 3.38 4.03 3.21 4.23 4.74 Tb 0.59 0.55 0.62 0.73 0.59 0.79 0.92 Dy 3.84 3.61 4.07 5.02 3.78 5.51 6.14 Ho 0.89 0.83 0.94 1.11 0.88 1.21 1.36 Er 2.55 2.40 2.79 3.39 2.62 3.58 3.89 Tm 0.390 0.371 0.419 0.478 0.389 0.542 0.591 Yb 2.66 2.58 2.80 3.41 2.63 3.68 3.98 Lu 0.399 0.389 0.421 0.490 0.402 0.552 0.588 Major elements in wt.%, trace elements in ppm. LOI = loss on ignition at 1100 ºC. MB = massive basalt, PB = pillow basalt. Mg# = 100*molar MgO/(MgO + FeOtotal). Damir slovenec and boško Lugović: Evidence of the spreading culmination in the Eastern Tethyan Repno oceanic domain... Geologia Croatica 441 In the Zr/TiO2 vs. Nb/Y diagram (WINCHESTER & FLOYD, 1977) for classification of altered extrusives, the analysed rocks with low Zr/TiO2 (0.006–0.007) and Nb/Y (0.14–0.16) plot in the field of subalkaline, tholeiitic andes- ite/basalt, and in this respect they are not distinguished from the Mt. Medvednica SSZ basalts (not shown). However, they are strongly geochemically separated in the Ti/Cr vs. Ni di- agram (Fig. 4) whereas in the Poljanica rocks, they plot ex- clusively in the field of high-Ti basalts of MORB affinity, whilst SSZ extrusives trend from high-Ti to low-Ti basalts of IAT and/or BABB signatures. The ocean ridge affinity of the Poljanica rocks is also deduced from their high Ti/V ra- tio (32.9–42.6), that is typical for MORB rocks derived from a slightly depleted mantle reservoir (Fig. 5). The multi-element abundance patterns normalized to N- MORB values for Poljanica volcanic rocks are displayed as a spider diagram in Fig. 6A. The rocks display a wide range of selective LILE enrichment consistent with their polyphase alterations. The unique feature of these rocks is their smooth and flat patterns in the profile from Nd to Lu which range from 0.8 to 1.3 times relative to N-MORB and, moreover, show a very slight HFSE positive anomaly [(Nb/La)n = 1.16– 1.34; (Ti/Gd)n = 1.00–1.16]. In that respect, six blocks of Poljanica basalts represent a geochemical exception concern- ing the whole suite of oceanic exstrusive rocks archived in the Kalnik Unit (compare with SLOVENEC et al., 2011). N-MORB normalized REE patterns of analysed rocks are displayed in Fig. 6B. Excluding a slight La-Ce enrichment, (most likely due to alteration), the REE show smooth and flat profiles in the Pr-Lu segment of pattern [(Pr/Lu)n = 1.08–1.26] at 0.8–1.4 times relative to N-MORB. Three out of seven samples (vs-94/1, vh-1001/1 and vs-307/1, Table 1) show fractionated patterns (> 1.2 times relative to N-MORB). This signature combined with a lack of negative HFSE anomalies in the spider diagram (Fig. 6A) show that the Poljanica basalts formed at an oceanic setting far away from the influence of any subduction related source, that is, at an evolved spreading ridge centre. A slight Eu anomaly (Eu/Eu* = 1.03–0.96) in the samples is typical for low accu mulation, or fractionation, of plagioclase, suggesting that the four unfractionated rock samples may represent near primary melts. The Nd and Sr isotopic compositions of two pillow basalts are shown in Table 3. Measured 143Nd/144Nd ratios are concordant in both samples (0.512975 and 0.512979) whilst the 87Sr/86Sr ratios show slight variations between 0.704057 and 0.704423, most likely due to different inten- sity of alterations. The initial εNd and Sr isotopic ratios calculated for an arbitrary age of 170 Ma vary from +6.21 to +6.27 and from 0.703365 to 0.703511, respectively, and plot within the mantle array in the field of recent MORB close to the PREMA (Fig. 7). Figure 6: (A) N-MORB normalized multielement patterns (SUN & McDONOUGH, 1989); (b) N-MORB normalized REE patterns (SUN & McDONOUGH, 1989) for the Mt. Medvednica N-MORB volcanic basaltic rocks. Gray shaded area represent compositional field of high-, medium- and low-Ti tholeiitic SSZ ba- salt suite from the Mt. Medvednica ophiolite mélange (SLOVENEC & LUGOVIĆ, 2009) plotted to stress their distinctive geochemical signatures. Table 3: Nd and Sr isotope data of N-MORB extrusive rocks from the Mt. Medvednica ophiolite mélange. Sample Location Rock group 143Nd/144Nda 147Sm/144Nd 87Sr/86Sra Nd(t) b 87Sr/86Sr(t) c Age (t)* vs-113/2 1 PB; high-Ti 0.512975 (8) 0.213788 0.704423 (9) +6.21 0.703511 170 Ma vh-1001/1 5 PB; high-Ti 0.512979 (6) 0.213589 0.704057 (10) +6.27 0.703365 170 Ma Location number corresponds to the locations in Fig. 1C. PB = pillow basalt. a Errors in brackets for Nd and Sr isotopic ratios are given at the 2-level. 147Sm/144Nd ratios calculated from the ICP-MS concentrations of Sm and Nd following equation: 147Sm/144Nd = (Sm/Nd)*[0.53151 + 0.14252*143Sm/144Nd]. bInitial eNd(t) calculated assuming IoCHUR = 0.512638, (147Sm/144Nd)o CHUR = 0.1966, and lSm = 6.54*10–12 a–1. c Initial 87Sr/86Sr(t) calculated using ICP-MS Rb and Sr concentrations and assuming lRb = 1.42*10–11 a–1. *Coresponding age for the initial eNd and initial Sr isotopic ratios. Geologia Croatica 65/3Geologia Croatica 442 7. DIsCUssION AND CONCLUsIONs Ophiolite mélanges that contain fragmented rocks similar to the lithologies of a coherent lithostratigraphic pile of an ophiolite complex, formed in an accretionary wedge in front of an overriding oceanic plate (e.g. FESTA et al., 2010). Rock fragments derived from the overriding plate were in- corporated in the accretionary wedge mainly through sedi- mentary processes, whilst the materials from the subducting slab were included by pealing off from a MORB-type sub- ducting slab (e.g. SACCANI & PHOTIADES, 2005). Ophi- olite mélanges are finally formed as tectonic mélanges dur- ing ophiolite emplacement, whereby various rocks from continental crust are ordinarily also included in the mélange as fault-bounded blocks. A systematic petrological and geo- chemical study of materials incorporated in mélanges pro- vides opportunity for detailed reconstruction of the geody- namic evolution of an oceanic realm, or its particular segment, as has been successfully done for the Repno oce- anic domain of the Meliata-Maliac-Dinaric-Vardar oceanic system (see Fig. 10 in SLOVENEC et. al., 2011). This work refers to terminal Jurassic spreading in the ROD and will improve the overall tectonomagmatic cartoon of the domain at its culminating spreading stage. As in the other two ophiolite mélange sectors (Mts. Samoborska Gora and Kalnik) of the Kalnik Unit, the oldest magmatic rocks in Mt. Medvednica are Illyrian-Fassanian alkali basalts, related to the terminal phase of intracontinental rifting that prograded to opening of an ensialic back-arc ba- sin in the ROD (SLOVENEC et al., 2011). The crustal suc- cession formed by onset of an ocean spreading ridge is com- pletely lacking in the Mt. Medvednica and Samoborska Gora ophiolite mélange, but was well documented in the Mt. Kalnik by fragments of E-MORB-type proto-oceanic crust formed during the Early and Late Ladinian, followed by Middle Carnian T-MORB-type crust; the oldest N-MORB- type crust formed in the Late Carnian (SLOVENEC et al., 2011) and the youngest in the Late Pliensbachian (LUGO- VIĆ et al., in rewiev), both related to an evolving oceanic spreading centre. As a rule, the N-MORB-type crust show supra-subduction signatures inherited from a subducting Pa- laeotethyan oceanic slab that diminish from Late Carnian to the Late Pliensbachian crustal rocks. It was inferred that slab break-off in the ROD might already have occurred in the Latest Pliensbachian. The initiation and onset of intraoceanic subduction is recorded in the Mt. Medvednica and Samoborska Gora sectors by abundantly distributed Late Bathonian high-Ti N-MORB-like extrusives, and Callovian medium- and low-Ti IAT lavas and composite fragments of SSZ upper crustal sequences (SLOVENEC & LUGOVIĆ, 2008; 2009; SLOVENEC et al., 2010). However, in Mt. Kalnik, ophiolite mélange fragments of SSZ crust are rare and marked by Late Bathonian medium-Ti basalts and Tithonian IAT-type amphibole gabbro, that is by now the youngest Jurassic SSZ crustal lithology in the ROD. Such a regular chemo stra- tigraphic progression (WHATMAN & STERN, 2011) of Late Bathonian to Tithonian SSZ upper crustal rocks in the ROD, clearly reflects progressive depletion of the mantle wedge involved in melting, influenced by enriched fluids released from the subducting plate. Assuming that this first SSZ crust in the ROD formed in the Late Bathonian the most evolved or, in other words, the youngest stage of Middle Jurassic ocean spreading evolution of the domain remains unsolved. The stratigraphic gap of ocean crust formation arising in the ROD between the Late Pliensbachian and the Late Figure 7: Initial 143Nd/144Nd – 87Sr/86Sr isotope ratios diagram for Mt. Medvednica N-MORB volcanic rocks and tholeiitic SSZ basalts (gray shaded; SLOVENEC & LUGOVIĆ, 2009) showing the main oceanic mantle reservoirs of ZINDLER & HART (1986). DM – depleted mantle, BSE – bulk silicate Earth, EMI and EMI II – enriched mantle, HIMU – mantle with high U/Pb ratio, PREMA – frequently observed PREvalent MAntle composition. The mantle array is defined by many oceanic basalts and a bulk Earth value for 87Sr/86Sr can be obtained from this trend. Data for back-arc basin basalts – BABB (gray shaded field) compiled from WILSON (1989) and references therein, COUSENS et al. (1994) and references therein, PEARCE et al. (1995) and EWART et al. (1998). Data for mid-ocean ridge basalts – MORB (solid line) compiled from WILSON (1989) and references there in and COUSENS et al. (1994), references therein and PEATE et al. (1997). Data for average N-MORB are from SUN & McDONOUGH (1989). Data for oceanic island arcs and active continental margins – IAB (broken line) compiled from WILSON (1989) and references therein, COUSENS et al. (1994) and references therein, PEARCE et al. (1995) and PEATE et al. (1997). Damir slovenec and boško Lugović: Evidence of the spreading culmination in the Eastern Tethyan Repno oceanic domain... Geologia Croatica 443 Bathonian, is now completed by the Latest Bajocian-Early Bathonian lavas from coherent slices with radiolarian cherts near the Poljanica locality (Fig. 1C). These volcanic rocks are geochemically more akin to Late Pliensbachian isotropic N-MORB-type gabbros, that were formed at a spreading ridge (LUGOVIĆ et al., in rewiev), than to Late Bathonian Figure 8: Discrimination diagrams for the N-MORB volcanic rocks from the Mt. Medvednica ophiolite mélange. (A) Ta/Yb – Th/Yb diagram (PEARCE, 1983). S – subduction zone enrichment; C – crustal contamination; W – within-plate enrichment. N-MORB and E-MORB are from SUN & MCDONOUGH (1989). Data for back-arc basin basalts (BABB) fields (Mariana, Lau-Tonga, Oman, East Scotia Ridge) are from PEARCE et al. (1984) and LEAT et al. (2000). (b) Th – Nb/16 – Hf/3 diagram (WOOD, 1980). A – normal mid-ocean ridge basalts (N-MORB); B – enriched MORB (E-MORB) and within-plate tholeiites (WPT); C – alkaline within-plate basalts (AWPB); D – calc-alkali basalts (CAB); E – island-arc tholeiites (IAT); 1 – crustal contamination; 2 – SSZ ophiolites trend; 3 – MORB ophiolites trend. Data for back-arc basin basalts – BABB (light gray shaded field) compiled from SAUNDERS & TARNEY (1979), WEAVER et al. (1979), JAHN (1986), IKEDA & YUASA (1989), GRIBBLE et al. (1998), LEAT et al. (2000). Fields for high-, medium- and low-Ti tholeiitic SSZ basalts from the Mt. Medvednica ophiolite mélange (SLOVENEC & LUGOVIĆ, 2009), Late Pliensbachian gabbros (LUGOVIĆ et al., in rewiev) and Late Carnian basalts (SLOVENEC et al., 2011) from Mt. Kalnik plotted for correlation constraints. Figure 9: Petrogenic model for N-MORB volcanic rocks from the Mt. Medvednica ophiolite mélange. Partial melting lines: DM – depleted mantle source, PM – primitive mantle source (KOSTOPOULOS & JAMES, 1992), OIB – enriched mantle source (CLAGUE & FREY, 1982). Model parameters = spinel-lherzo- lite source (ol57–opx25,5–cpx15–sp2,5), melting proportion = ol1,21–opx8,06–cpx76,37–sp14,36, distribution coefficients are from KOSTOPOULOS & JAMES (1992). Fractional crystallization lines: initial magma = 10% melting of DM and PM mantle source, respectively, fractionated mineral assemblage = ol30–cpx40– pl30, distribution coefficients are from CHEN et al. (1990). Field (gray shaded) for high-, medium- and low-Ti tholeiitic SSZ basalts from the Mt. Medved- nica ophiolite mélange (SLOVENEC & LUGOVIĆ, 2009) plotted for correlation constraints. Geologia Croatica 65/3Geologia Croatica 444 SSZ high-Ti N-MORB-like extrusives (SLOVENEC & LUGOVIĆ, 2009) (Fig. 5 and 8A–B). The Late Pliensbac- hian rocks show inherited negative HFSE anomalies, whilst these anomalies in the Late Bathonian extrusives are re- established during intraoceanic subduction (SLOVENEC et al., 2011). However, Poljanica extrusive rocks display very slight positive HFSE anomalies, which, combined with other geochemical parameters, suggest a different mantle source and comparatively peculiar geotectonic setting of formation. Their normalized element concentration patterns (Fig. 6A, 6B), high-Ti content (Fig. 4) and hosted clinopyroxene com- position (Fig. 3A–B) are accepted as being representative of melt generation at an N-MOR setting of the Eastern Tethyan ophiolites (e.g. SERRI, 1981; BECCALUVA et al., 1983). These mineralogical and geochemical signatures suggest that only suboceanic mantle, unaffected by subduction- related components, contributed to the formation of ROD crust during the Latest Bajocian-Early Bathonian. This is also supported by the Nd-Sr isotopic composition that is strictly akin to MORB (Fig. 7), and corresponds to the iso- topic composition of prevalent mantle composition (PREMA). The plot in the diagram Th-Yb vs. Ta/Yb reveals MORB sig- natures of the analyzed rocks that reflect a mantle source ranging in composition between enriched and depleted (Fig. 8A). The different mantle sources for Late Pliensbachian and Late Bathonian MORBs, that are obviously contaminated by subduction-related components (mantle wedge), compared to the subduction uncontaminated Latest Bajocian-Early Ba- thonian spreading ridge suboceanic mantle, are best illus- trated from this diagram. In Figure 8B, Th-Hf/3-Nb/16 is utilized for discrimination of geotectonic settings of mafic extrusive rocks, the Poljanica basalts plot in the field of N- MORB and form the trend typical of basalts from MOR re- latad ophiolites. The Poljanica basalts are undoubtely formed at a spread- ing ridge, from melts derived from uncontaminated suboceanic mantle (Figs. 7 and 8A). Different geochemical variables were used to infer the amount of partial melts extracted from a parental mantle source (e.g. PEARCE, 1983). The model implemented by KOSTOPOULOS and JAMES (1992), on relative concentrations of REE, revealed most reliable results in our case. In the La/Yb vs. La diagram wherein three potential mantle sources are considered (Fig. 9), analysed unfractionated rock samples plot between primitive mantle (PM) and depleted mantle (DM) sources. According to the model, the Poljanica basalts are compatible with appro xi mately 13–17% partial melting of a mantle source transitional between primitive and depleted MORB-type mantle. Jurassic mantle peridotite of such fertile composition, typical for Jurassic Central Dinaric ophiolite lherzolites (LUGOVIĆ et al., 1991; BAZYLEV et al., 2009), was never found in the ROD. In conclusion, based on geochemical and petrological data, the analysed Latest Bajocian-Early Bathonian basalts from the Mt. Medvednica ophiolite mélange sector of the Kalnik Unit, represent vestiges of the youngest Jurassic proper N-MORB crust formed at middle ocean ridge centre during culmination of spreading in the ROD as a part of the Meliata-Maliak-Dinaric-Vardar ocean system (Fig. 10). 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