The first record of ultramafic cumulates from the Mt . Kalnik ophiolite mélange in the SW part of the Zagorje-Mid-Transdanubian Zone ( NW Croatia ) : mineralogy , petrology , geochemistry and tectono-magmatic affinity

Ultramafic cumulate rocks represent the rarest allochthonous fragments of the Mesozoic oceanic lithosphere observed today in the Upper Jurassic to Lower Cretaceous mélange of Mt. Kalnik, located in the SW part of the Zagorje‒Mid-Transdanubian Zone (ZMTDZ). Poikilitic heteroadcumulate ultramafic rocks of Mt. Kalnik are represented by amphibole lherzolites/harzburgites and plagioclase lherzolites. Both were formed by in-situ processes within a magma chamber following the general crystallization sequence of: Al-chromite → Mg-rich olivine → enstatite ± augite → Ca-amphibole (pargasite ± edenite ± magnesiohornblende) → Ca-plagioclase (An82.6-87.4). Cumulate minerals are spinel and olivine as well as orthopyroxene and clinopyroxene which are usually enclosed in intercumulate phases such as amphibole and/or plagioclase that render an interstitial mesostasis. Rocks’ textural characteristics, mineral crystallization order and their phase chemistry are all suggestive of low-pressure sub-solidus crystallization in an open system. The low Ti content in augite and scant HFSE abundances suggest the studied rocks may have formed from a depleted mantle source. In addition to the medium to high degree of partial melting of the source, the parental process that gave rise to the Mt. Kalnik ultramafic cumulates also included a low degree of fractional crystallization. The segregation of oxidized Al-chromite and oikocrysts of pargasite and edenite in an early crystallization stage illustrates the formation of a cumulate sequence from volatile-rich magmas. These magmas usually have a high oxidation potential and are exclusively found in intra-oceanic subduction zones, predominantly in island arcs. The overall whole-rock geochemistry [e.g. (Nb/La)n = 0.25-0.34; (Ti/Gd)n = 0.49-0.89; (Th/Nb)n = 5.29-8.63; (La/Lu)cn = 0.57-0.68] together with a record of Ca-rich plagioclase (up to An87.4) and low Ti clinopyroxene (≤0.54 wt%) corroborate the supra-subduction tholeiitic nature of the magma source. Ultramafic cumulates from the ophiolitic mélange of Mts. Kalnik and Medvednica show common genetic features and geotectonic provenance. Comparison with analogous ultramafic lithotypes of the north-eastern segment of the ZMTDZ (the Szarvaskö Complex, Hungary), the ultramafic cumulates of Mts. Kalnik and Medvednica portray some subtle differences that may indicate their distinctive geotectonic provenance. Mts. Kalnik and Medvednica ultramafic cumulates represent the vestiges of a single Upper Jurassic intra-oceanic arc system formed in the western branch of the Meliata-Maliak segment of the Neotethyan oceanic realm. SHERVAIS, 2001; PEARCE et al., 2003; PEARCE, 2008; BORTOLOTTI et al., 2013; SACCANI & TASSINARI, 2015). Ultramafic rocks affiliated to ophiolite are commonly found in the Dinarides and Carpathians as a component of ophiolite complexes (e.g. BALLA, 1984; HOVORKA et al., 1985; PAMIĆ & DESMONS, 1989; LUGOVIĆ et al., 1991; MAJER, 1993; IVAN, 2002; HOECK et al., 2006). As integral parts of an ophiolite, ultramafic cumulates emerge as slices of oceanic plates that are now observed obducted and emplaced onto passive continental plate margins. More typically, these cumulates occur in the form of fragmented allochthonous bodies (olistoliths), embedded in the chaotic rock mixture usually referred to as an ophiolitic mélange (e.g. WAKABAYASHI & DILEK, 2003; FESTA et al., 2010). Ophiolite mélange represents a disordered tectono-sedimentary complex initially formed by tectonically activated sedimentary processes in the deep oceanic trench (accretionary wedge) developed in a forearc region in front of the leading edge of the overriding plate (RAYMOND, 1984; FESTA et al., 2010). Article history: Manuscript received June 19, 2018 Revised manuscript accepted September 07, 2018 Available online October 17, 2018

The goal of this research is to present for the first time the mineralogical, petrological and geochemical characteristics of cumulate ultramafic rocks from the ophiolitic mélange of Mt. Kalnik and to infer their petrogenesis by discussing a plausible geotectonic setting of their formation. In the future, this study will add to the existing knowledge on the geodynamic evolution of the oceanic lithosphere of the Dinaridic Tethys during Mesozoic time by correlating our findings with analogous rocks from the neighbouring ophiolitic complexes or mélanges from the ZMTDZ.

GEOLOGY OF MT. KALNIK
A simplified geological map and stratigraphic column of Mt. Kalnik is shown in Figure 2. Its surface geology comprises Neogene clastic rocks and parts of the heterogeneous ophiolitic mélange. The northern part of the Mt. Kalnik ophiolite mélange is thrust onto the Neogene-Pleistocene sedimentary succession (ŠIMUNIĆ et al., 1982). All other contacts of the ophiolite mélange exhibit a tectonic-erosional unconformity against the youngest Neogene and Pleistocene sedimentary rocks (Fig. 2). The central ridge of Mt. Kalnik is composed of Palaeocene carbonate breccia thrust over Neogene sedimentary rocks (ŠIMUNIĆ et al., 1981). The common constituents of the breccia are fragments of Triassic algal and stromatolithic limestone and dolomite, as well as Jurassic and Upper Cretaceous limestone. Several individual tectonic slices of Early Cretaceous(?) highly serpentinized mantle peridotites of island arc (IA) affinity, several hundred metres in diameter were exhumed along the mountain ridge tectonic zone (POLJAK, 1942;ŠIMUNIĆ et al., 1981), accompanied by a composite slice of serpentinized lherzolites (LUGOVIĆ et al., 2007) that are underlain by the mass of orthoamphibolites (ŠEGVIĆ et al., 2016). Amphibolite occurs in the form of hectometre-sized blocks placed within the ophiolitic mélange that is tectonically inserted in the Palaeogene sedimentary succession. Geochemical and petrological characteristics of mantle peridotites and amphibolites indicate a common tectono-metamorphic history. The mélange is predominantly composed of homogenous metre-to-hectometre-kilometre-sized ophiolitic blocks (mafic extrusives, subordinate gabbros), showing various geochemical signatures (E-, T-, N-MORB, IA) consistent with their distinct geotectonic formation setting during an age span from the Ladinian to the Bajocian (oceanic setting), and the Bathonian to the Late Oxfordian (suprasubduction setting; CRNKOVIĆ et al., 1974;VRKLJAN, 1989;PAMIĆ, 1997;VRKLJAN & GARAŠIĆ, 2004;SLOVENEC et al., 2011;LUGOVIĆ et al., 2015). However, some gabbroic blocks that appear as fault-bounded tectonic inclusions that were embedded in the mélange during ophiolite emplacement have been proved to represent evidence of back-arc Cretaceous magmatism (LUGOVIĆ et al., 2015). Two allochthonous and homogenous metre-sized blocks of ultramafic cumulates, that outcrop only rarely, are investigated in detail in this study. They have been recovered in the SE part of the Mt. Kalnik mélange in Kamešnica creek, about 1 km north of the central ridge of Mt. Kalnik (Fig. 2). In addition to ophiolitic blocks, mélange is also composed of the metre-to-hectometre-sized olistoliths of sedimentary rocks (greywackes, minor shales, Middle Triassic and Jurassic cherts, and scarce Triassic limestones), along with blocks of non-ophiolite (intracontinental rift) originated alkali basalts (e.g. ŠIMUNIĆ et al., 1982;SLOVENEC et al., 2011;Fig. 2). The blocks of Mesozoic rocks are embedded in a predominantly sheared continent-derived pelitic to silty matrix (Fig. 2). Based on matrix palynomorph assemblages, the accretionary age of the Kalnik Unit, i.e. ophiolite mélange is defined as Early Callovian to Late Valangian (BABIĆ et al., 2002). This time interval represents a period of accumulation of lithostratigraphically diverse material in an intra-oceanic trench (SLO-VENEC et al., 2011). Following the accretion, these rocks underwent emplacement onto the eastern continental margins of the Adria plate in Aptian to post-Palaeocene time (PAMIĆ & TOMLJENOVIĆ, 1998;PAMIĆ, 2002). There are arguments suggesting that the ZMTDZ was displaced by translation and rotation along the Zagreb-Zemplin lineament in a NE direction, finally reaching its present position during the Middle Miocene (PAMIĆ, 1997;TOMLJENOVIĆ et al., 2008;Fig. 1a-b).

ANALYTICAL TECHNIQUES
Twenty thin-sections of representative rock samples were analysed using an Olympus BH-2 polarization microscope installed at the Croatian Geological Survey (Zagreb, Croatia). The chemical composition of mineral phases from three samples were analysed at the Institute of Geosciences (University of Heidelberg, Germany) using a CAMECA SX51 electron microprobe equipped with five wavelength-dispersive spectrometers. The operating parameters included 15 kV accelerating voltage, 20 nA beam current, and ~ 1 μm beam size (~ 10 μm for feldspars). Counting times of 20 s on peak and 10 s on background on both sides of the peak were used for all elements. Limits of detection (LOD) were calculated as the minimum concentration required to produce count rates three times higher than the square root of the background (3s; 99 wt.% degree of confidence at the lowest detection limit). Natural minerals, oxides (corundum, spinel, hematite, and rutile), and silicates (albite, orthoclase, anorthite, and wollastonite) were used for calibration. The measurements relative error was less than 1%. Raw data were corrected for matrix effects using the PAP algorithm (POUCHOU & PICHOIR, 1984 implemented by CAMECA. Mineral phase formula calculations were done using a software package MINPET designed by Linda R. Richard.
Bulk-rock powders for chemical analyses of four samples were obtained from rock chips free of veins. The samples were analysed by ICP-OES 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%, respectively.

PETROGRAPHY AND MINERAL CHEMISTRY
The investigated rocks in hand specimen are characterised by their black to dark green colour. They are massive and dense while breaking unevenly. Sporadically, the rocks may be mottled by the pale spots of plagioclase.
The texture of the analysed cumulate ultramafic rocks is granular allotriomorphic to poikilitic, while the structure is homogenous ( Fig. 3), which is typical for the deepest part of an ophiolite sequence (e.g. MENZIES, 1973;ENGLAND & DAVIES, 1973). These rocks are characterised by a network of cumulate crystals found in an immediate contact. The intercumulus space is filled by interstitial melt that has continued to produce minerals long after the previously crystallized phases have settled. This led to the development of a reaction series between mineral phases which defines a heteroadcumulate texture (WAGER et al., 1960;WAGER & BROWN, 1968;IRVINE, 1982). Cumulate phases are spinel and olivine that are normally enclosed in intercumulus amphibole and/or plagioclase. Both intercumulus phases define an interstitial mesostasis ( Fig. 3a-b). Spinel represents the earliest cumulate phase that is readily hemmed in by olivine ( Fig. 3b and 3d). Alternatively, both phases are marked by a cotectic growth ( Fig. 3a and 3c). Orthopyroxene and clinopyroxene are rarely reported as cumuli, while more commonly they emerge along with the coarse oikocrystals of brown amphibole filling the intercumulus space and enclosing spinel and olivine ( Fig. 3c-d). The last intercumulus crystallization phase is plagioclase that occurs as mesostasis in the interstitial space between clinopyroxene and orthopyroxene (Fig. 3b). Accessory phases are leucoxene and tiny apatite reported in intercumulus plagioclase. Such textural char-   (modified after ŠIMUNIĆ et al., 1982 andHALAMIĆ, 1998). Legend: 1-Neogene sedimentary rocks; 2-Palaeogene sedimentary rocks; 3-Jurassic/Early Cretaceous ophiolite mélange with blocks of: 3a-basalt, 3b-gabbro, 3c-tectonite peridotite/ amphibolite, 3d-ultramafic cumulates and Triassic-Jurassic radiolarites, sandstones and shales (not separated on the map); 4-reverse or thrust faults; 5-normal faults; 6-discordance line, tectonic-erosion discordance; 7-quarry; 8-black arrow indicates the blocks of ultramafic cumulates in the Mt. Kalnik ophiolite mélange.   ene→1-36%, amphibole→10-33%, plagioclase→0-2% and spinel"0-0.6%. Based on the abundances of identified mineral phases (STRECKEISEN, 1974), the poikilitic heteroadcumulate ultramafites of Mt. Kalnik are classified as amphibole lherzolite/harzburgite and, to a lesser extent, plagioclase lherzolite.
acteristics are commonly reported in ophiolites and may stand for the post-cumulus crystallization of an intercumulus melt (e.g. TRIBUZIO et al., 1995TRIBUZIO et al., , 1999ROSS & ELTHON, 1997). The modal composition of analysed cumulate ultramafic rocks is as follows: olivine→40-70%, orthopyroxene→5-25%, clinopyrox- Primary mineral phases show different degrees of alterations. Olivine is partly serpentinized into reticular chrysotile (Fig. 3b). Within the olivine cracks one may often notice dispersed agglomerations of magnetite, which is yet another serpentinization product (Fig. 3a-c). Pyroxene is, along the rims, frequently uralitized or altered to chlorite, whereas plagioclase may be altered to saussurite or prehnite. Cumulate fine-grained spinel represents a cumulate phase representing virtually the only remaining fresh primary phase.

BULK ROCK CHEMICAL COMPOSITION
Chemical composition of the ultramafic cumulates from Mt. Kalnik is given in Table 2. The analysed rocks underwent a deuteric and sea-floor hydrothermal alteration which led to an increase in the loss on ignition (LOI £ 7.00 wt%). However, the preserved magmatic texture and only minor chemical disturbances of the major element content do not exhibit an appreciable impact on the original rocks' chemistry. The ultramafic cumulate nature of the analysed rocks may be clearly inferred from the CaO-MgO-Al 2 O 3 classification diagram (Fig. 5a). They are characterised by low contents of MgO (£ 25.16 wt%) and TiO 2 (£ 0.31 wt%), which defines the analysed cumulates as low Ti rocks (Fig. 5b). The contents of Fe 2 O 3total and Ni are elevated (13.86-15.96 wt% and 1121-1290 ppm, respectively), while the Mg# is moderately high (75.3-78.4). A low content of SiO 2 (37.06-40.63 wt%) is a typical feature of analysed intrusives suggesting their cumulate origin. Highfield strength elements (HFSE; Ti, P, Ta, Nb, Y and Th) as well as rare earth elements (REE) appeared to be relatively immobile during alteration. They maintained the typical igneous concentration levels and characteristic ratios (e.g. Zr/Hf = 41-44; Ti/Zr = 49-87; Nb/Y = 0.02-0.03; Sm/Nd = 0.3-0.4). Conversely, the large ion lithophile elements (LILE) showed a high degree of post-magmatic mobility and are therefore considered unsuitable for petrogenetic and geotectonic consideration.
The multi-element abundance patterns normalised to N-MORB are displayed in Fig. 6a, while the chondrite-normalised REE patterns are given in Fig. 6b. All rocks display a LILE enrichment which is consistent with post-magmatic alterations. Normalised patterns for a range from La to Lu are flat being at 0.2 to 0.6 times relative to N-MORB. A very low content of incompatible elements in cumulate ultramafites may actually reflect their cumulate origin (SAUNDERS et al., 1980;MEYER et al., 1989). Additionally, all analysed samples possess pronounced Only plagioclase lherzolite shows a faint negative Eu anomaly (Eu/Eu* = 0.92), which is typical for the minor accumulation of plagioclase. Relative depletion of LREE with regard to HREE defines a tholeiitic affinity of cumulate ultramafites from the ophiolite mélange of Mt. Kalnik (Fig. 6b).

DISCUSSION AND CONCLUSIONS
Fragments (i.e., blocks) of cumulate ultramafic rocks are the rarest member of the Mesozoic oceanic lithosphere documented in the ophiolitic mélange of Mt. Kalinik (Fig. 2). These metre-sized olistoliths originated from deep portions of oceanic crust and predominantly account for amphibole lherzolite and, to a lesser extent, amphibole harzburgite and plagioclase lherzolite. A lack of cumulate "layered" structures may suggest that the analysed rocks are not typical cumulates, and yet, the occurrence of heteroadcumulate texture visibly demonstrates the prevalence of cumulate processes at the time of formation of the Mt. Kalnik ophiolitic sequence. Cumulate ultramafites of Mt. Kalnik were formed in situ via crystallization in the magma chamber (CAMPBELL, 1978(CAMPBELL, , 1987. Petrographic evidence suggests the following general crystallization order: Al-chromite → Mg-rich olivine (chrysolite) → enstatite ± augite → Ca-amphibole (pargasite ± edenite ± magnesiohornblende) → Ca-plagioclase (bytownite) (Fig. 3). Spinel and olivine are the most common cumulate phases while the post-cumulate minerals are pyroxene, amphibole and plagioclase. Analysed rocks define a trend which is in accordance with the progressive removal of cumulate phases from the magma. Coarse oikocrysts of amphibole were formed from a magma that represents an intercumulus magmatic residuum. Amphibole frequently encloses other minerals. The crystallization of amphibole must have been sluggish thus enabling the development of several crystallization centres which ultimately gave rise to the formation of large poikilitic crystals of amphibole ( Fig. 3a; WAGER & BROWN, 1968).
which according to JAQUES & GREEN (1980) corresponds to medium to high degrees of partial melting of parental magma which leaves a mantle residue represented by harzburgite. A very low HFSE content (0.2 to 0.6 times relative to N-MORB) as well as a slight LREE depletion relative to HREE, along with a very low content of Ti in clinopyroxene indicate the origin of cumulates from a depleted source (PEARCE & NORRY, 1979;BEC-CALUVA et al., 1989;Fig. 5b and Fig. 6).
Chemistry of amphibole reflects a relatively complex process of the formation of cumulate ultramafite from Mt. Kalnik. Namely, the amphibole is characterised by a peculiar compositional trend (i.e. "pargasitic trend", Fig. 4e), indicative of a multifold formation sequence that firstly includes the crystallization of the primary pargasitic phase. Further cooling under magmaticsubmagmatic conditions enhanced deuteric alteration of the pri-mary cumulate assemblage giving rise to the appearance of magnesiohornblende. Finally, post-magmatic processes related to sea-floor hydrothermal activity facilitated the formation of secondary actinolite/tremolite. The presence of hydrous primary amphibole (pargasite) in the studied rocks indicates crystallization of the ophiolite sequence from volatile-rich magmas characterised by a high oxidation potential. Such magmas are exclusively found in the intra-oceanic subduction zones, predominantly in island arcs, where hydrous mantle melting was facilitated by the addition of volatiles from the subducting slab (CONRAD & KAY, 1984;DEBARI & COLEMAN, 1989;TATSUMI & EGGINS, 1995;KOEPKE & SEIDEL, 2004;KOCAK et al., 2005). Formation of amphibole-rich ultramafic cumulates in arc settings has been studied well, both empirically and experimentally (e.g. LAROCQUE & CANIL, 2010; KRAWCZYNSKI et al., 2012). Their occurrence is commonly linked to the middle to lower crust of volcanic arcs (DAVIDSON et al., 2007). Low content of TiO 2 (≤0.54 wt%) in augite and the presence of Ca-rich plagioclase is yet another characteristic of magmatic arc (e.g. BEARD, 1986;DEBARI & COLEMAN, 1989;PARLAK et al., 1996;ILBEYLI, 2008) and forearc (BALLANTYNE, 1992) settings in which analysed cumulates could have probably originated during the Upper Jurassic. However, chromite is still considered as a most reliable indicator of the geotectonic affiliation of intrusive parts of a cumulate sequence (e.g. KAMENETSKY et al., 2001). Chemistry of Al-chromite thus strongly suggests that analysed cumulates originated in an island-arc setting making a clear distinction from a possible forearc geotectonic setting ( Fig.  7a-b). This line of reasoning is further corroborated by cumulates' major element content (Na 2 O+K 2 O-MgO-FeO tot diagram; Fig. 7c), along with the negative Ta-Nb pair and Ti anomalies and relatively uniform depletion of HFSE and REE in multi-elemental plots normalised to N-MORB (Fig. 6a). These features are typical for suprasubduction zone (SSZ) magmas and therefore clearly indicate the influence of the subduction component.
Taking into account that Mesozoic (probably Upper Jurassic) ultramafite cumulates are rarely encountered in the ophiolite complexes/mélanges of the ZMTDZ there is a paucity of correlative mineralogical, petrologic and geochemical data. In addition to ultramafic olistoliths from the ophiolitic mélange of Mts. Kalnik and Medvednica (LUGOVIĆ et al., 2007), similar ultramafic lithotypes were reported at the north-easternmost part of the ZMTDZ in the Szarvaskö magmatic complex (Mts. Bükk) in Hungary (BALLA et al., 1983;BALLA, 1984;KUBOVICS, 1984;KUBOVICS & BILIK, 1984;DOWNES et al., 1990;HA-RANGI et al., 1996;JÓZSA, 1999;AIGNER-TORRES & KOLLER, 1999;Fig. 1a-b). Common characteristics of ultramafite cumulates from the three ophiolite complexes/mélanges is the presence of igneous intercumulus oikocrysts of amphibole, an early crystallization of Ti rich non-silicate minerals, low-grade (prehnite-pumpellyite facies) alteration overprint that took place during the Alpine orogeny and under a rather high oxidation state. The sequence of crystallization and mineral phase chemistry as well as rock bulk chemistry is very similar when comparing the cumulate lithotypes from the ophiolite mélange of Mts. Kalnik and Medvednica (Fig. 4). In general, ultramafic cumulates from all three localities within the ZMTDZ show similar normalised concentration patterns at different fractionation levels (Fig. 6), which point to a common origin but different degrees of evolution of the parental magma. Nonetheless, the absence of the negative Ta-Nb pair and Ti anomalies in ultramafites from the Szarvaskö complex and fractionation from an evolved basic melt distinguish these rocks (wherlite) from the cumulates of Mts. Kalnik and Medvednica whose geochemical particularities clearly testify to their formation in a supra-subduction arc setting ( Fig. 6 and 7). Considering the discussion presented herein and previous literature on the fragments of oceanic lithosphere from the ophiolite mélange of Mts. Kalnik and Medvednica (LUGOVIĆ et al., 2007;SLOVENEC & LUGOVIĆ, 2008;SLOVENEC et al., 2011;LUGOVIĆ et al., 2015) we suggest that ultramafic cumulates of the SW segment of the ZMTDZ originated from a single Mesozoic (most probably Upper Jurassic) oceanic domain. Subsequently, these rocks formed part of an intra-oceanic arc system formed in the western branch of the Meliata-Maliak segment of the Tethyan oceanic realm. Such a geodynamic scenario conforms well to the geotectonic reconstruction suggested by STAMPFLI & BOREL (2002.

ACKNOWLEDGEMENT
This work contributes to the scientific project "Mesozoic magmatic, mantle and pyroclastic rocks of north-western Croatia (grant no. 181-1951126-1141 to Da. S.)" carried out under the support of the Croatian Ministry of Science, Education and Sport. We thank B. LUGOVIĆ and H-P. MEYER for microprobe facilities and I. FIN for excellent polished thin sections. Finally, the critical comments and constructive reviews by M. PUTIŠ and an anonymous reviewer contributed significantly to the quality of this paper. Major elements in wt.%, trace elements in ppm. LOI = loss on ignition at 1100 o C. lhz = lherzolite, hzb = harzburgite; a-, p-= amphibole-or plagioclase-bearing assemblage. Mg# = 100*molar (MgO/(MgO+FeO total )).