OLIVINE WEBSTERITE VEINS CUTTING THE RABROVO SERPENTINITES (SOUTH MACEDONIA): NEW EVIDENCE OF THE ARC SETTING OF THE EAST VARDAR OPHIOLITES?

The study reports petrography and mineral chemistry data about an ultramafic vein that cuts the Rabrovo serpentinite (near Valandovo, Southern Macedonia). The serpentinite occurs as a block within a shearing zone of the East Vardar Zone tectonic mélange. The vein corresponds to medium-grained olivine websterite that sometimes displays cumulate-like textures. It is composed of low-Al orthopyroxene (Mg#[mol MgO*100/(MgO + FeO)]~85, Al2O3<2 wt.%), clinopyroxene (Mg# = 82–86), olivine (Mg#~84) and spinel (Cr#[mol Cr2O3/(Cr2O3 + Al2O3)]~0.4, which, according to geothermometric calculations, equilibrated at 750–850 C. Its pyroxene-rich modal composition suggests that this rock cannot represent a normal lithospheric mantle. Instead, it is supposed that it was formed via magmatic precipitations in the mantle lithosphere. In addition, the Rabrovo olivine websterite shows similar mineral chemical compositions to many other orthopyroxene-rich lithologies worldwide, which origin is commonly associated with subduction settings. Therefore, its presence is considered as additional evidence that the East Vardar ophiolites represent typical supra-subduction ophiolites.


INTRODUCTION
The Balkan Peninsula is known from the existence of several ophiolite belts which are part of a much broader ophiolite domain that can be traced from the Western Mediterranean to the Himalaya.From the west to the east they are: Dinaridic ophiolite belt, Western Vardar ophiolites and Eastern Vardar ophiolites (e.g.[1,2] and references therein).The geotectonic setting of all these ophiolite belts was mainly determined on the basis of the igneous members of ophiolites, mostly pillow lavas and diabase dykes, and only partly by studying depletion degrees of the underlying mantle peridotites [3][4][5].Only very rare studies from this region were aimed at investigating orthopyroxene-rich lithologies [6].On the other hand, it is very well known that ophiolites containing this lithology are commonly interpreted as originating from subductionrelated settings (e.g.[7]).
The East Vardar Zone has the most pronounced supra-subduction signature of all the Balkan ophiolite belts.This setting was first, at least indirectly, suggested by [3] Maksimović and Majer (1981) and [8] on the basis of the higher depletion extent found in the East Vardar peridotites with respect to the Dinaride and West Vardar peridotites.The most striking evidence came from a recent study of ophiolite-related volcanic rocks of the Demir Kapija ophiolites in south Macedonia [9].Moreover, based on the composition of mantle xenoliths entrained in Palaeogene mafic alkaline rocks of East Serbia, some authors [10,11] argued that the mantle underneath the present day East Serbia also possesses supra-subduction signatures.These authors further postulated that this subcontinental mantle slice may, in fact, represent suboceanic mantle portions that were accreted during the closure of the East Vardar Zone (see also [12]).One of the strong-est independent evidence for this hypothesis was derived from the presence and characteristics of a sub-group of olivine websterite xenoliths.[13] studied these orthopyroxene-rich xenoliths in detail and concluded that they represented lithospheric precipitates of boninite-like magmas similar to those commonly found in sub-arc settings.
In this study we report and discuss petrography and mineral chemistry of a pyroxene-rich vein cutting the serpentinite of Rabrovo (south Macedonia).The serpentinites geotectonically belong to the narrow East Vardar Zone ophiolite belt, and this is the first occurrence of orthopyroxene-rich lithology in this ophiolite zone.In a separate paper we shall present the full whole rock and mineral major and trace element geochemistry of this lithology, whereas in this study we focus on the major element compositions of main minerals.By comparing the compositions of the minerals from the Rabrovo websterite vein with those occurring in the olivine websterite xenoliths reported by [13], we shed more light on the significance of orthopyroxene-rich lithologies for determining the geotectonic setting of their host ophiolites.

GEOTECTONIC SETTING
The present day geology of Macedonia is generally interpreted as having resulted by collision between the continental margins of Adria and Eurasia [14][15][16].The central parts are dominated by the Pelagonides and the Vardar Zone s.l.The Pelagonides are composed of Precambrian rocks extending northward from the Skutari-Peć line to the Drina-Ivanjica metamorphic unit [2,[17][18][19][20][21].The Vardar Zone s.l. is represented by the Western and the East Vardar ophiolitic units (sensu [2]).The East Vardar ophiolites occur in the southeast part of Macedonia, where they are represented by the largest ophiolitic complex of Demir Kapija.The whole ophiolitic section is unconformably covered by the Upper Tithonian reef limestones, which are, in the northwest and the south-east covered by upper Eocene-Pliocene and Pliocene-Quaternary sediments, respectively.In the west, the East Vardar ophiolites have tectonic relationship with the Pelagonian unit, whereas the north-eastern tectonic contact with the Serbo-Macedonian Massif is characterized by a mylonite zone, and sporadically by tectonic mélange.Within this heterogeneous mélange-like zone occur numerous ophioliterelated rock associations, which are spatially detached from the main body of the Demir Kapija complex.They are represented by variably sized blocks of diabases, gabbros and serpentinites, often containing pyroxene-rich veins.

EXPERIMENTAL SECTION
Ten samples of the studied ultramafic vein were cut to produce chips for further scanning electron and microprobe investigations.The samples are first investigated petrographically using transmitted-light microscope in order to select the freshest rocks.The five freshest samples are studied on major element chemistry of the main minerals.Mineral chemistry was determined by electron microprobe (JEOL JXA 8900RL) at the Department of Geosciences, University of Mainz (Germany), using wavelength-dispersive analysis and a range of natural and synthetic standards.The data were corrected using the CITZAF procedure [22].Detection limits were between 0.01 and 0.07 wt%.Operating conditions were generally 15 kV (20 kV) accelerating voltage, 12 nA beam current, 1-5 µm beam diameter and 15-30 s counting time on peak.

Field occurrence and petrography of the orthopyroxene-rich veins
The studied pyroxene-rich veins are found cutting a serpentinite block from the tectonic mélange of the East Vardar Zone.The serpentinite crops out near Rabrovo, along the road Valandovo-Strumica.It appears as a ~1 km long and a few hundreds of meters wide lens-like body that exhibits sharp tectonic contacts with the adjacent medium -to lowgrade metamorphic rocks of the Serbo-Macedonian Massif (Figure 1; [23]).
The pyroxene-rich vein represents, in fact, an approximately 1-1.5 m thick veining zone (Figure 2a).The veining zone consists of numerous pyroxene-rich individual veins ranging in thickness from only a few millimeters to >15 cm.Both host serpentinite and pyroxene-rich veins are strongly sheared and at places mylonitized and no evidence of true magmatic contacts can be observed.Therefore, the veins sometimes have typical lens-like forms or dismembered bands that resemble typical boudins.
These suggest that the presently observed contacts resulted from shearing during mylonitization and that boudin-like structures are probably formed by stretching along the shear foliation and shortening perpendicular to this.The serpentinite is composed of rare relicts of olivine and pyroxene set in a fine-grained matrix of serpentine minerals.In this stage of investigation it is not clear whether they represent remnants of mantle peridotites or those originating from the bottom of oceanic crust.The pyroxenite veins have medium-to finegrained granular texture.At mm scale the phases are merely undeformed and display sharp and mostly curvilinear contact suggesting good equilibration.It is composed of variable proportions of olivine, orthopyroxene, clinopyroxene and spinel.Apart of these predominant phases a few grains of altered amphibole are also found in some veins.The modal composition is very variable, showing considerable differences at cm and even mm scale.The differences are mostly related to different proportions of olivine and two pyroxenes.Irrespectively to the wide range in modal composition, this rock differs from normal mantle peridotites by having less than 50 % olivine, and can be classified as olivine websterite.Orthopyroxene often predominates over clinopyroxene and forms the main crystal network (Figure 2b).It is subhedral and tabular in shape and usually displays tiny exsolution lamellae (Figures 2b, d).Clinopyroxene often appears filling interstitial spaces in-between orthopyroxene crystals and giving cumulitic characteristics (Figure 2c).It sometimes forms larger pools that enclose smaller orthopyroxene grains (Figure 2d).These textural relationships can suggest that orthopyroxene crystallized shortly before and/or partly simultaneously to clinopyroxene.Olivine is, most probably, an earlier phase that crystallized before the pyroxenes.Sometimes it appears as subhedral to almost idiomorphic crystal indicating free crystallization in open space (Figure 2b).It is rather fresh with serpenttinization developed only along cracks.Spinel is represented by tiny, subhedral and mostly equidimensional to slightly elongated grains.They are isolated and enclosed by more coarse-grained silicates (Figure 2c, d).

Mineral chemistry
The studied minerals from the Rabrovo olivine websterite veins have relatively uniform major element compositions.The results of microprobe investigations are given in Tables 1-4.The classifycation diagram of pyroxenes of Morimoto et al. [24] is given in Figure 3.
Orthopyroxene corresponds to enstatite with an average composition of En 85 Fs 14 W 1 .[25,26].In comparison to orthopyroxene from East Serbian mantle xenoliths, there is a clear compositional similarity with orthopyroxene from spinel-poor olivine websterite xenoliths (Figure 4).The origin of these olivine websterite xenoliths is interpreted in terms of lithospheric crystallization of high-Si-Mg boninite-like magmas [11].
Clinopyroxene shows narrow compositional transitions from diopside to augite with an average formula of En ~46 Fs ~8W ~46 .It is also characterized by relatively uniform Mg#s mostly between 85 and 86.Elements that commonly reflects the fertility of the magmatic source are present in very low concentrations, for instance: Al 2 O 3 = 1-1.6 wt.%, TiO 2 = 0.04-0.06wt.% and Na 2 O < 0.1 wt.%.Chromium contents are moderate and range between 0.3-0.4wt.% Cr 2 O 3 .In Figure 5 is given Mg# vs Al 2 O 3 (wt.%)plot for clinopyroxene.It is evident that the clinopyroxene that occurs in the studied olivine websterite veins is compositionally very similar to the clinopyroxene from olivine websterite mantle xenoliths derived from the East Serbian subcontinental mantle [11,13].

Geothermometric calculations
Temperatures were calculated using the olivine-spinel [28][29][30], clinopyroxene-only [31] and orthopyroxene-only [32] geothermometers for pressures ranging between 2 and 5 kbars.The results of geothermometric calculations are shown in Table 5.The average calculated temperature according to different geothermometers varies between 750 and 850 o C. plot for clinopyroxene; data for the composition of clinopyroxene from other lithologies are from: [11,13] for clinopyroxene from various East Serbian mantle xenoliths and [25] for clinopyroxene from Dinaride and West Vardar peridotites

DISCUSSION
The origin of the olivine websterite veins The olivine websterite veins found cutting the Rabrovo serpentinite are characterized by remarkably fertile compositions.Such compositions fall outside the main compositional trends of common mantle peridotites.These veins are ortho-and clinopyroxene-rich and all their Fe-Mg silicates have relatively low Mg# (< 87) to be in equilibrium with typical mantle silicates.These characteristics indicate that this lithology cannot be regarded as 'normal' upper mantle.It is generally accepted that such pyroxene-rich rocks are related to magmatic modifications of the upper mantle i.e. that they result from percolations and precipitations of mafic/ultramafic magma and formation of pyroxene-rich domains in the mantle lithosphere [33,34].Besides the formation of magmatic mafic and ultramafic veins, this process is also responsible for metasomatic refertilization of previously depleted lithospheric mantle.In such a way, previously variably depleted lherzolite/harzburgite mantle peridotite is transformed into clinopyroxene-rich lherzolite and, in some cases, even wherlite domains [35].However, the above mentioned modifications of the normal upper mantle material commonly produces clinopyroxene-rich lithologies and, additionally, such lithologies contain typically Fe-Ti-Al-Ca rich silicates.By contrast, the Rabrovo olivine websterite possesses a list of other characteristics that are unusual for lithologies having formed by modifications caused by mafic melts.First, the Rabrovo olivine websterite is rich in orthopyroxene (> 50 %vol.)that usually predominates over clinopyroxene.It is generally known that this mineral normally decrease in abundance during mafic metasomatism because this phase is not stable in Si-undersaturated magmas or those close to silica saturation [36].
The presence of orthopyroxene-rich mantle rocks have been reported by many authors [7,[37][38][39].The origin and evolution of these lithologies have been variously interpreted.Namely, by: (i) deserpentinization of the normal peridotitic upper mantle (e.g.[40]), (ii) metasomatism induced by percolating Si-rich melts/fluids [7,37], or (iii) direct crystallization from a silica saturated melt (e.g.[41]).The first two interpretations are not likely scenarios for the origin of melts from which the Rabrovo olivine websterite crystallized.On the contrary, these rocks show many characteristics that can be associated with crystallization of Si-rich lithospheric magmas.This is indicated from textural relationships that are found in the studied olivine websterite, at first place, from the presence of cumulate-like texture (see Figure 2c).Moreover, these rocks lack fibrous orthopyroxene or olivine relicts in orthopyroxene, which are commonly found after deserpentinization and/or fluid-induced metasomatic processes.Mineral chemistry data also support this conclusion.Namely, Mg# and NiO contents of the orthopyroxene from the Rabrovo olivine websterite is rather low (< 87 and <0.06 wt.%, respectively), to support an origin via deserpentinization and fluidinduced metasomatism.Therefore, it is more logical that the studied olivine websterite veins originated by crystallization of a primarily silica-and Mg-rich, presumably, boninite-like magma.Early crystallization of orthopyroxene, which is suggested by textural relationships, indicates silica-saturated or oversaturated melts, most probably with more than 53 wt.% SiO 2 (e.g.[42]).Higher modal abundance of clinopyroxene in combination with relatively low Mg# in olivine and pyroxenes and somewhat higher CaO contents in orthopyroxene, can suggest that these olivine websterites crystallized from evolved magmas.

Geodynamic implications
Notwithstanding which interpretation for the origin of the Rabrovo olivine websterite we adopt, it is very likely that the formation of these rocks was associated with a supra-subduction setting.
It is worth noting that the studied olivine websterite veins are found cutting a serpentinite block from the East Vardar Zone mélange.It is known that the East Vardar ophiolite is dominated by harzburgites and is considered to be the most depleted one in the Balkan Peninsula.As previously mentioned, [10,11,13] argued that orthopyroxenerich mantle xenoliths in Serbia also originated as lithospheric precipitates of arc-related magmas.They suggested that these high-Mg and high-Si magmas resulted from melting of a highly refractory harzburgitic source, most probably due to a H 2 O flux.Such melting processes are very common in fore-arc regions.Analogously, the formation of the studied olivine websterite vein could have resulted from melting of highly refractory mantle peridotites in the presence of H 2 O flux and depression of melting temperatures.The orthopyroxene-rich mantle xenoliths differ from the studied websterite by having abundant carbonate and displaying much higher Cr# values in their spinels.However, this cannot disturb the postulated petrogenetic analogy of these two lithologies.As shown by Cvetković and coauthors [10,11,13] the carbonate present in the East Serban orthopyroxene-rich mantle xenoliths is unrelated to their original composition but was likely introduced via later metasomatic processes.On the other hand, higher Cr#s in spinels from the xenoliths can be explained because the source from which this lithology originated was associated with higher degree of depletion.
The fact that there is a contrast in Fe-Mg distribution between the silicates of the olivine websterite and normal depleted peridotite suggests that these websterite domains did not reside long in the mantle before they were tectonically emplaced and reworked in the mélange.This is corroborated by a very large range temperature range that is obtained by thermometric calculations, which can suggest fast and incomplete equilibration.This, in turn, indicates that subduction processes that were responsible for subduction, the most likely candidate is the Mesozoic subduction related to closure of the Tethyan Ocean [21].

CONCLUSIONS
The presence of olivine websterite veins cutting the Rabrovo serpentinite block in the mélange is considered as evidence that the East Vardar Zone ophiolite belt formed in a suprasubduction setting.Textural relationships suggest that the olivine websterite vein represents relicts of magmatic precipitates at crustal or subcrustal (?) depths.The average temperature of equilibration is estimated to 750-850 o C. Low Mg# values in silicates of the studied olivine websterite suggests that these magmas did not reside long in the lithosphere.Major element chemical compositions of the main minerals composing the olivine websterite indicate that the studied veins formed by crystallization of silica saturated, MgO-rich primary magmas that commonly originate by melting of highly depleted peridotite in sub-arc settings.This study strongly suggests that the investigation of pyroxene-rich lithospheric rocks can be very useful palaeotectonic indicators, especially recording possible effects of subduction processes.

Figure 2 .
Figure 2. Field photos and BSE images of the studied olivine websterite.a -Field outcrop of the Rabrovo olivine websterite; the inset displays cm-thick interfingering of serpentinite and olivine websterite rocks; b -granular texture with subhedral to almost euhedral orthopyroxene and olivine and subordinate anhedral clinopyroxene; c -a cumulatelike texture with interstitial clinopyroxene surrounded by orthopyroxene; d -a larger clinopyroxene pool in the olivine websterite (right-hand side of the photo)

Table 5 .
Calculated temperatures according to different geothermometers