Mineralogical Evidence for Partial Melting and Melt-Rock Interaction Processes in the Mantle Peridotites of Edessa Ophiolite ( North Greece )

The Edessa ophiolite complex of northern Greece consists of remnants of oceanic lithosphere emplaced during the Upper Jurassic-Lower Cretaceous onto the Palaeozoic-Mesozoic continental margin of Eurasia. This study presents new data on mineral compositions of mantle peridotites from this ophiolite, especially serpentinised harzburgite and minor lherzolite. Lherzolite formed by low to moderate degrees of partial melting and subsequent melt-rock reaction in an oceanic spreading setting. On the other hand, refractory harzburgite formed by high degrees of partial melting in a supra-subduction zone (SSZ) setting. These SSZ mantle peridotites contain Cr-rich spinel residual after partial melting of more fertile (abyssal) lherzolite with Al-rich spinel. Chromite with Cr# > 60 in harzburgite resulted from chemical modification of residual Cr-spinel and, along with the presence of euhedral chromite, is indicative of late melt-peridotite interaction in the mantle wedge. Mineral compositions suggest that the Edessa oceanic mantle evolved from a typical mid-ocean ridge (MOR) oceanic basin to the mantle wedge of a SSZ. This scenario explains the higher degrees of partial melting recorded in harzburgite, as well as the overprint of primary mineralogical characteristics in the Edessa peridotites.


Introduction
Ophiolites are fragments of oceanic lithosphere that have been emplaced tectonically along continental margins in accretionary prisms during orogenic processes.They may be intact and almost complete or have an incomplete stratigraphy and frequently they are tectonically underlain by an ophiolitic mélange.Ophiolites provide important information for the evolution of ancient oceanic crust and mantle beneath spreading centers in mid-ocean ridge (MOR) and supra-subduction zone (SSZ) tectonic settings [1][2][3][4][5].Subduction-related and subduction-unrelated ophiolites form in a variety of tectonic settings [6].The chemical composition of ophiolitic rocks is commonly used for recognising a variety of different tectonic settings, as well as the nature of mantle sources.These tectonic settings include oceanic spreading centers, hot spots, backarc and forearc basins (supra-subduction zone environments), arcs and other extensional magmatic settings including those in association with plumes [3,[7][8][9][10][11][12].Furthermore, they provide information about magmatic, metamorphic and tectonic processes of the oceanic crust and upper mantle [3,13,14].Mantle rocks of ophiolitic origin provide significant information for the paleotectonic evolution of the oceanic Minerals 2019, 9, 120; doi:10.3390/min9020120www.mdpi.com/journal/mineralsMinerals 2019, 9, 120 2 of 24 lithosphere, partial melting, melt-rock interaction and mantle melt fractionation [15][16][17][18][19]. Specifically, the abyssal peridotites represent mantle residues produced by partial melting beneath MOR whereas the highly depleted SSZ-type peridotites represent mantle rocks that have experienced intense melting above a subduction zone.Serpentinisation is a common secondary process in these lithologies that can take place in both mid-ocean ridge and subduction zones and can strongly affect their primary mineralogical composition, thus obscuring any primary data that can provide petrogenetic information, as well to degrade their quality for several industrial applications [20][21][22][23].Spinel-group minerals have a wide range of compositions reflecting a residual or secondary origin and are normally resistant to serpentinisation, thus they are commonly used as petrogenetic and geotectonic indicators.Their chemical composition depends on the petrogenesis (crystallisation from melt, residue after partial melting) and physical conditions (temperature, pressure, oxygen fugacity) of their host peridotites [24][25][26][27][28][29][30][31].Additionally, the degree of partial melting can be calculated using the chemical composition of unaltered spinel-group minerals.
Ophiolitic rocks occur along two subparallel zones in Greece, the western (or external) and the eastern (or internal), which are thought to represent the sutures of Pindos and Axios (or Vardar) Oceans, respectively (although several authors argue for one ocean), which have been formed during progressive opening and closure of the Mesozoic Neo-Tethyan Oceanic domain (Figure 1).The ophiolitic complex of Edessa, in the internal zone, comprises a series of oceanic crust and mantle fragments linking the Veria-Naousa and Vermion ophiolite suites (to the S) to the Guevgeuli ophiolite (to the NE) (Figure 1).Despite the geological significance of the Edessa ophiolitic complex, few available comprehensive data on the geology and petrology of these rocks have been reported [10,32,33].
Minerals 2019, 9, x FOR PEER REVIEW 2 of 24 lithosphere, partial melting, melt-rock interaction and mantle melt fractionation [15][16][17][18][19]. Specifically, the abyssal peridotites represent mantle residues produced by partial melting beneath MOR whereas the highly depleted SSZ-type peridotites represent mantle rocks that have experienced intense melting above a subduction zone.Serpentinisation is a common secondary process in these lithologies that can take place in both mid-ocean ridge and subduction zones and can strongly affect their primary mineralogical composition, thus obscuring any primary data that can provide petrogenetic information, as well to degrade their quality for several industrial applications [20][21][22][23].Spinel-group minerals have a wide range of compositions reflecting a residual or secondary origin and are normally resistant to serpentinisation, thus they are commonly used as petrogenetic and geotectonic indicators.Their chemical composition depends on the petrogenesis (crystallisation from melt, residue after partial melting) and physical conditions (temperature, pressure, oxygen fugacity) of their host peridotites [24][25][26][27][28][29][30][31].Additionally, the degree of partial melting can be calculated using the chemical composition of unaltered spinel-group minerals.
Ophiolitic rocks occur along two subparallel zones in Greece, the western (or external) and the eastern (or internal), which are thought to represent the sutures of Pindos and Axios (or Vardar) Oceans, respectively (although several authors argue for one ocean), which have been formed during progressive opening and closure of the Mesozoic Neo-Tethyan Oceanic domain (Figure 1).The ophiolitic complex of Edessa, in the internal zone, comprises a series of oceanic crust and mantle fragments linking the Veria-Naousa and Vermion ophiolite suites (to the S) to the Guevgeuli ophiolite (to the NE) (Figure 1).Despite the geological significance of the Edessa ophiolitic complex, few available comprehensive data on the geology and petrology of these rocks have been reported [10,32,33].The aim of the present paper is to provide new data including field observations, mineral chemistry and specifically the chemical variability of spinel-group minerals of ultramafic rocks from the Edessa ophiolite complex, in order to shed more light into their petrogenesis and their tectonic setting.The aim of the present paper is to provide new data including field observations, mineral chemistry and specifically the chemical variability of spinel-group minerals of ultramafic rocks from the Edessa ophiolite complex, in order to shed more light into their petrogenesis and their tectonic setting.

Geological Setting
The Edessa ophiolite represents remnants of an oceanic lithosphere obducted onto Palaeozoic-Mesozoic marbles and schists during Upper Jurassic to Lower Cretaceous [33,35].The ophiolitic rocks include several tectonic units that are considered to be the northwards continuation of the Veria-Naousa ophiolite [32,36,37].
An ophiolitic matrix mélange formation occurs tectonically beneath the ophiolite near Ekklisochori village.It comprises a multi-coloured heterogeneous formation with a chaotic internal structure that consists of tectonic blocks of serpentinites, basalts, marbles and schists surrounded by a tectonised, serpentinitic matrix.It is structurally overlain by a local sub-ophiolitic metamorphic sole, composed of amphibolites and both formations are overthrust by upper mantle tectonites (Figure 2).These rocks at the lower part of the Edessa ophiolite comprise serpentinised harzburgite and minor lherzolite overlain by diorite, gabbro, diabase and basalt (Figure 2).The serpentinised harzburgite displays dark green colour and local relic pyroxenes with moderate to intense mantle deformation features, such as banding and foliation.Locally, it encloses lenses, pods or elongated bodies (up to few meters) of chromitite, which according to their texture and mode of occurrence, are classified as massive and disseminated podiform bodies.Small-scale irregular joints are also developed in the rocks with the presence of slickensides and striations.Moderately serpentinised lherzolite is an infrequent, medium grained rock, which is characterised by greenish black to dark green colour and conchoidal fracture.It occurs as relic, irregular bodies up to a few meters, surrounded by harzburgite (Figure 3a,b).Local lherzolite slivers are repeated in the mantle domain of Edessa due to a series of imbricated thrusts occurring through the area.Light to dark green veins of dunite (up to few cm thick) locally penetrate the lherzolite (Figure 3b).
Sparse gabbroic and dioritic dykes intrude the serpentinised harzburgite.The medium to coarse-grained, pale green dykes display chilled margins and have a thickness of up to 10 m.Some of the gabbroic dykes are intensely rodingitised.Massive gabbros are rare in the Edessa ophiolite; only near the villages of Agia Foteini and Krania massive gabbro were observed in tectonic contact with serpentinised harzburgite and diabase, respectively.Massive diabase and basalt are thrust over serepentinised harzburgite and pass upwards into basaltic pillow-lava flows.The diabase is usually dark green and locally shows low grade metamorphism characteristics.The thrust contacts of the massive diabase and the serpentinised harzburgite form cataclastic zones.
The ophiolitic rocks are unconformably overlain by middle Upper Cretaceous to Palaeocene sedimentary formations (conglomeratic limestone, brecciated limestone) and Neogene to Quaternary deposits (alluvial sediments, talus, travertine) (Figure 2).The serpentinised harzburgite is overthrust by flysch.Pliocene andesite and rhyolite have intruded the ophiolite (Figure 2).According to Sr-Nd isotopes, they are associated with a SSZ tectonic environment, in the Early Tertiary during the subduction of Africa beneath Eurasia [40].Sparse gabbroic and dioritic dykes intrude the serpentinised harzburgite.The medium to coarse-grained, pale green dykes display chilled margins and have a thickness of up to 10 m.Some of the gabbroic dykes are intensely rodingitised.Massive gabbros are rare in the Edessa ophiolite; only near the villages of Agia Foteini and Krania massive gabbro were observed in tectonic contact with serpentinised harzburgite and diabase, respectively.Massive diabase and basalt are thrust over serepentinised harzburgite and pass upwards into basaltic pillow-lava flows.The diabase is usually dark green and locally shows low grade metamorphism characteristics.The thrust contacts of the massive diabase and the serpentinised harzburgite form cataclastic zones.
The ophiolitic rocks are unconformably overlain by middle Upper Cretaceous to Palaeocene sedimentary formations (conglomeratic limestone, brecciated limestone) and Neogene to Quaternary deposits (alluvial sediments, talus, travertine) (Figure 2).The serpentinised harzburgite is overthrust by flysch.Pliocene andesite and rhyolite have intruded the ophiolite (Figure 2).According to Sr-Nd isotopes, they are associated with a SSZ tectonic environment, in the Early Tertiary during the subduction of Africa beneath Eurasia [40].
Rare Fe-Ni-Co sulphides with Cu (pyrite, pentlandite and millerite) and ilmenite are associated to spinel.Olivine and orthopyroxenes show strain lamellae, kink bands, undulose extinction, shearing and recrystallisation, all typical features of plastic deformation of upper mantle peridotites (Figure 4a,c,d).Olivine displays porphyroclastic grains (up to 2 cm long) and smaller neoblasts (Figure 4a,b).Orthopyroxene porphyroclasts (up to 3 cm long) are elongated and exhibit exsolution lamellae of clinopyroxene (Figure 4c).Olivine and orthopyroxene porphyroclasts are surrounded and partly replaced by neoblastic olivine (Figure 4b,c).The clinopyroxene occurs as porphyroclastic (0.5-1 cm long Figure 4a,b) and neoblastic grains in the recrystallised matrix.Spinel is of aluminous composition and forms anhedral to subhedral grains with lobate boundaries (Figure 4d,e).This Al-spinel is locally replaced by veins and rims of garnet, which is a solid solution dominated hydrogrossular and hydroandradite molecules (Figure 4e).The Al-spinel often displays thin rims of secondary magnetite.Local, fine-grained, euhedral Cr-spinel occurring along the cleavage of orthopyroxene porphyroclasts is thought to be of secondary origin (Figure 4e).Secondary phases also include serpentine, chlorite, tremolite and magnetite.Dunite veins (up to a few cm thick) occur in some lherzolitic samples with less deformed segregations of olivine penetrating the porphyroclastic lherzolite, as an indication of melt percolation.

Serpentinised Harzburgite
Sixty harzburgite samples were collected, which are intensely serpentinised.The samples were collected in a way to represent a more or less equal distribution in the peridotite exposure, in order to identify potential variations throughout the peridotite outcrops.Their primary mineralogical assemblage constitutes less than 5% of the mode and comprises relics of clinopyroxene, Cr-spinel and chromite (Figure 4f).Clinopyroxene appears as remnants of subhedral porphyroclasts in few samples.Two types of spinels are observed in these samples and even in the same thin section: Cr-spinel crystals, which are anhedral to subhedral with curved and lobate boundaries (Figure 4f,g) and chromite crystals, which are euhedral and rarely subhedral (Figure 4h).The Cr-spinel shows thick rims of Cr-bearing magnetite and Mn-bearing ferritchromit whereas the chromite displays thin rims of Cr-bearing magnetite (Figure 4g,h).Serpentine is the main alteration product showing mesh, ribbon, bastite, hourglass and interlocking textures (Figure 4f).Chlorite and magnetite are also secondary products in the harzburgite.Locally, quartz replaces serpentine, thus suggesting an incipient stage of listwaenitisation.

Analytical Methods
The mineralogical and textural characteristics of the samples were studied in polished-thin sections in optical and scanning electron microscopes (SEM).Mineral microanalyses were performed using a JEOL JSM-6300 SEM equipped with energy dispersive and wavelength spectrometers (EDS and WDS) and INCA software at the Laboratory of Electron Microscopy and Microanalysis, University of Patras.Operating conditions were accelerating voltage 25 kV and beam current 3.3 nA, with a 4 µm beam diameter.The total counting time was 60 s and dead-time 40%.Synthetic oxides and natural minerals were used as standards for our analyses.Detection limits are ~0.1% and accuracy better than 5% was obtained.
Al-spinel crystals occur in the lherzolite and display Cr# values ranging from 7.5 to 28.6 and Mg# [= 100 × Mg/(Mg + Fe 2+ )] values varying from 62.3 up to 80.3, thus resembling spinels occurring in abyssal peridotites (Figure 5).They are also rather enriched in NiO contents (0.24-0.68 wt.%) and show an Mg#-Cr# linear trend (Figure 5).Low TiO 2 values in these spinels are explained by the coexistence of ilmenite, as Ti preferentially enters the crystal lattice of the last.Secondary Cr-spinels in the lherzolite, which are associated to serpentinisation, along cleavage of orthopyroxenes, show lower Mg# (51.2-60.0)and higher Cr# values (31.3-41.8)than the Al-spinels.These Cr-spinels are not shown in Figure 5 and are not further considered for the igneous petrogenetic processes discussed below.
The harzburgite includes two different generations of spinel-group minerals.The older one comprises Cr-spinel whereas the younger includes magnesiochromite (Cr# > 60 and Mg > Fe 2+ ) and chromite (Cr# > 60 and Mg < Fe 2+ ).The magnesiochromite and chromite are considered as one group, therefore they will collectively be mentioned as chromite hereafter.Unaltered Cr-spinels show an inter-grain chemical variation with Cr# ranging from 30.8 to 58.5 and Mg# from 47.1 to 70.4 (Figure 5).The chromite crystals plot into two distinctive clusters on their Mg# vs. Cr# diagram: (i) a Cr#-poorer and Mg#-richer group (Cr# = 63.7-65.6,Mg# = 58.1-63.4),which are texturally associated to the Cr-spinels and (ii) a Cr#-richer and Mg#-poorer one (Cr# = 68.6-74.9,Mg# = 40.5-54.2),which are the aforementioned individual euhedral crystals.The last group of euhedral crystals shows a chemical core-to-rim zonation with decreasing Al 2 O 3 , MgO and Cr 2 O 3 and increasing FeO.The Cr-spinels straddle the abyssal-and arc-derived SSZ peridotite fields showing much resemblance with spinels from forearc peridotites whereas the chromites plot entirely in the field of SSZ-related peridotites with the Cr-richer and Mg-poorer chromites showing considerable similarities with spinels from boninitic melts (Figure 5).Ferritchromit and magnetite have formed along fractures and rims of Cr-spinels and are enriched in Cr, Mn and rarer Zn. show an Mg#-Cr# linear trend (Figure 5).Low TiO2 values in these spinels are explained by the coexistence of ilmenite, as Ti preferentially enters the crystal lattice of the last.Secondary Cr-spinels in the lherzolite, which are associated to serpentinisation, along cleavage of orthopyroxenes, show lower Mg# (51.2-60.0)and higher Cr# values (31.3-41.8)than the Al-spinels.These Cr-spinels are not shown in Figure 5 and are not further considered for the igneous petrogenetic processes discussed below.The harzburgite includes two different generations of spinel-group minerals.The older one comprises Cr-spinel whereas the younger includes magnesiochromite (Cr# > 60 and Mg > Fe 2+ ) and chromite (Cr# > 60 and Mg < Fe 2+ ).The magnesiochromite and chromite are considered as one group, therefore they will collectively be mentioned as chromite hereafter.Unaltered Cr-spinels show an inter-grain chemical variation with Cr# ranging from 30.8 to 58.5 and Mg# from 47.1 to 70.4 (Figure 5).The chromite crystals plot into two distinctive clusters on their Mg# vs. Cr# diagram: (i) a Cr#-poorer and Mg#-richer group (Cr# = 63.7-65.6,Mg# = 58.1-63.4),which are texturally associated to the Cr-spinels and (ii) a Cr#-richer and Mg#-poorer one (Cr# = 68.6-74.9,Mg# = 40.5-54.2),which are the aforementioned individual euhedral crystals.The last group of euhedral crystals shows a chemical core-to-rim zonation with decreasing Al2O3, MgO and Cr2O3 and increasing FeO.The Cr-spinels straddle the abyssal-and arc-derived SSZ peridotite fields showing much resemblance with spinels from forearc peridotites whereas the chromites plot entirely in the field of SSZ-related peridotites with the Cr-richer and Mg-poorer chromites showing considerable similarities with spinels from boninitic melts (Figure 5).Ferritchromit and magnetite have formed along fractures and rims of Cr-spinels and are enriched in Cr, Mn and rarer Zn.

Olivine
Representative olivine analyses from the lherzolite are listed in Table 4. Forsterite (Fo) contents range from 88.9 to 90.4, which are similar to Fo values of olivines from typical supra-subduction zone SSZ

Olivine
Representative olivine analyses from the lherzolite are listed in Table 4. Forsterite (Fo) contents range from 88.9 to 90.4, which are similar to Fo values of olivines from typical supra-subduction zone SSZ peridotites (Fo = 87-94 according to Pirard et al. [44]).The analysed olivines, both porphyroclasts and neoblasts (Table 4), show a rather similar composition having very similar FeO (8.87-10.25 wt.%) and MgO (44.54-48.62wt.%) contents.NiO contents ranging from 0.29 to 0.85 wt.%, are generally within the range of NiO concentrations for mantle olivines (0.25-0.51 wt.% according to De Hoog et al. [45]), with few exceptions, which show the higher NiO values.Few analyses of olivine neoblasts yielded NiO below detection limit.The NiO contents of olivine shows no correlation with the respective Fo values and a large number of several analyses are plotted in the field of abyssal peridotites (Figure 7).Again in the harzburgite we were unable to acquire reliable olivine analyses due to the lack of sizeable unaltered crystals.the range of NiO concentrations for mantle olivines (0.25-0.51 wt.% according to De Hoog et al. [45]), with few exceptions, which show the higher NiO values.Few analyses of olivine neoblasts yielded NiO below detection limit.The NiO contents of olivine shows no correlation with the respective Fo values and a large number of several analyses are plotted in the field of abyssal peridotites (Figure 7).Again in the harzburgite we were unable to acquire reliable olivine analyses due to the lack of sizeable unaltered crystals.Abyssal and supra-subduction zone (SSZ) fields from Sobolev et al. [46] and Ishii et al. [47], respectively.

Geothermobarometry
Equilibration temperatures and pressure for the studied lherzolite were determined using the two-pyroxenes (clinopyroxene-orthopyroxene) geothermobarometer of Brey and Köhler [48] (Supplementary Table S1).Care was taken to select orthopyroxene porphyroclasts free of clinopyroxene exsolution lamellae.We selected co-existing mineral phases for the estimation of temperature and pressure, which have a well-defined common boundary with no evidence of alteration and similar Mg#, which indicate that they have attained equilibrium.Application of the two-pyroxene geothermobarometer yielded calculated temperatures varying between 942 and 1070 °C, assuming a pressure range of 0.9-1.4GPa.Abyssal and supra-subduction zone (SSZ) fields from Sobolev et al. [46] and Ishii et al. [47], respectively.

Geothermobarometry
Equilibration temperatures and pressure for the studied lherzolite were determined using the two-pyroxenes (clinopyroxene-orthopyroxene) geothermobarometer of Brey and Köhler [48] (Supplementary Table S1).Care was taken to select orthopyroxene porphyroclasts free of clinopyroxene exsolution lamellae.We selected co-existing mineral phases for the estimation of temperature and pressure, which have a well-defined common boundary with no evidence of alteration and similar Mg#, which indicate that they have attained equilibrium.Application of the two-pyroxene geothermobarometer yielded calculated temperatures varying between 942 and 1070 • C, assuming a pressure range of 0.9-1.4GPa.

Alteration Features of Spinel-Group Minerals
Spinel-group minerals are generally more resistant to alteration and/or metamorphism as compared to olivine and pyroxene [49][50][51].Magnetite and Cr-bearing magnetite, are common alteration products of the spinel-group minerals in the two types of peridotites, whereas ferritchromit was observed only after Cr-spinel in harzburgite.Their formation is caused by a post-solidus state decomposition of the spinels in which Mg, Al and Cr diffuse out, hence Fe and other ions, such as Mn and Ni are enriched [52].The narrow alteration zones of ferritchromit and magnetites imply a limited diffusion of these ions.
Ferritchromit rims in Cr-spinel grains generally contain elevated concentrations of Mn, Zn and Co [53,54].The analysed harzburgitic Cr-spinel cores have lower contents of Mn and Zn compared to ferritchromit rims.In the absence of primary ilmenite in harzburgite, Cr-spinel is likely to be the favourable receptor for Mn.This may account for the observed high Mn-contents at the ferritchromit zone of harzburgite.Notable Mn-enrichments in the ferritchromit zones of harzburgitic Cr-spinel involve interaction of fluids that were likely rich in Mn [50, 54,55].Magnetite grows during serpentinisation from the excess Fe of the pre-existing olivine.Magnetite that rims spinels is a typical alteration product of spinels due to Cr and Al removal.
The occurrence of garnet at the rims and within micro-veins of Al-spinels in the lherzolite requires the existence of an influx characterised by substantial Ca-enrichment and Si-depletion.This has been documented within metasomatic lithologies known as rodingites [56][57][58] and indicates an initial stage of rodingitisation in associated gabbroic dykes.

Partial Melting of Mantle Peridotites
Partial melting of a mantle is controlled by various factors, such as pressure, temperature and water content, which can be used as diagnostic indicators of tectonic setting [59].Partial melting beneath a mid-ocean ridge occurs at dry conditions, whereas a mantle wedge normally experiences hydrous melting due to the dehydration of the subducted slab.Thus, forearc mantle peridotites undergo higher degrees of partial melting relative to the oceanic mantle beneath a mid-ocean ridge [16,60].Important information for mantle melting can be preserved by the modal mineralogy, mineral chemistry and whole-rock chemical composition.Clinopyroxene is generally considered to be the most rapidly consumed mineral specifically during the initial stages of partial melting [16,61].It is known that Al contents of pyroxene and spinel are sensitive to the partial melting of mantle peridotites and they systematically decrease as the peridotites become more depleted [62,63].Furthermore, forsterite contents of olivine and Cr# of spinel in peridotites are useful indicators of the degree of melting [41,42,64].
The variably depleted nature of the Edessa peridotites as evidenced by their mineralogical composition indicates that they are mantle residues after different degrees of melting.Variable amounts of remnant clinopyroxene occur in lherzolite, which gradually pass into harzburgites.The harzburgite has low clinopyroxene abundances (<5 vol.%) resembling mantle peridotites formed by high degrees of partial melting.On the contrary, the lherzolite has higher porphyroclastic clinopyroxene contents (5-15 vol.%) implying variable but small to moderate degrees of melt extraction [65,66].Lobate boundaries of Al-spinel grains in the lherzolite, along with their linear Mg#-Cr# evolution, as well as their resemblance to abyssal peridotite spinels support the hypothesis of a partial melting episode during their genesis.The refractory nature of olivine (with Fo = 88.8-90.4 and high NiO) and orthopyroxenes (with Mg# = 89.2-90.2) in the lherzolite, are compatible with a partial melting episode.Moreover, the harzburgitic clinopyroxenes are more refractory (richer in Mg# and poorer in Al) than those in the lherzolite, hence suggesting that progressively increasing degrees of partial melting are responsible for the evolution of harzburgite (Figure 6a).The relatively low to moderate Al 2 O 3 abundance in both orthopyroxene and clinopyroxene porphyroclasts of the lherzolites (Figure 6a,b), further advocate to the hypothesis that they have undergone considerable degrees of partial melting, thus representing upper mantle residues [67][68][69].These clinopyroxene porphyroclasts in the lherzolite are similar to residual clinopyroxene in MOR-type peridotites, which underwent dry partial melting [70].
Moreover, it has been suggested that spinels in mantle peridotites become increasingly Cr-rich along with melting and thus Cr# in residual spinels is a sensitive indicator for the extent of melting of their host peridotites [41,42,71].The Al-spinels from lherzolite show a broad range and a linear evolution of Cr# from 7.5 to 28.6, and Mg# from 62.3 to 80.3, strongly resembling spinels from peridotites forming in a MOR environment after low to moderate degrees of partial melting.The Cr-spinels in the harzburgite are thought to have a residual origin, as it is implied by their textural characteristics.They show higher Cr# than the Al-spinels, ranging from 30.9 to 58.5, and generally lower Mg# from 58.3 to 70.4, which are typical for spinel-group minerals from MOR-peridotites after high degrees of partial melting of an oceanic upper mantle source [64,72].
For the estimation of the degree of partial melting (F) for the lherzolite and harzburgite the Cr# in the spinel equation [F = 10 × ln(Cr#) + 24] proposed by Hellebrand et al. [71] was used for our calculations.Our calculations confirm that the lherzolite has experienced lower degrees of partial melting (<12%; see Table 1) whereas the harzburgite has undergone higher degrees of partial melting than the lherzolite ranging from 12% to 19% (Table 1).
The olivine spinel mantle array (OSMA) diagram (Figure 8) proposed by Arai [41,64] based on Fo of olivine and Cr# of coexisting spinel is extensively employed to characterise mantle-derived, spinel-bearing peridotites, especially mantle restites on the basis of melting trend for a fertile MORB mantle.The Fo-Cr# fractionation trend reflects the differentiation of magma including various processes such as crystallisation differentiation, magma mixing and assimilation.The spinel-olivine pairs in the lherzolite plot at the lower end of the abyssal peridotite field and overlap with the field of passive margin peridotites, further indicating the relatively fertile nature of the lherzolite.This plot is also consistent with the calculated low degrees of partial melting (<12%) mentioned above for the lherzolite (Figure 8 and Table 1).melting (<12%; see Table 1) whereas the harzburgite has undergone higher degrees of partial melting than the lherzolite ranging from 12% to 19% (Table 1).
The olivine spinel mantle array (OSMA) diagram (Figure 8) proposed by Arai [41,64] based on Fo of olivine and Cr# of coexisting spinel is extensively employed to characterise mantle-derived, spinel-bearing peridotites, especially mantle restites on the basis of melting trend for a fertile MORB mantle.The Fo-Cr# fractionation trend reflects the differentiation of magma including various processes such as crystallisation differentiation, magma mixing and assimilation.The spinel-olivine pairs in the lherzolite plot at the lower end of the abyssal peridotite field and overlap with the field of passive margin peridotites, further indicating the relatively fertile nature of the lherzolite.This plot is also consistent with the calculated low degrees of partial melting (<12%) mentioned above for the lherzolite (Figure 8 and Table 1).

Evidence of Melt-Rock Reaction
The petrographic features and the mineralogical compositions in the Edessa peridotites documented in this study cannot be explained only by partial melting mechanisms.It is argued above that the Edessa lherzolite shows evidence for mantle melting textures.However, most orthopyroxene porphyroclasts exhibit characteristic textures including curved boundaries and partial replacement by olivine neoblasts (Figure 4c), which are interpreted as a result of a later-stage incongruent dissolution of pyroxenes, caused by reaction between a percolating melt and the minerals [66,73].Fine-grained unstrained clinopyroxene and olivine occurring as interstitial crystals are thought to have been crystallised from these melts, which were trapped in the shallow mantle [74][75][76].These microstructures in the Edessa lherzolite support the hypothesis that a Si-poor basaltic melt passed through the lherzolite during its ascent in the shallow mantle and has interacted with primary minerals, resulting in resorption of orthopyroxene and the precipitation of fine-grained clinopyroxene and olivine.
Moreover, the band-like olivine aggregates occurring in a few samples of lherzolite, likely represent the melt migrating channels within the mantle.These dunite veins in the lherzolite may reflect the interaction of rising mantle melts with lherzolite, which traveled adiabatically upwards dissolving pyroxenes, thus becoming saturated in olivine [77].The observed dunite melt channels were formed upon olivine crystallisation and as soon as they were completely surrounded by olivine, melts effectively ceased to interact with their wallrocks, hence travelling upwards in a nearly closed-system.
Changes in the spinel-group minerals habits and composition may also reflect some variations in the peridotites [78].Apart from melt extraction mentioned earlier, the Cr# of modified spinel-group minerals has been shown to be a sensitive indicator for melt-peridotite reaction during porous flow and melt-wallrock reactions in the vicinity of intruding magmatic veins [79][80][81].Spinel formation by reaction with a migrating silicate liquid records enrichment in Cr and a concomitant decrease in Al.Unlike the Al-and Cr-spinels, whose chemical affinities are explained in terms of partial melting, the chromites in the harzburgite, which record enrichment in Cr and a concomitant decrease in Al, are likely the result of (re)crystallisation during melt-rock interactions, as it is implied by their generally euhedral habit.The Cr#-richer and Mg#-poorer chromites (Figure 5) resemble spinels from arc peridotites.Therefore, it is suggested that a Si-rich melt with boninitic affinity percolated through the Edessa mantle sequence producing the Cr-rich compositions of the euhedral chromite, as it commonly interpreted in studies of other similar environments [27,68,69].Furthermore, the local modification of chromite (the Cr#-poorer and Mg#-richer cluster described in Figure 5) is compatible with the impregnation by such a Cr-rich melt.The percolating melt likely became locally saturated with chromite, as a result of the rock-melt reaction, hence increasing the Cr#'s of the pre-existing residual Cr-spinels.The elevated Cr# and the low-Ti contents in the chromites are compatible with their crystallisation from a low-Ti, boninitic melt [60].

Geotectonic Implications
Ophiolite complexes represent fragments of oceanic lithosphere emplaced on the continental crust, and their origins have been assigned to a variety of tectonic settings [6].The modal and elemental composition of the mineral constituents of mantle peridotites are dependent on the geotectonic environment of their formation.Al-Mg and Cr-Mg relationships of the porphyroclastic pyroxenes, as well as Ni contents of olivine, support that lherzolites formed in a mid-oceanic spreading center by lower degrees of partial melting, whereas the more refractory nature of porphyroclastic clinopyroxenes in the harzburgites (Figure 6a,b) suggest an origin in a forearc setting by higher degrees of partial melting.Neoblastic clinopyroxenes in the lherzolite show lower Cr and Al contents than the porphyroclasts, which is interpreted as a result of their crystallisation presumably from a MORB-like melt, which impregnated the peridotite.Melts that evolve in a single environment are expected to be more differentiated leading normally to formation of olivine neoblasts, which are poorer in Fo than the porphyroclasts in a rock.However, MgO and FeO contents of olivine neoblasts in the lherzolite do not differ significantly from those of the porphyroclastic ones, thus showing similar Fo contents.
The Cr# in spinel-group minerals is a good indicator of the tectonomagmatic history of the host rock.The Al-spinel in the lherzolite and the Cr-spinel in the harzburgite display low Cr# (<60), typical of oceanic ophiolites (including back-arc basins [42]).It is argued above that the Al-spinels from the lherzolite are residues after low to moderate degrees of partial melting in a spreading environment.The origin of the residual Cr-spinels of the harzburgites (including the cpx-bearing ones) with Cr# clustering between 30.9 and 58.5 (Figure 5) require higher degrees of melting (up to 19%), which are consistent with an arc setting and hydrous conditions [82].The existence of the subhedral to anhedral chromites (Cr#-poorer-Mg#-richer in Figure 5) indicate a chemical modification of Cr-spinels after interaction with a Cr-rich melt and is in line with this interpretation.Crystallisation of euhedral chromites (Cr#-richer-Mg#-poorer in Figure 5) indicates that apparently this melt became saturated in Cr.This fact strongly favours the involvement of a reactive, percolating boninitic melt, which can only be generated in a mantle wedge above a subduction zone.
Therefore, it is proposed that the lherzolites were generated in the Mesozoic Tethys ocean spreading environment and underwent a first stage of melting followed by interaction with a Si-poor (presumably MORB-like) melt.Most likely, these rocks were subsequently migrated in the forearc region above a subduction zone and were subjected to more extensive melt extraction under hydrous conditions.In this environment a melt with boninitic affinity reacted with the rocks overprinting the lherzolite and the harzburgite.
The coexistence of both MOR-and SSZ-type peridotites has also been reported in other ophiolite suites, such as Troodos [83] and SW Turkey [63,69].These suites have a complex evolution involving both spreading and converging regimes similar to the Edessa peridotites.Previous studies from modern forearc settings have shown that those peridotites may have a variety of origins, ranging from abyssal to SSZ regimes and have experienced hydrous melting in the mantle wedge [16,25,60,[84][85][86][87].These peridotites display textural, mineralogical and compositional characteristics for multi-phase melting, depletion and enrichment processes in their petrogenesis similar to those observed in the Edessa peridotites [69,88].Abyssal peridotites might represent ocean lithosphere trapped above the subduction zone at initiation, or accretion to the upper plate during plate convergence at a later time [16].Furthermore, abyssal peridotites can also be juxtaposed with SSZ peridotites during collision of a spreading center with a subduction zone [89].

Conclusions
The Edessa ophiolite is dominated by peridotites.Crustal suite rocks include minor gabbro and diorite, as well as diabase and basalt.The mantle section is composed mainly of harzburgite and minor lherzolite.The lhezolite is considered as the product of a first stage melting that occurred probably in a mid-ocean ridge environment.The Al-and Cr-poor nature of the neoblastic clinopyroxenes and the rather rich in Fo-rich nature of neoblastic olivines in the lherzolite strongly support the hypothesis for impregnation from an Mg-rich and Si-poor melt, likely of MORB origin.Subsequently, harzburgites were formed by more complicated processes including higher degrees (12-20%) of melting and additional modification by melt-rock interaction, in a supra-subduction zone (SSZ) environment.The lherzolite contains Al-spinels with moderate Cr# values, which indicate varying degrees of melting up to 12%.The Cr# values of the Cr-spinels and chromite from the harzburgite are higher than the lherzolite.The harzburgite is interpreted as the product of at least two stages of melting, including an early anhydrous event followed by a hydrous one.The lower Al 2 O 3 and higher Mg# of the harzburgitic clinopyroxenes, as well the more refractory olivine in the harzburgite relative to those in the lherzolite are consistent with this interpretation and an origin in a SSZ environment.
The co-existence of both MOR-and SSZ-type peridotites in the Edessa ophiolite supports the hypothesis that they have experienced a complicated evolution in a spreading oceanic basin to eventually the mantle wedge above a subduction zone.This scenario appears to be common for most of the Neo-Tethyan ophiolites as they are highly heterogeneous in their mantle compositions, melt evolution patterns and collisional histories.

Figure 2 .
Figure 2. Geological and sampling map of the Edessa ophiolite (from IGME[38,39], modified after fieldwork and mapping by using ArcMap 10.1); rectangle in the inset shows the study area.

Figure 2 .
Figure 2. Geological and sampling map of the Edessa ophiolite (from IGME[38,39], modified after fieldwork and mapping by using ArcMap 10.1); rectangle in the inset shows the study area.

contents ( 2 .
38-4.58  wt.%) than those in the lherzolite.On an Al2O3 vs. Mg# and a Cr2O3 vs. Mg# diagrams the majority of the analysed porphyroclastic clinopyroxenes from lherzolite resemble those occurring in abyssal peridotites whereas the neoblastic ones are poorer in Cr (with overlaps) and Al (Figure6a,b).The harzburgitic diopsides show a more refractory nature and on a Cr2O3 vs. Mg# diagram they resemble clinopyroxenes originated in forearc peridotites (Figure6b).

Figure 6 .
Figure 6.Compositional variations of clinopyroxene and orthopyroxene in peridotites from the Edessa ophiolite.(a) Al2O3 vs. Mg# for cpx; (b) Cr2O3 vs. Mg# for cpx; (c) Al2O3 vs. Mg# for opx; (d) Cr2O3 vs. Mg# for opx.Fields of forearc and abyssal peridotites are from Lian et al. [43].Few pyroxene analyses with Al and/or Cr contents below detection limits are not plotted.

Figure 6 .
Figure 6.Compositional variations of clinopyroxene and orthopyroxene in peridotites from the Edessa ophiolite.(a) Al 2 O 3 vs.Mg# for cpx; (b) Cr 2 O 3 vs.Mg# for cpx; (c) Al 2 O 3 vs.Mg# for opx; (d) Cr 2 O 3 vs.Mg# for opx.Fields of forearc and abyssal peridotites are from Lian et al. [43].Few pyroxene analyses with Al and/or Cr contents below detection limits are not plotted.

Table 1 .
Representative electron microanalyses of spinel-group minerals from peridotites of the Edessa ophiolitic complex (-: below detection limit; *: degrees F of partial melting are not calculated for altered and chemically modified spinels).