armenian ophiolites 10 rolland

The Armenian Ophiolite: insights for Jurassic back-arc formation,

Lower Cretaceous hot spot magmatism and Upper Cretaceous

obduction over the South Armenian Block

Y. ROLLAND1*, G. GALOYAN1,2, M. SOSSON1, R. MELKONYAN2 & A. AVAGYAN2

1Universite de Nice-Sophia Antipolis, OCA, UMR GeoAzur, CNRS, Parc Valrose,

06108 Nice cedex 2, France2Institute of Geological Sciences, National Academy of Sciences of Armenia,

24a Baghramian avenue, Yerevan, 375019, Armenia

*Corresponding author (e-mail: [email protected])

Abstract: Similar geological, petrological, geochemical and age features are found in variousArmenian ophiolitic massifs (Sevan, Stepanavan and Vedi). These data argue for the presenceof a single large ophiolite unit obducted on the South Armenian Block (SAB). Lherzolite Ophiolitetype rock assemblages evidence a Lower–Middle Jurassic slow-spreading rate. The lavas andgabbros have a hybrid geochemical composition intermediate between arc and Mid OceanRidge Basalt (MORB) signatures which suggest they were probably formed in a back-arc basin.This oceanic sequence is overlain by pillowed alkaline lavas emplaced in marine conditions.Their geochemical composition is similar to plateau-lavas. Finally, this thickened oceanic crustis overlain by Upper Cretaceous calc-alkaline lavas likely formed in a supra-subduction zoneenvironment. The age of the ophiolite is constrained by 40Ar/39Ar dating experiments provideda magmatic crystallization age of 178.7+2.6 Ma, and further evidence of greenschist facies crys-tallization during hydrothermal alteration until c. 155 Ma. Thus, top-to-the-south obduction likelyinitiated along the margin of the back-arc domain, directly south of the Vedi oceanic crust, and wastransported as a whole on the SAB in the Coniacian times (88–87 Ma). Final closure of the basin isLate Cretaceous in age (73–71 Ma) as dated by metamorphic rocks.

The history of central and northern Neotethys can beinferred from the study of oceanic crust domainsobducted in the Armenian Lesser Caucasus. It isimportant to depict the geodynamic evolution ofoceanic domains that were formed in the Neo-Tethyan domain, as they provide key time andpalaeogeographic data in the Middle East BasinEvolution (e.g. Sengor & Yilmaz 1981; Tirrulet al. 1983; Ricou et al. 1985; Dercourt et al.1986; Ricou 1994; Okay & Tuysuz 1999; Stampfli& Borel 2001; Barrier & Vrielynck 2008). Further-more, they provide constraints on the timing ofoceanic closure and obduction, by the study of meta-morphic rocks associated to the ophiolites. Finally,their geometry is also important to infer the preser-vation potential of oil resources that could be con-tained underneath.

During the Mesozoic, the Southern margin of theEurasian continent has been featured by closure ofthe Palaeo-Tethys and opening Neo-Tethys Ocean(Fig. 1). Later on, subductions, obductions andmicro-plate accretions, ranging mostly from theCretaceous to the Eocene and finally continent–continent collision have occurred between Eurasia

and Arabia. The study of Armenian ophiolitesallows unravelling part of this complex history.Previous geological, petrological and geochemicalworks undertaken on those date back to late 1970sand 1980s, and have never been undertaken at thescale of the Armenian ophiolites. This work is par-ticularly difficult due to the very large number oftectonic and volcanic events that have occurredafter ophiolite obduction. The polyphased tectonichistory of the Lesser Caucasus region includes arc-continent accretion and subduction-exhumation inor above accretionary prisms followed by conti-nent–continent collision (e.g. Okay & Tuysuz1999; Rolland et al. 2007). The tectonic eventshave dissected the ophiolites (Avagyan et al.2005), which were further overlain by very thick(.1 km) sequences of volcanic rocks.

In this paper, we propose a synthesis of theresearch undertaken on Armenian ophiolites,based on recently published papers of individualophiolite zones and new geological, petrological,geochemical and 40Ar/39Ar geochronological dataobtained on these different zones. Three ophioliteshave been studied, located in NW Armenia

From: Sosson, M., Kaymakci, N., Stephenson, R. A., Bergerat, F. & Starostenko, V. (eds) Sedimentary BasinTectonics from the Black Sea and Caucasus to the Arabian Platform. Geological Society, London, Special Publications,340, 353–382. DOI: 10.1144/SP340.15 0305-8719/10/$15.00 # The Geological Society of London 2010.

(Stepanavan), north Armenia (Sevan) and centralArmenia (Vedi; Fig. 2). In the three zones, weshow the presence of three superposed levels oflavas corresponding to three distinct environments,from bottom to top: (1) back-arc; (2) ‘OceanIsland Basalt’ (‘OIB’)-like; and (3) arc. Moreover,we demonstrate that these ophiolite windowsshould correlate with each other and be part of a

unique obducted nappe above the South ArmenianBlock (or SAB, Knipper & Khain 1980; Zakariadzeet al. 1983). We propose that tectonic transport ofthis nappe onto the SAB occurred directly afterthe OIB ‘plateau’ event, in the Early Upper Cretac-eous, and shortly preceded the final oceanic closurealong the Eurasian margin in the latest Cretaceous(c. 73–71 Ma).

SeaBlack

i

i

i

i

i

ii

i

Albroz CCB

Albroz CCB: Albroz Carboniferous-Cimmerian belt

LC

LC

LC: Lesser Caucasus

Fig. 1. Tectonic map of the Middle East–Caucasus area, with main blocks and suture zones, after Avagyan et al.(2005), modified.

Y. ROLLAND ET AL.354

Geological setting

The ophiolites are located in the northern part ofthe Lesser Caucasus region (Fig. 1). They are situ-ated in three geographic zones (Figs 2–5).

(1) The Sevan–Akera zone at the northern rimof the SAB and at the southern edge of theEuropean active continental margin (Knipper1975; Knipper & Khain 1980; Adamia et al.1980). In the present paper, we presentdetailed mapping of the Stepanavan (NWArmenia, Fig. 3) and of the Sevan (northArmenia, Fig. 4) ophiolites.

(2) The Vedi zone (Fig. 5), disposed in a moresoutherly position, above the SAB (Knipper

& Sokolov 1977; Knipper & Khain 1980;Zakariadze et al. 1983), or within a suturezone eventually correlating with Central Iranor Alborz ophiolites (Sokolov 1977; Adamiaet al. 1981).

(3) The Zangezur zone situated along the Zange-zur fault (Aslanyan & Satian 1977, 1982),between the two domains, interpreted as anophiolite suture by Knipper & Khain (1980)and Adamia et al. (1981).

A companion paper written on the geology of theSevan ophiolite has already put up in detail thelithologies and radiometric age of this ophiolite(Galoyan et al. 2009). Main features are summar-ized below. The lithological assemblages found

Fig. 2. Sketch geological map of Armenia, with location of the studied areas: 1, Stepanavan area; 2, Sevan area; 3, Vediarea. 4 is the location of Zangezur ophiolites, located along a NNW–SSE striking fault.

ARMENIAN OPHIOLITES 355

agree with a Lherzolite Ophiolite Type (LOT;Nicolas 1989); these include the following.

(i) A high level of fractional crystallization in theseries, with cumulate olivine gabbros and twopyroxene gabbros overlain and intruded bygenerally amphibole-bearing gabbros, and fre-quent differentiated melts (diorites to plagio-granites). These most differentiated melts are

generally emplaced in ductile extensive shearzones cross-cutting the gabbros. This completedifferentiation series suggests low partialmelting levels and long-lived cooling as is pro-posed in LOT settings (Lagabrielle et al. 1984;Lagabrielle & Cannat 1990). Absolute radio-metric datings indicate oceanic crust emplace-ment in the Middle Jurassic, constrained at165–160 Ma by zircon U–Pb age of one

Fig. 3. Sketch geological map of the Stepanavan ophiolite (NW Armenia).


tonalite (160+4 Ma; Zakariadze et al. 1990)and by 40Ar/39Ar amphibole age on gabbro(165.3+1.7 Ma; Galoyan et al. 2009).

(ii) Rare pillow lavas, with compositions rangingfrom tholeiitic basalts to andesites. Thefeeding dyke swarm is reduced, as rare doleritedykes have been found crosscutting the series.The calc-alkaline affinity is also evidenced byNb–Ta negative anomalies, which agree withsome interaction with slab-derived component.These support a slow spreading rate in aback-arc setting.

(iii) Peridotites are frequent and often exhumed atsea-floor level. They are generally serpenti-nized, and witness further hydrothermal alter-ation when exhumed at sea-floor level(‘listwenites’). The lherzolitic nature of themantle-derived ultramafic rocks is then diffi-cult to assess. The previous petrographicalinvestigations on the serpentinized ultramaficssuggest that the protoliths was mantle-derivedwith various compositions ranging fromlherzolites to harzburgites and dunites (e.g.Melikyan et al. 1967; Harutyunyan 1967;Palandjyan 1971; Abovyan 1981; Ghazaryan1987; Zakariadze et al. 1990). Undeformedultramafics have a magmatic cumulative origin,shown by the poikilitic texture of olivineinclusions within large enstatite crystals (upto 10–15 mm; Palandjyan 1971). We observe

similar textures, together with layers, con-tained in magmatic pods cross-cutting the ser-pentinites in the Stepanavan area (Galoyanet al. 2007). These latter serpentinites arestrongly deformed and altered, thus it wasdifficult to unravel their origin. However, theductile character of deformation is in agree-ment with a mantle origin for these rocks.

(iv) Radiolarites are found as interlayers or asunconformably overlying the various abovelithologies. The fact that they overlie gabbros,plagiogranites and serpentinites shows thatthese rocks were uplifted and exhumed bynormal faults. Radiolarite datings undertakenin the different ophiolites all agree withoceanic accretion in the Middle–Upper Juras-sic (Danelian et al. 2007, 2008).

The ophiolitic sequences are weakly deformedwith anchizonal metamorphism. Only some out-crops show evidence of small shear zones ascribedto the ophiolite obduction in the Coniacian (Zakar-iadze et al. 1983). High pressure (HP) metamorph-ism is described in the Stepanavan region (Figs 2& 3), where blueschists (Aghamalyan 1981, 1998)outcrop in small km2 size tectonic windows belowthe ophiolite. Timing of metamorphism from radio-metric 40Ar/39Ar phengite datings indicates HPmetamorphic peak at c. 95 Ma, and medium

Fig. 4. Sketch geological map of the Sevan ophiolite (north Armenia), after Galoyan et al. (2009), modified.


pressure-medium temperature (MP-MT) retrogres-sion at 73–71 Ma (Rolland et al. 2007).

The ophiolite series are locally overlain frombottom to top by (1) alkaline lavas, which haveage ranges from 114 to 95 Ma (Baghdasaryanet al. 1988; Satian & Sarkisyan 2006) recently con-firmed by 40Ar/39Ar dating of 117.3+0.9 Ma(Rolland et al. 2009); and (2) Upper Cretaceousandesites and detrital series (Dali valley; Stepana-van; Galoyan et al. 2007). The alkaline lavas arealternatively interpreted as (1) intra-continental

rifting (Satian et al. 2005) in the Vedi area; and(2) plume-derived magmatism above the oceaniccrust before the obduction (Galoyan et al. 2007).The calc-alkaline series should be related tointra-oceanic arc emplacement above this oceaniccrust sequence and implies the presence of a subduc-tion zone between the ophiolite and the SAB, fea-tured by the Stepanavan blueschists (Rolland et al.2007). These two magmatic sequences closelypredate the ophiolite obduction onto the SABduring the Coniacian (Sokolov 1977).

Fig. 5. Sketch geological map of the Vedi ophiolite (Central Armenia).


Analytical methods

Mineral compositions were determined by electronprobe microanalysis (EPMA). The analyses are pre-sented in Figure 6. They were carried out using aCameca Camebax SX100 electron microprobe at15 kV and 1 nA beam current, at the Blaise PascalUniversity (Clermont-Ferrand, France). Naturalsamples were used as standards.

Thirty-seven samples of magmatic rocks fromthe Sevan, Stepanavan and Vedi ophiolites havebeen analysed for major, trace and Rare Earthelements (REE) (Table 1). Samples were analysedat the C.R.P.G. (Nancy, France). Analytical pro-cedures and analyses of standards can be foundon the following website (http://www.crpg.cnrs-nancy.fr/SARM).

Amphiboles were separated from the gabbrosample AR-05-110, from the Vedi ophiolite. Geo-chronology of amphiboles was undertaken by laser40Ar/39Ar dating. The results are presented inTable 2. Amphibole crystals were separated undera binocular microscope. The samples were then irra-diated in the nuclear reactor at McMaster Universityin Hamilton (Canada), in position 5c, along withHb3gr hornblende neutron fluence monitor, forwhich an age of 1072 Ma is adopted (Turner et al.1971). The total neutron flux density duringirradiation was 9.0 � 1018 neuton cm22. The esti-mated error bar on the corresponding 40Ar*/39ArK

ratio is+0.2% (1s) in the volume wherethe samples were set. Three amphibole grains(c. 500 mm in diameter) were chosen for analysison a laser UV spectrometer of Nice (GeosciencesAzur). Analyses were undertaken by step heatingwith a 50 W CO2 Synrad 48-5 continuous laserbeam. Measurement of isotopic ratios was donewith a VG3600 mass spectrometer, equipped witha Daly detector system; see detailed procedures inJourdan et al. (2004). The typical blank values forextraction and purification of the laser system arein the range 4.2–8.75, 1.2–3.9 and 2–6 cc STPfor masses 40, 39 and 36, respectively. The mass-discrimination was monitored by regularly analys-ing air pipette volume. Decay constants are thoseof Steiger & Jager (1977). Uncertainties on apparentages in Table 2 are given at the 1s level and do notinclude the error on the 40Ar*/39Ark ratio of themonitor. Uncertainties on plateau ages in Figure 7are given at the 2s level and do not include theerror on the age of the monitor.

Results

Field relationships

Synthetic logs are drawn on Figure 8, showingthe lithological associations and the structural

relationships in each zone. In Stepanavan (Fig. 8a,b), ophiolite sections exhibit abundant serpentinites,cross-cut by normal fault and shear zones in whichgabbronorites, gabbros and plagiogranites are intru-sive and deformed. Laterally, thick layers of pillowbasalts are observed which interlayer with radiolar-ites. At the top of the ophiolite section, a thin layerof alkaline lava flows is found. Above, these lavasare uncomformably overlain by Upper Cretaceousconglomerates and limestones, and calc-alkalinepillow basalts or graywackes. The ophiolitesequence is thrust over a blueschist facies meta-morphic sole, which outcrops in two km2 sizedtectonic windows.

In the Sevan area, sections are extremely vari-able laterally (Fig. 8c–e). Pillow lavas are rare,and serpentinites are frequently found at sea-floorlevel. Intense hydrothermal alteration (‘listwenites’)has transformed the uppermost part of exhumedserpentinites. Rare dolerites are observed. Largeintrusive pods of amphibole-bearing gabbros andplagiogranites are also exhumed and overlain byradiolarites. Normal faults are observed, and areinterpreted as the cause of such lateral variations,by vertical uplifting of footwall sections, and localinfilling of axial rift valleys, in agreement withthe LOT ophiolite model (e.g. Lagabrielle et al.1984; Lagabrielle & Cannat 1990). Locally, thicksequences of alkaline pillow lavas are observed.The ophiolite is locally eroded, and uncomformablyoverlain by conglomerates and soils, overlain by anUpper Cretaceous section of reef-limestones withgraywackes interlayers.

In the Vedi area, the ophiolite section is muchthinner (Fig. 8f–h). The basal tectonic contact isexposed, exhibiting top-to-the-south sense of shear.At the base, the ophiolite rests on a serpentinitelayer. The ophiolite is intensely sheared above thebasal contact with boudins of tholeiitic basalts(Fig. 8h). Laterally, the ophiolite consists mainlyof gabbros (Fig. 8g) or serpentinites, which suggestssimilar lithological features as in the Sevan area.However, the different parts of the ophiolite aredismembered and displaced from each other as aresult of obduction deformation. Above the ophio-lites, layers of radiolarites are found below a verythick section of alkaline pillow lavas (Fig. 8h).These alkaline lavas are amphibole-bearing. Pillowsare larger (metre-scale) than the ophiolite ones(several decimetre scale), and interlayer with thinpink limestones. At the front of the obduction, anolistolith formation with conglomerates and slidedblocks in a muddy matrix is present (Fig. 8f). Theage of the olistostrom is Coniacian–Santonian(see Sosson et al. 2010), it connects laterally toLower Coniacian series below the ophiolite, andwith Santonian reef limestones above the obduction.Therefore, the obduction age can be bracketed


to the Coniacian–Santonian (88–83 Ma). Laterally,the upper part of the ophiolite is made of kilometre-scale slided blocks, mainly comprised of alkalinepillow basalts and calc-schists. These blocks slideon a greenish mudstone rock, probably originatedfrom the alteration of the ophiolite itself. TheUpper coniacian uncomformity is variably markedby conglomerates, marls and reef limestones.

As emphasized in Galoyan et al. (2009) in theSevan area, and by Galoyan et al. (2007) inthe Stepanavan area, the lithologies found in allthe exposed Armenian ophiolites are in good agree-ment with the hypothesis of a slow expansion ratespreading centre, as described for the western Alpsophiolites (LOT; Nicolas & Jackson 1972; Nicolas1989). These LOT features include the following,as already stressed by Galoyan et al. (2009).

(1) A high degree of fractional crystallizationshown by coarse amphibole-bearing gabbrosand widespread plagiogranites.

(2) Both plutonic rock types are often strained inductile-to brittle conditions of amphibolite tolower greenschist metamorphic conditionsand often exhumed at sea-floor level, as forserpentinites. These tectonic features are inagreement with a slow-spreading system inwhich the overall morphology is dominatedby normal faults.

(3) The scarcity of basalt lava flows and therestricted dolerite dyke swarm are also in

agreement with a slow spreading ratesystem, featured by low partial melting levels.

The similar lithological and age features found inthe several Armenian ophiolites suggest that theywere part of the same oceanic crust section. Thishas to be confirmed by the comparison of geochem-ical data from each zone. The presence of three mag-matic series: ophiolite sensu stricto (tholeiitic),‘OIB’ (alkaline) and arc (calc-alkaline) in the samestructural position (from bottom to top, respect-ively) has been evidenced in the three zones.

Petrography and mineral chemistry

The field and microscopic analyses of the Armenianophiolite magmatic rocks have evidenced a continu-ous magmatic succession from ultramafic cumulates(wehrlites, websterites) to gabbros and plagiogra-nites, exhumed at sea-floor level and overlain bypillow-basalts.

Ophiolite plutonic rocks. Wehrlites are found inthe Stepanavan ophiolite and have a poikilitictexture showing numerous clinopyroxene crystalswith diopside composition (Wo45-47En48-50Fs2-4),included in large olivine Fo87-88 (.60–65%) por-phyric grains. Spinel is less than 5%, and is ofmagnetite composition.

Gabbros are the most abundant rocks in thecrustal complex. Their petrography evolves fromcumulate-banded olivine gabbros in their lower

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0.5 0.6 0.7 0.8 0.9 1.0

Na + Ca

Ti

G150 olivine gabbroAR-05-86 olivine gabbroAR-03-10 hornblende gabbroAR-03-02 diabase

AR-04-02 wehrliteAR-04-03 websteriteAR-04-04 websteriteAR-04-51 websteriteAR-04-53 anorthositeAR-04-45D hornblende gabbroAR-04-31 basaltic andesite

AR-05-112A hornblende gabbroAR-05-113 hornblende gabbroAR-05-103 gabbro-diabase

Tholeiitic Alkaline

Sevan

Stepanavan

Vedi

Fig. 6. Chemical compositions of studied clinopyroxenes plotted in the Ti v. (Naþ Ca) diagram of Letterier et al.(1982). Note that a majority of data plot in the Alkaline compositional field, and a minority is in the Tholeiitic part.


part towards more leucocratic plagioclase-richgabbros in the upper part (Abovyan 1981; Ghazar-yan 1987, 1994). The Cumulative banded olivinegabbros and websterites are described only inSevan and Stepanavan areas, while more leucocraticgabbros are widespread.

Olivine gabbros are fresh, massive, and fine- tomedium-grained (0.5–1 mm and 1.2–2 mm corre-spondingly). They have cumulate, ophitic texturesand consist of plagioclase (c. 60–65%; An68-74,An80-89), olivine (c. 5–10%; Fo72-76) and clino-pyroxene (c. 25–35%). Clinopyroxene is of augite(Wo39-44En45-48Fs11-13) and diopside (Wo45En44

Fs11) types. Some enstatitic orthopyroxenes (Wo2

En75Fs23) are also found rimming olivineporphyrocrysts.

Websterites have a granular texture with large2–8 mm porphyrocrysts of orthopyroxene (30–70%), clinopyroxene (70–30%) and olivine grains(0–35%; Fig. 9a). Orthopyroxenes are enstatite-rich(Wo1-5En59-84Fs11-37) and clinopyroxenes areaugites (Wo35-42En36-40Fs15-19) and olivine is rela-tively rich in forsterite (Fo84-88). Gabbronoriteshave a gabbroic texture, with plagioclase(10–60%, 1–3 mm), clino- and ortho-pyroxene.Plagioclase is of bytownite type (An80-85), whileorthopyroxenes (1–4 mm) are enstatites (Wo2-5

En59-61Fs34-37), and clinopyroxenes are augites(Wo35-42En36-40Fs15-19). Spinel is less than 5% andis of magnetite composition.

Mesocratic to leucocratic gabbros of the uppersection are massive, fine- to medium-grained andhave gabbroic (or gabbro-ophitic), subautomorpheto xenomorphe granular texture (0.5–4 mm), withplagioclase (c. 40–65%; An50-75, An72-93), clinopyr-oxene (8–45%; augite) and hornblende (0–40%)and lack olivine. Accessory minerals (1–10%) areapatite, titanomagnetite, ilmenite and rarely quartz.The hornblende-rich gabbros have coarse granulartextures (Fig. 9b), with c. 50–65% euhedral to sub-hedral plagioclase (An54-58) and (c. 35–50%) anhe-dral to subhedral amphibole (Fig. 10a–d). BrownTi-rich euhedral hornblende is presumed to be aprimary mineral as it appears in the centre of pheno-crysts; while a Ti-poor subhedral to xenomorphicgreen magnesio-hornblende (Leake et al. 1997) isthought to be a secondary phase related tohydrothermal alteration (Fig. 10d). The augite(Wo40-42 En39-47Fs11-14), diopside (Wo45-48

En40-44Fs8-15), and enstatite (Wo2En57Fs41) relicts(5–10%) are found in the magnesio-hornblendescrystals replacing the pyroxenes. However, it isnot related to shear zones and fractures, andshould also be a late magmatic mineral. In leuco-cratic gabbros, the clinopyroxene (augite Wo40-41

En33-35Fs18-19) content does not exceed 25%.Normal zoning is observed in plagioclase (fromAn85 to An60), which are frequently altered.

Clinopyroxenes have alkaline to slightly tholeiiticcompositions (0.8 , Naþ Ca , 0.9; Leterrieret al. 1982; Fig. 6). Pegmatitic gabbros crosscuttingthe plutonic sequence (Vedi zone) are composedof plagioclase and hornblende, and are mainlyaltered in chlorite, carbonate, sericite, albite,quartz, actinolite, and so on.

Diorites (well-known in Sevan and Vedi zone)which occur as small, apparently intrusive bodieswithin the gabbro units (Palandjyan 1971;Abovyan 1981; Ghazaryan 1987, 1994), have aporphyritic to subhedral granular (1–4 mm) textureand have relatively similar hornblende contents(5–30%) as in the gabbros. Plagioclase (c. 65–70%) is albite-rich (An34-38) and accessory minerals(quartz, opaque oxides) are rare. Amphibole grainsare magnesio-hornblendes in which actinolite andsometimes epidote aggregates are present. Laterallyand towards upper sections (Sevan zone) dioritesgrade into quartz-diorite (quartz 5–10%).

Plagiogranites (Fig. 10e, f) appear to be the mostdifferentiated component of the gabbro-dioriticintrusives. They form diffused segregations ordiscontinuous networks of veins with local coarsepegmatic, or hypidiomorphic to xenomorph granu-lar (0.5–4 mm) texture within and around gabbro-diorite intrusives. They are formed by 40–65%plagioclase (An15-30), 25–45% quartz, minorbiotite (,5%), ortho-amphibole (,5%; Stepana-van), K-feldspar (0–10%, microcline; Vedi zone;Fig. 9c) and accessory phases (titanomagnetite,hematite, sphene and apatite). Amphiboles arerarely preserved, commonly replaced by chlorite-and epidote-group minerals.

Ophiolite volcanic and subvolcanic rocks. Diabasesare present in several locations (Sevan and Step-anavan areas) as isolated dikes, crosscutting thelayered gabbros. They are generally altered (chlor-ite, epidote, carbonates) and have the subdoleritictexture composed of plagioclase (60–65%;An65-75) and two clinopyroxenes (augite Wo41-44

En44-47Fs11-13 and diopside Wo45En37Fs18).The volcanic rocks of the studied Armenian

ophiolites are present as pillowed and massive lavaflows and pillowed breccias. In general they arerelatively altered due to hydrothermalism but stillrelict igneous textures are preserved. The basaltsand basaltic andesites are vacuolar (1–5 mm, ves-icles are filled with carbonate-calcite, chlorite andquartz) and largely aphyric (intersertal, spilitic,microdoleritic and variolitic, up to 1.5–2 mm indiameter), composed mainly of albitized plagio-clase and/or plagioclase-clinopyroxene microlites,Ti-magnetite and hematite microlites, in a devitri-fied (calciteþ chlorite) groundmass (Fig. 9d).

Alkaline lavas. The alkaline basalts are foundin several locations of the three ophiolite zones.


Their structural position is as large massive pillow-lavas flows on top of the ophiolite section or asdykes of diabase cross-cutting it (Stepanavanarea), but their relationships with the ophiolite

(sensu stricto) pillow lavas remain often unclear.The first group of alkaline rocks displays largevacuoles (0.5–3 mm), filled with carbonates andrarely chlorites, and have both phyric and aphyric

Table 1. Representative whole-rock analyses of samples from the Sevan, Stepanavan and Vedi areas. Major

Groups no. Sevan ophiolitic series

Flasergabbro

Olivinegabbro

Olivinegabbro

Gabbro Gabbro-norite Hornblendegabbro

Diorite Diorite

Sample AR-03-25 AR-05-86 G150 AR-03-39 AR-03-24 AR-03-10 AR-04-218 AR-03-23

SiO2 45.36 48.09 48.39 49.49 50.60 50.68 55.09 57.41Al2O3 13.32 16.72 15.63 14.11 7.2 18.17 13.45 14.10Fe2O3 14.91 5.94 6.44 11.59 7.77 9.09 8.51 8.84MnO 0.14 0.11 0.12 0.18 0.17 0.16 0.15 0.14MgO 7.44 10.51 10.48 6.79 15.29 6.75 9.84 2.24CaO 10.89 14.1 16.65 9.38 17.65 9.26 8.76 4.92Na2O 3.15 1.65 1.16 3.52 0.48 3.21 2.59 6.36K2O 0.22 – – 0.29 – 0.15 0.17 0.12TiO2 2.55 0.27 0.29 1.32 0.20 0.36 0.24 0.87P2O5 0.31 0.02 0.04 0.14 0.06 0.05 0.04 0.16LOI 1.46 2.8 0.65 2.92 0.79 1.21 1.87 4.33Total 99.8 100.2 99.8 99.7 100 99.1 100.7 99.5

Mg# 52.1 79.3 77.9 56.1 81.1 61.6 71.6 35.6Rb 2.67 0.48 – 3.05 – 0.84 1.42 1.81Sr 231.7 102 101 189.5 58.2 303.7 207.4 212.5Y 61.81 6.41 7.24 30.05 7.84 11.93 7.82 25.22Zr 175.9 5.28 4.85 75.20 5.24 22.89 21.08 75.07Nb 4.51 0.08 – 1.55 – 0.50 0.32 1.83Ba 36.77 4.1 – 120.8 14.14 34.36 34.9 55.59Hf 4.25 0.22 0.21 2.12 0.20 0.81 0.76 2.27Ta 0.35 0.01 – 0.11 – 0.04 0.03 0.14Pb – – – – – – – 2.62Th 0.08 – – 0.29 – 0.14 0.07 0.75U 0.11 – – 0.09 – 0.07 0.04 0.25V 440.7 138 190 319.90 195.9 222.2 158.1 122.6Cr 35.43 802 412 94.50 810.3 104.3 562.7 136.9Co 26.81 40.2 41.7 34.77 41.88 33.71 39.6 15.76Ni 77.98 188 131 32.35 136.2 29.23 148.1 10.09Cu 18.16 102 111 52.59 142.3 44.81 15.63 21.02Zn 58.57 28.3 29.6 86.18 47.98 71.99 73.35 50.45La 14.89 0.33 0.36 3.39 0.28 1.60 1.44 4.62Ce 34.75 0.94 1.02 9.57 0.99 4.51 3.73 11.73Pr 5.057 0.18 0.19 1.57 0.22 0.74 0.57 1.82Nd 24.09 1.23 1.28 8.27 1.40 3.80 2.87 9.05Sm 7.38 0.58 0.61 2.93 0.67 1.34 0.94 2.91Eu 3.11 0.34 0.32 1.07 0.28 0.48 0.35 0.97Gd 9.32 0.87 0.98 4.01 1.07 1.70 1.13 3.68Tb 1.60 0.16 0.18 0.71 0.20 0.30 0.20 0.69Dy 10.47 1.14 1.24 4.89 1.36 1.92 1.31 4.60Ho 2.22 0.24 0.26 1.03 0.29 0.41 0.28 0.97Er 6.37 0.65 0.74 3.02 0.83 1.22 0.82 2.95Tm 0.94 0.10 0.11 0.46 0.13 0.19 0.13 0.47Yb 6.28 0.65 0.72 3.03 0.86 1.32 0.92 3.25Lu 0.95 0.09 0.11 0.48 0.13 0.21 0.15 0.52Eu/Eu* 1.15 1.45 1.28 0.95 1.01 0.97 1.03 0.91(La/Sm)N 1.27 0.36 0.37 0.73 0.26 0.75 0.97 1.00(La/Yb)N 1.60 0.34 0.34 0.76 0.22 0.82 1.06 0.96


oxides are in wt%, and trace elements and REE in ppm

Sevan alkaline series

Plagiogranite Diabase Trachy-andesite Basaltictrachyandesite

Andesite Basanite Trachybasalt Trachyandesite

AR-03-19 AR-03-02 G154 AR-03-17 AR-03-34 G142 AR-05-80 AR-03-33

74.91 46.02 53.70 54.27 55.48 40.63 43.80 51.5712.32 16.29 14.09 15.16 14.13 14.40 17.58 14.343.58 8.39 11.35 12.36 12.45 11.70 9.46 6.060.03 0.13 0.15 0.19 0.13 0.29 0.11 0.110.42 7.73 4.52 3.74 4.07 4.15 6.70 0.773.05 10.68 3.64 4.38 5.49 11.40 4.95 9.784.20 3.53 6.07 6.63 3.96 3.64 4.09 6.340.31 0.37 – – 0.61 1.42 2.24 0.560.21 1.26 1.36 1.33 1.17 2.06 1.68 1.980.04 0.17 0.12 0.15 0.11 0.48 0.44 1.080.57 4.61 4.85 1.78 2.18 10.01 8.89 6.65

99.6 99.2 99.9 100 99.8 100.2 99.9 99.3

20.2 66.6 46.2 39.5 41.6 43.4 60.5 21.72.22 13.97 – – 5.05 31.89 44.83 7.93

145.2 630.9 28.76 49.82 102.5 147.0 330.8 341.527.65 23.93 27.82 29.88 27.49 25.58 17.37 52.9171.64 127.4 81.43 85.60 54.39 153.4 131.4 411.22.15 2.95 1.77 1.44 1.69 40.63 17.95 49.22

65.83 285.3 16.09 19.01 16.65 166.7 299.0 168.72.48 2.91 2.36 2.38 1.63 3.57 2.86 9.030.10 0.24 0.14 0.12 0.12 2.99 1.19 3.801.32 1.51 1.61 – – 4.98 4.63 4.451.09 1.01 0.55 0.37 0.33 4.13 4.06 5.150.62 0.29 0.35 0.27 0.14 1.43 0.94 4.61

50.8 179.3 334.9 321.8 347.4 257.4 271.7 286.11464 277.0 – – 251.7 33.86 21.68 162.0

5.52 38.26 29.48 24.33 27.09 38.26 31.44 24.0637.19 54.43 8.95 5.14 16.09 34.29 28.76 13.83

6.91 58.20 102.8 15.61 5.23 61.86 54.48 18.349.98 66.89 67.20 80.0 19.68 100.1 91.55 92.745.38 7.07 4.25 3.91 2.69 32.41 29.46 48.54

12.66 18.32 11.14 10.93 7.17 64.63 59.03 107.11.78 2.71 1.67 1.81 1.15 7.64 6.92 13.388.36 12.68 8.76 9.35 6.17 29.89 27.11 56.552.68 3.53 2.93 3.26 2.32 6.02 5.21 12.980.67 1.36 1.14 1.15 0.79 1.97 1.63 4.143.50 3.95 4.04 4.18 3.36 5.58 4.45 12.430.64 0.67 0.71 0.75 0.62 0.82 0.62 1.844.33 4.21 4.78 4.99 4.34 4.69 3.42 10.270.94 0.84 1.01 1.06 0.95 0.90 0.61 1.872.94 2.45 2.99 3.11 2.82 2.48 1.64 4.800.46 0.36 0.46 0.47 0.44 0.35 0.22 0.643.30 2.39 3.16 3.22 2.98 2.33 1.44 4.050.53 0.37 0.51 0.51 0.47 0.36 0.22 0.600.67 1.12 1.01 0.95 0.86 1.04 1.03 1.01.26 1.26 0.91 0.75 0.73 3.39 3.56 2.351.10 2.00 0.91 0.82 0.61 9.39 13.83 8.09

(Continued )

(Fig. 9e) intersertal textures, with plagioclase mega-crysts (c. 5%; 0.5–2 mm), microliths and opaqueminerals (3–10%), surrounded by a calcite-chloritemesostase. The second group (Vedi and Stepanavan

zones) have doleritic (Fig. 9f ) to ophitic (gabbro-ophitic in central parts of lavas flows) textures, andare mainly composed of plagioclase (c. 40–55%;1–3 mm), clinopyroxene (10–30%; 1–4 mm),


amphibole (c. 25%; 1–3 mm) and accessoryTi-magnetite (.5–10%), apatite (c. 3%; prismatic,acicular, 0.5–1.5 mm) and rarely biotite. Apatitesare frequent. Vitreous interstices are filled by

carbonates or carbonate-chlorite assemblages.The tabular plagioclase laths show a transitionalzoning with bytownite to labrador (An72-60) orlabrador to andesine (An55-32) compositions. Thin

Table 1. Continued

Groups no. Stepanavan ophiolitic series

Websterite Hornblendegabbro

Hornblendegabbro

Plagiogranite Basaltictrachy-andesite

Basaltictrachy-andesite

Sample AR-04-03 AR-04-16 AR-04-45D AR-04-44 AR-04-20 AR-04-30

SiO2 53.24 47.30 53.77 75.35 51.53 48.55Al2O3 1.03 14.39 14.00 12.20 14.69 13.29Fe2O3 6.02 12.90 8.92 2.71 14.81 8.67MnO 0.15 0.21 0.15 0.03 0.23 0.15MgO 23.18 9.11 7.81 0.77 4.15 6.86CaO 16.52 10.14 6.98 2.05 4.86 10.49Na2O 0.12 2.93 3.34 5.03 5.74 4.74K2O – 0.19 2.42 – 0.18 0.24TiO2 0.05 1.18 0.16 0.11 1.62 1.08P2O5 0.03 0.07 0.05 0.02 0.13 0.11LOI 0.54 1.76 2.44 1.07 1.89 6.01Total 100.9 100.2 100.1 99.3 99.8 100.2

Mg# 90.0 60.4 65.6 38.1 37.7 63.1Rb – 1.12 30.17 0.58 1.4 7.94Sr 11.79 125.4 213.4 91.04 61.01 95.7Y 1.05 20.91 5.79 1.23 35.91 26.52Zr – 42.74 21.26 6.48 86.0 68.43Nb – 1.01 2.14 0.35 1.62 1.9Ba 3.71 32.71 228.1 20.58 19.58 21.73Hf – 1.21 0.63 0.15 2.54 1.83Ta – 0.08 0.21 – 0.13 0.15Pb – – 3.61 – 2.29 1.93Th – 0.18 1.27 0.02 0.43 0.19U – 0.05 0.43 0.01 0.12 0.09V 135.1 324.7 94.47 30.74 459.8 305.4Cr 2804 236.4 324.2 421.5 99.23 316.7Co 55.82 51.46 31.5 7.71 38.85 42.98Ni 361.3 78.0 101.7 24.86 22.76 109.1Cu 340.3 – 189.9 189.5 64.58 132.8Zn 25.68 60.28 60.33 23.13 130.6 81.13La – 2.40 3.06 2.42 4.23 2.53Ce 0.15 6.37 6.28 3.90 11.08 7.37Pr 0.02 1.08 0.65 0.42 1.88 1.31Nd 0.15 5.76 2.30 1.58 9.88 7.04Sm 0.08 2.09 0.49 0.29 3.44 2.55Eu 0.03 0.96 0.19 0.35 1.28 0.99Gd 0.13 2.98 0.55 0.24 4.69 3.49Tb 0.03 0.53 0.10 0.04 0.87 0.65Dy 0.18 3.53 0.81 0.22 5.93 4.34Ho 0.04 0.74 0.19 0.05 1.28 0.93Er 0.12 2.15 0.64 0.15 3.79 2.75Tm 0.02 0.32 0.12 0.03 0.58 0.42Yb 0.12 2.10 0.91 0.21 3.89 2.84Lu 0.02 0.32 0.17 0.04 0.61 0.44Eu/Eu* 0.85 1.17 1.12 4.05 0.98 1.02(La/Sm)N 0.0 0.72 3.94 5.25 0.77 0.63(La/Yb)N 0.0 0.77 2.27 7.92 0.73 0.60


Stepanavan alkaline series Stepanavan calk-alkaline series



Diabase Olivinebasalt



AR-06-02 AR-03-53 AR-04-05 AR-04-32 AR-04-40A AR-04-31

45.37 48.54 50.19 49.15 49.79 52.2014.27 15.01 13.91 18.53 15.80 17.0513.52 12.65 13.73 10.19 8.82 9.28

0.32 0.27 0.24 0.16 0.15 0.166.22 4.25 3.27 5.25 3.54 3.594.09 5.33 5.85 8.25 9.12 6.561.53 3.93 5.11 4.36 3.54 4.615.37 2.69 0.42 0.52 1.24 1.003.12 2.64 3.39 0.86 1.07 0.941.31 1.08 0.67 0.14 0.20 0.185.11 3.16 2.94 3.12 7.27 5.36

100.2 99.5 99.7 100.5 100.6 100.9

49.8 42.3 34.0 52.7 48.4 44.545.3 33.17 7.62 9.61 18.12 18.45

157 322.6 198.8 520.3 303.8 282.340.6 44.41 51.24 16.0 24.4 24.19

268 294.4 373.5 44.4 99.18 95.352.8 57.95 42.33 2.14 2.29 3.32

608 578.3 156.6 133.9 239.1 213.65.97 6.51 8.01 1.25 2.69 2.613.97 4.20 3.24 0.17 0.18 0.267.04 2.54 2.24 7.22 3.42 5.665.39 5.98 4.65 0.72 1.46 1.671.34 1.46 1.20 0.19 0.67 0.58

172 94.35 201.6 241.5 279.1 263.7– 25.77 – 21.05 31.91 73.83

26.3 16.75 31.93 29.51 29.05 27.366.5 – – 15.55 22.38 18.95

20.4 9.48 14.94 12.6 188.1 170.9177 137.1 152.7 150.9 86.53 100.0

50.3 50.59 40.02 4.93 7.87 8.6996.1 107.0 85.12 11.46 18.51 18.0512.1 12.97 10.85 1.74 2.68 2.5353.3 53.32 45.27 8.35 12.58 11.6511.2 11.26 10.35 2.36 3.47 3.15

3.87 4.08 3.39 0.94 1.13 1.0410.5 10.41 10.3 2.60 3.89 3.57

1.50 1.52 1.63 0.44 0.65 0.618.11 8.69 9.50 2.80 4.09 3.961.49 1.57 1.80 0.57 0.85 0.833.75 4.16 4.93 1.62 2.46 2.460.51 0.57 0.70 0.24 0.38 0.383.18 3.64 4.51 1.60 2.54 2.540.45 0.56 0.69 0.25 0.39 0.401.09 1.15 1.0 1.15 0.94 0.952.83 2.83 2.43 1.31 1.43 1.74

10.68 9.38 5.99 2.07 2.09 2.31

(Continued )

rims of pure albite (Ab – 98%) are also present.The clinopyroxenes are generally chloritized,but its relics are still recognizable and belong todiopside (Wo49En35Fs16). The amphibole is a

kaersutite (Leake et al. 1997), with zonation fromkaersutite to ferro-kaersutite from core to rim,respectively. Some samples show abundant calcite-filled veins and pockets.


Table 1. Continued

Groups no. Vedi ophiolitic series Vedi alkaline series

Hornblendegabbro

Diorite Plagio-granite

Basalt Basalticandesite

Basalt Trachy-basalt

Basaltictrachyan-

desite

Trachy-dacite

Sample AR-05-113 AR-05-110 AR-05-111 AR-05-114 AR-05-106 AR-05-78 AR-05-104 AR-05-102 AR-04-75

SiO2 45.09 58.57 70.45 47.5 48.3 44.58 44.64 50.39 59.61Al2O3 21.24 16.03 14.69 16.17 15.03 12.52 15.41 16.2 17.48Fe2O3 4.32 6.66 4.44 8.76 10.14 9.36 11.99 7.76 7.64MnO 0.07 0.11 0.08 0.15 0.16 0.12 0.14 0.13 0.12MgO 7.88 5.18 1.06 8.46 5.97 2.63 4.85 5.05 1.11CaO 14.29 7.08 3.87 8.57 8.01 15.53 7.85 7.16 1.89Na2O 2.12 3.94 4.47 3.99 3.96 3.83 4.24 3.24 6.37K2O 0.17 0.47 0.2 0.72 0.18 – 0.96 1.96 2.4TiO2 0.16 0.33 0.43 0.93 1.2 2.35 3.67 2.36 0.72P2O5 – 0.04 0.09 0.09 0.12 0.33 0.85 0.64 0.25LOI 5.08 2.08 1.04 4.78 7.14 9.17 5.04 5.07 2.14Total 100.4 100.5 100.8 100.1 100.2 100.4 99.6 99.9 99.7

Mg# 79.7 62.7 34 67.6 56.0 37.8 46.6 58.4 23.16Rb 2.41 4.12 1.11 3.51 4.14 0.63 10.48 27.1 64.89Sr 492.7 254 161 134.6 110.1 153.5 926 643 260.4Y 3.70 10.1 13.1 20.78 26.91 24.94 36.26 22.4 58.18Zr 4.30 42.3 86.5 54.9 73.98 160.8 318.8 260 680.5Nb 0.08 0.49 0.59 0.69 2.46 23.22 67.52 43.3 82.24Ba 131.6 57.5 33.5 141.7 16.2 1097 444.9 659 422Hf 0.16 1.3 2.35 1.48 2.04 3.91 6.81 6.03 14.76Ta – 0.04 0.05 0.07 0.20 1.76 4.88 3.29 5.99Pb – 1.1 1.12 – – 1.53 1.25 6.71 4.30Th – 0.42 0.48 0.15 0.22 2.085 4.60 8.39 12.28U – 0.12 0.13 0.06 0.10 0.643 1.18 1.8 2.78V 89.5 185 44.5 211.3 238.3 240.7 219.8 135 5.26Cr 785.2 123 9.1 416.7 324.7 50.2 4.22 136 66.9Co 30.98 22.8 6.6 42.44 49.08 28.9 31.4 64.3 4.98Ni 130.7 42.4 6.4 195.5 122.6 26.28 21.81 126 –Cu 92.4 18.3 – 14.05 81.78 28.41 50.82 42.2 9.93Zn 24.06 50.7 39 65.52 94.28 106.8 146.8 134 167.9La 0.231 2.09 2.91 1.96 3.07 18.44 49.35 50.7 74.8Ce 0.71 4.84 6.15 6.25 8.51 39.11 109.7 91.8 142.6Pr 0.14 0.72 0.81 1.09 1.47 5.02 14.15 9.83 15.84Nd 0.78 3.74 4.48 5.9 7.97 21.85 59.18 42.9 59.79Sm 0.36 1.20 1.45 2.15 2.88 5.5 12.52 8.47 12.64Eu 0.24 0.42 0.69 0.9 1.12 1.93 4.15 2.67 3.68Gd 0.53 1.46 1.81 3.0 4.02 5.72 10.96 7.36 11.69Tb 0.10 0.25 0.32 0.53 0.70 0.85 1.48 0.99 1.877Dy 0.64 1.65 2.11 3.48 4.67 4.85 7.84 5.01 10.97Ho 0.14 0.34 0.46 0.74 0.97 0.88 1.32 0.81 2.08Er 0.38 1.01 1.35 2.08 2.78 2.30 3.26 1.92 5.77Tm 0.06 0.16 0.21 0.32 0.42 0.31 0.42 0.25 0.86Yb 0.35 1.14 1.52 2.15 2.83 1.95 2.49 1.46 5.84Lu 0.06 0.18 0.25 0.33 0.45 0.29 0.35 0.22 0.88Eu/Eu* 1.64 0.97 1.31 1.08 1.0 1.05 1.08 1.03 0.93(La/Sm)N 0.40 1.10 1.26 0.57 0.67 2.11 2.48 3.77 3.73(La/Yb)N 0.44 1.24 1.29 0.62 0.73 6.39 13.39 23.44 8.64


A few dacitic dyke-like bodies crop out amongthe basaltic pillow flows in the Vedi valley. As inthe pillow basalts, plagioclase is the main mineralphase and Fe-oxides (c. 5–10%) are the accessoryminerals (Fig. 9g). Some plagioclase unzonedphenocrysts (1–2 mm, oligoclase-andesine?) aredistributed in the fine-grained (,0.2–0.5 mm)hyalopilitic or micro-cryptocrystalline devitrifiedgroundmass made of albitic plagioclase, opaquemicrolites, and carbonate-quartz-chlorite aggre-gates. There are also thin hydrothermal veins madeof calcite.

Calc-alkaline lavas of Stepanavan zone.These consist of large pillow-lavas of basalticand basaltic andesitic compositions with micro-cryptocrystalline (Fig. 9h) to intersertal textures oflarge andesine-oligoclase plagioclase phenocrysts(2–7 mm) and microliths, and minor augite(Wo36-38En42-43Fs13-15) clinopyroxenes. Theselavas overlie Upper Cretaceous limestones, uncom-formably lying on the ophiolite s.s.

Major–trace–REE geochemistry

The geochemical analyses of the ophiolitic rocksfrom the Sevan ophiolite are of relatively alkalinecomposition in comparison to MORB. Majorelement data of pillow- lavas, diabase and gabbrosshow that they have predominantly basalt totrachybasalt compositions.

Major elements. Major element analysis of plutonicrocks ranges from gabbros to granites (plagiogra-nites) with intermediate dioritic compositions(Fig. 11a). These magmatic rocks appear to definea large trend (Le Maitre et al. 1989). Similarly, inthe alkali v. iron and magnesium (AFM) diagram(Fig. 11b) most rocks lie close to the limitbetween the tholeiitic and calc-alkaline fields.

† Overall, the rocks of the ophiolitic suite areenriched in MgO and more depleted in TiO2,K2O and P2O5 relative to the alkaline suite(Figs 11 & 12; Table 1). The volcanic rocksfrom the different studied areas plot in the

Table 2. 40Ar/39Ar dating results of single grain AR-05-110 gabbro amphiboles from the Vedi ophiolite

Step Laserpower(mW)

Contamin.atmosph.

(%)

39Ar (%) 37ArCa/39ArK

40Ar*/39ArK Age (Ma+1s)

AR-05-110(A) (K103)1 500 99.437 1.03 16.751 0.237 9.792+116.9462 620 73.967 0.58 5.709 3.169 126.879+159.4323 690 134.382 0.54 7.676 2 2+ 24 850 93.450 0.91 17.798 0.599 24.696+125.7975 1050 38.568 11.19 34.521 3.440 137.322+10.2566 1150 19.355 39.90 40.983 3.920 155.682+5.7107 1200 24.516 16.76 44.907 3.601 143.488+6.7328 1300 17.488 5.37 35.012 4.116 163.105+17.0089 2500 17.374 23.73 37.852 4.009 159.066+4.931

AR-05-110(B) (K119)1 500 82.734 1.70 13.380 30.304 178.902+22.9432 585 83.526 1.36 18.310 15.052 91.078+30.8633 670 29.902 9.58 50.630 30.333 179.068+4.3644 760 17.905 27.30 48.774 29.942 176.868+2.3425 806 15.376 23.98 43.418 30.465 179.810+2.6046 860 20.078 7.09 42.179 29.367 173.631+5.4727 1800 13.653 28.99 40.579 30.589 180.506+2.321

AR05 110(C) (K132)1 484 81.313 1.51 12.110 6.266 243.436+50.8942 520 70.539 1.35 11.216 3.010 121.065+52.8383 600 30.721 8.14 45.281 4.824 190.263+10.9234 660 22.277 17.99 38.256 4.168 165.539+4.8345 710 32.287 12.26 46.619 3.678 146.840+8.2846 785 25.007 18.02 52.601 4.278 169.727+10.0537 862 33.799 11.28 62.023 5.175 203.359+6.9198 2000 18.441 29.44 34.698 4.251 168.686+2.481


Fig. 7. 40Ar/39Ar age spectra and isochrons undertaken on single amphibole (hornblende) grains of a gabbro sample(AR-05-110) from Vedi ophiolite.


Fig. 8. Representative geological logs of the Stepanavan, Sevan and Vedi ophiolites.

AR

ME

NIA

NO

PH

IOL

ITE

S369

Fig. 9.


same compositional range (from basalts to ande-sites and trachyandesites), and are relativelyricher in Na2O than the plutonic rocks of thesame series.

† The studied alkaline lavas from different zonesplot in the same range, varying compositionallyfrom basanite-trachybasalt to basaltic trachyan-desite and trachyandesite, and are clearly inthe calc-alkaline/alkaline domain of the AFMdiagram (Fig. 11a, b). One of the most significantfeatures of the alkaline lavas is their higher TiO2,K2O and P2O5 contents.

† The arc-type calc-alkaline lavas, having trachy-basalt and basaltic trachyandesite composi-tions in TAS diagram (Fig. 11), plot essentiallyin a transitional position between ophioliticand alkaline domains in Harker’s diagrams(Fig. 12), except lower TiO2 and higher Al2O3,depend on the abundance of plagioclase insuch rocks.

Regarding now the spread of compositional vari-ations in major elements within series, it appearsthat SiO2, Al2O3, MgO, correlate relatively wellwith variations in TiO2, an element considered asmore immobile during alteration processes (e.g.Staudigel et al. 1996), (Fig. 12). Other elements,and particularily Na have relatively scatteredcompositions, even in individual magmatic suites,which could be ascribed to alteration or relativelycomplex magmatic processes. This is also supportedby thin section observations and previous studies ofthe Armenian ophiolites (e.g. Palandjyan 1971;Abovyan 1981; Ghazaryan 1994), which indicatethat the whole magmatic sequence has been affectedby oceanic low-temperature hydrothermal altera-tion events. These processes induced modificationof the most lithophile elements, as revealed by theincrease of LOI (loss on ignition) in whole-rockchemistry (Table 1).

Trace elements. High field-strength elements(HFSE) are not mobilized during alteration and

their contents reflect, without ambiguity, thoseof their parental magma (Staudigel et al. 1996).Trace elements contents confirm the presence ofthree clearly distinct magmatic suites, as definedin previous section.

† Basalts and gabbros of the ophiolite suite showstrong enrichments in LILE (Large Ion Litho-phile Elements: Ba, Rb, K and Th) are closeand up to ten times MORB values and bear nega-tive anomalies in Nb-Ta and Ti (Fig. 13a, b),which is generally indicative of volcanic islandarc environments (Taylor & McLennan 1985;Plank & Langmuir 1998).

† Overall, the concentrations of each element inthe alkaline basalts exceed those in the basaltsfrom ophiolitic series (Fig. 13c). Moreover, alka-line series basalts are characterized by highabundances of LILE, high field strengthelements (Nb, Ta, Zr and Ti), and light rare-earthelements (LREE).

† The calc-alkaline suite rocks show strongdepletions in Nb and Ta, relative to Th and La,and slight Ti negative anomaly (Fig. 13d).They globally show slightly stronger enrich-ments in LREE and LILE relative to the ophio-lite suite rocks.

These differences in normalized element patternssupport that these basalts are not petrogeneticallyrelated and most likely derived from melts formedin different tectonic settings: (1) N-type MORB(and/or Back-arc basin type); (2) Ocean-islandand/or within-plate alkali basalts; and (3) volcanicisland arc.

REE geochemistry. In the chondrite-normalized rareearth element (REE) diagrams (Fig. 13), analysedophiolite basalts and gabbros have flat and parallelREE spectra in chondrite-normalized plots ((La/Yb)N ¼ 0.6–0.9), showing some slight depletionsin LREE and a slight enrichment in MREE(Fig. 13e, f ). No extensive Eu anomalies were

Fig. 9. (Continued) Microphotographs of representative magmatic rock types from different ophiolite complex.Plutonic and volcanic ophiolite series: (a) subautomorph granular texture of a cumulate banded websterite (sampleAR-04-36, Stepanavan area, Cheqnagh valley); (b) coarse-grained hornblende gabbro with normally zoned plagioclases(sample AR-05-110, Vedi area, massif of Qarakert); (c) xenomorph granular texture of a microcline (Mc) bearingplagioclase rich leucogranite (sample AR-05-109, in the same massif ); (d) aphyric, intersertal (spilitic) and varioliticbasalt composed of mainly albitized plagioclase, Ti-magnetite and hematite microlites, in a devitrified groundmass(sample AR-05-106, Vedi area, Khosrov valley). Alkaline series: (e) aphyric, intersertal basalt, totally devoid ofphenocrysts, and composed of carbonatized plagioclase microlites and opaque minerals (c. 5%) in a chlorite-carbonategroundmass (sample AR-05-80, Sevan area, Tsapatagh valley); (f ) doleritic texture in a trachybasalt composed ofplagioclase, chloritized clinopyroxene, kaersutite (Krs), Ti-magnetite and apatite (sample AR-05-104, Vedi area);(g) phyric trachydacite with a hyalopilitic to cryptocrystalline texture (sample AR-04-75, Vedi valley). Calc-alkalineseries: (h) olivine-bearing, plagioclase phyric (15–40%) basalt with a microcrystalline (plagioclase, quartz, opaqueminerals) texture from pillow lavas suite (sample AR-04-32, Stepanavan area, Herher valley), in which the olivinephenocrysts are entirely pseudomorphosed to quartz and rims of iron oxides. From a–c under crossed nichols, and d–hunder parallel nichols.


Fig. 10. Microphotographs of plutonic rocks from the Vedi ophiolite. (a) Gabbro (sample AR-05-112A) composed ofeuhedral to subhedral plagioclase (Pl) with normal zoning with green euhedral amphibole (Amph), containingclinopyroxene inclusions (Cpx). (b) In the same sample, subhedral plagioclase coated by anhedral amphibole. (c) Ingabbro sample AR-05-110, detailed back-scattered image showing zoning in large amphibole crystals, frommagnesio-hornblende (1), to edenite (2) and pargasite (3) toward the rim. (d) Subhedral amphibole crystallized over aprevious anhedral amphibole core (in sample AR-05-110), with some interstitial quartz (Q). (e) Plagiogranite (sampleAR-05-111) formed by subhedral plagioclase, de anhedral quartz and little proportion of chloritizedþ epidotized (Ep)amphibole. (f ) Leucocratic plagiogranite (sample AR-05-109), showing anhedral granular structure of plagioclase,K-feldspar (with microcline twining, Mcc) and quartz. Observations are under parallel (//) or crossed (�) nicols all atthe same scale.


(a) (b)GabbrosDioritesPlagiograniteOphiolitic lavasAlkaline lavas

GabbrosPlagiograniteOphiolitic lavasAlkaline lavasArc-type lavas

GabbrosDioritesPlagiograniteOphiolitic lavasAlkaline lavasTrachydacite

Fig. 11. Plots of magmatic rocks (ophiolite, ‘OIB’ and arc series) in the (a) (Na2OþK2O) v. SiO2 (Le Maitre et al.1989) and (b) AFM (Irvine & Baragar 1971) diagrams.

GabbrosDioritesPlagiograniteOphiolitic lavasAlkaline lavas

GabbrosDioritesPlagiograniteOphiolitic lavasAlkaline lavasTrachydacite

GabbrosPlagiograniteOphiolitic lavasAlkaline lavasArc-type lavas

Fig. 12. Harker variation diagrams showing the compositions of the three (ophiolitic, alkaline and calc-alkaline) series.


observed (Eu/Eu* ¼ 0.95–1.15), which show thatalmost no plagioclase fractionation has occurred.Thus, plagioclase likely remained stagnant andwas enriched in the final liquid. The concentration

of REE varies from 8 to 30 times chondrite compo-sitions in volcanic rocks, and from 1 to 15 times inthe gabbros. Only one gabbro sample (sampleAR-03-25) shows an extreme 60 times-chondrite

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 13. Trace and REE plots of the three studied magmatic suites. The multi-element spiderdiagrams are normalized tothe N-MORB values of Sun & McDonough (1989), and REE plots are normalized to the Chondrite values of Evensenet al. (1978). Patterns for the studied magmatic rocks: ophiolitic volcanic (a, e) and plutonic (b, f ) series; OIB typealkaline series (c, g), and arc type calk-alkaline series (d, h).


REE concentration, which may be explained byfluid alteration as it displays a flaser–structure.These features are interpreted as a result ofextreme crystal fractionation involving plagioclase,clinopyroxene, orthopyroxene and, to a lesserextent, olivine accumulation (Pallister & Knight1981).

The websterite and gabbronorite have the lowestconcentrations of REE (0.1–0.9 and 1–5 timeschondrite respectively) with patterns characterizedby depletion in LREE (Fig. 13f). One hornblendegabbro (sample AR-04-45D from Stepanavanophiolite) is characterized by LREE enrichment((La/Yb)N ¼ 2.27) and some depletion in MREE(a convex downward pattern) with smaller positiveEu anomalies (Eu/Eu* ¼ 1.12).

The REE patterns of the diorites (6-20 timeschondrite) and plagiogranites are parallel to thoseof the gabbros, with smaller enrichment in LREE((La/Yb)N ¼ 1.1). While the plagiogranite fromStepanavan (sample AR-04-44) is characterized bymore depletion in the middle to heavy REE com-pared to other plagiogranites, and strongly positiveEu anomalies (Eu/Eu* ¼ 4.05) due to its cumulat-ive nature that ascribed to high plagioclase contents.

In contrast, chondrite-normalized REE patternsof alkaline lavas (Fig. 13g) show huge LREE enrich-ments and HREE depletion ((La/Yb)N ¼ 6–14),being characteristic of intraplate continentalbasalts, as compared to ophiolite lavas. Meanwhile,no extensive Eu anomalies were observed (Eu/Eu* ¼ 0.95–1.15). The pattern of a trachydacites(sample AR-04-75) is parallel to those of thebasanite-trachyandesite series having the highestoverall REE concentration.

Chondrite-normalized REE patterns of calc-alkaline lavas are strongly parallel and form anarrow domain (Fig. 13h). They have similarHREE contents as volcanics of previous serieswith significantly more depleted LREE contentsthan alkaline series rocks ((La/Yb)N ¼ 2.1–2.3).

These differences of trace elements behaviourbetween the three studied series further supportthat these basalts are petrogenetically unrelatedand, most likely derived from melts formed indifferent tectonic settings.

40Ar/39Ar dating

Three analyses have been done on amphibole singlegrains from one gabbro sample (AR-05-110) fromthe Vedi ophiolite, which is described in the ‘Petro-graphy and mineral chemistry’ section. These arepresented in Table 2 and Figure 14.

In the first dating (k103), a plateau age is definedby the four last steps at 154.7+6.9 Ma (2s) com-prising 86% of 39Ar. Further, using all the steps, an

isochron age of 154.4+8.1 Ma (MSWD ¼ 1.4) isobtained. The initial 40Ar/36Ar ratio is of 264+7shows a slight shift from the air value which shallbe ascribed to the large error bars in the lowtemperature steps.

In the second experiment (k119) a well-constrained plateau of 178.7+2.6 Ma (2s) isobtained, with 97% of 39Ar. The average 37ArCa/39ArK ratio is high as for the latter experiment,c. 40 in low temperature steps, decreasing steadilyto c. 30 in high-temperature steps. An isochron isobtained using the five steps used in the estimateof the plateau age, with an initial 40Ar/36Ar ratioclose to the atmospheric value. Even with all ofthe steps, including the lower temperature ones,except step 2 (which has a large error and representsonly 1.4% of degased 39Ar) we calculate a similarwithin-error isochron age of 177.6+2.6 Ma.

The third experiment (k132) provides a moredisturbed Ar spectra, which does not provide anyplateau due to one high-temperature step, featuredby a higher age. However, we obtain a weightedaverage age of 172+6 Ma, using steps 4–8, or apseudo-plateau age of 167.3+6.6 when excludingstep 7. The calculated isochron age including allsteps and that obtained only with the HT stepsprovide a similar within error age of about 160 Ma(see Fig. 7). The initial 40Ar/36Ar ratio variesfrom 346+7 with all steps to 318+3 with thehigh temperature ones, which is slightly lowerthan the air value, and might be ascribed to somedisturbance of the Ar system.

Age variations within samples are ascribed tothe contribution of finely inter-fingered mineralcomponents within the dated amphiboles, follow-ing the works done by Villa et al. (2000). In thethree datings, 37ArCa/

39ArK ratios are close theobtained electron micro probe (EMP) values of theamphiboles, with some slight variations. The clear-est case is the experiment K119, which also showsthe flatter age pattern. In this sample, the slightdecrease from higher 37ArCa/

39ArK in low temp-erature steps towards lower 37ArCa/

39ArK inhigher temperature steps is ascribed to core-rimvariations in amphibole. Indeed, the petrographicanalyses show the presence of several mineral gen-eration (see Fig. 10c, d) and EMP analyses showhigher Ca/K ratios in the rim (actinolite) v. the inthe (hornblende) core of minerals. Therefore, it islikely that the amphibole rim contributes more tothe low temperature steps, while the core contrib-utes more to the high temperature steps, as featuredin Villa et al. (2000) and Rolland et al. (2006). Suchprocess of amphibole recrystallization may explainthe large range in obtained ages within the samesample. Plateau ages are always similar (withinerror) to isochron ages, with initial 40Ar/36Arratios very close to the air value. Thus, all the ages


obtained here have a geological meaning. However,the slight disturbances observed in initial 40Ar/36Arratio of the most disturbed Ar spectra (K132) is agood argument to explain part of the age spreadby some fluid-rock interaction process. The flatterAr spectrum (k119) also provided the older age(178.7+2.6 Ma). It is therefore probable that thisage is very close to the initial magmatic crystal-lization age. This age is also in agreement withother geological data, comprising palaeontologi-cal age of the pelagic limestones (Sokolov 1977)and radiolarian interbedded in the pillow lavas(Danelian et al. 2008) all attributed to the MiddleJurassic period. Therefore, the perturbed (k 132)and c. 155 Ma (k103) amphiboles agree for alater or long-lasting alteration process, related tothe crystallization of greenschist to epidote amphi-bolite minerals within very heterogeneous anddiscrete zones.

In conclusion this is the first Ar-dating under-taken on the plutonic part of the Vedi ophiolite.The initial crystallization age of the gabbro is atthe limit between the Lower and Middle Jurassic(Toarcian-Aalenian), and predates shortly the depo-sition of radiolarites and limestones. This age is alsoslightly older than similar Ar-dates obtained on onegabbro from the Sevan ophiolite (Galoyan et al.2009), and other evidence for the formation of theSevan ophiolite in the Middle Jurassic (Zakariadzeet al. 1990).

Discussion

Ophiolites of the Lesser Caucasus region ofArmenia are generally separated into three distinctzones: (1) the Sevan–Akera zone in the North(Knipper 1975; Adamia et al. 1980); (2) the Zange-zur zone in the SE (Aslanyan & Satian 1977;Knipper & Khain 1980; Adamia et al. 1981) ofArmenia, respectively; and (3) the Vedi zone tothe south (Knipper & Sokolov 1977; Zakariadzeet al. 1983).

Due to the importance of Cenozoic volcanismthat covered most of the surface of Armenia(Fig. 2), it is still difficult to conclude if whetherthe different ophiolites correlate with each other,or if they represent various suture zones delimitatingseveral continental micro-blocks. As emphasized inthe following discussion, we will show further thatthese ophiolites show some similarities and differ-ences in their age, structure, lithological successionsand geochemical features; but these features remaincompatible with an origin from a sole oceanicdomain. This domain opened in the Lower–Middle Jurassic and has registered several phasesof magmatic emplacement, evidenced in each ofthe different investigated geographic zones. These

correlations provide insights into the evolution ofthe Tethyan domain, and in particular allow us topropose a geodynamical model for the obductionof the ophiolite over the Armenian block.

In the following discussion, we will evaluate thefollowing points.

† Petrographically and geochemically, the Arme-nian ophiolites are similar to island-arc tholei-ites. Such geochemical features are typical foroceanic crust, formed on a back-arc settingwith the melting of a shallow asthenosphericsource contaminated by slab-derived fluids(Tarney et al. 1981; Saunders & Tarney 1984).Such a hypothesis has already been proposedfor ophiolitic gabbros from Turkey (Kocaket al. 2005), but has to be further evaluatedconsidering isotopic compositions and partialmelting rates constraints.

† Lower Cretaceous Alkaline lavas of variablethicknesses overlain this ophiolitic sequence.Their origin has to be considered. (i) Do theyalso derive from the same ophiolitic series?(ii) Did they formed in an island-arc setting orin an oceanic plateau environment? The occur-rence of alkaline magmatism prior to obductionin the the Late Lower Cretaceous may be ofsignificant importance for the obduction modelof the ophiolite crustal sequence.

† Finally, the calc-alkaline lavas are Upper Creta-ceous in age. Then, these volcanic arc-relatedseries may be formed during closure of theNeo-Tethys Ocean. Their geochemical featurewill be considered to evaluate this hypothesis.

Significance of Armenian ophiolites:

MOR or back-arc setting?

We could not find any outcrops of the Zangezurophiolites. These might be very thin stretchedrocks, which we believe mark the base of a majorthrust in SE Armenia. Petrographically and geo-chemically, the Armenian ophiolite rocks fromthe different studied localities (Stepanavan, Sevanand Vedi) share the same lithological and petrologi-cal features. They have geochemical features inter-mediate between MOR basalts and island arctholeiites. However, as shown by field relationshipsin the ‘Geological setting’ section, it is clear thatthese ophiolites were emplaced at oceanic spreadingcentres. The association of exhumed serpentinites,gabbros and plagiogranites at sea-floor levels, over-lain by radiolarites is the result of intra-oceanic tec-tonics (Lagabrielle et al. 1984), typical of LOTophiolites (Nicolas & Jackson 1972; Nicolas 1989;Lagabrielle & Cannat 1990). In this context, thepaucity of sea-floor lava spreading explains thatdeep sections of the oceanic crust are exhumed by


extensional faults. Faults and shear zones haveguided magmatic infiltration, which explains theintense hydrothermal activity observed in the ser-pentinites (‘listwenites’) and the pervasive altera-tion of lavas and diabase dykes. In addition,Armenian ophiolitic series are shown to be tholeiiteswith slight calc-alkaline character, ranging frombasalts to basaltic andesites and basaltic trachyande-sites. Spider diagrams show clear Nb–Ta negativeanomalies (Fig. 13a, b), LILE enrichments and flatto slightly LREE-enriched spectra. These obser-vations do not support a geochemical ‘normal’ophiolitic crust and are more probably in agreementwith typical volcanic arc settings, in which enrich-ments in LILE, LREE result from slab fluids/melts contamination (Pearce et al. 1984). Forthese reasons it appears most likely that the Arme-nian ophiolites were emplaced in a back-arcsetting with contamination provided by slab fluids.We did not find any evidence of the associatedUpper Jurassic volcanic arc system, which wesuggest might have been subducted or erodedduring the obduction.

The very strong (greenschist to epidote amphi-bolite facies) and long-lasting (.20 Ma) hydrother-mal imprint which is evidenced in the 40Ar/39Ardating experiments (Fig. 7) is also in agreementwith a LOT environment. In such context, the timefor magmatic crystallization and hydrothermalismis longer due to slow-spreading rate. In the presentcase, a time of .23 Ma for greenschist facies hydro-thermalism may suggest accretion rates ,1 cm a21.The time of formation of the Vedi (178.7+2.6 Ma,this work) and Sevan (165.3+1.7 Ma; Galoyanet al. 2009) ophiolites, constrained by 40Ar/39Ar ongabbro amphibole, show that the age of oceaniccrust is older in the southern leading edge of theobducted sequence than on its northern side. Thisage difference is also seen in the age of radiolariansthat are younger in Sevan and Stepanavan than inVedi (Danelian et al. 2007, 2010). Such differenceis in agreement with the Vedi ophiolite being veryclose to the rim of the former back-arc basin, theobduction may then have been triggered along themargin of the back-arc domain, directly south ofthe Vedi oceanic crust.

Origin of alkaline lavas

Mineral chemistry and geochemistry of the alkalinevolcanic series of Sevan, Stepanavan and Vediophiolites is similar to that of OIBs. As shown inthe Mineral Chemistry section, pyroxenes bear analkaline composition. The alkaline lava samplesshow strong enrichments in incompatible elements(up to 100 times chondrite values). In the Vediarea, Satian et al. (2005) already pointed out thealkaline character of the lava series, which they

interpreted as intra-continental rifting. However,these lavas were emplaced above, and formedafter the ophiolites. An age of 117.3+0.9 Ma wasrecently obtained with the 40Ar/39Ar datingmethod undertaken on single-grain amphibole bylaser step-heating (Rolland et al. 2009). Moreover,they are interstratified and overlain by reef lime-stone, which suggests a shallow marine environ-ment just after emplacement. Thus, we interpretthese series as resulting from a plume event thatoccurred in an intra-oceanic setting. This plumewas sufficiently large to overlain the various ophio-lite zones present in Armenia, over a surface of.5000 km2, thus this plume event might haveformed a plateau, which by itself may explain theobduction (Rolland et al. 2009).

Such alkaline magmatism is widely documentedin the Middle-East region, along the Arabian andIndian platforms, in relationship with the formationof the Neo-Tethys ocean (Lapierre et al. 2004).Similar Cretaceous alkaline series are found abovethe Iranian ophiolite (Ghazi & Hassanipak 1999),and in Turkey (Tuysuz et al. 1995; Tankut et al.1998). However, it is still difficult to relate thesealkaline events due to their geographical and tem-poral distance, the paucity of radio-chronologicaland Sr, Nd, Pb isotopic data.

Reconstruction of the ‘ophiolite’ history

From all the available geological data, we canpropose the following model for the evolution ofthe Armenian Ophiolite (Fig. 14).

† The SAB is of Gondwanian origin according tolithological associations found in central andSE Armenia (Knipper & Khain 1980; Kazminet al. 1987; Aghamalyan 2004). Therefore, it islikely that the Sevan oceanic basin opened as aresponse to the north-dipping subduction ofNeo-Tethys (Fig. 14). Emplacement of theophiolite occurred in the Early to Middle Juras-sic (Galoyan et al. 2009) in an intra-oceanicback-arc setting between the Armenian blockand the active Eurasian margin. The older ageof the Vedi ophiolite, with respect to that ofSevan implies that it should be at the rim of theback-arc system.

† Emplacement of an Oceanic Plateau above theback-arc oceanic crust during the late LowerCretaceous (114–102 Ma).

† The calc-alkaline lavas unconformably overlainthe ophiolite and related alkaline series(Galoyan et al. 2007). These lavas have similargeochemical features as volcanic arc series.Their emplacement is bracketed in the UpperCretaceous, as for the high pressure metamorph-ism constrained in the Stepanavan area (Melik-setyan et al. 1984 and references therein),


+

+

+

+

+++

++

S

Lower–Upper Jurassic

South Armenian Block

Intra-oceanic subductionArc volcanism

Sevan/StepanavanVedi

Slab fluid metasomatism

Back-arc extensionand decompressional melting

+

+

+

+

+ + +

+ + + +

+ +

+

S N

Late Lower Cretaceous(Albian-Aptian, 120–115 Ma)


Intra-oceanic subduction Arc volcanism (reduced)

Eurasian Active Margin

Vedi

+

+

+

+

+ + +

+ +

S

CenomanienUpper Cretaceousc. 102–95 Ma


Volcanic arc subduction

Sevan/ Stepanavan

Vedi

Slab retreat

Subduction ofthe arc series (Stepanavan Blueschists)c. 95–90 Ma

Erosion

Hot spot Alkaline magmatism Crustal thickening

+

+

+

++ ++

++ ++

++

+

SN

Turonian - Lower Coniacian95–88 Ma


Obduction

Eurasian Margin

Sevan/Stepanavan

Vedi

Slab retreat

Erosion Erosion

Oceanic plateau

Sevan/ Stepanavan

Frontal flysch sequence

Shallow to pelagiccarbonated sedimentation+grawackes

+

+

+

++ ++

++ +++

++

SN

Santonien83 Ma


ObductionStoppedUpper ConiacianUncomformity

Eurasian Margin

Sevan/Stepanavan

VediErosion Erosion-slumps

Frontal molasse sequence

Blocking of the subduction at c. 73–71 Ma

Arc volcanism

Slab retreat

+ +

+ +

+

N


?

++

++

+

N


??

(1)

(2)

(3)

(4)

(5)

?

Fig. 14.


constrained at c. 95–90 Ma (Rolland et al.2007). Therefore this magmatic event can berelated to the N-dipping subduction of the Neo-Tethys ocean below the Sevan–Akera back-arcprior to the obduction of the Armenian Ophio-lites onto the SAB.

† Then, the SAB enters the subduction zone inthe Cenomanian (102–95 Ma), which triggersa ‘collision’ with the thickened plateau. Duringthis process, the volcanic arc has probablybeen subducted below the Oceanic plateauand metamorphosed in the blueschist facies(Rolland et al. 2007). The large variety oflithologies comprising metabasites, marls andconglomerates in a pelitic matrix, within theStepanavan blueschists, is in agreement withsuch a scenario.

† The obduction of the ‘ophiolite’ section overthe SAB is further constrained by the LowerConiacian frontal flysch sequences, foundbelow and in front of the Vedi obductedsequence. The calc-alkaline series found abovethe Stepanavan ophiolite show that a volcanicarc was active during this time above theobducted sequence.

† The end of the obduction is constrained by UpperConiacian fauna in sediments unconformablyoverlying the ophiolite. The subduction belowthe Eurasian margin may stop at 73–71 Ma, asshown by 40Ar/39Ar age of MT-LP metamorph-ism in the Stepanavan blueschists and thegeneral tectonic uplifting of the region, wit-nessed by erosion and absence of sedimentaryrecord during the Late Cretaceous-Palaeocene(Rolland et al. 2007). This 73–71 Ma event isthus interpreted as the insight of ‘collision’.

Conclusions: geodynamic significance of

Armenian ophiolites

(1) The Armenian ophiolites show evidence forthe obduction of a single oceanic crustsequence above the SAB. Similar geological,petrological, geochemical and age featuresare found in the studied Armenian ophioliticmassifs (Sevan, Stepanavan and Vedi).

(2) The age of the ophiolite is constrained by40Ar/39Ar dating experiments undertaken ongabbro amphibole in the Vedi area, which

provided a magmatic crystallization age of178.7+2.6 Ma, and further evidence ofgreenschist facies crystallization duringhydrothermal alteration until c. 155 Ma. Ascompared to the Sevan ophiolite, whereoceanic crust formation is dated at165.3+1.7 Ma with the same method, theoceanic crust sequence in the Vedi area is sig-nificantly older, suggesting that the initiallateral oceanic age relationships arepreserved.

(3) The oceanic crust sensu stricto corresponds toa Lherzolite Ophiolite Type (LOT), formed inthe Early-Middle Jurassic by slow-spreadingaccretion. The long-time span of alteration(.20 Ma) recorded in 40Ar/39Ar ages ofdated amphiboles is also suggestive of sucha slow spreading environment. In addition,the hybrid arc-MORB geochemical signatureof the ophiolite rocks strongly suggests theyformed in a back-arc basin by melting of anasthenosphere source contaminated by sub-ducted slab-derived products.

(4) Alkaline volcanic series with OIB-typegeochemical features are found above theophiolite sequence in each of the studiedareas. Hot-spot related magmatism may haveled to the formation of Oceanic island(s) oreven Oceanic plateau(s), with significantcrustal thickening dated to the Albian–Ceno-manian (114–95 Ma) like in many other partsof the world.

(5) These alkaline series are also locally overlainby calc-alkaline volcanic series, which werelikely formed in a supra-subduction zoneenvironment. Further evidence of this subduc-tion is provided by blueschists series dated at95–90 Ma. Therefore Plateau formation andvolcanic arc formation shortly pre-dated theobduction, which occurred in the Conia-cian–Santonian (88–83 Ma). The obductionwas followed by final collision of the SABwith the Eurasian margin at c. 73–71 Ma.

This work was supported by the Middle East BasinsEvolution project jointly supported by a consortiumincluding oil companies and the CNRS. Many thanks tothe MEBE programme coordinators Eric Barrier andMaurizio Gaetani for their support and encouragements.Special thanks to Marie-Francoise Brunet for her

Fig. 14. (Continued) Geodynamic reconstruction of the ophiolite formation from the Lower Jurassic to the UpperCretaceous periods. (1) Formation of the ophiolite in a back-arc setting between the SAB and the Active Eurasianmargin; (2) Formation of OIB-type series above a hot-spot in the late Early Cretaceous; (3) Obduction of the ophioliteinitiated directly after hot-spot magmatism in the early Late Cretaceous leading (4) to its emplacement above the SABfrom the Early Coniacian to Santonian times (5). The final blocking of the north-dipping subduction below the Eurasianmargin, and insight of the collision at 73–71 Ma is constrained by the study of blueschists in the Stepanavan area.


inalterable patience and Analytical data were acquiredwith the help of the Geosciences Azur Laboratory, inwhich we thank M. Fornari and G. Feraud for their invol-vement during data acquisition. We also thank the supportof the French Embassy at Yerevan for the MAE PhD grantgranted to G. Galoyan. This paper was improved bydetailed reviews undertaken by R. Hebert and P. Agard.

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