Implications for the Obduction Process

Lithos 112 (2009) 163–187

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Jurassic back-arc and Cretaceous hot-spot series In the Armenian ophiolites — Implications for the obduction process

Yann Rolland a,⁎, Ghazar Galoyan a,b, Delphine Bosch c, Marc Sosson a, Michel Corsini a, Michel Fornari a, Chrystèle Verati a a Géosciences Azur, Université de Nice Sophia Antipolis, CNRS, IRD, Parc Valrose, 06108 Nice cedex 2, France b Institute of Geological Sciences, National Academy of Sciences of Armenia, 24a Baghramian avenue, Yerevan, 375019, Armenia c Géosciences Montpellier, CNRS UMR-5243, Université de Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France article info abstract

Article history: The identification of a large OIB-type volcanic sequence on top of an obducted nappe in the Lesser Caucaus of Received 2 July 2008 Armenia helps us explain the obduction processes in the Caucasus region that are related to dramatic change Accepted 16 February 2009 in the global tectonics of the Tethyan region in the late Lower Cretaceous. The ophiolitic nappe preserves Available online 10 March 2009 three distinct magmatic series, obducted in a single tectonic slice over the South Armenian Block during the Coniacian–Santonian (88–83 Ma), the same time as the Oman ophiolite. Similar geological, petrological, Keywords: geochemical and age features for various Armenian ophiolitic massifs (Sevan, Stepanavan, and Vedi) argue Nd–Sr–Pb isotopes for the presence of a single large obducted ophiolite unit. The ophiolite, shows evidence for a slow-spreading Armenian ophiolite Back-arc oceanic environment in Lower to Middle Jurassic. Serpentinites, gabbros and plagiogranites were exhumed Obduction by normal faults, and covered by radiolarites. Few pillow-lava flows have infilled the rift grabens. Oceanic plateau The ophiolite lavas have hybrid geochemical composition intermediate between Arc and MORB signatures: 143 144 87 86 Tethys (La/Yb)N =0.6–0.9; (Nb/Th)N =0.17–0.57; ( Nd/ Nd)i = 0.51273–0.51291; ( Sr/ Sr)i =0.70370– Lesser Caucasus 207 204 208 204 206 204 0.70565; ( Pb/ Pb)i =15.4587–15.5411; ( Pb/ Pb)i =37.4053–38.2336; ( Pb/ Pb)i =17.9195– 18.4594. These compositions suggest they were probably formed in a back-arc basin by melting of a shallow asthenosphere source contaminated by a deeper mantle source modified by subducted slab-derived products. 87Sr/86Sr ratios and petrological evidence show that these lavas have been intensely altered by mid-oceanic hydrothermalism as well as by serpentinites, which are interpreted as exhumed mantle peridotites.

The gabbros have almost the same geochemical composition as related pillow-lavas: (La/Yb)N =0.2–2.3; 143 144 87 86 207 204 (Nb/Th)N =0.1–2.8; ( Nd/ Nd)i =0.51264–0.51276; ( Sr/ Sr)i =0.70386–0.70557; ( Pb/ Pb)i = 208 204 206 204 15.4888–15.5391; ( Pb/ Pb)i =37.2729–37.8713; ( Pb/ Pb)i =17.6296–17.9683. Plagiogranites show major and trace element features similar to other Neo-Tethyan plagiogranites (La/Yb)N =1.10–7.92; (Nb/ 143 144 Th)N =0.10–0.94; but display a less radiogenic Nd isotopic composition than basalts [( Nd/ Nd)i = 87 86 0.51263] and more radiogenic ( Sr/ Sr)i ratios. This oceanic crust sequence is covered by variable thicknesses of unaltered pillowed OIB alkaline lavas emplaced in marine conditions. 40Ar/39Ar dating of a single-grain amphibole phenocryst provides a Lower Cretaceous age of 117.3±0.9 Ma, which confirms a distinct formation age of the OIB lavas. The geochemical composition of these alkaline lavas is similar to plateau-lavas [(La/ 143 144 87 86 Yb)N =6–14; (Nb/Th)N =0.23–0.76; ( Nd/ Nd)i =0.51262–0.51271; ( Sr/ Sr)i =0.70338–0.70551; 207 204 208 204 206 204 ( Pb/ Pb)i =15.5439–15.6158; ( Pb/ Pb)i =38.3724–39.3623; ( Pb/ Pb)i =18.4024–19.6744]. They have significantly more radiogenic lead isotopic compositions than ophiolitic rocks, and fit the geochemical compositions of hot-spot derived lavas mixed with various proportions of oceanic mantle. In

addition, this oceanic+plateau sequence is covered by Upper Cretaceous calc-alkaline lavas: (La/Yb)N =2.07– 144 143 87 86 2.31; (Nb/Th)N =0.08–0.15; ( Nd/ Nd)i =0.51271–0.51282; ( Sr/ Sr)i =0.70452–0.70478), which were likely formed in a supra-subduction zone environment. During the late Lower to early Upper Cretaceous period, hot-spot related magmatism related to plateau events may have led to signiﬁcant crustal thickening in various zones of the Middle-eastern Neotethys. These processes have likely hindered subduction of some of the hot and thickened oceanic crust segments, and allowed them to be obducted over small continental blocks such as the South Armenian Block. © 2009 Elsevier B.V. All rights reserved.

⁎ Corresponding author. Tel.: +33 4 92 07 65 86. E-mail address: [email protected] (Y. Rolland).

1. Introduction density (Cloos, 1993; Abbot and Mooney, 1995). The emplacement of oceanic plateaus has a great inﬂuence either on the slab dip, but also The role of Oceanic Plateaus in the obduction processes of oceanic on the cessation of subduction and on the onset of obduction as is crust has still not been clearly established. We understand that their proposed for the Ontong–Java plateau (Petterson et al., 1997). The larger crustal thickness and buoyancy as compared to ‘standard’ ability of an oceanic plateau to resist subduction and eventually be oceanic crust does not allow them to subduct, in particular when they transported onto continental crust depends on both crustal thickness reach subduction zones soon after their formation (e.g., Ben-Avraham and plateau age (Kerr and Mahoney, 2007). The older a plateau, the et al., 1981; Cloos, 1993; Abbot and Mooney, 1995; Abbot et al., 1997; cooler and thus the less buoyant it will be. Alternative hypotheses for Kerr and Mahoney, 2007). However, the reasons for oceanic crust obduction involve rapid inversion of tectonic plate motions and rapid obduction onto continental margins are still debated: (i) is obduction continental convergence (e.g., Agard et al., 2007). Obduction is driven by subduction of continental crust? Or (ii) does it result from ascribed to the presence of young oceanic crust in the hanging-wall the intrinsic nature of the oceanic crust? In the ﬁrst case, ophiolites are of the subduction zone, as a result of subduction initiation at the Mid- obducted due to the mechanical coupling of continental crust with the Oceanic Ridge (e.g., Boudier et al., 1988; Nicolas, 1989); or to scalping dense subducting slab (e.g., O'Brien et al., 2001; Guillot et al., 2003). of oceanic lithosphere (e.g., Agard et al., 2007 and references therein). Continental subduction may be facilitated by the thinned margins of The case of Armenian ophiolites (Lesser Caucasus) is peculiar as continental domains following earlier phases of divergence rifting recent investigations (Galoyan et al., 2007, 2009; Rolland et al., in that precede oceanic crust emplacement (Guillot and Allemand, press) have shown the presence of slow-spreading ophiolites in 2002). In the second case, a lower density of oceanic lithosphere several locations. Further, the ophiolites were tectonically transported might result from intra-oceanic hot-spot and magmatic arc events, above the South Armenian Bloc or SAB (Zakariadze et al., 1983). which will lead to crustal thickening and a decrease in lithosphere Although some blueschists are locally found, these affect oceanic

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. / Lithos 112 (2009) 163–187 165 crust-derived rocks which underwent intra-oceanic subduction and windows correlate with each other and be part of a unique obducted exhumation within accretionary prisms (Rolland et al., 2009). In nappe. Tectonic transport of this nappe onto the SAB can be dated to the contrast, the underthrusted Armenian continental crust appears not Coniacian–Santonian (88–83 Ma; Sokolov, 1977; Sosson et al., in press). to have been metamorphosed by any subduction event. Therefore, the Finally, the influence of oceanic plateau event in oceanic lithosphere obduction of the Armenian ophiolites might be explained by the rheology and its role in the obduction process is discussed. intrinsic nature of the oceanic crust. However, the slow-spreading nature of the ophiolites, and in particular the fact that exhumed 2. Geological setting mantle forms a large part of the reconstructed ophiolite is rather in agreement with a relatively dense oceanic lithosphere. During the Mesozoic, the Southern Margin of the Eurasian In this paper, we report new geochemical data, including major and continent has been featured by closure of the Palaeo-Tethys and trace elements and Nd, Sr, Pb isotopes, on magmatic series from several opening of the Neo-Tethys Ocean (e. g.; Sengör and Yilmaz, 1981; Armenian ophiolites (i.e. Stepanavan (NW Armenia), Sevan (N Tirrul et al., 1983; Ricou et al., 1985; Dercourt et al., 1986; Stampfli and Armenia), Vedi (central Armenia); Fig. 2). We identify three superposed Borel, 2002, Fig. 1). Later on, subductions, obductions, micro-plate levels of lavas corresponding to three distinct environments: (1) back- accretions, ranging mostly from the Cretaceous to the Eocene, and arc, (2) ‘OIB’-like and (3) arc. Moreover, we suggest that these ophiolite finally continent–continent collision have occurred between Eurasia

Fig. 2. Sketch geological map of Armenia, with location of the studied area: 1 — Stepanavan area; 2 — Sevan area; 3 — Vedi area. 166 Y. Rolland et al. / Lithos 112 (2009) 163–187 and Arabia. The study of Armenian ophiolites allows us to unravel part tions ranging from lherzolites to harzburgites and dunites (e.g., of this complex history. The ophiolites are located in the northern part Melikyan et al.,1967; Harutyunyan,1967; Palandjyan,1971; Abovyan, of the Lesser Caucasus region (Fig. 1). The Lesser Caucasus lies south of 1981; Ghazaryan, 1987; Zakariadze et al., 1990). Undeformed the Great Caucasus range, between the Black and Caspian seas. Here, ultramafics have intrusive cross-cutting relationships and bear a the ophiolitic belt separates the SAB from the active Eurasian margin. cumulative magmatic origin, shown by poikilitic texture of olivine The SAB is correlated westwards to the Taurides-Anatolide blocks, inclusions within large enstatite crystals (up to 10–15 mm; which were separated from Gondwana in the Early Mesozoic (Lower- Middle Jurassic) and accreted to the Eurasian margin in the Late Palandjyan, 1971). We have observed similar textures, together Mesozoic periods (Upper Cretaceous). The Gondwanian nature of the with cumulative strata, contained in magmatic pods cross-cutting SAB is shown by the Proterozoic age of the basement crust and the age serpentinites in the Stepanavan area (Galoyan et al., 2007). These and lithologies of the overlying sedimentary series (Aghamalyan, latter serpentinites are strongly deformed and altered. The ductile 2004; Sosson et al., in press). The active Eurasian margin is formed by character of deformation is in agreement with a mantle origin for a thick volcanic arc sequence formed above an active margin, resting these rocks. on a Paleozoic (Caledonian to Hercynian) crystalline basement (iv) Radiolarites are found interlayered or disconformably overlying (Adamia et al., 1981). The ophiolites are situated in three geographic the various lithologies described earlier. The fact that they overlie zones (Fig. 2): gabbros, plagiogranites and serpentinites shows that these rocks were (1) The Stepanavan ophiolite situated in NW Armenia. uplifted and denuded by normal faults. Radiolarite datings under- (2) The Sevan ophiolite located in NNE Armenia. taken in the different ophiolites all agree with oceanic accretion in the (3) The Vedi zone, disposed in a more southerly position, in the Middle–Upper Jurassic (Danelian et al., 2007, 2008). centre of Armenia. The two first ophiolites are interpreted to correlate with each other The ophiolitic sequences are weakly deformed with anchizonal along the Sevan–Akera suture zone at the northern rim of the SAB, and metamorphism. Only some outcrops show evidence of small shear at the southern edge of the European active continental margin zones ascribed to the ophiolite obduction in the Coniacian–Santonian (Knipper, 1975; Adamia et al., 1980). The Vedi ophiolite is diversely (Sokolov, 1977; Zakariadze et al., 1983). HP metamorphism is interpreted as being an obducted sequence above the SAB (Knipper described in the Stepanavan region (Fig. 2), where blueschists outcrop and Sokolov, 1977; Zakariadze et al., 1983), or within a suture zone in small km2-size tectonic windows below the ophiolite. Timing of correlating with Central Iran and Alborz ophiolites (Sokolov, 1977; metamorphism from radiometric 40Ar/39Ar phengite datings indicates Adamia et al., 1981). It is generally believed that the different ophiolite a HP metamorphic peak at ca 95 Ma, and MP–MT retrogression at 73– locations may represent suture zones, and thus feature several paleo- 71 Ma (Rolland et al., 2009). subduction zones (Aslanyan and Satian, 1977; Knipper and Khain, The ophiolite series are locally overlain by (1) alkaline lavas, which 1980; Adamia et al., 1981; Aslanyan and Satian, 1982). have a Lower Cretaceous age, though with very large error bars, A companion paper written on the geology of the Sevan ophiolite ranging from 120 to 95±20 Ma, (Baghdasaryan et al.,1988; Satian and has already put up in details the lithologies and radiometric age of this Sarkisyan, 2006); and (2) Upper Cretaceous andesites and detrital ophiolite (Galoyan et al., 2009). Main features are summarized below; series (Dali valley; Stepanavan; Galoyan et al., 2007). The alkaline lavas these include: are alternatively interpreted as (1) intra-continental rifting (Satian et al., 2005) in the Vedi area, and (2) plume-derived Ocean Island (i) A high level of fractional crystallisation in the series, with magmatism above the oceanic crust before the obduction (Galoyan cumulate olivine gabbros and two pyroxene gabbros overlain and et al., 2007, 2009). The calc-alkaline series are ascribed to intra-oceanic intruded by amphibole-bearing gabbros and more differentiated arc emplacement above this oceanic crust sequence and implies the melts (diorites to plagiogranites). These melts are maximally presence of a subduction zone between the ophiolite and the SAB, differentiated and are generally emplaced in ductile extensive featured by the Stepanavan blueschists (Rolland et al., 2009). These shear zones cross-cutting the gabbros. This complete differentia- two magmatic sequences closely predate the ophiolite obduction onto tion series suggests small percent partial melts and long-lived the SAB during the Coniacian–Santonian (Sokolov, 1977). cooling as is proposed to occur in slow spreading ophiolite settings (Lagabrielle et al., 1984; Lagabrielle and Cannat, 1990). Absolute 3. Analytical methods radiometric datings indicate oceanic crust emplacement in the Mineral compositions were determined by electron probe micro- Middle Jurassic, constrained at 165–160 Ma by zircon U–Pb age of analysis (EMP). The analyses are presented in Figs. 4 and 6. They were one tonalite (160±4 Ma; Zakariadze et al., 1990) and by 40Ar/39Ar carried out using a Cameca Camebax SX100 electron microprobe at amphibole age on gabbro (165.3±1.7 Ma; Galoyan et al., 2009). 15 kV and 1 nA beam current, at the Blaise Pascal University (ii) Rare pillow lavas are found, with compositions ranging from (Clermont-Ferrand, France). Natural samples were used as standards. tholeiitic basalts to andesites. The density of the feeding dyke For 40Ar/39Ar dating of the alkaline suite, fresh amphibole grains swarms is reduced, as rare dolerite dykes have been found were separated from the Vedi ophiolite unaltered sample AR-05-104. crosscutting the series. The slight calc-alkaline composition is also Geochronology of amphiboles was performed by laser 40Ar/39Ar evidenced by Nb–Ta negative anomalies, which agree with some dating. Results are presented in Table 1 and Fig. 7. Amphibole crystals slab-derived contamination. These geochemical features support were separated under a binocular microscope. The samples were then slow spreading in a back-arc setting. irradiated in the nuclear reactor at McMaster University in Hamilton (Canada), in position 5c, along with Hb3gr hornblende neutron (iii) Peridotites are frequent and often exhumed as a result of intra- fluence monitor, for which an age of 1072 Ma is adopted (Turner et al., oceanic extension. They are generally serpentinized, and witness 1971). The total neutron flux density during irradiation was 9.0×1018 further hydrothermal alteration when exhumed at the contact with neutron cm− 2. The estimated error bar on the corresponding 40Ar⁎/ marine water (‘listwenites’). The mineralogical nature of the mantle- 39 ArK ratio is ±0.2% (1σ) in the volume where the samples were set. derived ultramafic rocks is still difficult to assess. The previous Three amphibole grains (~500 μm in diameter) were chosen for petrographical investigations on the serpentinized ultramafics analysis by the laser UV spectrometer in Géosciences Azur laboratory suggest that protoliths were mantle-derived with various composi- at the Nice University. Analyses were done by step heating with a Y. Rolland et al. / Lithos 112 (2009) 163–187 167

Table 1 Summary of amphibole 40Ar/39Ar dating results from the trachybasalt samples AR-05-104 and AR-05-70.

39 37 39 40 39 Step Laser power (mW) Atmospheric cont (%) Ar (%) ArCa/ ArK Ar⁎/ ArK Age (Ma±1σ) Amphibole AR-05-104, J=44.55, plateau age: 117.3±0.9 Ma (92.4% 39Ar), isochron age: 117.5±0.8 Ma (MSWD: 0.78) 1 400 99.99 0.26 8.84 ––± – 2 500 95.85 0.88 9.90 4.18 132.1 ± 41.2 3 550 92.07 0.46 2.24 1.19 38.5 ± 27.3 4 650 24.82 6.00 4.15 3.48 110.6 ± 3.5 5 718 8.79 6.40 5.02 3.65 115.8 ± 1.9 6 750 4.17 22.09 5.46 3.64 115.6 ± 1.3 7 800 0.00 9.24 5.54 3.78 119.7 ± 1.6 8 1111 0.81 54.68 5.75 3.70 117.5 ± 0.5

Amphibole AR-05-70 (1), J=4.57, plateau age: –, isochron age: 107.8±18 Ma (MSWD: 0.66) 1 340 99.99 4.44 1.44 ––± – 2 460 96.12 25.09 3.92 2.41 15 ± 17 3 560 87.34 52.41 3.08 6.94 42 ± 7 4 640 70.78 12.68 7.08 11.05 67 ± 16 5 640 58.83 2.15 46.80 29.02 171 ± 87 6 1111 62.59 3.23 8.09 38.33 223 ± 55

Amphibole AR-05-70 (2), J=35.20, plateau age: –, isochron age: 114.5±37 Ma (MSWD: 15) 1 380 144.64 6.23 2.35 ––± – 2 500 113.84 17.22 1.31 ––± – 3 772 92.47 16.80 1.40 0.90 37 ± 36 4 1093 50.17 53.82 4.52 4.18 166 ± 10 5 1190 123.34 0.71 194.72 ––± – 6 4000 86.24 5.22 30.61 16.57 584 ± 187

50 W CO2 Synrad 48–5 continuous laser beam. Measurement of 4.1.1. Description of the ophiolitic units isotopic ratios was done with a VG3600 mass spectrometer, equipped In Stepanavan (Fig. 3A, B), ophiolite sections exhibit abundant with a Daly detector system; see detailed procedures in Jourdan et al. serpentinites, cross-cut by normal fault and shear zones in which (2004). The typical blank values for extraction and purification of the gabbro-norites, gabbros and plagiogranites are intrusive and laser system are in the range 4.2–8.75, 1.2–3.9, and 2–6cm3 STP for deformed (see Galoyan et al., 2007, 2009 for details and cross- masses 40, 39 and 36, respectively. The mass-discrimination was sections). Laterally, thick layers of pillow basalts are observed which monitored by analyzing air pipette volume at regular intervals. Decay interlayer and are covered by radiolarites. On top of the ophiolite constants are those of Steiger and Jäger (1977). Uncertainties in section, a thin layer of alkaline lava flows is found. Above, these lavas apparent ages in Table 1 are given at the 1σ level and do not include are eroded and unconformably covered by Upper Cretaceous 40 39 the error on the Ar⁎/ Ark ratio of the monitor. conglomerates and limestones, and calc-alkaline pillow basalts or Thirty-seven samples of magmatic rocks from the Sevan, Stepana- graywackes. The ophiolite sequence is thrusted over a blueschist facies van and Vedi ophiolites have been analyzed for major and trace metamorphic sole. elements including Rare Earth Elements (REE; Table 2). Samples were In the Sevan area, sections are extremely variable laterally (Fig. 3C–E; analyzed at the C.R.P.G. (Nancy, France). Analytical procedures and see Galoyan et al., 2009 for details and cross-sections). Pillow lavas are analyses of standards can be found on the following website (http:// rare, and serpentinites were frequently exhumed. Intense hydrothermal www.crpg.cnrs-nancy.fr/SARM). alteration (‘listwenites’) has transformed the uppermost part of For isotope measurements, powdered samples were weighed to exhumed serpentinites. The feeding doleritic dyke swarm is extremely obtain approximately 100 to 200 ng of Sr, Nd and Pb. A leaching step with scarce. Large intrusive pods of amphibole-bearing gabbros and 6N HCl during 30 min at 65 °C was done before acid digestion. After plagiogranites are also exhumed and covered by radiolarites. Normal leaching, residues have been rinsed three times in purified milli-Q H2O. faults are observed, and are interpreted as the cause of such lateral Sr, Nd and Pb blanks for the total procedures were less than 50 pg, 15 pg variations, by vertical uplifting of footwall sections, and local infilling of and 30 pg, respectively. Lead isotopes were measured by multi-collector axial rift valleys, following the model of slow-spreading ophiolite (e.g., inductively-coupled plasma mass spectrometry (MC-ICP-MS; VG Plasma Lagabrielle et al., 1984; Lagabrielle and Cannat, 1990). Locally, thick 54) at the Ecole Normale Supérieure in Lyon (ENSL). Details about isotope sequences of alkaline pillow lavas are observed. The ophiolite is locally chemical separations and analytical measurements including reproduci- eroded, and unconformably covered by basal conglomerates and soils, bility, accuracy and standards, can be found in Bosch et al. (2008) and and an Upper Cretaceous section of limestones comprising graywackes references therein. The Nd and Sr isotopic data were measured on a interlayers. Finnigan MAT261 multicollector mass spectrometer at the Geochemical In the Vedi area, the ophiolite section is much thinner (Fig. 3F–H, Laboratory, Paul Sabatier University of Toulouse. 87Sr/86Sr was normal- see Galoyan et al., 2009 for details and cross-sections). The basal ised to 8.3752, NBS standard was measured to 0.710250 (±15). 143Nd/ tectonic contact is exposed, with the top oriented to the south sense of 144Nd ratio was normalised to a value of 146Nd/144Nd of 0.71219; measure shear. At the base, the ophiolite rests on a serpentinite layer by a of Rennes standard was 0.511965 (±12). tectonic contact. The ophiolite is intensely sheared above the basal contact with boudins of tholeiitic basalts (Fig. 3H). Laterally, the 4. Results ophiolite consists mainly of gabbros (Fig. 3G) or serpentinites, which suggests a similar lithology as in the Sevan and Stepanavan areas. 4.1. Field relationships However, the different parts of the ophiolite are dismembered and displaced from each other as a result of obduction deformation. Above Synthetic logs are drawn on Fig. 3, showing the lithological the ophiolites, layers of radiolarites are found below a very thick associations and the structural relationships in each of the three section of alkaline pillow lavas (Fig. 3H). This section is of variable studied ophiolitic zones. thickness depending on the location. This may be due to lateral 168

Table 2 Representative whole-rock analyses of samples from the Sevan, Stepanavan and Vedi areas, major oxides are in wt.%, and trace elements and REE in ppm.

Groups Sevan ophiolitic series Sevan Alkaline series Stepanavan ophiolitic series No. Flaser Olivine Olivine Gabbro Gabbro- Hornblende Diorite Diorite Plagiogranite Diabase Trachy- Basaltic Andesite Basanite Trachybasalt Trachyandesite Websterite Hornblende Hornblende gabbro gabbro gabbro norite gabbro andesite tracyandesite gabbro gabbro Sample AR-03-25 AR-05-86 G150 AR-03-39 AR-03-24 AR-03-10 AR-04-218 AR-03-23 AR-03-19 AR-03-02 G154 AR-03-17 AR-03-34 G142 AR-05-80 AR-03-33 AR-04-03 AR-04-16 AR-04-45D

SiO2 45.36 48.09 48.39 49.49 50.60 50.68 55.09 57.41 74.91 46.02 53.70 54.27 55.48 40.63 43.80 51.57 53.24 47.30 53.77 Al2O3 13.32 16.72 15.63 14.11 7.20 18.17 13.45 14.10 12.32 16.29 14.09 15.16 14.13 14.40 17.58 14.34 1.03 14.39 14.00 Fe2O3 14.91 5.94 6.44 11.59 7.77 9.09 8.51 8.84 3.58 8.39 11.35 12.36 12.45 11.70 9.46 6.06 6.02 12.90 8.92 MnO 0.14 0.11 0.12 0.18 0.17 0.16 0.15 0.14 0.03 0.13 0.15 0.19 0.13 0.29 0.11 0.11 0.15 0.21 0.15 MgO 7.44 10.51 10.48 6.79 15.29 6.75 9.84 2.24 0.42 7.73 4.52 3.74 4.07 4.15 6.70 0.77 23.18 9.11 7.81 CaO 10.89 14.1 16.65 9.38 17.65 9.26 8.76 4.92 3.05 10.68 3.64 4.38 5.49 11.40 4.95 9.78 16.52 10.14 6.98 Na2O 3.15 1.65 1.16 3.52 0.48 3.21 2.59 6.36 4.20 3.53 6.07 6.63 3.96 3.64 4.09 6.34 0.12 2.93 3.34 K2O 0.22 ––0.29 – 0.15 0.17 0.12 0.31 0.37 –– 0.61 1.42 2.24 0.56 – 0.19 2.42 TiO2 2.55 0.27 0.29 1.32 0.20 0.36 0.24 0.87 0.21 1.26 1.36 1.33 1.17 2.06 1.68 1.98 0.05 1.18 0.16 P2O5 0.31 0.02 0.04 0.14 0.06 0.05 0.04 0.16 0.04 0.17 0.12 0.15 0.11 0.48 0.44 1.08 0.03 0.07 0.05 LOI 1.46 2.8 0.65 2.92 0.79 1.21 1.87 4.33 0.57 4.61 4.85 1.78 2.18 10.01 8.89 6.65 0.54 1.76 2.44 Total 99.8 100.2 99.8 99.7 100.0 99.1 100.7 99.5 99.6 99.2 99.9 100 99.8 100.2 99.9 99.3 100.9 100.2 100.1 Mg# 52.1 79.3 77.9 56.1 81.1 61.6 71.6 35.6 20.2 66.6 46.2 39.5 41.6 43.4 60.5 21.7 90.0 60.4 65.6 163 (2009) 112 Lithos / al. et Rolland Y. Rb 2.67 0.48 – 3.05 – 0.84 1.42 1.81 2.22 13.97 –– 5.05 31.89 44.83 7.93 – 1.12 30.17 Sr 231.7 102 101 189.5 58.2 303.7 207.4 212.5 145.2 630.9 28.76 49.82 102.5 147.0 330.8 341.5 11.79 125.4 213.4 Y 61.81 6.41 7.24 30.05 7.84 11.93 7.82 25.22 27.65 23.93 27.82 29.88 27.49 25.58 17.37 52.91 1.05 20.91 5.79 Zr 175.9 5.28 4.85 75.20 5.24 22.89 21.08 75.07 71.64 127.4 81.43 85.60 54.39 153.4 131.4 411.2 – 42.74 21.26 Nb 4.51 0.08 – 1.55 – 0.50 0.32 1.83 2.15 2.95 1.77 1.44 1.69 40.63 17.95 49.22 – 1.01 2.14 Ba 36.77 4.1 – 120.8 14.14 34.36 34.9 55.59 65.83 285.3 16.09 19.01 16.65 166.7 299.0 168.7 3.71 32.71 228.1 Hf 4.25 0.22 0.21 2.12 0.2 0.81 0.76 2.27 2.48 2.91 2.36 2.38 1.63 3.57 2.86 9.03 – 1.21 0.63 Ta 0.35 0.01 – 0.11 – 0.04 0.03 0.14 0.10 0.24 0.14 0.12 0.12 2.99 1.19 3.80 – 0.08 0.21 Pb –––––– – 2.62 1.32 1.51 1.61 ––4.98 4.63 4.45 –– 3.61 Th 0.08 ––0.29 – 0.14 0.07 0.75 1.09 1.01 0.55 0.37 0.33 4.13 4.06 5.15 – 0.18 1.27 U0.11––0.09 – 0.07 0.04 0.25 0.62 0.29 0.35 0.27 0.14 1.43 0.94 4.61 – 0.05 0.43 V 440.7 138 190 319.9 195.9 222.2 158.1 122.6 50.8 179.3 334.9 321.8 347.4 257.4 271.7 286.1 135.1 324.7 94.47 Cr 35.43 802 412 94.50 810.3 104.3 562.7 136.9 1464 277.0 –– 251.7 33.86 21.68 162.0 2804 236.4 324.2

Co 26.81 40.2 41.7 34.77 41.88 33.71 39.6 15.76 5.52 38.26 29.48 24.33 27.09 38.26 31.44 24.06 55.82 51.46 31.5 – 187 Ni 77.98 188 131 32.35 136.2 29.23 148.1 10.09 37.19 54.43 8.95 5.14 16.09 34.29 28.76 13.83 361.3 78.0 101.7 Cu 18.16 102 111 52.59 142.3 44.81 15.63 21.02 6.91 58.20 102.8 15.61 5.23 61.86 54.48 18.34 340.3 – 189.9 Zn 58.57 28.3 29.6 86.18 47.98 71.99 73.35 50.45 9.98 66.89 67.20 80.0 19.68 100.1 91.55 92.74 25.68 60.28 60.33 La 14.89 0.33 0.36 3.39 0.28 1.60 1.44 4.62 5.38 7.07 4.25 3.91 2.69 32.41 29.46 48.54 – 2.40 3.06 Ce 34.75 0.94 1.02 9.57 0.99 4.51 3.73 11.73 12.66 18.32 11.14 10.93 7.17 64.63 59.03 107.1 0.15 6.37 6.28 Pr 5.057 0.18 0.19 1.57 0.22 0.74 0.57 1.82 1.78 2.71 1.67 1.81 1.15 7.64 6.92 13.38 0.02 1.08 0.65 Nd 24.09 1.23 1.28 8.27 1.40 3.80 2.87 9.05 8.36 12.68 8.76 9.35 6.17 29.89 27.11 56.55 0.15 5.76 2.30 Sm 7.38 0.58 0.61 2.93 0.67 1.34 0.94 2.91 2.68 3.53 2.93 3.26 2.32 6.02 5.21 12.98 0.08 2.09 0.49 Eu 3.11 0.34 0.32 1.07 0.28 0.48 0.35 0.97 0.67 1.36 1.14 1.15 0.79 1.97 1.63 4.14 0.03 0.96 0.19 Gd 9.32 0.87 0.98 4.01 1.07 1.70 1.13 3.68 3.50 3.95 4.04 4.18 3.36 5.58 4.45 12.43 0.13 2.98 0.55 Tb 1.60 0.16 0.18 0.71 0.20 0.30 0.20 0.69 0.64 0.67 0.71 0.75 0.62 0.82 0.62 1.84 0.03 0.53 0.10 Dy 10.47 1.14 1.24 4.89 1.36 1.92 1.31 4.60 4.33 4.21 4.78 4.99 4.34 4.69 3.42 10.27 0.18 3.53 0.81 Ho 2.22 0.24 0.26 1.03 0.29 0.41 0.28 0.97 0.94 0.84 1.01 1.06 0.95 0.90 0.61 1.87 0.04 0.74 0.19 Er 6.37 0.65 0.74 3.02 0.83 1.22 0.82 2.95 2.94 2.45 2.99 3.11 2.82 2.48 1.64 4.80 0.12 2.15 0.64 Tm 0.94 0.10 0.11 0.46 0.13 0.19 0.13 0.47 0.46 0.36 0.46 0.47 0.44 0.35 0.22 0.64 0.02 0.32 0.12 Yb 6.28 0.65 0.72 3.03 0.86 1.32 0.92 3.25 3.30 2.39 3.16 3.22 2.98 2.33 1.44 4.05 0.12 2.10 0.91 Lu 0.95 0.09 0.11 0.48 0.13 0.21 0.15 0.52 0.53 0.37 0.51 0.51 0.47 0.36 0.22 0.60 0.02 0.32 0.17 Eu/Eu⁎ 1.15 1.45 1.28 0.95 1.01 0.97 1.03 0.91 0.67 1.12 1.01 0.95 0.86 1.04 1.03 1.0 0.85 1.17 1.12 (La/Sm)N 1.27 0.36 0.37 0.73 0.26 0.75 0.97 1.00 1.26 1.26 0.91 0.75 0.73 3.39 3.56 2.35 0.0 0.72 3.94 (La/Yb)N 1.60 0.34 0.34 0.76 0.22 0.82 1.06 0.96 1.10 2.00 0.91 0.82 0.61 9.39 13.83 8.09 0.0 0.77 2.27 Table 2 (continued )

Groups Stepanavan ophiolitic series Stepanavan alkaline series Stepanavan calk-alkaline Vedi ophiolitic series Vedi Alkaline series series No. Plagiogranite Basaltic trachy- Basaltic trachy- Basaltic trachy- Basaltic Diabase Olivine Basaltic Basaltic Hornblende Diorite Plagiogranite Basalt Basaltic Basalt Trachybasalt Basaltic Trachydacite andesite andesite andesite trachy-andesite basalt trachy- trachy- gabrro andesite trachyandesite andesite andesite Sample AR-04-44 AR-04-20 AR-04-30 AR-06-02 AR-03-53 AR-04- AR-04- AR-04- AR-04- AR-05-113 AR-05- AR-05-111 AR-05- AR-05- AR-05- AR-05-104 AR-05-102 AR-04-75 05 32 40A 31 110 114 106 78

SiO2 75.35 51.53 48.55 45.37 48.54 50.19 49.15 49.79 52.20 45.09 58.57 70.45 47.5 48.3 44.58 44.64 50.39 59.61 Al2O3 12.20 14.69 13.29 14.27 15.01 13.91 18.53 15.80 17.05 21.24 16.03 14.69 16.17 15.03 12.52 15.41 16.2 17.48 Fe2O3 2.71 14.81 8.67 13.52 12.65 13.73 10.19 8.82 9.28 4.32 6.66 4.44 8.76 10.14 9.36 11.99 7.76 7.64 MnO 0.03 0.23 0.15 0.32 0.27 0.24 0.16 0.15 0.16 0.07 0.11 0.08 0.15 0.16 0.12 0.14 0.13 0.12 MgO 0.77 4.15 6.86 6.22 4.25 3.27 5.25 3.54 3.59 7.88 5.18 1.06 8.46 5.97 2.63 4.85 5.05 1.11 CaO 2.05 4.86 10.49 4.09 5.33 5.85 8.25 9.12 6.56 14.29 7.08 3.87 8.57 8.01 15.53 7.85 7.16 1.89 Na2O 5.03 5.74 4.74 1.53 3.93 5.11 4.36 3.54 4.61 2.12 3.94 4.47 3.99 3.96 3.83 4.24 3.24 6.37 K2O – 0.18 0.24 5.37 2.69 0.42 0.52 1.24 1.00 0.17 0.47 0.2 0.72 0.18 – 0.96 1.96 2.4 TiO2 0.11 1.62 1.08 3.12 2.64 3.39 0.86 1.07 0.94 0.16 0.33 0.43 0.93 1.2 2.35 3.67 2.36 0.72 P2O5 0.02 0.13 0.11 1.31 1.08 0.67 0.14 0.20 0.18 – 0.04 0.09 0.09 0.12 0.33 0.85 0.64 0.25 LOI 1.07 1.89 6.01 5.11 3.16 2.94 3.12 7.27 5.36 5.08 2.08 1.04 4.78 7.14 9.17 5.04 5.07 2.14 Total 99.3 99.8 100.2 100.2 99.5 99.7 100.5 100.6 100.9 100.4 100.5 100.8 100.1 100.2 100.4 99.6 99.9 99.7

Mg# 38.1 37.7 63.1 49.8 42.3 34.0 52.7 48.4 44.5 79.7 62.7 34 67.6 56.0 37.8 46.6 58.4 23.16 163 (2009) 112 Lithos / al. et Rolland Y. Rb 0.58 1.4 7.94 45.3 33.17 7.62 9.61 18.12 18.45 2.41 4.12 1.11 3.51 4.14 0.63 10.48 27.1 64.89 Sr 91.04 61.01 95.7 157 322.6 198.8 520.3 303.8 282.3 492.7 254 161 134.6 110.1 153.5 926 643 260.4 Y 1.23 35.91 26.52 40.6 44.41 51.24 16.0 24.4 24.19 3.70 10.1 13.1 20.78 26.91 24.94 36.26 22.4 58.18 Zr 6.48 86.0 68.43 268 294.4 373.5 44.4 99.18 95.3 4.30 42.3 86.5 54.9 73.98 160.8 318.8 260 680.5 Nb 0.35 1.62 1.9 52.8 57.95 42.33 2.14 2.29 3.32 0.08 0.49 0.59 0.69 2.46 23.22 67.52 43.3 82.24 Ba 20.58 19.58 21.73 608 578.3 156.6 133.9 239.1 213.6 131.6 57.5 33.5 141.7 16.2 1097 444.9 659 422 Hf 0.15 2.54 1.83 5.97 6.51 8.01 1.25 2.69 2.61 0.16 1.3 2.35 1.48 2.04 3.91 6.81 6.03 14.76 Ta – 0.13 0.15 3.97 4.20 3.24 0.17 0.18 0.26 – 0.04 0.05 0.07 0.20 1.76 4.88 3.29 5.99 Pb – 2.29 1.93 7.04 2.54 2.24 7.22 3.42 5.66 – 1.1 1.12 ––1.53 1.25 6.71 4.30 Th 0.02 0.43 0.19 5.39 5.98 4.65 0.72 1.46 1.67 – 0.42 0.48 0.15 0.22 2.085 4.60 8.39 12.28 U 0.01 0.12 0.09 1.34 1.46 1.20 0.19 0.67 0.58 – 0.12 0.13 0.06 0.10 0.643 1.18 1.8 2.78 V 30.74 459.8 305.4 172 94.35 201.6 241.5 279.1 263.7 89.5 185 44.5 211.3 238.3 240.7 219.8 135 5.26 Cr 421.5 99.23 316.7 – 25.77 – 21.05 31.91 73.83 785.2 123 9.1 416.7 324.7 50.2 4.22 136 66.9

Co 7.71 38.85 42.98 26.3 16.75 31.93 29.51 29.05 27.36 30.98 22.8 6.6 42.44 49.08 28.9 31.4 64.3 4.98 – 187 Ni 24.86 22.76 109.1 6.5 ––15.55 22.38 18.95 130.7 42.4 6.4 195.5 122.6 26.28 21.81 126 – Cu 189.5 64.58 132.8 20.4 9.48 14.94 12.6 188.1 170.9 92.4 18.3 – 14.05 81.78 28.41 50.82 42.2 9.93 Zn 23.13 130.6 81.13 177 137.1 152.7 150.9 86.53 100.0 24.06 50.7 39 65.52 94.28 106.8 146.8 134 167.9 La 2.42 4.23 2.53 50.3 50.59 40.02 4.93 7.87 8.69 0.231 2.09 2.91 1.96 3.07 18.44 49.35 50.7 74.8 Ce 3.90 11.08 7.37 96.1 107.0 85.12 11.46 18.51 18.05 0.71 4.84 6.15 6.25 8.51 39.11 109.7 91.8 142.6 Pr 0.42 1.88 1.31 12.1 12.97 10.85 1.74 2.68 2.53 0.14 0.72 0.81 1.09 1.47 5.02 14.15 9.83 15.84 Nd 1.58 9.88 7.04 53.3 53.32 45.27 8.35 12.58 11.65 0.78 3.74 4.48 5.9 7.97 21.85 59.18 42.9 59.79 Sm 0.29 3.44 2.55 11.2 11.26 10.35 2.36 3.47 3.15 0.36 1.20 1.45 2.15 2.88 5.50 12.52 8.47 12.64 Eu 0.35 1.28 0.99 3.87 4.08 3.39 0.94 1.13 1.04 0.24 0.42 0.69 0.90 1.12 1.93 4.15 2.67 3.68 Gd 0.24 4.69 3.49 10.5 10.41 10.3 2.60 3.89 3.57 0.53 1.46 1.81 3.0 4.02 5.72 10.96 7.36 11.69 Tb 0.04 0.87 0.65 1.50 1.52 1.63 0.44 0.65 0.61 0.10 0.25 0.32 0.53 0.70 0.85 1.48 0.99 1.877 Dy 0.22 5.93 4.34 8.11 8.69 9.50 2.80 4.09 3.96 0.64 1.65 2.11 3.48 4.67 4.85 7.84 5.01 10.97 Ho 0.05 1.28 0.93 1.49 1.57 1.80 0.57 0.85 0.83 0.14 0.34 0.46 0.74 0.97 0.88 1.32 0.81 2.08 Er 0.15 3.79 2.75 3.75 4.16 4.93 1.62 2.46 2.46 0.38 1.01 1.35 2.08 2.78 2.30 3.26 1.92 5.77 Tm 0.03 0.58 0.42 0.51 0.57 0.70 0.24 0.38 0.38 0.06 0.16 0.21 0.32 0.42 0.31 0.42 0.25 0.86 Yb 0.21 3.89 2.84 3.18 3.64 4.51 1.60 2.54 2.54 0.35 1.14 1.52 2.15 2.83 1.95 2.49 1.46 5.84 Lu 0.04 0.61 0.44 0.45 0.56 0.69 0.25 0.39 0.40 0.06 0.18 0.25 0.33 0.45 0.29 0.35 0.22 0.88 Eu/Eu⁎ 4.05 0.98 1.02 1.09 1.15 1.0 1.15 0.94 0.95 1.64 0.97 1.31 1.08 1.0 1.05 1.08 1.03 0.93 (La/Sm)N 5.25 0.77 0.63 2.83 2.83 2.43 1.31 1.43 1.74 0.40 1.10 1.26 0.57 0.67 2.11 2.48 3.77 3.73 (La/Yb)N 7.92 0.73 0.60 10.68 9.38 5.99 2.07 2.09 2.31 0.44 1.24 1.29 0.62 0.73 6.39 13.39 23.44 8.64 169 170 .Rlade l ihs12(09 163 (2009) 112 Lithos / al. et Rolland Y. – 187

Fig. 3. Representative geological logs of the Stepanavan, Sevan and Vedi ophiolites. Y. Rolland et al. / Lithos 112 (2009) 163–187 171 variations in the amount of erupted lavas, or may be explained by tectonic scalping of the ophiolite upper part during obduction. These alkaline lava ﬂows are well exposed and preserved in the Vedi ophiolite. They are made of amphibole-bearing basaltic pillows. The pillows are larger (metre scale) than the ophiolites ones (several decimetre scale), and interlayer with thin pink limestones. At the front of the obduction in the Vedi zone, an olistolith formation exhibits conglomerates and slided blocks in a muddy matrix (Fig. 3F). The olistostrom age is Coniacian– Santonian (nanofossils, Carla Muller, com. pers.), and it connects progressively below and above with Lower and Upper Coniacian reef limestones, respectively. Therefore, the obduction age can be bracketed to the Coniacian–Santonian, which agrees with former estimates (Sokolov, 1977). Laterally, always in the Vedi zone, the upper part of the ophiolite is made of kilometre scale slided blocks, mainly comprised of alkaline pillow basalts and calc-schists. These blocks slide on a greenish mudstone rock, probably originated from the ophiolite Fig. 4. Chemistry of Cr-spinel from Armenia with respect to the compositional ﬁelds of alteration. The Upper Coniacian uncomformity is variably marked by Abyssal peridotites (1; Brynzia and Wood, 1990) and arc-related peridotites (Mariana conglomerates, marls and reef limestones. seamount peridotites (2) and dunites (3); Parkinson and Pearce, 1998).

4.1.2. General features of the Armenian ophiolites: evidence for LOT features As emphasized in Galoyan et al. (2009) in the Sevan area, and by Gabbros are the most abundant rocks in the crustal complex, and Galoyan et al. (2007) in the Stepanavan area, the lithologies found in are found in each ophiolite zone (Galoyan et al., 2007, 2009). Their all the exposed Armenian ophiolites are in good agreement with the petrography evolves from cumulate-banded olivine gabbros in their hypothesis of a slowly expanding spreading centre, as described for lower part towards more leucocratic plagioclase-rich gabbros in the the western Alps ophiolites and exposed earlier in Section 2 (Nicolas upper part (Abovyan, 1981; Ghazaryan, 1987, 1994). The cumulative and Jackson, 1972; Nicolas, 1989). banded olivine gabbros and websterites are found locally, only in the The similar lithological and age features found in the several Sevan and Stepanavan areas, while more leucocratic gabbros are Armenian ophiolites suggest that they were part of the same oceanic widespread in the three zones. crust section. This has to be confirmed by the comparison of Olivine gabbros found in Stepanavan and Sevan ophiolites (Galoyan et geochemical data from each zone. The presence of three magmatic al., 2007, 2009) are fresh, massive, and fine- to medium-grained (0.5 to series: ophiolite s.s. (tholeiitic), ‘OIB’ (alkaline) and arc (calc-alkaline) 2 mm). They have cumulate, ophitic textures and consist of plagioclase in the same structural position (from bottom to top, respectively) has (~60–65%; An68–74,An80–89), olivine (~5–10%; Fo72–76), and clinopyrox- been evidenced in the three zones. Isotopic geochemistry on the three ene (~25–35%). Clinopyroxene is of augite (Wo39–44En45–48Fs11–13) series will allow us to constrain the nature of sources and to identify and diopside (Wo45En44Fs11) types. Some enstatite orthopyroxenes the magmatic processes that existed prior to ophiolite obduction. (Wo2En75Fs23) are also found rimming olivine porphyrocrysts. Websterites found in Stepanavan and Sevan ophiolites (Galoyan 4.2. Petrography and mineral chemistry et al., 2007, 2009) have a granular texture with large 2–8mm porphyrocrysts of orthopyroxene (30–70%), clinopyroxene (70–30%) The field and microscopic analyses of Armenian ophiolite mag- and olivine grains (0–35%; Fig. 5A). Orthopyroxenes are enstatite-rich matic rocks show a continuous magmatic succession from ultramafic (Wo1–5En59–84Fs11–37) and clinopyroxenes are augites (Wo35–42En36– cumulates (wherlites, websterites) to gabbros and plagiogranites, 40Fs15–19), olivine is relatively rich in forsterite (Fo84–88). Gabbronor- cross-cutting intensely altered serpentinites in each of the three ites (from Stepanavan; Galoyan et al., 2007) have a gabbroic texture, studied ophiolites. All these lithologies were exhumed in the footwall with plagioclase (10–60%, 1–3 mm), clino- and ortho-pyroxene. below normal faults and covered by pillow-basalts. Plagioclase is of bytownite type (An80–85), while orthopyroxenes (1– 4 mm) are enstatites (Wo2–5En59–61Fs34–37), and clinopyroxenes are 4.2.1. Serpentinites augites (Wo35–42En36–40Fs15–19). The study of serpentinite mineralogy is difficult due to intense Mesocratic to leucocratic gabbros of the upper section found in the serpentinization. However, EMP analysis of chromiferous spinel relicts three ophiolites (Galoyan et al., 2007, 2009; Rolland et al., in press)are from Sevan Tsapatagh area (sample AR-05-80 in Galoyan et al., 2009; massive, fine- to medium-grained and have gabbroic (or gabbro-ophitic), Fig. 3)reflects still unaltered mineral compositions. EMP analyses xenomorphic granular texture (0.5–4 mm), with plagioclase (~40–65%; show a very narrow compositional range in Cr# (Cr/Cr+Al=0.71– An50–75, An72–93), clinopyroxene (8–45%; augite) and hornblende (0– 0.73) and Mg# (Mg/Mg+Fe=0.58–0.59) ratios (Fig. 4). These Cr# 40%), without any olivine. Accessory minerals (1–10%) are apatite, compositions are more elevated than those of abyssal peridotites titanomagnetite, ilmenite and rarely quartz. The hornblende-rich gabbros (Brynzia and Wood, 1990) and agree with a fore-arc peridotites (Galoyan et al., 2009; Rolland et al., in press) have coarse granular composition (Parkinson and Pearce, 1998). However, Mg# values are textures (Fig. 5B), with ~50–65% euhedral to subhedral plagioclase slightly lower than those of Parkinson and Pearce (1998), which is (An54–58)and(~35–50%) anhedral to subhedral amphibole. Some brown ascribed to high partial melting in such context. However, we cannot Ti-rich euhedral hornblende is presumed to be a primary mineral; while a exclude any hydrothermal process, as the primary nature of chromites Ti-poor subhedral to xenomorphic green magnesio-hornblende (Leake et is uncertain. al., 1997) is thought to be a secondary phase formed by hydrothermal alteration as it replaces generally the brown type. The augite

4.2.2. Ophiolite plutonic rocks (Wo40–42En39–47Fs11–14), diopside (Wo45–48En40–44Fs8–15), and enstatite Wehrlites are found in the Stepanavan ophiolite (see Galoyan et al., (Wo2En57Fs41) relicts (5–10%) are found in the crystals of magnesio- 2007). They have a poikilitic texture showing numerous clinopyrox- hornblendes that replace the pyroxenes. However, it is not related to ene crystals with diopside composition (Wo45–47En48–50Fs2–4), shear zones and fractures, and is thus a late magmatic mineral. In included in large olivine Fo87–88 (N60–65%) porphyric grains. leucocratic gabbros (Galoyan et al., 2009), the clinopyroxene (augite 172 Y. Rolland et al. / Lithos 112 (2009) 163–187

Wo40–41En33–35Fs18–19) content does not exceed 25%. Normal zoning is are composed of plagioclase and hornblende, and are mainly altered observed in plagioclase (from An85 to An60), which is frequently into chlorite, carbonate, sericite, albite, quartz, actinolite, etc. altered. Clinopyroxenes have alkaline to slightly tholeiitic compositions Diorites occur as small intrusive bodies within the gabbro units in (0.8bNa+ Cab0.9; Leterrier et al., 1982, Fig. 6). Pegmatitic gabbros Sevan and Vedi zones (Palandjyan, 1971; Abovyan, 1981; Ghazaryan, crosscutting the plutonic sequence (Vedi zone; Rolland et al., accepted) 1987, 1994). They have a porphyritic to subhedral granular (1–4 mm) Y. Rolland et al. / Lithos 112 (2009) 163–187 173

Fig. 6. Chemical compositions of studied clinopyroxenes plotted in the Ti vs. (Na+Ca) diagram of Leterrier et al. (1982). Note that a majority of data plot in the Alkaline compositional ﬁeld, and a minority is in the Toleiitic part.

texture and have relatively similar hornblende contents (5–30%) as albitized plagioclase and/or plagioclase–clinopyroxene microlites, Ti- gabbros. Plagioclase (~65–70%) is albite-rich (An34–38) and accessory magnetite and hematite microlites, in a devitrified (calcite+chlorite) minerals (quartz, opaque oxides) are rare. Amphibole grains are groundmass (Fig. 5D). magnesio-hornblendes in composition, sometimes rimmed by actinolite fringes and epidote aggregates. In Sevan zone, diorites grade 4.2.4. Alkaline lavas into quartz-diorites (quartz 5–10%), laterally and upwards in the The alkaline basalts are found in the three zones on top of the series. ophiolite section as large massive pillow-lavas or as diabase dykes, but Plagiogranites are found in the three zones (Galoyan et al., 2007, their relationships with the ophiolite (s.s.) pillow lavas remain unclear. 2009; Rolland et al., in press). They appear to be dioritic intrusives The first group of alkaline rocks displays large vesicles (0.5–3mm),filled most differentiated components, forming diffuse segregations or with carbonates and rarely chlorites, and have both phyric and aphyric discontinuous networks of veins. Plagiogranites have local coarse (Fig. 5E). They have intersertal textures, with plagioclase megacrysts pegmatic, or hypidiomorphic to xenomorph granular (0.5–4 mm) (~5%; 0.5–2 mm), microliths and opaque minerals (3–10%), surrounded textures. They are formed by 40–65% plagioclase (An15–30), 25–45% by a calcite–chlorite mesostase. The second group (Vedi and Stepanavan quartz, minor biotite (b5%), ortho-amphibole (b5%; Stepanavan), K- zones, e.g., samples AR-05-70 and AR-05-104 dated by 40Ar/39Ar) have feldspar (0–10%, microcline; Fig. 5C) and accessory phases (titano- doleritic (Fig. 5F) to ophitic textures. They are mainly composed of magnetite, hematite, sphene and apatite). plagioclase (~40–55%; 1–3 mm), clinopyroxene (10–30%; 1–4mm), amphibole (~25%; 1–3 mm) and accessory Ti-magnetite (N5–10%), 4.2.3. Ophiolite volcanic and subvolcanic rocks apatite (~3%; prismatic, acicular, 0.5–1.5 mm) and rarely biotite. Apatites Diabases are present in several locations (Sevan and Stepanavan are present in the plagioclase crystals and in the vitreous interstices, areas; Galoyan et al., 2007, 2009) as isolated dikes, crosscutting the which are filled by carbonates or carbonates-chlorites. The tabular layered gabbros. They are generally altered (chlorite, epidote, carbo- plagioclase laths show a transitional zoning with bytownite to labrador nates) and have a subdoleritic texture composed of plagioclase (60–65%; (An72–60) or labrador to andesine (An55–32) compositions. Thin rims of An65–75)andtwoclinopyroxenes(augiteWo41–44En44–47Fs11–13 and pure albite (Ab — 98%) are also present. Clinopyroxenes are generally diopside Wo45En37Fs18). chloritized, but still preserve diopside compositions (Wo49En35Fs16). The volcanic rocks of the Armenian ophiolites we studied are The amphibole is a kaersutite (Leake et al., 1997), with zoning from present as pillowed and massive lava flows and pillowed breccias. In kaersutite to ferro-kaersutite from core to rim. Some samples show general, they show signs of hydrothermal alteration but relict igneous abundant calcite-filled veins and pockets. textures are preserved. In the three locations basalts and basaltic Afewdacitic sills and dikes occur among the basaltic pillow lava andesites are vesicular (1–5 mm, filled with carbonate-calcite, chlorite flows in the Vedi valley. As in the pillow basalts, plagioclase is the and quartz) and largely aphyric (intersertal, spilitic, microdoleritic main mineral phase and Fe-oxides are present (~5–10%; Fig. 5G). and variolitic, up to 1.5–2 mm in diameter), composed mainly of Some 1–2 mm large unzoned plagioclase phenocrysts of oligoclase-

Fig. 5. Microphotograph of representative magmatic rock types from the Armenian ophiolite complex. Plutonic and volcanic ophiolite series: (A) subautomorph granular texture of a cumulate banded websterite (sample AR-04-36, Stepanavan area, Cheqnagh valley; see Galoyanetal.,2007); (B) coarse-grained hornblende gabbro with normally zoned plagioclases (sample AR-05- 110, Vedi area, massif of Qarakert, see Rolland et al., in press-b); (C) xenomorph granular texture of a microcline (Mc) bearing plagioclase rich leucogranite (sample AR-05-109, in the same massif, see Rolland et al., accepted); (D) aphyric, intersertal (spilitic) and variolitic basalt composed of mainly albitized plagioclase, Ti-magnetite and hematite microlites, in a devitrified groundmass (sample AR-05-106, Vedi area, Khosrov valley, see Rolland et al., accepted). Alkaline series: (E) aphyric, intersertal basalt, totally devoid of phenocrysts, and composed of carbonatized plagioclase microlites and opaque minerals (~5%) in a chlorite–carbonate groundmass (sample AR-05-80, Sevan area, Tsapatagh valley, see Galoyan et al., 2009); (F) doleritic texture in a trachybasalt composed of plagioclase, chloritized clinopyroxene, kaersutite (Krs), Ti-magnetite and apatite (sample AR-05-104, Vedi area, see Rolland et al., accepted); (G) phyric trachydacite with a hyalopilitic to cryptocrystalline texture (sample AR-04-75, Vedi valley, see Rolland et al., accepted). Calc-alkaline series: olivine-bearing, plagioclase phyric (15–40%) basalt with a microcrystalline (plagioclase, quartz, opaque minerals) texture from pillow lavas suit (sample AR-04-32, Stepanavan area, Herher valley, see Galoyan et al., 2007), in which the olivine phenocrysts are entirely pseudomorphosed to quartz and rims of iron oxides. From (A) to (C) under crossed nichols, and (D) to (H) under parallel nichols. Scale bar is for all photographs. 174 Y. Rolland et al. / Lithos 112 (2009) 163–187 andesine compositions are distributed in the fine-grained devitrified groundmass made of albitic plagioclase, opaque microlites, and carbonate-quartz-chlorite aggregates.

4.2.5. Calc-alkaline lavas of Stepanavan zone They consist of large pillow-lavas of basaltic and basaltic andesitic compositions with micro-cryptocrystalline (Fig. 5H) to intersertal textures formed of large phenocrysts (2–7 mm) and microliths of andesine- oligoclase plagioclase, and minor augite (Wo36–38En42–43Fs13–15) clinopyroxenes. These lavas overlie Upper Cretaceous limestones, unconformably lying on the ophiolite stricto sensu.

4.3. 40Ar/39Ar dating

Complementary to previously published datings (Galoyan et al., 2009) obtained from the ophiolite, which span the Middle Jurassic, we provide here the first unambiguous dating of the alkaline suite. In the Stepanavan and Sevan regions, the alkaline lavas have been deformed and altered in the late collisional evolution, so that preserved and unaltered amphibole-bearing lavas were only sampled in the Vedi area. Three analyses have been done on amphibole single grains from two trachybasalt samples (AR-05-70 and AR-05-104) from the Vedi ophiolite, which is described in Section 4.2 though only one is considered fully successful. These datings are listed in Table 1 and the successful one is presented in Fig. 7. In the two datings for sample AR-05-70 (Table 1), the 39Ar content was very low, so no plateau age can be calculated. However, from the 36Ar/40Ar versus 39Ar/40Ar plots, it was possible to estimate isochron ages, with large errors, of 108±18 Ma (MSWD: 0.66) and 115±37 Ma (MSWD: 15). The very low 39Ar content is interpreted as a consequence of the low K content of amphibole, which likely resulted from pyroxene destabilisation in this sample. In the dating of sample AR-05-104, a well-constrained plateau of 117.3±0.9 Ma (2σ) was obtained, with 92% of released 39Ar (Fig. 7A). 37 39 The average ArCa/ ArK ratio is similar as the EPM value of the amphibole from ~40 in low temperature steps, decreasing steadily to ~30 in high temperature steps (Fig. 7B). An isochron age of 117.5± 0.8 Ma (MSWD: 0.77) is obtained using the five steps of the plateau age estimate (steps 3–7), with an initial 40Ar/36Ar ratio close to the 40 36 atmospheric value [( Ar/ Ar)0 =238±4%; Fig. 7C]. Including the step 1 of lower temperature we calculate a similar within-error isochron age of 117.5±0.8 Ma (2 σ). The above age of 117.5±0.8 Ma is more precise than the previous ages determined by the whole-rock K–Ar method (Baghdasaryan et al., 1988) which ranged between 114 and 97 Ma. We interpret these younger K–Ar ages as resulting from alteration of the vitreous matrix. These ~118 Ma Albian ages are also in agreement with age estimates undertaken by Satian and Sarkisyan (2006) who provided 40 39 whole-rock Rb/Sr errorchron ages between 120 and 95 Ma. The Fig. 7. Ar/ Ar amphibole dating results of trachybasalt sample AR-05-104 from the Vedi alkaline suite. Alkaline sequence is thus undoubtedly younger than the ophiolite by about 50 Ma, unlike some generally admitted views that all the alkaline, calc-alkaline and tholeiitic series part of the obducted sequence were formed in the same mid-oceanic context (Sokolov (Fig. 8A). These magmatic rocks plot in a large domain comprised 1977; Knipper and Khain, 1980). between alkaline and tholeiitic tendencies of the TAS diagram (Le Maitre et al., 1989). In the AFM diagram (Fig. 8B) most rocks lie close 4.4. Major-trace-REE geochemistry to the limit between the tholeiitic and calc-alkaline fields.

The geochemical analyses of the ophiolitic rocks from the Sevan 1. Overall, the rocks of the ophiolitic suite are enriched in MgO and ophiolite are of relatively alkaline composition in comparison to more depleted in TiO2,K2O and P2O5 relative to the alkaline suite MORB. Major element data of pillow — lavas and plutonic rocks show (Figs. 8–10; Table 1). Compared to the plutonic rocks of the same that they have predominantly basalt to trachybasalt compositions. series, the volcanic rocks from the different areas plot in the same compositional range (from basalts to andesites and trachyande-

4.4.1. Major elements sites) and are slightly Na2O richer. Major element analysis of plutonic rocks ranges from gabbros to 2. The alkaline lavas from different zones plot in the same range, granites (plagiogranites) with intermediate dioritic compositions varying compositionally from basanite-trachybasalt to basaltic Y. Rolland et al. / Lithos 112 (2009) 163–187 175

Fig. 8. Plots of magmatic rocks (ophiolitic, alkaline and calc-alkaline series) in the (A) (Na2O+K2O) vs. SiO2 (Le Maitre et al., 1989) and (B) AFM (Irvine and Baragar, 1971) diagrams.

trachyandesite and trachyandesite, and are clearly in the calc- 1. Basalts and gabbros of the ophiolite suite show strong enrichments alkaline/alkaline domain of the AFM diagram (Fig. 8A, B). One of in LILE (Large Ion Lithophile Elements: Ba, Rb, K and Th), up to ten the most signiﬁcant features of the alkaline lavas is their higher times MORB values. They have negative anomalies in Nb–Ta and Ti

TiO2,K2O and P2O5 contents. (Fig. 11A, B), which is generally indicative of volcanic island arc 3 The arc-type calk-alkaline lavas, have trachybasalt and basaltic environments (e.g. Taylor and McLennan 1985; Plank and Lang- trachyandesite compositions in TAS diagram (Fig. 8A). They occupy muir, 1998). However, trace element contents remain low relative a transitional position between ophiolitic and alkaline domains in to volcanic arc lavas, which indicates a setting with little fractional

Harker's diagram (Fig. 9), except lower TiO2 and higher Al2O3, crystallization, in agreement with a back-arc setting (e.g., Galoyan which depend on the abundance of plagioclase in such rocks. et al., 2009). 2. Overall, the concentrations of each element in the alkaline basalts Regarding the spread of compositional variations in major largely exceed the concentrations in the basalts from ophiolitic elements within the series, it appears that only rough correlations series (Fig. 11C). Moreover, alkaline series basalts are characterized can be seen in the plots of SiO vs. other oxides (Fig. 9). Even the most 2 by high abundances of LILE, high field strength elements (Nb, Ta, Zr immobile elements during alteration processes, such as Al O , MgO 2 3 and Ti) and light rare-earth elements (LREE). and TiO (e.g., Staudigel et al.,1996) do not show any clear correlations 2 3 The calc-alkaline suite rocks show strong depletions in Nb and Ta, (Fig. 9). In particular, Large Ion Lithophile Elements (LILE) such as Na relative to Th and La, and slight Ti negative anomaly (Fig. 11D). They and K have scattered compositions, even in individual magmatic globally show slightly stronger enrichments in LREE and LILE than suites. Such variations are ascribed to a combination of alteration and the ophiolite suite rocks. magmatic processes. The occurrence of a long-lasting hydrothermal event, ascribed to the slow-spreading oceanic environment is These differences in normalized element patterns support that indicated by scattered 40Ar/39Ar ages within individual gabbro these basalts are not petrogenetically related and were most likely samples (Galoyan et al., 2009; Rolland et al., in press). Thus, we derived from melts formed in different tectonic settings: (1) A back- ascribe the most important part of the elemental variability in the arc setting with slow-spreading rates, (2) Ocean-island within-plate ophiolitic suite (gabbros, diorites, plagiogranites and ophiolitic lavas) setting and (3) volcanic island arc. to spilitization process in an oceanic environment while variations in Differences within the different suites can be related to magmatic the alkaline and calc-alkaline volcanic rocks is ascribed to some processes such as fractional crystallization and magma mixing or to magmatic cause (variations in the source components, as is high- alteration processes. To analyse the importance of alteration, some lighted by the isotopic compositions, Section 4.5). trace and two major elements are plotted versus Zr and Th (Fig. 10). Zr Thin section observations and previous studies of the Armenian is an incompatible element (for basaltic to andesitic lavas) well- ophiolites (e.g., Palandjyan, 1971; Abovyan, 1981; Ghazaryan, 1994) known to remain stable during alteration or weathering processes, so have shown that the whole ophiolitic sequence apart from the it was used as reference to test the mobility of the other trace elements alkaline lavas has been affected by oceanic low-temperature (Fig. 10). The trace element composition of both ophiolitic series and hydrothermal alteration events. These processes induced modifica- arc-type lavas plots in a restricted range of values for Zr (0–127 ppm), tion of the whole-rock chemistry, as revealed by the increase of LOI while alkaline lavas with higher Zr are characterized by a large (Table 1). compositional range (131–411 ppm; Table 2). This compositional spread is ascribed to various levels of fractional crystallization with 4.4.2. Trace elements one trachydacite sample having very high Zr content (681 ppm). High field strength elements (HFSE) are not mobilized during The concentration of zirconium normally increases in response to alteration and their contents reflect, without ambiguity, those of their magmatic processes such as fractional crystallisation, except for the parental magma (Staudigel et al., 1996). Contents in these trace most differentiated lavas in which it has fractionated. Enrichment in Zr elements confirm the presence of three clearly distinct magmatic is positively correlated with that of major elements as Ti, but there is suites, as defined in the previous section. no clear correlation with SiO2,whichareascribedtoaslight 176 Y. Rolland et al. / Lithos 112 (2009) 163–187

Fig. 9. Harker variation diagrams showing the compositions of the three (ophiolitic, alkaline and calc-alkaline) series.

fractionation of Si within each series. Compositional variations in SiO2 some slight depletions in LREE and a slight enrichment in MREE and TiO2 in alkaline lavas correlate well with variations in Zr (Fig. 10), (Fig. 11E, F). No extensive Eu anomalies were observed (Eu/ while traces and REE such as Nd and Th contents show slight positive Eu⁎=0.95–1.15), which show that plagioclase has remained almost correlation with Zr contents, which are ascribed to fractionation of stagnant, and is enriched in the final liquid. The concentration of REE these elements. Such process is also shown by the composition of for volcanic rocks varies from 8 to 30 times chondrite and for gabbros ultramafic cumulative plutonic rocks (websterite from Stepanavan varies between 1 and 15 times chondrite, for exception a flaser gabbro and gabbronorite from Sevan), which have lower trace element — 60 times (sample AR-03-25). These features are interpreted as a contents than associated lavas due to their cumulative origin. In result of extreme crystal fractionation involving plagioclase, clinopyr- contrast, Sr does not correlate well with Zr, and part of scattering of Sr oxene, orthopyroxene and, to a lesser extent, to olivine accumulation data may be due to its mobility due to alteration processes of (Pallister and Knight, 1981). plagioclase. This is confirmed by a similar mobility of Ca, as shown in The websterite and gabbronorite have the lowest concentrations of the Ti vs. Ca diagram (Fig. 9). REE (0.1–0.9 and 1–5 times chondrite respectively) with patterns characterized by depletion in LREE (Fig. 11F). One hornblende gabbro 4.4.3. REE geochemistry (sample AR-04-45D from Stepanavan ophiolite) is characterized by

In the chondrite-normalized rare earth element (REE) diagrams, LREE enrichment ((La/Yb)N =2.27) and some depletion in MREE (a analysed ophiolite basalts and gabbros have ﬂat and parallel REE convex downward pattern) with smaller positive Eu anomalies (Eu/ ⁎ spectra in chondrite-normalized plots [(La/Yb)N =0.6–0.9], showing Eu =1.12). Y. Rolland et al. / Lithos 112 (2009) 163–187 177

The diorites REE patterns (6–20 times chondrite) and plagiogra- Chondrite-normalized REE patterns of calc-alkaline lavas are nites are parallel to those of the gabbros, with smaller enrichment in strongly parallel and form a narrow domain (Fig. 11H). They have

LREE ((La/Yb)N =1.1). The most differentiated plagiogranite (sample similar HREE contents as volcanics of previous series with signiﬁcantly AR-04-44 from Stepanavan) is characterized by more depletion in the more depleted LREE contents than alkaline series rocks [(La/Yb)N = middle to heavy REE compared to other plagiogranites, and strongly 2.1–2.3]. positive Eu anomalies (Eu/Eu⁎=4.05) ascribed to high plagioclase These differences in trace elements contents between the three contents. Such features indicate a cumulative effect of plagioclase. studied series further support that these basalts are petrogenetically In contrast, chondrite-normalized REE patterns of alkaline lavas unrelated and, most likely derived from melts formed in different (Fig. 11G) show huge LREE enrichments and HREE depletions [(La/ tectonic settings.

Yb)N =6–14], being representative of intraplate continental basalts, as compared to ophiolite lavas. Meanwhile, no extensive Eu anomalies are 4.5. Nd, Sr, Pb isotope geochemistry observed (Eu/Eu⁎=0.95–1.15). One trachydacite sample (AR-04-75) is featured by signiﬁcant enrichments in trace elements, which is 4.5.1. Ophiolite series explained by a high degree of fractional crystallization, as its REE Initial ɛNdi values of the ophiolitic lavas from the different studied pattern is parallel to those of the basanite–trachyandesite series. zones range from +5.9 to +9.5 (Table 3) intermediate between

Fig. 10. Major and trace elements vs. Zr and Th diagrams. Major and trace elements are chosen to investigate the effects of alteration on the Sr, Nd and Pb isotopic systems; explanations in the text. 178 Y. Rolland et al. / Lithos 112 (2009) 163–187

The initial Pb isotopic ratios in ophiolitic rocks range from 37.273 to 38.234 for 208Pb/204Pb, from 15.459 to 15.541 for 207Pb/204Pb and from 17.630 to 18.459 for 206Pb/204Pb. In the Pb–Pb isotope diagrams both ophiolitic volcanic and plutonic rocks plot on or close the MORB domain (Fig. 12B).

4.5.2. Alkaline series Overall, alkaline lavas show, in comparison to ophiolite rocks,

lower initial ɛNdi values ranging from +2.1 to +4.0 but have a similar 87 86 143 144 87 86 range of ( Sr/ Sr)i ratios. In ( Nd/ Nd)i vs. ( Sr/ Sr)i diagram (Fig. 12A) most of these rocks are located in the OIB ﬁeld. In the 208 204 206 204 207 204 206 204 ( Pb/ Pb)i vs. ( Pb/ Pb)i, and ( Pb/ Pb)i vs. ( Pb/ Pb)i diagrams these samples overlap various speciﬁc OIB provinces such as Kerguelen, Samoa and Society and Marquises (Fig. 12B).

4.5.3. Calc-alkaline series

For the calc-alkaline lavas from Stepanavan, the initial ɛNdi values 87 86 and the ( Sr/ Sr)i ratios range from +3.8 to +5.9 and from 0.7045 to 0.7048, respectively. Thus ɛNdi ratios appear to be intermediate between those of ophiolitic and alkaline rocks, and Sr isotopic ratios plot in the same range as these two series (Fig. 12A).

5. Discussion

Ophiolites of the Armenian Lesser Caucasus region are generally separated into three distinct zones: (1) The Sevan-Akera zone in the North (Knipper, 1975; Adamia et al., 1980), (2) The Zangezur zone in the centre (Aslanyan and Satian, 1977; Knipper and Khain, 1980; Adamia et al., 1981) and (3) The Vedi zone in the south (Knipper and Sokolov, 1977; Zakariadze et al., 1983). Due to the importance of Cenozoic volcanism spread over most of Armenia (Fig. 2), it is still difficult to conclude only from geological mapping whether the different ophiolites correlate with each other, or if they represent various suture zones delimitating several continental micro-blocks. For this reason, we have undertaken field investigations in various ophiolites: Stepanavan (Galoyan et al., 2007) and Sevan (Galoyan et al., 2009), along the northern rim of Armenia; and the Vedi ophiolite, in the centre of Armenia. As emphasized in the following discussion, the use of Nd, Sr and Pb isotopes in complement to conventional major and trace element data allow us to correlate the ophiolites with each other. These ophiolites show some similarities and differences in their structure and lithological successions, but these features remain compatible with a single oceanic domain origin. This domain opened in the Lower-Middle Jurassic and has undergone several phases of magmatic emplacement, for which we find evidence in each of the geographic zones investigated. These correlations provide insight into the evolution of the Tethyan domain, and in particular allow us Fig. 10 (continued). to propose a geodynamical model for the obduction of the ophiolite over the Armenian block. In the following discussion, we will evaluate the following points: typical MORBs and OIBs values and indicate a source region that experienced long-term depletion in LREE (Fig. 11E). The initial (87Sr/ 1. Petrographically and geochemically, the Armenian ophiolites are 86 Sr)i ratios range from 0.7037 to 0.70565 for these lavas, which are similar to island-arc tholeiites. Such geochemical features are significantly too high for typical tholeiitic MORB lavas and thus, not typical for oceanic crust, formed in a back-arc setting with melting considered as the primary magmatic Sr isotope signature of these of a shallow asthenospheric source contaminated by slab-derived rocks. The gabbros have ɛNdi (+4.3 to +6.5) and initial Sr isotopic fluids (Saunders and Tarney, 1984). Such a hypothesis has already values (0.70386 to 0.70557) in the same range as volcanic rocks. been proposed for ophiolitic gabbros from Turkey (Kocak et al., In the Nd–Sr isotope diagram both ophiolitic volcanic and 2005), but has to be evaluated considering isotopic compositions plutonic rocks exhibit a significant increase of the initial Sr ratios and partial melting constraints. 87 86 relatively to MORB (Fig. 12A). Moreover, this increase of ( Sr/ Sr)i 2. Alkaline lavas of variable thickness have covered this ophiolitic ratios positively correlates with Sr, Ba, Rb and K2O contents. This sequence. Their origin has to be considered. (i) Do they also derive shift towards 87Sr/86Sr radiogenic ratios is commonly attributed to from the same ophiolitic series? (ii) Did they form in an island-arc exchange between rocks and seawater during oceanic crust setting or (iii) in an oceanic island/plateau environment? The hydrothermal alteration (e.g. McCulloch et al., 1981; Kawahata source of alkaline lavas will be discussed below regarding the Sr– et al.,2001). Nd–Pb isotopic data. The occurrence of alkaline magmatism prior Y. Rolland et al. / Lithos 112 (2009) 163–187 179

Fig. 11. Trace and REE plots of the three studied magmatic suites. The multi-element spider diagrams are normalized to the N-MORB values of Sun and McDonough (1989), and REE plots are normalized to the Chondrite values of Evensen et al. (1978). Patterns for the studied magmatic rocks: ophiolitic volcanic (A, E) and plutonic (B, F) series; OIB type alkaline series (C, G), and arc type calk-alkaline series (D, H). 180

Table 3 Sr, Nd and Pb isotopic analyses of samples from ophiolitic complexes of Armenia.

Locality Sevan Stepanavan Vedi age (Ma) 165 165 165 165 165 165 117 117 117 165 117 117 95 95 165 165 165 117 117 117 sample AR-03-25 AR-03-24 AR-03-10 AR-04-218 AR-03-02 G154 G142 AR-05-80 AR-04-44 AR-04-30 AR-03-53 AR-04-05 AR-04-32 AR-04-32 AR-05-113 AR-05-114 AR-05-106 AR-05-104 AR-05-78 AR-04-75 (206 Pb/204 Pb) 18.2577 17.9321 n.d. n.d. n.d. 18.8172 19.1457 18.61 n.d. n.d. 19.2349 19.2791 n.d. n.d. 17.8966 18.0686 18.4101 20.6925 19.2867 n.d. 2σ 0.00049 0.00084 0.00062 0.00046 0.00092 0.00068 0.00051 0.00057 0.00065 0.00051 0.00076 0.00047 (238 U/204 Pb) 11.1328 10.2463 13.7665 18.2699 12.7305 36.8567 34.3464 10.274 5.7363 10.6564 62.4439 26.8011 206 204 ( Pb/ Pb)i 17.9683 17.6658 18.4594 18.8478 18.4024 18.6339 18.719 17.6296 17.9195 18.1073 19.6744 18.8497

(207 Pb/204 Pb) 15.5033 15.5344 n.d. n.d. n.d. 15.559 15.5681 15.5803 n.d. n.d. 15.5901 15.5926 n.d. n.d. 15.5524 15.4661 15.497 15.6653 15.5651 n.d. 163 (2009) 112 Lithos / al. et Rolland Y. 2σ 0.00055 0.00084 0.00061 0.00061 0.00123 0.00061 0.00035 0.00065 0.00064 0.00049 0.0005 0.00055 (235 U/204 Pb) 0.0818 0.0753 0.1011 0.1342 0.0935 0.2707 0.2523 0.0755 0.0421 0.0783 0.4587 0.1968 207 204 ( Pb/ Pb)i 15.4888 15.5211 15.5411 15.5536 15.5702 15.5609 15.5654 15.5391 15.4587 15.4897 15.6158 15.5439 (208 Pb/204 Pb) 37.9444 37.8028 n.d. n.d. n.d. 38.4167 39.1236 38.6686 n.d. n.d. 39.5224 39.6069 n.d. n.d. 37.7996 37.9304 38.1116 40.6747 39.2226 n.d. 2σ 0.0018 0.0025 0.0025 0.002 0.0034 0.002 0.0013 0.0024 0.0017 0.0015 0.0016 0.0021 (232 Th/204 Pb) 8.8411 8.4704 22.1422 54.4859 56.9259 155.9984 138.024 63.7004 63.5101 64.1466 252.2401 89.804 208 204 ( Pb/ Pb)i 37.8713 37.7328 38.2336 38.8401 38.3724 38.7107 38.8887 37.2729 37.4053 37.5546 39.3623 38.7553 143 Nd/144 Nd 0.512957 0.512957 0.512911 0.512848 0.512929 0.512966 0.512754 0.512773 0.51275 0.51315 0.512696 0.51275 0.51282 0.5129 0.512989 0.512965 0.513007 0.512709 0.512813 0.512765 2σ 0.000011 0 0.000044 0.000039 0.000039 0.000038 0.000008 0.000008 3.4E−05 0.00003 0.00001 6E−06 3.7E−05 4E−05 0.000042 0.000007 0.00006 0.00008 0.000011 0.000037 (147 Sm/144 Nd) 0.18522 0.29064 0.21337 1.80575 0.16855 0.20223 0.12176 0.11619 0.11069 0.21883 0.12767 0.13822 0.17125 0.1668 0.22032 0.22032 0.21848 0.1279 0.15218 0.12781 143 144 ( Nd/ Nd)i 0.51276 0.51264 0.51268 0.5109 0.51275 0.51275 0.51267 0.51269 0.51263 0.51291 0.51261 0.51265 0.51271 0.5128 0.51275 0.51273 0.51277 0.51262 0.51271 0.51268 ɛNdi 6.47 4.25 4.98 −29.80 6.27 6.28 3.26 3.7 4.04 9.53 2.05 2.86 3.8 5.87 6.35 5.88 6.74 2.3 4.01 3.39 (87 Sr/86 Sr) 0.704433 0.70401 0.703875 0.703134 0.704589 0.705061 0.70636 0.704072 0.70471 0.706076 0.705421 0.70568 0.70485 0.7048 0.705745 0.70509 0.70591 0.70453 0.704272 0.705331

2σ 0.000011 0.00001 0.000053 0.000048 0.000048 0.000012 0.0001 0.000011 4.6E−05 0.000039 0.000011 9E−06 4.8E−05 4E−05 0.000013 0.00001 0.00001 0.000009 0.00001 0.00005 – 187 (87 Rb/86 Sr) 0.1326 0.0058 0.008 1.2024 0.064 0.5785 0.6276 0.392 0.0184 0.3765 0.2974 0.1109 0.0534 0.1725 0.0754 0.0754 0.1087 0.0327 0.6009 0.7208 87 86 ( Sr/ Sr)i 0.70412 0.70399 0.70386 0.70031 0.70444 0.7037 0.70543 0.70349 0.70466 0.70519 0.70498 0.70551 0.70478 0.7045 0.70557 0.70491 0.70565 0.70448 0.70338 0.70427 ɛSri −2.61 −4.39 −6.39 −56.69 1.88 −8.55 14.97 −12.56 5.12 12.59 8.57 16.15 5.54 1.91 17.92 8.62 19.15 1.47 −14.11 −1.59 Notes: isotopic data (2σ error) are corrected for in situ decay assuming a mean age of 165 Ma of the ophiolite (Galoyan et al., 2009), 117 Ma for the alkaline series (this paper) and 95 Ma for the calc-alkaline series from palaeontogical and Ar-Ar dating (Rolland et al., in press). i: initial ratios calculated at 165, 104 and 95 Ma respectively. εNdi calculated with actual (143Nd/144Nd)CHUR =0.512638 and (147Sm/144Nd)CHUR=0.1967 (Wasserburg et al., 1981). εSri calculated with actual (87Sr/86Sr)CHUR=0.7045 and (87Rb/86Sr) CHUR=0.0827 (Wasserburg et al., 1981). Pb isotopic ratios measured with external precision of ca. 250-300 ppm for the 206, 207, 208Pb/204Pb ratios. Y. Rolland et al. / Lithos 112 (2009) 163–187 181

Fig. 12. Plots of the magmatic rocks in (A) Sr–Nd; (B) 206Pb/204Pb vs. 208Pb/204Pb and 207Pb/204Pb isotopic diagrams, with the ﬁelds of MORB and of various OIB contexts. NHRL, Northern Hemisphere Reference Line (Hart, 1984).

to obduction in the late Lower Cretaceous may be of importance for fore, it is likely that the ophiolitic rocks were produced by melting of a the obduction model of the ophiolite crustal sequence. depleted mantle source contaminated by hydrothermal slab-derived 3. Finally, the calk-alkaline lavas are Upper Cretaceous in age. These fluids in a back-arc basin environment. volcanic arc-related series likely formed during closure of the Neo- To estimate the level of slab-derived contamination in the Tethys ocean. Their geochemical features will be considered to formation of the ophiolitic rocks, mixing curves have been drawn on evaluate this hypothesis. Fig. 13 between different components: depleted mantle pole (MORB), Enriched Mantle 1 and Enriched Mantle 2 [EM1 and EM2, respec- 5.1. Significance of Armenian ophiolites: MOR or back-arc setting? tively; Zindler and Hart (1986), Salters and White (1998) and Hanan et al. (2000)]. This isotopic modelling suggests that basaltic ophiolite Armenian ophiolitic series are shown to be of slight alkaline to lava composition results from contamination of a typical MORB by a tholeiitic character, ranging from basalts to basaltic andesites and basaltic mixed source composed of 1–4% EM2 and 2–5% EM1. Such degrees of trachyandesites. Spider diagrams show clear Nb–Ta negative anomalies contamination appear to be relatively high in a back-arc setting, and (Fig. 11A, B), LILE enrichments and flat to slightly LREE-enriched spectra. suggest the participation of subducted slab sediments in the source Their isotopic compositions are significantly more radiogenic in 87Sr/86Sr (EM1) and a possible fertile E-MORB type source (EM2). and slightly less radiogenic in Nd isotopes than typical MORB compositions (Fig. 12A). These observations do not support a geochemical 5.1.2. Partial melting estimates “normal” ophiolitic crust and are more probably in agreement with Fig. 14 shows calculated REE composition for melts produced by a typical volcanic arc settings, in which enrichments in LILE, LREE result depleted mantle type source (MORB) partial melting (Fig. 14A) and by from slab fluids/melts contaminations (Pearce et al., 1984). an enriched mantle source (EM2) partial melting (Fig. 14B). The REE patterns of the ophiolitic suite fit those of melts produced by the partial 5.1.1. Source components non-modal batch melting from 4% to 10% of a spinel-bearing mantle, The isotopic compositions of the ophiolitic magmatic rocks lie at which mineralogical compositions spinel 5%, olivine 55%, orthopyrox- the limit of the MORB domain and overlap the OIB field (Fig. 12A, B). ene 20% and clinopyroxene 20%, using the partition coefficient factors Nevertheless, their flat REE spectra together with their “enriched” of McKenzie and O'Nions (1991), Johnson (1994), and Nikogosian and isotopic character suggest partial melting from a spinel-bearing Sobolev (1997). Such composition is rather similar to that of a slightly mantle with small percent partial melts similar as to those known depleted mantle source (e.g., Juteau and Maury, 1997). for slow spreading ridges (Lagabrielle, 1987). This hypothesis will be tested by a non-modal batch partial melting model in the following 5.2. Origin of alkaline lavas: source components and geodynamic section. Further, the measured Nb–Ta negative anomalies combined significance? with slight LILE enrichments are indicative of a volcanic arc setting. Finally, the emplacement depth of pillow lava flows was clearly Mineral chemistry and geochemistry of the alkaline volcanic series abyssal as shown by the deposition of radiolarite interlayers. There- of Sevan, Stepanavan and Vedi ophiolites is similar to that of OIBs. As 182 Y. Rolland et al. / Lithos 112 (2009) 163–187

Fig. 13. Isotopic diagrams showing mixing curves between the different mantle end-members. 143Nd/144Nd vs. 206Pb/204Pb (A) and 87Sr/86Sr (B) isotopic diagrams; and 206Pb/204Pb vs. 87Sr/86Sr (C) and 207Pb/204Pb (B). Compositions of end-members used in the calculation of mixing curves (after Hart, 1984; Zindler and Hart, 1986; Sun and McDonough, 1989; 87 86 143 144 206 204 207 204 208 204 Eisele et al., 2002) are the following. HIMU: ( Sr/ Sr)=0,703; ( Nd/ Nd)=0,51285; ( Pb/ Pb)=21,5; ( Pb/ Pb)=15,82; ( Pb/ Pb)=40; [Sr]=120; [Nd]A =6,5; [Pb]=0,4. EM2 pole: (87Sr/86Sr)=0,71682; (143Nd/144Nd)=0,51216; (206Pb/204Pb)=18,99; (207Pb/204Pb)=15,65; (208Pb/204Pb)=39,5; [Sr]=218; [Nd]=34; [Pb]=25. EM1 pole: (87Sr/86Sr) =0,705; (143Nd/144Nd)=0,5122; (206Pb/204Pb)=16,8; (207Pb/204Pb)=15,45; [Sr]=513; [Nd]=33; [Pb]=3,5; DMM: (87Sr/86Sr)=0,7022; (143Nd/144Nd)= 0,513075; (206Pb/204Pb)=17,3; (207Pb/204Pb)=15,4; (208Pb/204Pb)=37,5; [Sr]=11,3; [Nd]=1,12; [Pb]=0,0489. The mixing curves equations are from Faure (1986).

Fig. 14. Modelling effect of non-modal batch melting of several mantle sources, and comparison with ophiolite and alkaline REE spectra compositional domains obtained in this study. Calculated composition (A) of the depleted peridotite ‘ophiolite’ source: olivine 55%, orthopyroxene 20%, clinopyroxene 20% and spinel 5%; and (B) of the enriched peridotite ‘alkaline’ source: olivine 54%, orthopyroxene 20%, clinopyroxene 20%, garnet 2% and spinel 4% (Salters and Stracke, 2004). Y. Rolland et al. / Lithos 112 (2009) 163–187 183

Fig. 15. Geodynamic reconstitution of the Lesser Caucasus in the Middle Jurassic to Upper Cretaceous periods. 184 Y. Rolland et al. / Lithos 112 (2009) 163–187 shown in the Mineral Chemistry section, pyroxenes are slightly 4. After this, the SAB enters the subduction zone in the Turonian (95– alkaline. The alkaline lava samples (Fig. 14) show strong enrich- 88 Ma), which triggers a “collision” with the thickened oceanic ments in incompatible elements (up to 100 times chondrite crust. During this process, part of the volcanic arc has probably values). Partial melting degree calculation suggests that they may been subducted below the obducted oceanic sequence and meta- be derived from ~20% non-modal batch melting of an enriched morphosed in the blueschist facies (Rolland et al., 2009). The spinel-garnet-bearing mantle source characterized by La and Lu large variety of lithologies comprising metabasites, marls and concentrations 3.3 times and 1.8 times the chondrite mantle, conglomerates in a pelitic matrix, within the Stepanavan blues- respectively. The isotopic composition of these lavas plots in the chists, is in agreement with such a scenario. field of OIBs in agreement with an enriched alkaline mantle source. 5. The obduction of the ‘ophiolite’ section over the SAB is further In the isotopic plots of Fig. 13, the obtained isotopic data plots constrained by the Lower Coniacian frontal flysch sequences, mainly on the DMM-EM2 mixing curve, with ~2–3% of EM2, ~5– found below and in front of the Vedi obducted sequence. The calc- 15% HIMU and almost no EM1 contamination which suggests that alkaline series found above the Stepanavan ophiolite show that subduction-derived contamination may not be envisaged. The a volcanic arc was active during this time above the obducted isotopic compositions of the studied Armenian alkaline series are sequence. thus rather in agreement with an OIB-type source. In the Vedi area, 6. The end of the obduction is constrained by Upper Coniacian fauna Satian et al. (2005) already pointed out the alkaline character of in sediments unconformably overlying the ophiolite. Blocking of the lava series, which they interpreted as volcanic series formed in the subduction below the Eurasian margin may stop at 73–71 Ma, an intra-continental rift. These lavas were emplaced above, and as shown by Ar–Ar age of MT-LP metamorphism in the Stepanavan formed ~50 Ma after the ophiolites. Moreover, they are inter- blueschists and the general tectonic uplifting of the region, stratified and overlain by shallow marine reef limestone. Thus, all witnessed by erosion and absence of sedimentary record during these features are in agreement with a plume event that occurred the Upper Cretaceous–Paleocene (Rolland et al., 2009). This 73– in an intra-oceanic setting. 71 Ma event is thus interpreted as the insight of ‘collision’. Such alkaline magmatism is widely documented in the Middle- East region, along the Arabian and Indian platforms, in relationship 5.4. Implications of a plateau/OIB event on the ophiolite obduction? with the formation of the Neo-Tethys ocean (e.g. Lapierre et al., 2004). Similar Cretaceous alkaline series are found above the Iranian The alkaline series show features of Plume-related magmas. What ophiolite (Ghazi and Hassanipak, 1999), and in Turkey (Norman, is the significance of such plume magmatism, and what is its 1984; Tüysüz et al., 1995; Tankut et al., 1998; Okay, 2000). However, it consequence with the obduction and preservation of the Armenian is still difficult to relate these alkaline events due to their geographical ophiolites? and temporal distance and to paucity of radio-chronological and Sr, Only 3% of the current oceanic floor is composed of plume- Nd, Pb isotopic data. related crust, of which oceanic plateaus are the largest part (Petterson et al., 1997). In Armenia, this alkaline event seems to 5.3. Reconstruction of the ‘ophiolite’ history be relatively large due to the presence of alkaline lavas over the ophiolite in all the studied sections. This large size is thus in From all the available geological data, we propose the following agreement with an oceanic plateau event, in which large volumes of model for the evolution of the Armenian Ophiolite (Fig. 15): lavas are erupted during volcanic emplacement in a small time range. 1. The SAB is of Gondwanian origin according to lithological Only several examples of obducted plateau series have been associations found in central and SE Armenia (Knipper and claimed worldwide, amongst which the Wrangellia terrane of Alaska Khain, 1980; Kazmin et al., 1987; Aghamalyan, 2004). Therefore, and British Columbia (e.g. Richards et al., 1991), and Gorgona Island in it is likely that the Sevan oceanic basin opened in response to the N- Columbia (Duncan and Hargreaves, 1984; Storey et al., 1991), but such dipping subduction of Neotethys to the south of Eurasia (Fig. 15— examples remain relatively uncommon. The paucity of obducted stage 1). The continuation of Paleotethys in the area is a matter of plateau sequences may be explained by the fact that they are not easily debate, as the westward continuation of the Cimmeride orogenic recognized in the geological records. However, their potential in the system is not identified in the Lesser Caucasus (e.g., Sengör, 1984, blocking or reversion of polarity of subduction zones has been noted 1990). Emplacement of the ophiolite occurred in the Lower-to in numerous cases (e.g. Petterson et al., 1997; Kerr et al., 2003; Kerr Middle Jurassic (Galoyan et al., 2009). The older age of the Vedi and Mahoney, 2007). For instance large oceanic plateaus can cause the ophiolite (178.7±2.6 Ma; Rolland et al., accepted), with respect to reversal of subduction polarity, as did the Ontong Java Plateau that of Sevan (160–165 Ma, Zakariadze et al., 1990; Galoyan et al., (Coleman and Kroenke, 1981). Cloos (1993) calculated that basalt- 2009) implies that it was at the southern rim of the back-arc dominated oceanic plateau crust must exceed 17 km thickness to system. The structural setting of the ophiolite obduction indicates survive subduction, and about 30 km to cause any significant clearly that oceanic crust of the back-arc basin was formed between ‘collisional’-type deformation. the SAB and the Eurasian active margin. TheArmenianophiolitesshowevidencefortheobductionofa 2. Emplacement of an Oceanic Island/Plateau above the back-arc oceanic single oceanic crust sequence above the SAB, as similar geological, crust during the late Lower Cretaceous (40Ar/39Ar age of 117.5±0.8 Ma petrological, geochemical and age features are found in the three in this paper; Fig. 15—Stage 2). studied Armenian ophiolitic massifs (Sevan, Stepanavan, and Vedi). 3. The calc-alkaline lavas disconformably overlie the ophiolite and The oceanic crust s.s. corresponds to a slow-spreading ophiolite related alkaline series (Galoyan et al., 2007). These lavas have formed in the Lower-Middle Jurassic in a back-arc basin by 4–10% similar geochemical features as volcanic arc series, including the melting of a shallow asthenosphere spinel-bearing source con- isotopic Sr–Nd composition. Their emplacement is bracketed in the taminated by subducted slab-derived products. Alkaline volcanic Upper Cretaceous, as for the high pressure metamorphism series with OIB-type geochemical features are found above the constrained in the Stepanavan area (Meliksetyan et al., 1984 and ophiolite sequence in each of the studied areas, which late Lower references therein), constrained at about 95–90 Ma (Rolland et al., Cretaceous age has been constrained above by the 40Ar/39Ar 2009). Therefore this magmatic event can be related to the method on amphibole at 117.3±0.9 Ma. Therefore, this Armenian subduction of Neo-Tethys ocean prior to the obduction of the ‘plume event’ shortly predates the Coniacian–Santonian (88– Armenian Ophiolites onto the SAB. 83 Ma) obduction of the Armenian ophiolitic sequence. Therefore, Y. Rolland et al. / Lithos 112 (2009) 163–187 185 the thickened and hot oceanic plate had a low density when it Abbot, D., Drury, R., Mooney, W., 1997. Continents as lithological icebergs: the importance of buoyant lithospheric roots. Earth and Planetary Science Letters overrode the SAB continental margin, which suggests that the 149, 15–27. plume event has likely played an important role in the obduction Abovyan, S.B., 1981. The mafic–ultramafic complexes of the ophiolitic zones in process. Armenian SSR. Izvestia Academy of Science Armenian SSR. 306 pp. (in Russian). Adamia, S., Bergougnan, H., Fourquin, C., Haghipour, A., Lordkipanidze, M., Ozgül, N., The original width and thickness of the Armenian plume related Ricou, L., Zakariadze, G., 1980. The Alpine Middle East between the Aegean and the series are difficult to assess. However, this volcanic series covered Oman traverses. 26th International Geological Congress Paris C5, 122–136. each of the studied Armenian ophiolites, which suggests a N104 km2 Adamia, S., Bergougnan, H., Fourquin, C., Haghipour, A., Lordkipanidze, M., Ozgül, N., surface regarding the initial ophiolite surface prior to horizontal Adamia, S., Chkhotua, T., Kekelia, M., Lordkipanidze, Shavishili, I., Zakariadze, G., 1981. Tectonics of the Caucasus and the adjoining regions: implications for the 5 6 2 shortening, and N10 –10 km if they can be correlated to similar evolution of the Tethys ocean. Journal of Structural Geology 3, 437–447. settings in Turkey and Iran. 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Tectonophysics series is within range of a period of major oceanic plateau formation – fi 151, 275 296. in the late Lower Cretaceous; tting precisely the age of formation of Brynzia, L.T., Wood, B.J., 1990. Oxygen thermobarometry of abyssal spinel peridotites; one of the largest plateaus, the Ontong Java plateau (Tarnudo et al., the redox state and C–O–H volatile composition of the Earth's sub-oceanic upper 1991). These alkaline series are also locally covered by an arc- mantle. American Journal of Science 290, 1093–1116. Cloos, M.,1993. Lithospheric buoyancy and collisional orogenesis: subduction of oceanic derived calc-alkaline volcanic sequence, which was likely formed in plateaus, continental margins, island arcs, spreading ridges, and seamounts. a supra-subduction zone environment. Further evidence of this Geological Society of America Bulletin 105, 715–737. subduction is provided by blueschists series dated at 95–90 Ma Coleman, P.J., Kroenke, L.W., 1981. Subduction without volcanism in the Solomon Islands Arc. Geo-Marine Letters 1, 129–134. (Rolland et al., 2009). Danelian, T., Galoyan, G., Rolland, Y., Sosson, M., 2007. Palaeontological, (Radiolarian) Therefore both oceanic Plateau and volcanic arc formations shortly Late Jurassic age constraint for the Stepanavan ophiolite, Lesser Caucasus, Armenia. pre-dated the obduction, which occurred in the Coniacian–Santonian Proceedings of the 11th International Congress, Athens, May 2007. Bulletin of the – Geological Society of Greece, vol. 37. (88 83 Ma; Sokolov, 1977). Crustal thickening related to plateau and Danelian, T., Asatryan, G., Sosson, M., Person, A., Sahakyan, L., Galoyan, G., 2008. Discovery arc events are thought to have increased crustal buoyancy (e.g., Cloos, of two distinct Middle Jurassic Radiolarian assemblages in the sedimentary cover of 1993; Abbot and Mooney, 1995; Abbot et al., 1997; Kerr and Mahoney, the Vedi ophiolite, Lesser Caucasus, Armenia. Comptes Rendus Palevol 7, 327–334. 2007). Such low buoyancy likely hindered subduction of the oceanic Dercourt, J., Zonenschain, L.P., Ricou, L.E., Kazmin, V.G., Le Pichon, X., Knipper, A.L., Grandjacquet, C., Sbortshikov, I.M., Geyssant, J., Lepvrier, C., Pechersky, D.H., Boulin, crust and allowed it to be obducted over the SAB continental crust. J., Sibuet, J.C., Savostin, L.A., Sorokhtin, O., Westphal, M., Bazhenov, M.L., Lauer, J.P., Such process is not unlikely in other obduction contexts of the Peri- Biju-Duval, B., 1986. Geological evolution of the tethys belt from the Atlantic to the – Tethyan region, especially in the Caucasus–Middle East segment, but pamirs since the LIAS. Tectonophysics 123, 241 315. fi Duncan, R.A., Hargreaves, R.B.,1984. Plate Tectonic Evolution of the Caribbean Region in the the obducted ophiolite sections have to be analysed in detail to nd if Mantle Reference Frame. Geological Society of America Memoir 162, 81–93. whether alkaline series of similar age and geochemical signatures may Eisele, J., Sharma, M., Galer, S.J.G., Blichert-Toft, J., Devey, C.W., Hofmann, A.W., 2002. The role be present. of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot. Earth Planetary Science Letters 196, 197–212. Evensen, N.M., Hamilton, P.J., O'Nios, R.K., 1978. Rare earth abundances in chondritic Acknowledgements meteorites. Geochimica et Cosmochimica Acta 42, 1199–1212. Faure, G., 1986. Principles of isotope geology, 2nd edition. Wiley, New York. Galoyan, G., Rolland, Y., Sosson, M., Corsini, M., Melkonyan, R., 2007. Evidence for This work was supported by the Middle East Basins Evolution superposed MORB, oceanic plateau and volcanic arc series in the Lesser Caucasus, project jointly supported by a consortium including oil companies Stepanavan, Armenia. Comptes Rendus Geosciences 339, 482–492. and the CNRS. Many thanks to the MEBE program coordinators E. Galoyan, G., Rolland, Y., Sosson, M., Corsini, M., Billo, S., Verati, C., Melkonyan, R., 2009. Geochemistry and 40Ar/39Ar dating of Sevan Ophiolites, Lesser Caucasus, Armenia): Barrier and M. Gaetani for their support and encouragements, and M. evidences for Jurassic Back-arc opening and hot spot event between the South F. Brunet for coordinating the project. Analytical data were acquired Armenian Block and Eurasia. Journal of Asian Earth Sciences 34, 135–153. with the help of the Geosciences Azur Laboratory, in which we thank doi:10.1016/j.jseaes.2008.04.002. fi L. Vacher and J.P. Goudour for their involvement during data Ghazaryan, H.A., 1987. Strati ed gabbros of ophiolitic series of the south-eastern part of the Sevan mountain range. Tipomorfizm and the Parageneses of the Minerals of acquisition. We also thank the support of the French Embassy at Armenian SSR. 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