Lithos 112 (2009) 163–187 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos 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). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.02.006 164 Y. Rolland et al. / Lithos 112 (2009) 163–187 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.