Jurassic back-arc and Cretaceous hot-spot series In the Armenian ophiolites — Implications for the obduction process
Introduction
The role of Oceanic Plateaus in the obduction processes of oceanic crust has still not been clearly established. We understand that their larger crustal thickness and buoyancy as compared to ‘standard’ oceanic crust does not allow them to subduct, in particular when they reach subduction zones soon after their formation (e.g., Ben-Avraham et al., 1981, Cloos, 1993, Abbot and Mooney, 1995, Abbot et al., 1997, Kerr and Mahoney, 2007). However, the reasons for oceanic crust obduction onto continental margins are still debated: (i) is obduction driven by subduction of continental crust? Or (ii) does it result from the intrinsic nature of the oceanic crust? In the first case, ophiolites are obducted due to the mechanical coupling of continental crust with the dense subducting slab (e.g., O'Brien et al., 2001, Guillot et al., 2003). Continental subduction may be facilitated by the thinned margins of continental domains following earlier phases of divergence rifting that precede oceanic crust emplacement (Guillot and Allemand, 2002). In the second case, a lower density of oceanic lithosphere might result from intra-oceanic hot-spot and magmatic arc events, which will lead to crustal thickening and a decrease in lithosphere density (Cloos, 1993, Abbot and Mooney, 1995). The emplacement of oceanic plateaus has a great influence either on the slab dip, but also on the cessation of subduction and on the onset of obduction as is proposed for the Ontong–Java plateau (Petterson et al., 1997). The ability of an oceanic plateau to resist subduction and eventually be transported onto continental crust depends on both crustal thickness and plateau age (Kerr and Mahoney, 2007). The older a plateau, the cooler and thus the less buoyant it will be. Alternative hypotheses for obduction involve rapid inversion of tectonic plate motions and rapid continental convergence (e.g., Agard et al., 2007). Obduction is ascribed to the presence of young oceanic crust in the hanging-wall of the subduction zone, as a result of subduction initiation at the Mid-Oceanic Ridge (e.g., Boudier et al., 1988, Nicolas, 1989); or to scalping of oceanic lithosphere (e.g., Agard et al., 2007 and references therein).
The case of Armenian ophiolites (Lesser Caucasus) is peculiar as recent investigations (Galoyan et al., 2007, Galoyan et al., 2009, Rolland et al., in press) have shown the presence of slow-spreading ophiolites in several locations. Further, the ophiolites were tectonically transported above the South Armenian Bloc or SAB (Zakariadze et al., 1983). Although some blueschists are locally found, these affect oceanic crust-derived rocks which underwent intra-oceanic subduction and exhumation within accretionary prisms (Rolland et al., 2009). In contrast, the underthrusted Armenian continental crust appears not to have been metamorphosed by any subduction event. Therefore, the obduction of the Armenian ophiolites might be explained by the intrinsic nature of the oceanic crust. However, the slow-spreading nature of the ophiolites, and in particular the fact that exhumed mantle forms a large part of the reconstructed ophiolite is rather in agreement with a relatively dense oceanic lithosphere.
In this paper, we report new geochemical data, including major and trace elements and Nd, Sr, Pb isotopes, on magmatic series from several Armenian ophiolites (i.e. Stepanavan (NW Armenia), Sevan (N Armenia), Vedi (central Armenia); Fig. 2). We identify three superposed levels of lavas corresponding to three distinct environments: (1) back-arc, (2) ‘OIB’-like and (3) arc. Moreover, we suggest that these ophiolite windows correlate with each other and be part of a unique obducted nappe. Tectonic transport of this nappe onto the SAB can be dated to the Coniacian–Santonian (88–83 Ma; Sokolov, 1977, Sosson et al., in press). Finally, the influence of oceanic plateau event in oceanic lithosphere rheology and its role in the obduction process is discussed.
Section snippets
Geological setting
During the Mesozoic, the Southern Margin of the Eurasian continent has been featured by closure of the Palaeo-Tethys and opening of the Neo-Tethys Ocean (e. g.; Sengör and Yılmaz, 1981, Tirrul et al., 1983, Ricou et al., 1985, Dercourt et al., 1986, Stampfli and Borel, 2002, Fig. 1). Later on, subductions, obductions, micro-plate accretions, ranging mostly from the Cretaceous to the Eocene, and finally continent–continent collision have occurred between Eurasia and Arabia. The study of Armenian
Analytical methods
Mineral compositions were determined by electron probe microanalysis (EMP). The analyses are presented in Fig. 4, Fig. 6. They were carried out using a Cameca Camebax SX100 electron microprobe at 15 kV and 1 nA beam current, at the Blaise Pascal University (Clermont-Ferrand, France). Natural samples were used as standards.
For 40Ar/39Ar dating of the alkaline suite, fresh amphibole grains were separated from the Vedi ophiolite unaltered sample AR-05-104. Geochronology of amphiboles was performed
Field relationships
Synthetic logs are drawn on Fig. 3, showing the lithological associations and the structural relationships in each of the three studied ophiolitic zones.
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
Acknowledgements
This work was supported by the Middle East Basins Evolution project jointly supported by a consortium including oil companies and the CNRS. Many thanks to the MEBE program coordinators E. Barrier and M. Gaetani for their support and encouragements, and M.F. Brunet for coordinating the project. Analytical data were acquired with the help of the Geosciences Azur Laboratory, in which we thank L. Vacher and J.P. Goudour for their involvement during data acquisition. We also thank the support of the
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