Petrology, geochemistry and Sm-Nd analyses on the Balkan-Carpathian Ophiolite (BCO - Romania, Serbia, Bulgaria): Remnants of a Devonian back-arc basin in the easternmost part of the Variscan domain

The pre-Alpine basement of the Southern Carpathians/Western Balkans contains four ophiolitic massifs dismembered by Alpine tectonics, which define the Balkan-Carpathian Ophiolite (BCO) for which the tectonic setting and age of formation are still debated (Precambrian or Early Devonian). In this contribution, we demonstrate that, in light of a Pre-Alpine restoration, the four massifs belonged to a unique slice of very complete, obducted oceanic lithosphere and we re-evaluate its tectonic setting. Large chromitite volumes with Al-rich spinel compositions (Cr# = 0.39-0.48), as well as major and trace geochemical results on basalts (slightly enriched N-MORBs with low negative Nb anomaly associated with calk alkaline BABBs), point to a formation in a back-arc basin. Mantle spinel composition (Cr# = 0.49-0.51) and melting modeling indicate mean melting extents of 8.5-11% favouring intermediate spreading rate. New Sm-Nd dating on lower gabbroic rocks give a whole rock isochron, interpreted as the age of formation of the BCO crust at 409 +/- 38 Ma, thus confirming an Early Devonian oceanic crust. The previous 563 Ma U-Pb zircon age can be interpreted as casual inheritance indicating the proximity of an old continental lithosphere. Taking into account the lithological evidences and paleocontinental affinities of the two recognized terranes separated by the BC oceanic basin (Balkans and Sredna Gora) and by analogy with other Variscan ophiolites in Western/Central Europe, we suggest that the BC ophiolite belong to the similar to 400 Ma ophiolites group obducted between West and East Galatia and belonging to the southern Variscan suture. However, the BC ophiolite is the only one of this group obducted to the north and not involved in the Lower Allochthon/ophiolite/Upper Allochthon thrust pile, likely explaining its exceptional preservation. Finally, we tentatively propose a new unifying tectonic model where different terrane drift rates and highly oblique displacements create two Rheic branches, the Rheic and the Galicia-Brittany-Massif Central. (C) 2017 Elsevier Ltd. All rights reserved.