Early Triassic basalts from the North Dobrogea orogen (Romania): Evidence of MORB-type magmatism in the Cimmerides

The North Dobrogea Orogen (NDO) is located in the Carpathian foreland between the Moesian and Scythian Platforms (East Romania) and represents the westernmost end of a Cimmerian orogenic belt, which extends eastwards up to the Asian Cimmerides. Various geodynamic models have been suggested for the Triassic-Jurassic evolution of this belt, the main are: (1) a short-lived failed rift (abandoned branch of the Carpathian triple junction); and (2) a fragment of a back-arc basin developed above a north-dipping Triassic subduction zone, active during the closure of the Paleo-Tethys ocean. The NDO is characterized by the widespread occurrence of Late Permian – Late Triassic (and minor Jurassic) volcanic and subvolcanic rocks. The controversial geodynamic interpretations of the evolution of this orogenic belt are a consequence of a still-poor knowledge of the petrogenetic history of these rocks. In particular, one of the most debated points concerns the occurrence of MOR-type oceanic basalts (i.e., ophiolites) in the Cimmerides. A Permo-Triassic phase of crustal thinning of the Hercynian basement is suggested by the occurrence of a still poorly known alkaline magmatism. The Triassic magmatism developed through the intrusion of tholeiitic dykes in the Hercynian basement of the Macin Zone, and extrusion of pillow basalts (Niculitel Formation) in a rifted basin with a thinned crust, as suggested by the facies characteristics of carbonate rocks interbedded with basalts. Geochemical studies on mafic dykes emplaced in different Hercynian basement rocks, as well as on the Early-Middle Triassic Niculitel Formation basalts have been performed in order to provide new constraints for the geotectonic setting of this belt. Low field strength elements and REE contents indicate that Niculitel basalts display compositions very similar to those of E-MORBs, whereas Macin basalts display compositions intermediate between E- and N-MORBs. This suggests that both basaltic series are derived from a MORB-type asthenospheric mantle source variably influenced by a plume-type component, where the less enriched character of the Macin dykes reflects a lesser influence of plume source on magma composition with respect to the Niculitel basalts. In fact, both Niculitel Formation and Macin Zone basalts are consistent with low degrees of partial melting of mantle sources ranging from a theoretical mixed plume - MORB mantle source to a depleted MORB mantle source. Modern chemical analogues are found in the South West Indian and American-Antarctic Ridges, where composition of basalts range from pure plume-type ocean island basalts (OIBs) to pure MORBs, depending on the influence of the Bouvet mantle plume on MORB source. Regardless of the geochemical differences related to the variations of the plume component, a common geodynamic setting can be postulated for the origin of the two basaltic series. According to the hypothesis of aborted rift, the evolution of the mantle sources, started with a plume activity followed by the uprise of primitive asthenospheric mantle. By contrast, in the back-arc basin model, the plume activity may have played a major role in weakening the lithosphere and preparing the back-arc spreading. The occurrence of MORB-type basalts in the NDO does not necessarily imply that these basalts represent an ophiolitic sequence related to an oceanic spreading steady-state. Our data indicate that the Macin and Niculitel basalts originated in an extensional tectonic setting in which the transition from alkaline to E-MORB magmatism was a consequence of the mantle plume evolution through time.