Carpathian peri-Gondwanan terranes in the East Carpathians (Romania): A testimony of an Ordovician, North-African orogeny

Abstract

The eastern branch of the Romanian Carpathians – the East Carpathians – is essentially an Alpine thrust and fold belt made up in its median part by a Crystalline–Mesozoic zone. This, in turn, is built up by several Alpine nappes (top to bottom): the Wildflysch, Bucovinian, Subbucovinian and Infrabucovinian. In the basement of the Bucovinian and Subbucovinian nappes the following Variscan tectonic units have been identified (top to bottom): Rarău, Putna, Pietrosu Bistriţei and Rodna. The Infrabucovinian nappes comprise the Rarău nappe only. The Alpine nappes have an eastward vergence, opposite to the Variscan ones (present coordinates). In terms of pre-Variscan terranes distribution, the Rarău nappe involved the Bretila terrane basement and its late Paleozoic cover, Putna the Tulgheş terrane basement, Pietrosu Bistriţei the Negrişoara terrane basement and Rodna the Rebra terrane basement. These terranes originated along northwestern Gondwana margin during some Ordovician thermotectonic events. They do not represent Cadomian terranes and we call them Carpathian-type terranes. Two igneous protoliths from Bretila terrane basement (i.e. Anieş orthogneiss and Hăghimaş granitoid) yield U/Pb LA-ICP-MS zircon ages of 462±3Ma and 469.2±6.5Ma, respectively. An orthogneiss from Tulgheş terrane basement yield 462.6±3.1Ma; the Pietrosu porphyritic orthogneiss from Negrişoara terrane basement yield 461.1±5.2Ma; and the Nichitaş orthogneiss from Rebra terrane basement yield 447.9±2.8Ma. All these ages suggest the magma crystallization time. Two paragneisses from the Rebra terrane basement show a detrital zircon age distribution characteristic of a NE-African provenance. Regarding the tectonic settings, the lithology of the Bretila terrane suggests a magmatic arc on a continental margin, while of the Tulgheş terrane suggests a back arc environment, and those of the Rebra and Negrişoara terranes suggest a passive continental margin. An Ordovician metamorphism of medium grade (staurolite–kyanite zone) affected the basements of Bretila, Negrişoara and Rebra terranes, whereas a low grade (chlorite to biotite zone) event affects the Tulgheş terrane. With regard to the Variscan orogeny, the existence of a Paleotethys suture is proposed within the metamorphic basement of the East Carpathians. In this interpretation, the Bretila terrane was the upper plate, the Rebra and Negrişoara terrane pair formed the lower plate and the Tulgheş terrane was a component of the suture. The Variscan thermotectonic events reflect isothermal decompression with andalusite+cordierite in the basement of the Rebra terrane and retrogression in the basement of the other terranes.