Origin and evolution of the Sebes Lotru terrane, South Carpathians (Romania)

The Sebes Lotru terrane (SLT) is the largest basement unit of the South Carpathians and of the Dacia superterrane. It is also one of the largest basement terranes in Alpine Europe. It represents the metamorphosed basement of the Alpine Getic thrust sheet and is covered in places by unmetamorphosed Alpine rocks. Its relationships with most of the other basement units in the South Carpathians is tectonic and is related to the assembly of a Mesozoic fold–thrust belt. Only its juxtaposition against greenschist facies rocks to the north and east in a small area of the Cibin and Lotru mountains is documented to be Permian, possibly early Triassic. The SLT was metamorphosed to amphibolite facies during the Variscan orogeny. Previous regional zircon (detrital and igneous) U–Pb studies have shown a continuous Ediacaran–early Silurian magmatic arc, with some additional inherited ages at 1 Ga and older than 2 Ga. A major 470–460 Ma zircon age peak dwarfs all other ages in the zircon archive and reflects a flare-up event at that time. However useful, these ages are insufficient in unravelling the full origin of these rocks without a petrological context and there are few constraints on the timing of their metamorphism. This paper provides new whole-rock geochemical and radiogenic isotopic (⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd) data from the SLT basement ortho-rocks of the Cibin, Sebes and Lotru mountains in the central South Carpathians. New Sm–Nd garnet ages are also reported for the spatially extensive Negovanu mica schist and U–Pb zircon ages for pegmatitic granites, most of which cross-cut the metamorphic fabric. Most of the rocks are mafic and intermediate with calc-alkaline compositions and trace element patterns typical of subduction-related magmatic rocks. The isotopic ratios are relatively non-radiogenic at the time of igneous crystallization. Overall, the trace elements and isotopes of the magmatic rocks are consistent with a transitional island arc origin. The presence of various inherited Precambrian age peaks in the volcano-sedimentary detrital record demonstrates proximity to the larger Gondwana mass. The Negovanu Mare schist is probably a cover sequence that rests unconformably over the island arc assemblages. They were metamorphosed together at c. 340–323 Ma in the area of the Cibin Mountains. The pegmatitic granites were emplaced at c. 323–317 Ma based on new zircon U–Pb age dates on three of the samples presented here. The SLT is similar to other basement terranes identified elsewhere in Europe and has a peri-Gondwanan origin. Its arcs are probably related to the development and closure of the Rheic Ocean. The location of the terrane at the time of its major pulse of magmatism was close to modern-day North Africa. The SLT was juxtaposed against another Cambro-Ordovician terrane free of Variscan metamorphism during the latest Permian. The exact evolution and history of docking to larger continental masses during the Paleozoic remains unresolved for the SLT and nearby terranes.