Nature and age of pre-Variscan eclogite protoliths from the Low-to Medium-Grade Metamorphic Complex of north-central Sardinia (Italy) and comparisons with coeval Sardinian eclogites in the northern Gondwana context

U-Pb zircon data on retrogressed eclogites sampled in the Giuncana locality from the Sardinian Medium-Grade Metamorphic Complex yielded a weighted age of 454 +/- 6 Ma. This is in agreement with U-Pb zircon ages of 453-460 Ma obtained from eclogites from the High-Grade Metamorphic Complex. The Giuncana eclogites are very similar to the other well-known Sardinian eclogites. All of the Sardinian eclogites show positive K, Rb, Ba, U and Pb anomalies and negative Nb, La, Ce and Sr anomalies. Th is depleted in the Giuncana eclogites and enriched in those from Punta de Li Tulchi and Punta Tittinosu. All these data reveal clear crustal contamination of the Sardinian Ordovician mantle. REE patterns typical of normal mid-ocean ridge basalt (N-MORB) characterize all of the Sardinian eclogites. The supply of crustal and calc-alkaline materials to the Sardinian mantle during the Ordovician is further confirmed by the fact that most Sardinian eclogites plot on the left side and well above the mantle array in a Th/Yb v. Nb/Yb diagram. In the general Variscan framework of northern Gondwana, the Sardinian eclogites are witness to the most recent back-arc basins generated by the northward opening of the Rheic Ocean.