Geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block 4Q2 Q3

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.

Kyanite-bearing eclogites from the Migmatite Complex of Golfo Aranci area, NE Sardinia, have been investigated using microstructural analyses and thermodynamic modelling in order to define their early metamorphic evolution and P-T path. The eclogites underwent a clockwise P-T path recorded by the different composition of garnet core, mantle, and rim. P-T pseudosection modelling allowed to estimate P-T conditions in the range T = 580-630°C and P = 1.5-2.00 GPa for the formation of garnet core, between T = 620-690°C and P = 2.0-2.3 GPa for garnet mantle, and within T = 650-700°C and P = 1.4-2.1 GPa for garnet rim. The first part of the P-T path is characterized by a prograde increase in P and T in the eclogite facies that occurred between the growth of the garnet core and mantle. After the peak pressure, recorded by the garnet mantle composition, the rocks underwent moderate temperature increase and significant pressure decrease until they reach the peak temperature near to the eclogite/upper granulite-facies transition, recorded by the garnet rim composition. The P-T trajectory proposed for the early stage of the metamorphic evolution of the kyanite-bearing eclogites is similar to already existing observations from the Sardinian Low-to Medium-Grade Metamorphic Complex and introduce new constraints on their prograde eclogite-facies evolution. The role of the new prograde path is then discussed in the light of the Variscan orogen.

The Posada Valley amphibolites in the inner zone of Variscan Sardinia are characterized by garnet porphyroblasts containing epidote, titanite, quartz, clinopyroxene, amphibole, and plagioclase concentrated in the garnet core. The matrix is made up of amphibole and plagioclase and finegrained clinopyroxene and plagioclase. The amphibolites recorded an HP stage, a metamorphic re-equilibration, and a third stage under the greenschist facies. P -T conditions of stage I (530-650°C/0.9-1.3 GPa for garnet core and 570-690°C/1.0-1.4 GPa for garnet rim) were obtained by P -T pseudosection modelling. The P -T conditions for stage 2 (T = 600-700°C/P = 0.5-0.8 GPa) were obtained by applying the same approach to the most retrogressed samples. The Posada Valley amphibolites underwent a cooler evolution, with a thermal peak lower than that recorded by the northward eclogites from the high-grade metamorphic complex of Sardinia.The proposed geodynamic scenario starts with the subduction of oceanic crust under the peri-Gondwanan terrane in Upper Devonian. The Posada Valley amphibolites were subducted up to maximum depths of 50 km. Subsequently, they reached the subduction channel or were detached from the slab before the eclogite facies conditions were reached.

A geological map at 1:10,000 scale is presented that shows the distribution of the metamorphic rock units in the area of Montigiu (Mt.) Nieddu (NE Sardinia). These units belong to the High-Grade Metamorphic Complex in the Inner Zone of the Variscan chain of Sardinia. The rocks include different types of migmatite, amphibolite, and retrogressed eclogites. The geological map and cross section show the complex tectonic and metamorphic setting of the area, whereas the metamorphic assemblages and deformation framework represent a valuable tool for the reconstruction of the P-T metamorphic evolution for the crustal sectors involved in the Variscan orogeny. The metamorphic rock units reported in the map derived from the lower and upper continental plates involved in the Variscan collision and were tectonically juxtaposed during the early Carboniferous exhumation stage.