Ordovician thermometamorphic events: implications in the correlations between the External Zone (Mt. Filau- Settiballas area) and the High Grade Metamorphic Complex (Tamarispa Area) in Sardinia (Italy).

In SW Sardinia, the Sulcis-Iglesiente Complex forms part of the External Zone of the Sardinian Variscan chain. In the southernmost Sulcis, the Mt. Filau-Settiballas complex records an exceptional convergence of events quite scarce in other Variscan basement rocks. Costamagna et al., (2016) reported granitic intrusions (Filau Granite, FG) and dikes of the Middle Ordovician age (457 ± 0,17 Ma), associated termometamorphic aureole (Settiballas Spotted Schist, SSS) with marble lens containing Grt, Ves e Wo. The Complex is surrounded and superposed by the Bithia Formation (BF) dated to Middle Cambrian. In the Tamarispa area (NE of Sardinia), in the High Grade Metamorphic Complex (Axial Zone), there are several calc-silicate lenses containing Wo, decimetric Grt, Di e Cc (Franceschelli et al., 2021) in contact with San Lorenzo-Tanaunella Ordovician Orthogneiss (453 ± 8 Ma; Rossi et al., 2009) and within the Brunella Biotitic Gneiss. Structural and petrographic evidences show that the Mt. Filau complex forms an integral part of the SW Sardinia CambrianOrdovician succession, which was intruded and metamorphosed by a Middle-Late Ordovician granite. BF could have been the SSS protolith, as well as Brunella Biotitic Gneiss protolith could have been quite analogous to SSS itself. Moreover, FG and San Lorenzo-Tanaunella Orthogneiss have similar emplacement ages. In the Settiballas-Mt. Filau area, the host rocks are affected by Variscan greenschist-facies conditions, whereas the Brunella area attained amphibolite-facies conditions. The pre-Variscan history of southern Sardinia is preserved because the HT/LP metamorphism of the SSS is of higher grade than that in the adjacent BF. However, in northern Sardinia, the same geological framework was overprinted by the Variscan event under amphibolite-facies conditions. The diameters of garnet crystals in calc-silicate lenses in the Tamarispa area (10 to 30 cm), are distinct from other areas in the High Grade Metamorphic Complex suggesting that the growth did not probably result from the Variscan orogenic overprint, but from an older event instead. In the SettiballasMt. Filau ML, garnet growth is more clearly related to older contact metamorphism that resulted from the emplacement of the Ordovician FG (Costamagna et al., 2016). Despite differences in term of dimension (mm to cm size in the SW, dm size in the NE), analyses on garnet samples performed by Costamagna et al. (2016), Franceschelli et al. (2021) and University of Firenze shows a high compositional homogeneity (not zoned) and a significant chemical affinity (Al0.3-1.2Py0.4-0.5Grs87-91Sps0.06-0.2Adr7.4-8.4) between the investigated areas. Age and dynamics of emplacement (Middle-Late Ordovician extensive arc magmatic event), garnets chemical affinities and lithotypes interrelationship highlight a strong correlation between the High Grade Metamorphic Complex sequence in NE and the Cambrian-Ordovician sequence cropping out in SW.