The Sardinian basement is one of the best preserved segments of the Southern European Variscan belt in the Mediterranean area, as in this area it has not been deformed significantly during the later Alpine deformation (Carmignani et al., 2001). The Sardinian Variscan basement is characterized by several deformation phases that testify the long evolution of Gondwanan and Avalonian margins (Stampfli et al., 2002) from the rifting and evolution of the Rheic Ocean (Cambrian) to the continental collision and final collapse (Upper Carboniferous). The basement of Sardinia is composed by Carboniferous magmatic and sedimentary rocks in the south and south-western part of the island and by metasedimentary Cambro-Lower Carboniferous sequences in the central northern areas, with an increasing in metamorphic grade and intensity deformation moving toward the inner zone of chain (from south-south west to north/north-east) (Carmignani et al., 1994; 2001 and references therein).
The collisional structural frame is broadly characterized by three main tectono-metamorphic complexes (Figs. 1, 2 and 3; Carmignani et al., 1982; 1994; 2001):
A thrust and fold belt foreland consisting of a sedimentary succession ranging in age from upper (?) Vendian to lower Carboniferous which crops out in the southwestern part of the island (External zone: Iglesiente and Sulcis);
A SW verging nappe stack which equilibrated mostly under greenschist facies conditions, consisting of a Palaeozoic sedimentary sequence bearing a thick continental arc-related volcanic suite (the so called "Nappe zone");
An inner zone ("Inner" or "Axial zone") characterised by medium- to high-grade metamorphic rocks that consists of two different metamorphic complexes: a) A medium grade, chiefly metapelitic complex, consisting of micaschists and paragneisses bearing ky±stau±grt (Franceschelli et al., 1982; mineral abbreviation according to Kretz, 1983) and including quartzites and grt-bearing amphibolites boudins with N-MORB chemical affinity (Cappelli et al., 1992; Oggiano & Di Pisa, 1992; Cortesogno et al., 2004). In some areas eclogitic bodies are well preserved (e.g. South-West Gallura) (Eclogite B: Cortesogno et al., 2004). b) A polymetamorphic high-grade complex (HGMC) made up of anatexites and metatexites that developed from the northern part of the island to Corsica. Decametric amphibolitic bodies were recognized in this complex. They testify the complex history of this sector of the chain as the eclogitic paragneisses relics (Eclogite B: Cortesogno et al., 2004) registered a strong re-equilibration under the granulite facies (Miller et al., 1976; Ghezzo et al., 1979, 1982; Di Pisa et al., 1992; Franceschelli et al., 1998, 2002; Cortesogno et al., 2004; Giacomini et al., 2005) and afterward under medium pressure and temperature conditions (amphibolite facies).
U/Pb dating on zircons from the eclogitic bodies constrained the evolution of the HGMC. (Cortesogno et al., 2004; Giacomini et al., 2005). According to these authors the emplacement of mafic protolith is dated at ~460 Ma whereas the eclogitic peak could be placed at 403±4 Ma (Cortesogno et al., 2004). A different age of resetting of the U/Pb system in zircon at 352±3 Ma was instead interpreted from Giacomini et al. (2005), as the age of amphibolitic metamorphism.
The contact between these two metamorphic complexes is well exposed along the Posada Valley (Figs. 2 and 3) (Elter et al., 1990) as well as in Southern Gallura and Asinara island (Oggiano & Di Pisa, 1992; Carosi et al., 2004), and it corresponds to a wide transpressive shear belt (Carosi & Palmeri, 2002), which is interpreted by several authors as a Hercynian suture zone (Posada-Asinara Line, PAL: Cappelli et al., 1992; Carmignani et al., 1994), between the Gondwana and Armorica continents affected by Late Variscan shear zones (Elter et al., 1990).
The three study transects are portions of the Internal Nappes (Inner zone: Carmignani et al., 1994) (Figs. 1, 2, and 3), translated toward the SW during the main collisional event (D1) and located just south of the Posada-Asinara suture. Micaschists and porphyroblastic paragneisses intruded by granodioritic orthogneisses and granitic augen gneisses (458±31 Ma and 441±33 Ma, respectively, Ferrara et al., 1978; ~457 Ma, Helbing & Tiepolo, 2005) crop out in the southern regions of study areas, whereas migmatites and migmatitic gneisses with bodies of amphibolites in the northern areas (Figs. 2 and 3).