The metamorphic basement of Sardinia is a fragment of the southern European Variscan Belt and represents the continuation of the Maures, Montagne Noire and Pyrénées, France (Arthaud and Matte, 1966; Westphal et al., 1976; Arthaud and Matte, 1977; Ricci and Sabatini, 1978).
The Sardinian Variscan Belt (Figure 1, 2) is generally divided into four tectono-metamorphic zones with decreasing grade from NE to SW: 1) Inner zone or "Axial zone", with medium- to high-grade metamorphic rocks and migmatites (northern Sardinia) of Precambrian (?) to lower Paleozoic age; 2) Internal Nappe zone (central-northern Sardinia), with crystalline units consisting of low- to medium-grade metamorphic rocks; 3) External Nappe zone (central-southern Sardinia), with low-grade metamorphic rocks; 4) External zone (southern Sardinia, Iglesiente, Sulcis), with very low- to low-grade metamorphic rocks. The Internal and External Nappe zones consist of Paleozoic metasedimentary and metavolcanic successions.
Figure 1. Tectonic sketch map
Tectonic sketch map of the Variscan Belt in Sardinia (after Carmignani et al., 2001, modified).
Figure 2. Geological sketch cross-section
Geological sketch cross-section throughout the Variscan basement of Sardinia from Barbagia to the Gallura region (after Carmignani et al., 1989 and Carosi and Palmeri, 2002, modified ). 1: Variscan granitoids; 2: External Nappes; 3: Internal Nappes; 4: phyllites, micaschist and gneiss; 5: orthogneiss, 6: augen gneiss; 7: mainly mylonite and amphibolite lenses; 8: migmatite with lenses of metabasite with eclogite facies relics; 9: regional schistosity; 10: thrusts. The regional distribution of some index minerals in pelitic rocks from Barbagia to Gallura is also shown. Mineral abbreviations according to Kretz (1983).
The Inner zone or "Axial zone" includes two metamorphic complexes: the High Grade Metamorphic Complex (HGMC) of North Sardinia, consisting of gneisses and migmatites with a metamorphic grade reaching the sillimanite + K-feldspar isograd. Bodies and lenses of mainly Ordovician granitic-granodioritic orthogneisses (Di Simplicio et al., 1974; Ferrara et al., 1978), mafic and ultramafic metamorphic rocks showing eclogite to granulite relics and calcsilicate nodules are frequently embedded within the HGMC.
The second metamorphic complex in the axial zone is made up of low- to medium-grade metamorphic rocks (L-MGMC), largely outcropping adjacent to the southern side of the Posada-Asinara Line (Figure 1, 2), in central-southern Asinara, northern Nurra, Anglona and the northern Baronie regions. The Posada-Asinara line is considered by Cappelli et al. (1992) and Carmignani et al. (2001) as a "South Hercynian Suture Zone" between the Armorica and Gondwana plate margin.
The Internal Nappe zone includes metasandstones, quartzites and phyllites outcropping in the Monti del Gennargentu ("Postgotlandiano "Auct. p.p"), Goceano and Nurra. Near the area of contact with metavolcanic rocks, metaconglomerates outcrop in Goceano, Barbagia and Baronie. Micaschists and paragneisses of the internal Units outcrop in Barbagia and Baronie. Internal Nappes also include acidic metavolcanic rocks ("Porfiroidi"), with related metarkoses and microcline-bearing quartzites, as well as mafic and intermediate (andesite) metavolcanic rocks of probable Middle-Ordovician age. As regards the Upper Ordovician to Lower Carboniferous sequences, internal nappes are characterised by Siluro-Devonian graphite-rich phyllites, marble and calcschist intercalations and metadolerites and metagabbros attributed to the Lower Carboniferous. Sequences in the southern part of Nurra and Baronie can easily be correlated with the Cambro-Ordovician successions of the External zone.
The External Nappe zone is characterised by Middle Ordovician magmatism, which produced the thick metavolcanic sequences occurring above Cambro-Ordovician metapsammites. The Variscan sedimentary cycle started at the beginning of the Upper Ordovician (Caradoc), with marine transgression on a previously-destroyed volcanic arc. Silurian black phyllites show a more widespread, homogeneous distribution, indicative of an extended pelagic environment, while Devonian platform carbonate gradually decreases in thickness towards the internal areas of the chain. The deepest units of the External Nappe zone are the Castello Medusa, Riu Gruppa and Monte Grighini Units, respectively reaching the biotite and medium-grade zones. The Monte Grighini represents the core of greatest tectonic culmination in the Nappe zones: the Flumendosa Antiform. On its southern side, from bottom to top, are the Gerrei and Genn'Argiolas units; on the northern side, the Nappe sequence, from bottom to top, is comprised of the Gerrei and Meana Sardo Units and the Barbagia Low-Grade Metamorphic Complex.
The External zone corresponds to the oldest, southwestern part of Sardinia. In southern Sulcis, the Cambro-Ordovician sequence of northern Sulcis-Iglesiente overlies a metamorphic complex including, from bottom to top, Monte Filau orthogneisses, Monte Settiballas micaschists and the Bithia formation (Fm). The latter is made up of alternating metapsammites and metapelites, with metabasite and carbonate intercalations (Junker and Schneider, 1980, 1983). Some authors (e.g. Minzoni, 1981) consider the Settiballas micaschists part of a Precambrian basement. A Precambrian to Early Cambrian age is also attributed by Naud (1979) to the Bithia Fm, owing to its position below the Early Cambrian Nebida group (Junker and Schneider, 1980, 1983; Cocozza, 1980) and to the appearance of schistosity unknown in overlying Cambrian formations (Carosi et al., 1995). Monte Filau orthogneisses, on the contrary, are lower Ordovician granitoid (Delaperrière and Lancelot, 1989) rocks intruded within Settiballas micaschists.
The Cambrian-Early Ordovician sequence is made up, from bottom to top, of the Nebida Group, Gonnesa Group and Iglesias Group (Pillola, 1991). The Nebida Group is a deltaic terrigenous sequence (metasandstone and metasiltite), with rare interlayered lagonal oolitic limestones containing Early Cambrian Archeocyaths.
The Gonnesa Group is mainly composed of metadolostones at the base and grey stratified metalimestones at the top. The Gonnesa Group is interpreted as a thick platform carbonate succession (arid tidal flat environment) (Boni and Cocozza, 1978; Boni and Gandin, 1980; Barca et al., 1987) showing gradual heteropic transition to nodular limestones (extensional deep basins).
The base of the Iglesias Group is made up of marly nodular crystalline limestone (Campo Pisano Fm) and the top of phyllites and metasiltites interbedded with minor massive fine-grained metasandstones (Cabitza Fm). This framework and the lack of Neoproterozoic to Early Ordovician calc-alkaline rocks indicates that Sardinia belonged to a passive continental margin from the Late Neoproterozoic to the Early Ordovician.
Cambrian-Early Ordovician sequences are separated from the overlying Upper Ordovician-lower Carboniferous sedimentary cycle by sharp angular unconformity (Sardic phase) and overlain by a transgressive (Martini et al., 1991) polygenic metaconglomerate, the Puddinga Auct.