A first systematic attempt at reconstructing the deformation phases of the Variscan chain in Northern Sardinia was published by Carmignani et al. (1979, 1982a) and Elter et al. (1986). Three main deformation phases were distinguished: D1, D2 and D3. The first phase, recognisable in the southern part of northern Sardinia, is characterised by southwards recumbent kilometre-sized isoclinal folds with penetrative axial plane schistosity S1. The second deformation phase D2 produced folds with E-W trending axes. In the southern part of northern Sardinia, D2 folds are open and characterised by strain-slip S2 schistosity. Going northwards, D2 folds gradually become more and more closed, finally turning into northwards recumbent tight isoclinal folds with a southwards dipping penetrative S2 schistosity that almost completely obliterated older D1 structural features. The third deformation phase D3 generated chevron, box or kink folds, with N-S trending axes locally associated with strain-slip or fracture schistosity.
A slightly more complex framework was proposed for the Baronie region (Posada area) by Elter in Oggiano and Di Pisa (1992, 155-157). The main difference is the addition of a D4 and D5 deformation phase. The D4 phase generated a mylonitic complex characterised by continuous, gradual transition from an S-C structure to an ultramylonitic one (Elter, 1987). The late D5 phase, deforming all previous structures in Elter's reconstruction, produced, according to Helbing (2003), a kilometre-sized flexure with a sub-horizontal axis parallel to the orogenic trend.
Several papers have been published on the Variscan deformations in Anglona, western Gallura, Nurra and Asinara: Elter et al. (1990); Franceschelli et al. (1990); Oggiano and Di Pisa (1992); Carmignani et al. (1992, 1994, 2001); Carosi and Oggiano (2002); Carosi et al. (2004).
The tectonic setting of central and southern Sardinia has been discussed by Carmignani and Pertusati (1977), Dessau et al. (1983), Carosi et al. (1990), Carosi and Pertusati (1990), Carosi et al. (1992a,b), Musumeci (1992), Carmignani et al. (1994) and Conti et al. (1998, 1999).
Conti et al. (2001) have recently tried to reconstruct the tectonometamorphic evolution of the Sardinian Variscan segment as a whole, correlating deformation events in the central, southeastern and southwestern parts of Sardinia. Three deformation phases (D1, D2 and D3) were identified by Conti et al. (2001, Figure 3). D1 deformation can be subdivided into four different phases: Gerrei, Meana, Sarrabus and Flumendosa. The Gerrei phase is characterised by mylonitic deformation confined to narrow zones beneath the main thrust planes, kilometre-sized southward vergent isoclinal folds and metamorphic recrystallisation under greenschist facies conditions. The result is the generation of steeply-dipping Sa foliation parallel to S0 stratigraphic contact. However, Sa/S0 surfaces are cut by a flat-lying Sb crenulation cleavage parallel to the Meana Sardo thrust surface, more and more penetrating as it approaches the Meana Sardo thrust. Sb was considered to be related to the Meana Sardo phase leading to the S-directed emplacement of the Meana Sardo and Barbagia Units.
Figure 3. Metamorphic zonation
Metamorphic zonation in metapelite and metapsammite from the Variscan basement of northern Sardinia, (from Franceschelli et al., 1982b, modified). The locations of the main outcrops of metabasite with eclogite and granulite facies relics are also shown. See text for explanation and data sources.
The Sarrabus phase marks a drastic change in tectonic transport from S-directed to W-directed, identified by Conti et al. (2001) within the Arburese and Sarrabus Units, respectively west and east of the Campidano graben. This means, according to Conti et al. (2001), a 90° rotation of the tectonic trasport direction, taking place during the Early Carboniferous.
The final phase, named by Conti et al. (2001) the Flumendosa phase, brought a renewed N-S shortening phase, characterised by E-W to ESE-WNW trending upright folds, and generated the Flumendosa and Gennargentu antiforms and the Barbagia synform.
The D2 deformation, named the Riu Gruppa phase, took place during exhumation and produced normal faulting, NW-SE trending folds and crenulation cleavage. The last D3 deformation is revealed by NE-SW folds and crenulation cleavage.
As regards SW Sardinia, an initial tectonic framework was proposed by Arthaud (1963), who distinguished four deformation phases in the Cambro-Ordovician sequences of SW Sardinia:
1) Sardic phase producing E-W trending open folds; 2) first Variscan phase generating E-W trending folds; 3) second and main Variscan phase yielding N-S trending folds; 4) third Variscan phase producing weak deformation structures with variable orientation.
Conti et al. (2001) have recently questioned the pre-Variscan age of the E-W trending open folds on the basis of a careful field review of existing data. The authors note that contact between the Middle Ordovician conglomerate (Puddinga Auct.) and the underlying Iglesias syncline and Gonnesa anticline consists of a steep reverse fault surface of Variscan age and cannot be considered a Caledonian unconformity (Sardic phase) between pre-Variscan and Variscan sediments.
In the evolution of the Sardinian Variscan basement, a key role was played by a composite network of shear zones (Elter et al., 1986,1990, 1999; Elter and Ghezzo, 1995) displaying complex evolution from early HT/LP stages to late MT-LT/LP conditions associated with changes in the sense of movement. Two shearing events were distinguished: an early shear event (ESE) and a late shear event (LSE). Five shear zones (SZ) associated with synkinematic HT-LP metamorphism were active during the ESE: 1) Barrabisa; 2) Golfo Aranci; 3) Porto Ottiolu; 4) Siniscola-Mamone; 5) Posada Valley. The LSE gave rise to two sub-systems of strike-slip faults, the first associated with synkinematic intrusions and the second with synkinematic retrograde metamorphism. LSE shear zones from south to north are: 1) Monte Grighini; 2) Ottana-Monte Senes; 3) Barrabisa II; 4) Posada Valley II; 5) Anglona; 6) Porto Ottiolu II; 7) Golfo Aranci II.
As regards metamorphic grade, the Golfo Aranci shear zone is the only one showing structural features related to an extensional phase with amphibolite facies conditions (Elter and Ghezzo, 1995; Elter et al., 1999), while other shear zones are characterised by cataclastic to mylonitic products, generated for the most part under greenschist facies conditions.
Three main magmatic cycles have been distinguished in the Variscan basement of Sardinia by Memmi et al. (1983), Di Pisa et al. (1992) and Franceschelli et al. (2003). The first one led to the emplacement of basaltic to rhyolitic calc-alkaline volcanic rocks during the Middle Ordovician (pre-Caradoc cycle). Metamorphosed porphyritic acid rocks of this cycle are known as "porphyroids" and derived from original rhyolitic, rhyodacitic to dacitic lava flows and domes or ignimbrites, widespread in Sarrabus, Gerrei, Sarcidano, Goceano and Nurra. A radiometric age of 474 ± 13 Ma (Table 1) has ben obtained for the Lula porphyroids by Helbing and Tiepolo (2005).
These volcanic products are considered late orogenic calc-alkaline crustal rocks. In some regions (Sarcidano, Goceano), the rocks of the pre-Caradoc cycle show a clear andesitic (Serra Tonnai Fm) to sub-alkaline basaltic affinity. The first cycle lasted from the Arenig to Caradoc (Carosi et al., 1987). Ordovician acidic plutonic and basic volcanic rocks, at present orthogneiss or amphibolite, occur in the northern Sardinia.
A second cycle attributed to the Silurian by Memmi et al. (1983) and to the Caradoc-Ashgill by Di Pisa et al. (1992) gave rise to alkaline within-plate basalts (Beccaluva et al., 1981) emplaced as subvolcanic body sills and dykes, in Goceano, Gerrei, Iglesiente-Sulcis and Sarcidano.
The third cycle, considered by Di Pisa et al. (1992) and Garbarino et al. (2005) of Devonian (?) to Carboniferous age, produced within-plate alkali basalts now outcropping as metavolcanic rocks embedded in the Palaeozoic sequences of Sulcis, Sarrabus, and metagabbbros and metadolerites intruded into black metapelites at Nurra. Franceschelli et al. (2003) demonstrated that Nurra metagabbros emplaced during the extensional phase prior to Variscan continent-continent collision originated from an ocean island alkali basalt-like asthenospheric mantle enriched with incompatible elements but devoid of a crustal component.
A Middle Devonian episode (387 ± 2 Ma) with calc-alkaline acid products of orogenic affinity was reported by Garbarino et al. (2005) in southern Sulcis. However, further geochemical and geochronological data need to prove the existence of this episode.
During the Variscan tectono-metamorphic event, widespread plutonic activity (Di Vincenzo et al., 1994; Carmignani and Rossi, 1999) led to the formation of one of the largest batholiths in SW Europe. The intrusive sequence consists of: 1) an earlier syn-tectonic Mg-K calc-alkaline association (northwestern Corsica) emplaced at ~330-345 Ma; 2) a late- to post-tectonic high-K calc-alkaline association cropping out in Corsica and Sardinia (Rossi and Cocherie, 1991) emplaced from 310 to 280 Ma. 3) Peraluminous association. The high-K calc-alkaline late-tectonic intrusions range in composition from gabbro and diorite to leucogranite, whereas post-tectonic intrusions consist of leucogranites. According to Tommasini et al. (1995), the source of late orogenic Variscan gabbros is a subcontinental mantle contaminated by 5% of material subducted 450 Ma ago during a previous Ordovician cycle. The high-K calc-alkaline association also includes highly-peraluminous granitoids (Di Vincenzo et al., 1994 and references therein). The latter mainly consist of granodiorites and monzogranites, with minor tonalites and leucogranites. During the Upper Carboniferous-Permian, ignimbrites, lava flows and subvolcanic bodies were emplaced.