In the last ten years, a new, more complex geodynamic model has been proposed for the history of the Variscan Belt. It is based on the hypothesis that the "Hun Superterrane (HS)" (Stampfli, 1996, 2000), a ribbon-like assemblage of basement blocks, separated in Siluro-Devonian times from the northern Gondwana margin and collided in the Late Devonian-Carboniferous with Laurussia or Laurussia derived-fragments.
Major review papers on the HS model are those by von Raumer (1998), Stampfli et al. (2002) and von Raumer et al. (2002, 2003), to which the reader is referred for more details and bibliography.
The HS model proposes that Variscan orogeny took place between the Early Silurian to Early Permian, overprinting two previous orogenic cycles. The first cycle is that of Cadomian- Pan African orogeny (0.54-0.60 Ga), linked to the opening of the Iapetus Ocean, recognizable only in the Erzgebirge Massif (Saxo-Thuringian zone) and in the adjacent Lusatia plutons (Tichomirowa et al., 2001), in Miranda do Duro (Spain) (Lancelot et al., 1985) and Bormes orthogneiss, Maures (Maluski and Gueirard, 1978).
The Cadomian event was followed by a Neoproterozoic to Cambrian cycle that shows the persistence of an active continental margin setting, still preserved in the Alpine External Massifs and in Penninic and Austroalpine Nappes.
The surprising evolutionary parallelism between pre-Variscan orogenic cycles and the persistence of a volcanic arc parallel to the future Variscan belt in front of the Gondwana margin has led to the hypothesis that starting in Neoproterozoic times, an extremely long, ribbon-like alignment of microplates repeatedly detached from and collided with Gondwana.
At the Cambrian/Ordovician boundary, the appearance of bimodal volcanism, alkaline granites and rhyolites and within-plate basalts testify to the initial rifting stages of a thickened crust leading to the opening of the Rheic Ocean. This rifting event is recognizable in Vanoise (Briançonnais), in the Penninic nappes (Guillot et al., 2002) and in the Maures Massif (Seyler, 1983).
The Late Cambrian-Early Ordovician opening of the Rheic Ocean is revealed by several ophiolite associations occurring in the Alpine External Massifs and in some Austroalpine nappes (Schaltegger et al., 2002 and references therein).
Acidic magmatism from the same tectonic setting was active during the Middle to Late Ordovician. Metavolcanics and orthogneisses, once S granites and rhyolites, occur in Sapey, Ruitor gneisses (Briançonnais) (Guillot et al., 2002) and in the Argentera Massif (Rubatto et al., 2001).
Lower to Middle Ordovician (500-460 Ma) mafic-ultramafic rocks reveal ocean crust formation during the opening of the Rheic Ocean that separated Gondwana from the Hun Superterrane. The slight age difference between early mafic-ultramafic associations (496-459 Ma, Rubatto et al., 2001, Schaltegger et al., 2002 and references therein) and late to post-orogenic acidic magmatism (471- 443 Ma, Rubatto et al., 2001; Guillot et al., 2002) emphasizes the very short duration of the Rheic Ocean rifting episode and of the related Ordovician orogenic cycle.
A comparison of the Corsica-Sardinia microplate with previously-described HS microplates (Figure 13) reveals that, up to now, Corsica-Sardinia shows no clear evidence of a Cadomian-Pan African orogenic event (580-540 Ma).
Figure 13. Baltica and Gondwana
Geodynamic evolution between Baltica and Gondwana during the Early Paleozoic (modified from von Raumer et al., 2002). The paleogeographic position of Sardinia at the Gondwana margin and subsequently in the Hun Superterrane can explain the magmatic evolution from Early Middle-Ordovician calcalkaline magmatism to Late Ordovician-Silurian alkaline volcanism. 1: continental crust; 2: oceanic crust; 3: lithospheric mantle; 4: accretionary prism; 5: calc-alkaline volcanics; 6: calc-alkaline and tholeiitic intrusives; 7: alkaline volcanics.
In Sardinia, metabasite intercalations of Late Precambrian to Early Cambrian age are known in the Bithia Fm, consisting of a regressive terrigenous sequence which evolved on a continental margin (Carmignani et al., 2001 and references therein). According to Tucci (1983), these metabasites are metandesites originated during a Precambrian continental rifting event (Carmignani et al., 2001).
Cortesogno et al. (2005) reveal the existence within the External nappe zone (Gerrei, Goceano) of andesitic-dacitic lava flows having calc-alkaline affinity, associated with rhyolites. The latter, on a geochemical basis, are considered by the Authors as comparable to the Ligurian Gneiss amphibolite and Sardinia Gneiss Complexes of the inner zone (as proposed by Cortesogno et al., 2004). All these rocks are attributed by Cortesogno et al. (2005) to a "Neoproterozoic(?) Cambro-Ordovician rifting and ocean spreading event emphasized by "bimodal tholeiitic basalt and rhyolite volcanism". Actually, discarding the unlikely Neoproterozoic to Cambrian age of this event, Cortesogno et al. (2005) claim, at least, the existence of an Early Ordovician rifting event in pre-Variscan Sardinia.
Comparable but slightly older bimodal volcanism (the Leptyno-amphibolitic complex by French authors) characterises the pre-Variscan basement of the Maures Massif, Provence, France (Seyler and Boucarut, 1979; Seyler and Crevola, 1982; Seyler, 1983, 1986), a Variscan microplate with which most geodynamic reconstructions associate the Corsica-Sardinia microplate.
However, most metabasites and orthogneisses from Sardinia yielded protolith ages ranging from 470-460 Ma to 450-440 Ma (Cortesogno et al., 2004; Palmeri et al., 2004; Giacomini et al., 2005a; Helbing and Tiepolo, 2005). The Middle to Late Ordovician orthogneisses of northern Sardinia and the coeval metavolcanites known as "Porfiroidi" in southern Sardinia are generally considered, on geochemical and isotopic grounds, as calc-alkaline rocks, defined by Carmignani et al. (1994) as "Andean type" rocks.
The bimodal nature of some Ordovician magmatism has attracted the attention of Giacomini et al. (2005b), who attributed the "nearly coeval felsic and mafic magmatic rocks" to "an arc-back-arc setting of the Sardinia Corsica microplate".
A new Late Ordovician-Early Silurian detachment of the HS from the Gondwana margin caused the opening of the PaleoTethys Ocean and, northwards, the diachronous subduction of the Rheic Ocean crust below the various HS microplates The leading edge of the HS was represented by NW Iberia, the southern part of Armorica, the Massif Central, Vosges, Schwarzwald, Alpine External Massifs, Penninic and Austroalpine Nappes, Bohemian Massif and Erzgebirge. The southern passive margin (Stampfli et al., 2002; von Raumer et al., 2003) of the European Hunic Terranes includes the central-eastern Pyrénées, Montagne Noire, Alboran plate, Corsica-Sardinia, Maures, Adria plate, Carnic and Julian Alps, Dinarides and Hellenides.
From the Late Ordovician to the end of the Devonian, while the northern HS leading edge was affected by the subduction of ocean crust-producing eclogites (440-360 Ma), the southern passive edge underwent only extensional tectonics and rifting processes that supplied subvolcanic bodies of within-plate alkali basalt affinity (Ricci and Sabatini, 1978; Memmi et al., 1983; Di Pisa et al., 1992; Franceschelli et al., 2003). At the Devonian/Carboniferous boundary, the southern passive margin became active owing to northwards subduction of the Paleo-Tethys Ocean crust below the southern margin (see Stampfli et al., 2002, Figure 4).
The great uncertainty regarding the age of eclogite genesis in NE Sardinia is linked to a lack of reliable data on the metamorphic zircons involved in the HP event. However, the position of Sardinia within the group of microplates belonging to the southern passive margin of the HS during its early northwards migration towards Laurussia suggests that the HP event took place in Sardinia later than in central-northern Variscan microplates and, probably, when the southern margin changed into an active one. For this reason, the Early Visean age proposed by Giacomini et al. (2005a) seems to be plausible.
After the early HP metamorphic stage, a first Variscan phase of continental collision between the northern HS margin and Laurussia or Laurussia-derived microplates took place between 360-320 Ma, with peak metamorphism in the 340-320 Ma range (Bussy et al., 1996; Dobmeier, 1998; Giorgis et al., 1999; Bussy et al., 2000; Rubatto et al., 2001) at the end of the first Variscan deformation phase.
From the previous paragraph on Variscan metamorphic events and geochronological data in Variscan Sardinia (southern margin), a likely age of 355-335 Ma may be attributed to the first D1-M1 Variscan phase, with the attainment of the thermal peak at 340-335 Ma. These values are comparable to those obtained for the first Variscan phase in northern HS microplates. The complex evolution of Variscan magmatism may be described as follows.
The oldest evidence of continental collision has been found in southern Brittany and the Massif Central, where metamorphism and anatexis with granitoid emplacement is dated at 380 Ma ago (Matte, 1986). After this early granite magmatism, the first important magmatic cycle took place approximately between 350 and 330 Ma; it was characterised by high K-Mg contents interpreted as the signature of a wet subcontinental mantle contaminated by crustal and older subducted material. This magmatism is well known from the Bohemian Massif, through Schwarzwald, Vosges, the Massif Central, Tauern Window, Alpine External Massifs and Corsica (see von Raumer, 1998 and references therein), but not in Sardinia.
After the attainment of peak metamorphism at the end of the first deformation phase, subsequent rapid exhumation accompanied by the upwelling of a more primitive asthenospheric mantle and by the thinning of the crust and lithosphere gave rise to a second deformation phase and to HT-LP metamorphism in all Variscan basement blocks. This second HT-LP phase took place between 330 Ma and 300 Ma in the basement blocks of the northern leading edge of the Hun Superterrane.
In Sardinia, the second D2-M2 metamorphic event may be dated at 335-320 Ma (Di Vincenzo et al., 2004), comparable to the 330-300 Ma range of the event in northern HS microplates.
As regards the tectonic evolution of Variscan Sardinia, Elter et al. (1999) proposed the following reconstruction. After 350-344 Ma, the age of Barrovian metamorphism, marking the end of collision, extensional tectonics driven by mantle underplating, started in the axial zone, while compression was still active in the external one. Extensional tectonics migrated southwards in the Nappe and External zones, producing detachment along low-angle shear faults, with consequent exhumation of midcrustal rocks.
Late high-K calc-alkaline magmatism producing granites, granodiorites, diorites and rhyolites took place in the Alpine External Massifs between 307 ± 1 Ma (Mont Blanc, Bussy and Hernandez, 1997) and 292 ± 11 Ma (Gotthard, Oberli et al., 1981).
The roll-back of the Benioff plane in the late stages of Variscan orogeny strengthened the post-orogenic extensional tectonics favourable to the emplacement of late- to post-orogenic calc-alkaline to andesitic ignimbrites and lava flows in a "Central European Lower Permian Depression" (Benek et al., 1996).
This late to post-orogenic magmatism took place from the Late Permian (308 Ma, Aiguilles, Rouges, Capuzzo and Bussy, 1998) to Middle Permian (268 Ma, Dora Maira, Bussy and Cadoppi, 1996) in a wide region including Harz and the Alpine External Massifs, the Ligurian Briançonnais, Corsica and the Monte Rosa Massif.
The late- to post-orogenic high-K calc-alkaline magmatism of Sardinia and the post-orogenic one yielded respective age values in the range of 310-280 Ma and 290-280 Ma (Di Vincenzo et al., 2004), very similar to those obtained for the same type of magmatism in the Alpine External Massifs and in the Central European Lower Permian Depression (see above).
It must be emphasized that the Corsica-Sardinia batholith is one of the largest among those in the Variscan Belt (≈ 12000 Km2 and probably tens of thousands of cubic kilometres). The time interval of its genesis from early melting (migmatites of 344 ± 7 Ma age; Ferrara et al., 1978) to the emplacement of post-orogenic Early Permian igneous rocks (290-280 Ma) covers a range of 55-65 Ma, by far the longest life for a Variscan batholith.
The remarkable spatial and temporal dimensions that characterise the genesis of the Corsica-Sardinia batholith may be explained as follows. The southern edge of the Hun Superterrane, including Sardinia, was colliding with the huge Gondwana Supercontinent, far larger than the microplates (Lizard, south Portuguese, Moravo-Silesian, Harz, Giessen; Stampfli et al., 2002) which, after detachment from Laurussia, were moving against the northern HS leading edge. This difference in the dimensions of colliding plates on opposite sides of the Hun Superterrane could explain why, as compared with the batholiths generated within the northern HS edge, the Corsica-Sardinia one is characterised by remarkably greater volumes and longer gestation times.
According to Arthaud and Matte (1975), the latest orogenic tectonic phase (305-270 Ma) produced a conjugate system of strike-slip faults consisting of NW-SE dextral faults with displacement up to 50 km and of NE-SW sinistral faults. The first dextral movement of about 150-300 km brought Corsica-Sardinia from contiguity with Provence to a position adjacent to NE Spain, as proven by the parallelism between the Early Permian paleomagnetic declination of NE Spain and Sardinia. The Cretaceous opening of the Bay of Biscay by means of a 20° anticlockwise rotation of NE Spain and Sardinia along the north Pyrenean sinistral shear zone brought Sardinia near to the Maures Massif. Finally, an Oligo-Miocene anticlockwise rotation of 30° brought Sardinia to its present-day position.
It is our hope that this paper has provided a useful synthesis regarding Variscan Sardinia.