The Sardinian Variscan belt
The Variscan basement in Sardinia shows a prominent NW-SE trend (Carmignani et al., 1979; 1994 and references therein) characterized by nappes, tectono-metamorphic zoning and shortening similar to those developed in continent-continent collision type orogen.
It is composed of Carboniferous magmatic rocks and Cambrian – Lower Carboniferous igneous-sedimentary sequence with metamorphic grade increasing from South to North.
The collisional structural frame results in three different structural zones (Carmignani et al., 1979; 1982; 1994) (Figure 1):
i) a foreland "thrusts-and-folds" belt consisting of a metasedimentary sequence ranging in age from upper (?) Vendian to lower Carboniferous, which crops in SW part of the island, with a very low grade to greenschist facies metamorphic imprint;
ii) a SW verging nappe building (central Sardinia) which equilibrated mainly under greenschist facies conditions, consisting of a Palaeozoic sedimentary sequence bearing a thick continental arc-related volcanic suite of Ordovician age;
iii) an inner zone ("axial zone") (northern Sardinia and southern Corsica) characterised by medium- to high-grade metamorphic rocks with migmatites and by abundant late-Variscan intrusions.
According to Carmignani et al. (1994) the inner zone consists of two different metamorphic complexes:
A) a polymetamorphic high-grade complex made up of anatexites and metatexites hosting minor amphibolite bodies which equilibrated in HT-LP conditions which corresponds to the northernmost part of the island and extends to Corsica. In spite of this late re-equilibration, in places granulite relic assemblages of high-intermediate P and unknown age are still detectable (Miller et al., 1976; Ghezzo et al. 1979; Franceschelli et al., 1982; Di Pisa et al. 1993).
B) a medium grade, chiefly metapelitic complex, consisting of micaschists and paragneisses bearing kyanite±staurolite±garnet (Franceschelli et al., 1982) and including quartzites and N-MORB metabasalts boudins (Cappelli et al., 1992).
The contact between these two complexes is well exposed along the Posada Valley (Elter, 1987) as well as in Southern Gallura and Asinara Island (Oggiano & Di Pisa, 1992; Carmignani & Oggiano, 1997) and it coincides with a wide transpressive shear belt (Carosi & Palmeri, 2002) and it is affected by late Variscan shear zones (Elter et al., 1990).
Thrusting of complex A onto complex B has been inferred in places where the contact is not complicated by late Variscan retrograde dextral strike-slip shear (Oggiano & Di Pisa, 1992).
Within the collisional frame the high-grade migmatitic complex has been regarded as a crustal nappe comparable to the inner crystalline nappe of the French Massif Central and the high strained complex B has been regarded by Cappelli et al., (1992) as the Sardinia segment of the south Variscan suture zone which re-equilibrated under intermediate pressure amphibolitic conditions. As a matter of fact some of the metabasalts embedded within the high-strain kyanite bearing micaschists and in the high-grade complex retain clear relics of eclogitic assemblages (Miller et al., 1976; Cappelli et al., 1992, Oggiano & Di Pisa, 1992; Cortesogno et al., 2004; Giacomini et al., 2005).
However, the presence of a suture in northern Sardinia separating the low- to medium-grade metamorphic rocks of Gondwanian origin from the high-grade metamorphic rocks belonging to the Armorica microplate, as proposed by Cappelli et al., (2002) has been recently questioned by several authors mainly on the basis of the presence of ophiolites and presence of similar Ordovician orthogneisses and similar evolution both south and north of the Posada-Asinara Line (Helbing & Tiepolo, 2005; Giacomini et al., 2005, 2006; Franceschelli et al., 2005).
Moreover, Sardinia has been recently attributed by several authors to the “Hun Superterrane” (HS) (Stampfli et al., 2000, 2002; von Raumer, 1998; von Raumer et al., 2002, 2003; Franceschelli et al., 2005; Giacomini et al., 2006), a ribbon-like continent detached from the northern margin of Gondwana during Silurian-Devonian times. In this picture the main subduction of oceanic crust happened (below the Gondwana continent) at the northern margin of the HS (e.g. NW Iberia) during Late Ordovician to Devonian producing eclogites at nearly 440-360 Ma, whereas the southern margin (to which Corsica-Sardinia belonged) underwent extensional tectonics leading to the opening of the Paleo-Tethys. Only at the Devonian/Carboniferous boundary the southern passive margin of the HS became active with the subduction of the Paleo-Tethys crust northward below the southern margin of HS (Stampfli et al., 2002; Giacomini et al., 2006). During this stage continental crust in Sardinia underwent the main phase of southward migrating deformation and prograde Barrovian-type metamorphism.
In places this collisional frame is complicated by the occurrence of a neovariscan (300 Ma) HT-LP re-equilibration affecting both the metamorphic complexes (Del Moro et al., 1991; Oggiano & Di Pisa 1992). This late HT-LP metamorphic evolution has been related by Oggiano & Di Pisa (1992) to the post-collisional gravitative collapse of the chain, chiefly on the base of its age and of some meso and micro-structural evidences but, alternatively, it could be related to late Variscan intrusions.
The geochronological data in Nurra and in western Gallura the available Ar-Ar data on amphibole and muscovite yielded ages close to 350 Ma (Del Moro et al., 1991). In north-eastern Sardinia an upper limit to the collision-related metamorphism could be represented by the age of 344 ± 7 Ma (Rb-Sr age of isotopic exchange blocking among different compositional domains on a banded gneiss; Ferrara et al., 1978). More recent data yielded ~ 330 Ma (U-Pb dating on zircons; Palmeri et al., 2004), 350-320 Ma (U-Pb datings on zircons; Giacomini et al., 2006) and at 330-340 Ma (Ar-Ar on white micas; Di Vincenzo et al., 2004) for the collision related metamorphism. It is worth noting that U-Pb dating on zircons (Palmeri et al., 2004; Cortesogno et al., 2004; Giacomini et al., 2005) suggest a HP event bracketed between ~ 450 and ~ 350 Ma in the retrogressed eclogites of northern Sardinia.
The D2 transpressional deformation is constrained at 310-320 Ma (Ar-Ar on white micas on S2 foliation; Di Vincenzo et al., 2004). The upper limit of the age of the deformation is constrained by the crosscutting Carboniferous granitoids at ~ 290-311 Ma (Rb-Sr whole rock isochron; Del Moro et al., 1975).
The structural and metamorphic evolution of the inner zone is well exposed along the three transects provided in the field trip (Figure 1 and Figure 2).
Figure 2. Geological cross sections
Geological cross sections (1-4) throughout the Variscan basement of northern Sardinia in the three study areas (see Geological map of Northern Sardinia, Carosi et al., 2005).
1- Variscan granitoids; HGMC: High-Grade Metamorphic Complex:
2- Migmatites;
3- High-grade mylonites; MGMC: Medium-Grade Metamorphic Complex:
4- Eclogites relics;
5 and 6- Orthogneisses;
7- Augen gneisses;
8- Metasedimentary complex in amphibolites facies;
9- Metasedimentary complex with HT-LP metamorphic overprinting;
10- Mylonitic micaschists in amphibolites facies;
11- Phyllonite belt; L-MGMC: Low- to Medium Grade Metamorphic Complex:
12, 13 and 14- Internal Nappe;
15- External Nappe.
(Modified after Carmignani et al., 1979; Oggiano & Di Pisa, 1992; Carosi & Oggiano, 2002; Carosi & Palmeri, 2002; Carosi et al., 2004).
The D1 collisional event is well-recorded in the study transects producing SW facing folds, top to the S and SW shear zones and the main fabric in the low- grade metamorphic rocks in southern part of the section (Carmignani et al., 1979; Simpson, 1989; Franceschelli et al., 1990; Carosi & Oggiano, 2002; Montomoli, 2003).
Recent structural investigations highlighted the occurrence of a regional D2 transpressional deformation related both to NNE- SSW direction of compression and to a NW-SE shear displacement (Carosi & Oggiano, 2002; Carosi & Palmeri, 2002; Carosi et al., 2004, 2005). The deformation is continuous and heterogeneous, showing a northward strain increase, indicated by progressively tighter folds, occurrence of F2 sheath folds and vorticity analysis. D2 transpression is characterized by the presence of a crustal-scale shear deformation overprinting previous D1 structures, related to nappe stacking and top-to-the S and SW “thrusting”. The L2 prominent stretching lineation points to an orogen-parallel extension and to a change in the tectonic transport from D1 to D2 (Figure 3). Orogen-parallel extension could be attributed to the position of Sardinia close to the NE part of the Cantabrian indenter during the progressive evolution of the Ibero-Armorican arc (Carosi et al., 1999; Conti et al. 2001; Carosi & Oggiano, 2002; Carosi & Palmeri, 2002) or to a general progressive curvature of the belt, as well as to the presence of an irregular collided margin. It has been suggested that D1 phase developed during initial frontal collision whereas D2 deformation characterized the progressive effect of horizontal displacement during the increasing curvature of the belt. The Nurra-Asinara transect is a clear example of heterogeneous transpressional deformation partitioned in the space (Carosi et al., 2004; Iacopini, 2005). We have detected a switch in the attitude of L2 stretching lineation, going northward, from nearly sub-horizontal and parallel to A2 fold axes to down-dip in the northern part of the Asinara Island according to theoretical models of transpression proposed by Tikoff & Teyssier, (1994).
Figure 3. Stereographic projections
Stereographic projections (Schmidt equal area projection, lower hemisphere) of the main structural elements in the study areas (from Carosi et al., 2005).
A. Nurra-Asinara zone; B. Anglona-SW Gallura zones; C. Baronie zone. A2: axes of F2 folds; S2: second phase schistosity; L2: stretching lineation. A3 and A4 are referred to later fold axes.