The Variscan basement in Sardinia is characterized by a polyphase deformation and metamorphic history. In northern Sardinia the main pervasive foliation at the regional scale is the S2 foliation that increases in intensity moving from south to north (Franceschelli et al., 1982; 1989; Carmignani et al., 1994). The oldest S1 foliation is clearly observable only at the outcrop scale in the southern portion of the Internal Nappes whereas going northward, it was progressively transposed by the D2 deformation phase (Fig. 3) and it is recognizable only as relics at the microscopic scale. Subsequently, two systems of later folds (F3 and F4) affected the S2 foliation.
Figure 3. Geological cross section
Geological cross section (1-4) throughout the Variscan basement of northern Sardinia in the three study areas. Same legend as in Figure 1. 1: Asinara Island, 2: Nurra region; 3: Baronie region; 4: SW Gallura region; HGMC: High Grade Metamorphic Complex; MGMC: Medium Grade Metamorphic Complex: L-MGMC: Low to Medium Grade Metamorphic Complex (Modified after Carmignani et al. (1979), Di Pisa & Oggiano, (1992), Carosi & Oggiano, (2002), Carosi & Palmeri, (2002), and Carosi et al., (2004)).
The D1 collisional event is well-recorded in the low-grade metamorphic rocks of the southern portion of the Nurra-Asinara transect, where it is associated with the development of an S1 foliation, axial plane of SW facing folds and shear zones (Carmignani et al., 1979; Franceschelli et al., 1990; Simpson, 1999; Carosi & Oggiano 2002; Montomoli, 2003; Carosi et al., 2004) (Figs. 2 and 3).
Moving to the north, D2 strain increases and completely transposes S1 foliation and related structures (cross-section n. 2 in Fig. 3). In southern Nurra, F1 folds are meter to decimeter in size and show variable opening angles (from 30° - 40° up to isoclinal). They show thickened hinges and stretched limbs and commonly belong to class 2 of Ramsay (1967).
A low grade S1 foliation, with syn-kinematic crystallization of quartz, muscovite, paragonite, chlorite and oxides (Carmignani et al., 1979) develops parallel to F1 axial planes..
Late D1 ductile/brittle shallowly dipping shear zones have been recognized in phyllites and quartzites (Simpson, 1999; Montomoli, 2003). Shear planes strike from N130 to N150°E and dip 30-40° to the NE (Fig. 2). C-S fabric indicates a top-to-the S and SE sense of shear. Late D1 shear zones are affected by S2 crenulation cleavage and they are characterized by abundant sigmoidal quartz veins. According to Simpson (1999), quartz veins are due to dehydratation reactions during prograde metamorphism. During non-coaxial deformation they assume a sigmoidal shape.
Fluid inclusions trapped in quartz veins along secondary trails oriented both at high and low angles with respect to the shear zone boundaries, highlight the presence of aqueous carbonic fluids (Montomoli, 2003). Bulk composition and densities, calculated on the basis of microthermometric and Raman data, point out to two main types of fluid inclusions: a first type is characterized by density values ranging between 0.84 and 0.87 g/cm3 while a second type has density values between 0.88 and 0.92 g/cm3 (Montomoli, 2003). The computed isochores for both fluid inclusion types do not match the P-T peak conditions proposed for the area by Franceschelli et al., (1990), suggesting that fluid inclusions have been trapped or at least re-equilibrated during a retrograde metamorphic pattern. In particular, during their trapping, the host rocks experienced a lowering pressure with respect to the peak metamorphic conditions estimated for the D1 tectonic phase (Franceschelli et al., 1989, 1990).
The decreasing pressure values at the time of late D1 shear zones activity suggest that during crustal stacking the nappe pile continued to undergo overall compression causing the development of shear zones and allowing the exhumation of the hanging wall rocks.
Going towards the northernmost part of the study sections, D2 deformation shows a gradual strain increase up to the complete transposition of sedimentary bedding and S1 foliation (Fig. 3). S1 foliation is recognized only at the microscopic scale in D2 microlithons or as inclusion trails in post-D1 Barrovian porphyroblasts such as: plagioclase, biotite, garnet, staurolite and kyanite.
Mineral crystallized along the S1 foliation allowed to constrain the P-T conditions during prograde metamorphism. Moreover, syn-D1 celadonite-rich white mica, inside plagioclase in the garnet zone, allowed Di Vincenzo et al., (2004), to date the S1 foliation in northern Sardinia at 330-340 Ma.
This tectonic phase is characterized by a heterogeneous deformation affecting the previous syn-collisional fabric. D2 deformation is partitioned into domains alternating prevailing folding and shearing deformation (Carmignani et al., 1979; Simpson, 1999; Carosi & Oggiano, 2002; Carosi et al., 2004). This is particularly well recognizable along the Nurra-Asinara transect.
In low strain areas, to the south, D2 deformation is characterized by a crenulation cleavage axial planar of F2 folds. Moving northward, approaching the Posada-Asinara Line strain is predominantly characterized by non-coaxial deformation giving rise to the development of protomylonites, mylonites and phyllonites in which folding is only a secondary effect.
The F2 folds are well-detectable in the southern portion of the Nurra peninsula and in the southern part of the Baronie area. The Nurra-Asinara transect offers good outcrops showing the progressive development of the D2 deformation (Figs. 2 and 3) (Carmignani et al., 1979; Simpson, 1999; Carosi & Oggiano, 2002; Carosi et al., 2004).
The D2 deformation is characterized in the Nurra- Asinara and Anglona-Gallura zone by F2 folds with E-W to NW-SE trending axes (Figs. 4) and S to SW moderately to steeply dipping axial planes.
In the Nurra area, the F2 folds have centimetric to decimetric size and a well-developed S2 axial plane foliation that represents the main planar element recognized in the area. The F2 fold vergence is toward the north, which is opposite to the vergence of F1 folds (Fig. 3). In the Nurra area the F2 similar fold geometry changes from open (in the south) to isoclinal (in the north), with interlimb angles decreasing from 60° to 1-2°, whereas the fold geometry (Ramsay, 1967) varies from class 1C to class 3 (see Carosi et al., 2003). Sheath folds have been detected in central and northern Nurra (Carosi & Oggiano, 2002). In central Asinara Island F2 fold geometry varies from class 1C to 2. A2 axes trend mostly NW-SE plunging to the SE in the Nurra area and to the NW in the Asinara Island (Fig. 4a).
Figure 4. Stereographic projections
Stereographic projections (Schmidt equal area projection, lower hemisphere) of main structural elements for the study areas (a: Nurra-Asinara zone; b: Anglona-SW Gallura zones; c: Baronie zone). A2: axes of F2 folds; S2: schistosity; L2: stretching lineation. A3 and A4 are referred to later fold axes.
In the Tula-Erula area (Anglona region; Fig. 2), the D2 deformation phase is highlighted by rare F2 isoclinals folds with axis scattering from NW-SE to NNE-SSW with a maximum in the N-S direction (Fig 4b). Their plunge is less than 30 degree mainly to NNW (Fig. 4b).
In the Badesi-Giagazzu area (SW Gallura region; Fig. 2) the F2 axis orientation shows a NW-SE trend, plunging 10-20° toward both the SE and the NW (Fig. 4b). In this area from SW to NE, it is possible to observe a strain increase causing different F2 fold geometries. The F2 folds are chevron and kink in the SW region, whereas in the phyllonitic zone (toward NE), they become isoclinals (Fig. 5) with parallel limbs.
Figure 5. F2 tight folds in phyllonites
F2 tight folds in phyllonites from SW Gallura close to the boundary between the Medium Grade Metamorphic Complex and the High Grade Metamorphic Complex.
In the Baronie zone the F2 fold axes are still parallel to the stretching lineation but show a different trend with a mean ENE-WSW strike direction. F2 folds have moderately dipping axial plane (Fig. 4C).
In the northwestern areas the strike of S2 foliation ranges from NW-SE to WNW-ESE, dipping 45-50° both to the NE and the SW (Fig. 4). In the Nurra-Asinara island, within the phyllite complex, the S2 foliation is a spaced crenulation cleavage and is composed by white mica, chlorite and biotite. In paragneiss and micaschist it is defined by quartz, muscovite, biotite, chlorite, oxides, garnet and albite/oligoclase porphyroblasts and show a transition to a mylonitic foliation defined by ribbon quartz.
In the Badesi-Giagazzu area (SW Gallura region; Fig. 2), the S2 foliation shows a transition from an F2 axial plane foliation, in the SW area, to a S2 mylonitic foliation in the phyllonites level. In this area S2 strike from NE-SW to NW-SE and dips to the North.
In the Baronie zone, the S2 foliation shows NE-SW direction, which is moderately to steeply dipping toward the NW (Fig 4c).
In the all the northwestern and central area, the L2 object lineation (sensu Piazolo & Passchier, 2001 and reference therein) is represented both by grain and stretching lineations. It is well defined in the gneisses and quartzites and it trends N100-N170 (Fig. 4), parallel to F2 fold axes and plunges a few degrees toward the NW and the SE. In the Baronie zone it mostly trends E-W (Fig 4c).
In fine-grained and porphyroblastic paragneiss, mantled porphyroclasts with sigmoidal and delta-type geometry, mica-fish, shear band cleavage and symmetric - structures have been observed in sections parallel to the lineation and orthogonal to the foliation plane.
The S2 mylonitic foliation is generally steeply dipping and has been folded by later folds so that it dips to the NE in the Baronie and Nurra section, whereas it dips to the SW in Anglona-Gallura and Asinara Island (Figs. 2 and 3). In the Nurra peninsula, between Isola dei Porri and Punta Falcone, sheath folds have been observed associated with top to the NW shear sense indicators. In Asinara Island up to the Cala d'Oliva orthogneiss, the D2 microstructures are poorly preserved because of a HT/LP overprint, and only locally micafish and shear band cleavage have been observed, confirming a top-to-the NW shear sense. In the Baronie and SW Gallura areas, mylonites with top-to-the SE sense of shear are the prominent D2 features.
However, in the SW Gallura NNW-SSE striking shear zones with a sinistral sense (top-to-the NNW) of shear have been recently detected in the migmatites and migmatite gneisses of the High Grade Metamorphic Complex (Fig. 7) (Carosi et al., 2005). The relation between dextral and sinistral shear zone is not clear in the field but they point to a more complex shear zone evolution in this sector of the belt.
Figure 6. S-C' fabric in mylonites from SW Gallura
S-C' fabric in mylonites from SW Gallura (MGMC); dextral sense of shear: a) outcrop view; b) Shear deformation affects Barrovian mineral assemblage in the MGMC (CP, field of view nearly 2 mm).
In the metatexitic complex (HGMC) in the Asinara Island, S2 foliation is steeply dipping and strikes between N160° and N180° (as indicated by a maximum in S2 plot within Fig. 4a), whereas in the central and northern Asinara, the mean direction is of N 110-120° (Fig. 4a). In the central area within the andalusite, sillimanite bearing micaschist complexes, the foliation becomes a mylonitic fabric defined by both ribbon quartz and microlithons indicating a top-to-the-SE sense of shear.