P-T estimates for the metamorphic rocks of the Stilo Unit (Aspromonte Massif, Calabria) and correlations with analogue Sardinian Variscan crystalline complexes.

The Variscan continental belt fragmented during Alpine tectonic phases is currently dispersed in the whole circum-Mediterranean area. The Sardinia-Corsica block (SCB) and Calabria-Peloritani Orogen (CPO) represent remnants of this chain. Although there is an agreement in the literature that they have been strictly connected for most of their tectonic evolution before the opening of the Tyrrhenian Sea, the corresponding arguments seem to be poor (Alvarez & Shimabukuro, 2009). The PT path here presented is a contribution to the knowledge of the Variscan Orogeny in the southern part of the belt, which has been intensely reworked by Alpine tectonics in the Calabrian region. The metapelitic sequence of the intermediate/upper crust of the Stilo Unit (SU) exposed in the Aspromonte Massif (southern Calabria) is weakly affected by Alpine deformation (Fazio et al., 2008) and well preserves the earlier Variscan tectono-metamorphic evolution. Moreover, the inferred PT path allows us to establish possible correlations with similar rocks exposed at different places of the southern European Variscides. In particular, a comparison has been made with similar metapelites in Calabria along the eastern and southern borders of the Serre Massif (Stilo Unit s.s.), as well as in northern Sardinia (Nurra and Baronie areas). Indeed, a similar metamorphic zonation in both crystalline basements of Calabria (Aspromonte and Serre massifs) and northern Sardinia (Franceschelli et al., 1982), showing a complete prograde sequence from the chlorite to sillimanite zone, has been recognised. The SU is the uppermost nappe of the Alpine edifice forming the Aspromonte Massif (Pezzino et al., 2008, Heymes et al., 2010) consisting of phyllites grading downwards (i.e., northward due to the southward tilting of the whole tectonic pile) to amphibolite-facies schists (Graeßner & Schenk, 1999). Occasionally, Cambrian to Carboniferous protholith ages have been documented (Bouillin et al., 1984). PT estimates of principal metamorphic episodes of the SU (Aspromonte Massif), inferred by means of PT pseudosections, suggest a steep clockwise loop (Fig. 1a). The path consists of an initial syn-D1 episode (M1: 360-600°C at 0.35-0.6 GPa depending on the metamorphic zone, varying from Chl to St+And zone) linked to a thickening phase producing folds associated to a pervasive axial plane foliation (Fig. 1b, S1); these early structures are successively affected by crenulations (M2: syn-D2s blastesis of Qtz, Wm, Chl, Bt). A syn-D3 shear phase (M3: 450-550°C – 0.5-0.9 GPa) probably coeval with the emplacement of granitoid bodies along extensional shear zones followed. A final (M4) static crystallization episode (Wm, Bt, sporadic Crd, and overgrowth of And on St) produces a clear metamorphic aureole in the host rocks testified by the widespread occurrence of spotted schists close to the intrusive bodies. According to the tectonic model proposed by Angì et al. (2010), this PT path shows strong similarity with the trajectory reconstructed for analogous crystalline rocks of the Serre Massif (central Calabria, Fig. 1a). Similar clockwise PT paths, typical of collisional belts, have been also depicted for the Variscan metamorphism of Sardinia. They have been attributed to late thermal relaxation following the initial thickening stage (Franceschelli et al., 2005), advising a rough correspondence with Calabrian metamorphic rocks. Focusing our attention on metapelitic sequences that show a strict compositional and metamorphic similarity with those of Calabria, we believe that the greenschist to amphibolite facies units exposed in north Sardinia (Nurra and Baronie areas) could be analogue candidates of the Calabrian SU rocks. The PT trajectories reconstructed for the Nurra area show very similar peak pressure conditions and a temperature range, from Chl- to And-zone, overlapping the SU thermal estimates; this suggests a possible correlation between them. In the Baronie area, except for higher pressure estimates (about 0.9 - 1.1 GPa), again the thermal range (420-580°C) is comparable with that of the SU (550°C ca.), supporting the hypothesis of a potential continuum between metamorphic zonation of the north-eastern Sardinia coast to that occurring in the southern Calabria Aspromonte region. Aiming at solving the question of the match between the Variscan geological history of Sardinia and Calabria, we attempted to compare existing PT paths (e.g., Carmignani et al., 2001; Carosi et al., 2004; Franceschelli et al., 2005, and references therein) for the SCB, as well as for the CPO (e.g., Graeßner & Schenk, 1999). Nevertheless, caution should be taken when such correlations are attempted, because PT estimates obtained by means of different techniques based on different thermodynamic datasets can be rather different, definitely weakening their consistency. In order to do this evaluation, we propose to carry out similar investigations on the Sardinian side using the PT pseudosection approach. PT estimates obtained with the same technique, based on the same thermodynamic database, will provide an improved accuracy and a good basis for future comparisons. Several further correlations based on different aspects (petrographic, mineralogical, structural, geological, geochemical and geochronological features) between the Calabrian and Sardinian sides can be made. In this view, regional scale shear zones of Sardinia (e.g. Posada-Asinara Line, PAL) should have their continuation on the Calabrian side. One of such shear zones has been partially studied (Fazio, 2005) in the metapelite sequence of the SU in the Aspromonte Massif, and future investigations could allow us to better compare it with Sardinian shear zones. The occurrence of HP key minerals could also give additional evidence of a similarity between the two exposed crystalline basements: Ky relics containing fibrolitic sillimanite in the Asinara migmatites (Oggiano & Di Pisa, 1998) have been also found within St-micaschists near Palizzi village (southern Calabria, Grande, 2008). Other analogies have been observed between structural features of Sardinian and Calabrian basement rocks. Three main Variscan deformation phases have been recognized in Sardinia (Conti et al., 1999), the principal ones are: D1a, b, which is a composite compressional phase, with early SW and later W-verging folds and associated S1a, b schistosity; D2, strike slip, either transpression and transtension, with S2 schistosity transposing S1 within NW-SE shear zones. Later, D3 produced kilometre-long antiformal structures with an axis roughly parallel to the orogenic trend; the very late stage of deformation (D4, extensional) was coeval with thermal doming and denudation of metamorphic core-complexes (Casini & Oggiano, 2008). The main deformational events recorded in the SU of the Aspromonte Massif (Fig. 1b) roughly resemble those observed within the Sardinian metamorphic rocks: compressional (D1) and shear (D3) events appear comparable to D1a, b and D2 Sardinian phases. A further folding episode (D4) possibly related to the late D3 Sardinian phase produces metre- to decametre-sized chevron folds. Further evidence for validating the supposed match between the two crystalline basements of northern Sardinia and southern Calabria could arise from a detailed study evaluating the petrological signatures of granitoid bodies outcropping in the north-eastern Sardinian (Giacomini et al., 2006) and western Calabrian coast (Rottura et al., 1993, Fiannacca et al., 2008). Moreover, a comparison between migmatitic complexes widely occurring north of the PAL (Posada-Asinara Line) in Sardinia and similar rocks outcropping near the village of Scilla (at the Calabrian Tyrrhenian coast) could furthermore support the hypothesis of a continuous Variscan metamorphic chain presently exposed in Sardinia and Calabria. In this contribution we presented only preliminary data and further studies and useful constraints, such as geochronological ones, are needed to prove such hypothesis making more forceful correlations between the margins of Sardinia and Calabria. Nevertheless, the PT estimates of a small portion of the Variscan belt presented here are a useful contribution to the reconstruction of its tectono-thermal evolution.