Variscan metamorphism in Sardinia, Italy: review and discussion

M. Franceschelli, M. Puxeddu, and G. Cruciani

The Sardinian segment of the Variscan chain is divided into four tectono-metamorphic zones from NE to SW: 1) Inner zone, with medium- to high-grade metamorphic rocks and migmatites; 2) Internal Nappe zone, with low- to medium-grade rocks; 3) External Nappe zone, with low-grade metamorphic rocks; 4) External zone, with very low- to low-grade rocks. Metabasite lenses, with eclogite and granulite relics, occur in the migmatites.

Pre-Variscan ages are attributed to the E-W trending folds below the Sardic unconformity (SW Sardinia). Five Variscan deformation phases have been distinguished, the principal ones being: D1, compressional, with SW-vergent folds and S1 schistosity; D2, extensional, with S2 schistosity transposing S1. Later, a composite network of post-D3, syn-D4 shear zones was active from an older HT/LP shear event to a younger MT-LT/LP one. The late D5 phase produced kilometric flexures with an axis parallel to the orogenic trend.

For Inner zone sequences, a complete prograde sequence has been recognised, from the chlorite zone to the sillimanite + K-feldspar one. The Nappe zone shows decreasing metamorphic grade from bottom to top of the Nappe pile, as well as in each single unit. The Variscan P-T-t path is the clockwise loop typical of continental collision. For the Barrovian stage, thermobarometers yielded a Ppeak of ≈1.2 GPa (end of D1) and Tpeak of ≈750°C (early D2). For HP/HT metamorphic relics, calculations indicated: eclogite stage, T ≈ 550°-700°C, P ≈ 1.3-1.7 GPa; granulite stage, T ≈ 650°-900°C, P ≈ 0.8-1.2 GPa; amphibolite stage: T ≈ 550°-650°C, P ≈ 0.3-0.7 GPa. Pre-Variscan igneous rocks consist of Lower to Middle Paleozoic metabasites with MORB or WPB affinity and Middle Ordovician calc-alkaline intrusive and effusive rocks.

The Corsica-Sardinia microplate belongs to the southern passive margin of the Hun Superterrane of the Swiss authors. The beginning of Variscan continental collision is marked by a probable eclogite-producing HP event. Ages of 355-335 Ma and 335-320 Ma for the first two Variscan metamorphic phases and 310-280 Ma and 290-280Ma for late-post orogenic Variscan magmatism in Sardinia match those of the same events in the northern Hun Superterrane active leading margin.

The Variscan Corsica-Sardinia batholith is made up of late-orogenic calc-alkaline intrusive rocks and post orogenic leucogranites. The greater volumes (surface area≈ 12000 Km2) and long gestation times of the Corsica-Sardinia batholith as compared to those of the northern Hun Superterrane batholiths is attributed to the collision between the southern Hun Superterrane margin, including Corsica-Sardinia, and the huge Gondwana Supercontinent.