Franceschelli, M., Puxeddu, M. and Cruciani, G. 2005.   Variscan metamorphism in Sardinia, Italy: review and discussion. In: (Eds.) Rodolfo Carosi, R Dias, David Iacopini, and Gideon Rosenbaum, The southern Variscan belt, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 19, paper 2, doi:10.3809/jvirtex.2005.00121

Variscan metamorphism in Sardinia, Italy: review and discussion

M. Franceschelli

Dipartimento di Scienze della Terra, Università di Cagliari, Via Trentino 51, 09127 Cagliari, Italy


M. Puxeddu

Istituto di Geoscienze e Georisorse, CNR Pisa, Via Moruzzi 1, 56124 Pisa, Italy

G. Cruciani

Dipartimento di Scienze della Terra, Università di Cagliari, Via Trentino 51, 09127 Cagliari, Italy


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.

Keywords: Sardinian chain, metamorphic evolution, P-T-t path, tectonic setting, Variscan Orogeny