The Ordovician of Sardinia (Italy): from the ‘Sardic Phase’ to the end-Ordovician glaciation, palaeogeography and geodynamic context

This review illustrates the most important features of the Ordovician successions of the Sardinian basement. We focus on the stratigraphy and tectonic structures in the tectonic units of the External and Nappe zones of the Variscan basement. The Ordovician successions are characterized by unconformities related to tectonic events ascribed to the Sardic and Sarrabese phases. The different durations of the unconformity related gaps in the External (17 myr) and Nappe (6 myr) zones, recent work on the trilobite fossil content, and the occurrence of a volcanic arc only in the Nappe Zone (Sarrabus and Gerrei units) highlight significant discrepancies suggesting that these domains did not share the same geodynamic setting and palaeogeographical position during the Ordovician. This implies they were amalgamated only in Variscan times. Whereas for the external and nappe zones the Ordovician features are clear, the high-grade metamorphic Inner Zone, where numerous Ordovician ortho- and para-gneiss occur, more detailed studies are needed to define a complete framework for the Ordovician evolution of Sardinia. The present revision of data for the best-preserved succession of Sardinian tectonic units suggests that at least two distinct terranes, which did not share the same Ordovician evolution, were only amalgamated during the Variscan Orogeny.