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 Ordovician succession of the Sardinian basement. We focus on stratigraphy and tectonic structures in the tectonic units of External and Nappe zones of the Variscan basement. The Ordovician successions are characterized by unconformities related to tectonics events ascribed to the Sardic and Sarrabese phases. The different time gap of the unconformity-related gaps in the External (17 Ma) and Nappe (6 Ma) zones, the recent work on 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 palaeogeographic 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, on the contrary the high-grade metamorphic Inner Zone, where numerous Ordovician ortho- and para-gneiss, needs more detailed studies to define a complete framework of the Ordovician evolution of Sardinia. The present revision of data on the best-preserved succession of the Sardinian tectonic units suggests that at least two distinct terranes, that did not share the same Ordovician evolution, were amalgamated only during the Variscan Orogeny.