Contact metamorphism in Middle Ordovician arc rocks (SW Sardinia, Italy): new paleogeographic contraints

In the early Cambrian Bithia Formation in the Variscan foreland of Sardinia, a Middle Ordovician granitic intrusion (478–457 Ma) is hosted by marly metasedimentary rocks that were affected by high-temperature (HT) metamorphism. A detailed structural–petrographical transect was conducted through the granitic intrusion and its host rocks. Field data and relationships between HT/low-pressure (LP) mineral assemblages in the metasedimentary rocks (Grt + Wo + Ves in carbonate lenses and And in pelite) demonstrate that the study area was affected by a polyphase HT overprint (I: T = 520–620°C at XCO2 = 0.1, P: 0.2–0.4 GPa; and II: T = 600–670°C at XCO2 = 0.1, P = 0.2–0.4 GPa) that pre-dates the Variscan tectonic, metamorphic, and igneous phases. In the Canigò or Canigou Massif (Eastern Pyrenees), the Somail Massif (Montagne Noire), and the Ruitor Massif (Internal Massifs, NW Alps), Middle Ordovician orthogneiss with relict igneous textures are deciphered despite being overprinted by Variscan amphibolite-to-granulite-facies metamorphism and subsequent Alpine low-grade metamorphism. Comparisons of associated igneous and metasedimentary rocks in the Sardinia foreland with the High-Grade Metamorphic Complex in the Variscan Axial Zone and the Canigou Massif indicate a convergent Middle Ordovician evolution that was overprinted by HT Variscan metamorphism.